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#1
I posted this in the wrong forum at first, but we're good now

I bought a used 6505+ 112 for $200 about a year ago,and I've been using it as my project amp, experimenting with caps and resistors for different tones, eventually creating an actually decent clean channel with a variety of mods found online. However, I still feel like it isn't right. Paired with my Blackstar 2x12 it just doesn't have that percussive WHOMP I'm looking for (blame that on my recent Metallica phase). I don't use the internal speaker (it just sucks), and with my old JCM900 4x12 cab (since sold) this was slightly better, but still not quite. This is especially noticeable going from clean to dirty (the clean has so much low end that the lead channel just sounds thin). I know part of this is due to, well, playing a 6505, in addition to using a 2x12 cab (I justify this due to a bad back). One solution is converting the open-back cab to a closed-back, which should help with some low-end, but this is only one piece of the equation, which brings me to my next point:

It's common knowledge that the 5150/6505 line of amps were based on the legendary Soldano SLO-100. I also read somewhere that the early Mesa Dual Rectifiers were based on this design as well. After looking at schematics, I was shocked to see just how similar in design these preamps really are. The tonestacks--save for the mid frequencies, and one cap and resistor--are virtually identical to one another. The older 2-channel Dual Recs are REALLY close to the SLO, especially the lead channels (the only main differences being the FX loop placement BEFORE EQ on the Soldano, and the 680pF cap on the Mesa's treble pot vs. the 470pF on the SLO). The Peavey, of course, has some different gain staging, and the input stage is slightly different, which I feel is where the more-compressed, tighter tone is coming from in part).

For example, from the input on the 6505+ (combo and head), you see R9 (a 10K resistor), C22 (a 39pF capacitor), C19 (.1uF cap), R55 (1meg resistor), and R2 (68K resistor). The SLO only has the 1meg and 68K resistors. The Mesa only has the 1meg. I've heard that removing C19 and R22 can improve dynamics and picking response. Another example would be C5 on the 6505+ 112, which is a 470pF cap. On the SLO and original 5150, its a .0022uF cap and 470K resistor in parallel, which results in more low end and a thicker voice than the 470pF cap in the 5150II/6505+. The Mesa has the .0022uF cap with a 680K resistor in parallel, which I hypothesize in theory should result in even more low end than the SLO (I could be wrong). Changing these areas to Mesa specs could make the tone and response of this amp less compressed and more organic.

Don't be fooled here, I'm not trying to turn a 6505+--much less the 112 combo--into a Dual Rectifier (it simply won't happen); rather, it's more of an attempt to land somewhere between the two. We all know that a large part of the Recto tone is what happens after the preamp. I will be doing the mods in about 1-2 weeks, when all the components are here. Who else is interested in this, that way I can detail everything I do for my fellow UGers? Sharing is caring, after all.
Last edited by dkennedy88 at Jun 13, 2016,
#2
KailM will be with you shortly. Please hold the line.

EDIT:

Another example would be C5 on the 6505+ 112, which is a 470pF cap. On the SLO and original 5150, its a .0022uF cap and 470K resistor in parallel, which results in more low end and a thicker voice than the 470pF cap in the 5150II/6505+.

I've done this exact mod to my own 6505+ and it does add quite a lot more bass end. Palm mutes are especially more percussive and it's something you feel more than you hear. I'd try running the 0.022 cap and 470K resistor in parallel with C5 and see if you like it. It's a very simple mod.

All the other little mods that you've mentioned are all very simple and reversible. So there's no harm in trying all of them to see what you like.
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Last edited by T00DEEPBLUE at May 23, 2016,
#3
Quote by T00DEEPBLUE
I've done this exact mod to my own 6505+ and it does add quite a lot more bass end. Palm mutes are especially more percussive and it's something you feel more than you hear. I'd try running the 0.022 cap and 470K resistor in parallel with C5 and see if you like it. It's a very simple mod.

All the other little mods that you've mentioned are all very simple and reversible. So there's no harm in trying all of them to see what you like.


I have the 470K with .0022 cap right now, and it does help. I did a lot of the Atomium Amps mods to the amp, using what I could to translate from the 6505+ head to the 112. Look for the mods on Tumblr to show you most of what I did. I didn't do all of these. I'm still not happy with the tone. It's almost too middy and not thick-sounding enough. Which is why I have this idea. I plan to mod as much of the preamp to Boogie specs as I can without gutting the amp. That way, if I don't like it, I can always revert the lead channel back to stock, or close to it. I feel there is minimal risk, considering I only paid $200 for this amp, and if it doesn't work out, I may just buy a Mark III or an early Rec. Part of my mods include switching the mid pots on both channels to 20K linear (lead is 50K, rhythm is 10K). The Mesa spec is 25K but I can't find one that will be a direct drop-in, so 20K it is. It would also be replacing the cap on the lead treble pot from 470pf to 680pf, and changing one resistor on the rhythm channel (which one I can't recall without the schematic in front of me), then removing a resistor in the rhythm channel. Other than those four things, the tonestacks on the 6505+ is identical to that of a '93-ish Dual Rec.

Past that, I think removing a lot of the extra caps/resistors immediately after the input should cause the signal to be less compressed and thus allowing more low end. I also plan to bias V4 a bit hotter, hopefully it tames some of the pissed-off killer bee fizz.

I ordered all the caps and resistors today, should be here Wednesday, ordering pots within the week.
#4
I think in light of those mods you're doing, if you still don't like how the amp sounds, then the problem is that you simply aren't dialing it in right imo.

I find one thing that is immensely helpful is an MXR 10-band in the effects loop. It pretty much solves the problem of controlling the midrangey nature of the amp.
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Last edited by T00DEEPBLUE at May 23, 2016,
#5
Quote by T00DEEPBLUE
I think in light of those mods you're doing, if you still don't like how the amp sounds, then the problem is that you simply aren't dialing it in right imo.

I find one thing that is immensely helpful is an MXR 10-band in the effects loop. It pretty much solves the problem of controlling the midrangey nature of the amp.


Could be. It also could be that I like to f--- with things. Could be both. Haha.
#6
I can agree with that. I fuck with this amp as well.
you see R9 (a 10K resistor), C22 (a 39pF capacitor), C19 (.1uF cap), R55 (1meg resistor), and R2 (68K resistor).

I've heard that removing C19 and R22 can improve dynamics and picking response.

I've only just noticed this, but do you mean C22, not R22? R22 is a big 100K resistor coming from the second half of V4.
Part of my mods include switching the mid pots on both channels to 20K linear (lead is 50K, rhythm is 10K). The Mesa spec is 25K but I can't find one that will be a direct drop-in, so 20K it is.

A much, much easier way of achieving what you're trying to do (with the lead channel at least) is soldering a 47K resistor in parallel with R48 instead of replacing the pot.
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Last edited by T00DEEPBLUE at May 23, 2016,
#7
Oops... It's R22 on the original 5150 head. On the 112, it's R2. That was a typo. The resistor in question in a 68K resistor in the input.

What I plan on doing is removing everything on that input except R55, to mirror what the signal sees in the input of the Mesa.
Last edited by dkennedy88 at May 23, 2016,
#8
R2? Are you sure? If that's removed, how is the signal supposed to go through to the grid of V1? Or are you putting a jumper in where R2 once was?

Not sure if you're still confusing the combo schematic for the head.
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Last edited by T00DEEPBLUE at May 23, 2016,
#9
It is R2, the 68K grid resistor. If you look at this excerpt from the 112 schematic, you see this in the input:



The plan is to bypass R9, C19, and R2 with jumpers and remove C22, then keep R55 in there to match what is shown in the input of this schematic for the Dual Rec:



I may keep R2 and R55, which is the SLO spec. Next thing: I noticed that on the Recto schematic there is a 47K resistor in series with the 1uF at C31, which is in parallel with a 1.82K resistor (R291). This is coming from pin 3 of V1A. This is also shown on V2A and V3A. The Peavey just has the 1uF in parallel with the 1.82K resistor. What does this do? I see it's on a relay.
Last edited by dkennedy88 at May 23, 2016,
#10
Thanks for clarifying. I might just try this myself and see if I like it.
Next thing: I noticed that on the Recto schematic there is a 47K resistor in series with the 1uF at C31, which is in parallel with a 1.82K resistor (R291). This is coming from pin 3 of V1A. This is also shown on V2A and V3A. The Peavey just has the 1uF in parallel with the 1.82K resistor. What does this do? I see it's on a relay.

I'm afraid I wouldn't have a clue. I'm not familiar with Recto circuits.
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Last edited by T00DEEPBLUE at May 23, 2016,
#11
Also, note R110 in the Recto schematic. it's a 680K resistor in parallel with C9, which is a .0022uf cap. It's similar to C5 in the 112, where the common mod (that you and I have both done) is to replace that with the .0022uf cap with 470K resistor, which is the old 5150 and SLO spec. Wonder why Mesa went with the 680K resistor...more low end maybe, my guess. I'm also not familiar with Mesa circuits, but other than this, and some of the other details I've pointed out, I was surprised to see the similarities in the preamps.

EDIT: Wait, wait, wait. I think the 47K resistor in series with the 1uF cathode cap is to slightly shift the bypass frequencies downward in the circuit, my sources on this being the WarpedMusician blog. The LDR just switches the cap on or off depending on channel (in this case, always on unless you were on the Orange channel in clean mode).
Last edited by dkennedy88 at May 23, 2016,
#12
Aye, it is quite interesting to see the exact same thing be represented on a different amp.

Thank you so far for providing me so many options for modding this amp. It's given me plenty of things to look forward the next time I come home from work.
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#13
Quote by T00DEEPBLUE
Aye, it is quite interesting to see the exact same thing be represented on a different amp.

Thank you so far for providing me so many options for modding this amp. It's given me plenty of things to look forward the next time I come home from work.


Any time. I kinda went into mad scientist mode yesterday, and pretty much spent the whole day looking at the schematics for the 6505+ 112, 6505+ head, 5150 head, SLO, and Dual Recto, trying to see what was different in each. Figured I'd share to some extent with the class

I currently have an old PODxt in my loop for delays, reverbs, among other things, such as a master volume while practicing at home (so I can push the preamp tubes at low volumes!), and for the 4-band parametric EQ, which I have set to boost the super low frequencies. I'm trying to get out of this, as for some reason this sounds artificial. I think I may just have to break down and get some T75s for my cab. That speaker was the tits in my Marshall cab.
Last edited by dkennedy88 at May 23, 2016,
#14
IIRC your current cab is open-backed? Sealing the back of that cabinet is going to dramatically increase the bass end. Even doing something as simple as backing your cabinet against a wall is going to make the cabinet sound more like it's a closed-back. I'd really strongly recommend you try that first before going much further because it's going to make your cabinet sound completely different and these high gain amps were built with running a closed-back cabinet in mind anyway.

And presumably you've already considered modding the effects loop?
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#16
Quote by T00DEEPBLUE
IIRC your current cab is open-backed? Sealing the back of that cabinet is going to dramatically increase the bass end. Even doing something as simple as backing your cabinet against a wall is going to make the cabinet sound more like it's a closed-back. I'd really strongly recommend you try that first before going much further because it's going to make your cabinet sound completely different and these high gain amps were built with running a closed-back cabinet in mind anyway.

And presumably you've already considered modding the effects loop?


My cab is open-back with Seventy 80s.

Here is a comprehensive list of mods I have done thus far:

FX LOOP:
--.022uf cap in parallel with 100k resistor at R86, to make FX loop sound more transparent, even though it really isn't.

RHYTHM CHANNEL:
--Replaced 6.8nf cap at C36 with a 100pf (removes boxy sound from rhythm channel at lower gain settings, thus enabling better cleans).
--Replaced 470K at R91 with a 1meg, 6.8K at R69 with a 10K, and 68nf at C25 with a .047uf to counteract volume loss from the above C36 mod.
--Removed 47K at R13 for more bass punch in cleans (may put a 33K in instead. There's almost too much low end on the cleans).
--Replaced .022uf at C34 with a .001uf for a slightly tighter crunch.
--Removed 100pf cap at C47 to, along with the above C34 mod, causes the crunch mode to better resemble that of a Marshall.
--Replaced the 2.2K resistors at R52 and R53 with 2K to make these preamp tubes run hotter.

LEAD CHANNEL:
--Removed .001uf cap at C30 (the Warren Haynes SLO mod, if you will).
--Replaced 470pf at C5 with a 470K and .0022uf in parallel.
--Replaced .001uf at C26 with a .022uf. Both this and the above C5 mod are standard 5150/SLO/Dual Rec values.
--Replaced the 37.4K at R12 with a 39K; again, standard values of the previously mentioned 3 amps.
--Replaced 33K at R25 with a 47K and two 330pf caps, all in parallel, for thicker mids. I don't like this much and will go back to stock value.

MISC:
--Replaced 6.8nf caps at C37 and C74 with .01uf caps to shift the resonance controls downward. Don't like this either, sometimes you barely notice it's even there.
--Replaced 220K at R14 with 200K. Makes power tubes run hotter.
Last edited by dkennedy88 at May 23, 2016,
#17
Quote by Ippon
Step-by-step details with lots of pics please.

I have one of those still in the box when they closed out for $300. The 6505+ head is a beast as is.


I may not take pictures per se, as I lack a camera that will do it justice. However, I probably will post pictures of the schematics and PCB layouts to help along.
#18
Quote by dkennedy88
My cab is open-back with Seventy 80s.


I'd start with some different speakers and close that bad boy in before modding the amp. . . .

How about some V30's or clones?

Or a K100 and a V30 if you want that massive bottom end.

Or just pickup a Mesa Rectifier 2x12. . .
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#19
Quote by metalmingee
I'd start with some different speakers and close that bad boy in before modding the amp. . . .

How about some V30's or clones?

Or a K100 and a V30 if you want that massive bottom end.

Or just pickup a Mesa Rectifier 2x12. . .


The whole reason I started this thread is because I figured I'd get with some fellow UGers and brainstorm a way to modify a common, albeit flawed, amplifier that isn't the norm that you see every day on either the Peavey forum, or Tumblr, or wherever you search on the interwebz. Most of the people who buy these amps do so because of the price tag. Let's be real here. I know I did--$250 for a tube amp large enough to gig with that actually sounded ok? Sign me up! So many of us are on a budget and with priorities (and in my case, a truck that is eating my lunch with maintenance and repairs, plus a wife and a toddler) so we can't just go out and simply buy a Mesa when we want more low end, or spend more than we paid for the whole rig just on speakers. Those of us in this ballpark have 2 options: either deal with it and make do with what we have or explore other methods to get the tone we want. I fall into the latter.

I used to play this amp through a Marshall 4x12 cabinet loaded with G12T-75s. T75s are known for their scooped mids and hyped low end, but even with this cabinet it just could not put out the low end that I needed from it. My old Randall head used to rattle walls with this cab, but still be bright and defined, which used to get me tons of compliments on my tone. That being said, I think that justifies the following principle:

Spending $25 on amp mods and another $25 on closing the back of my cabinet (which includes tolex) > Spending $200+ on new speakers that might not even get me there. Simple Economics 101.
Last edited by dkennedy88 at May 24, 2016,
#20
Quote by dkennedy88
My cab is open-back with Seventy 80s.

There's your problem. No wonder you hate how the amp sounds!

The Seventy 80 is more or less a modeling amp speaker with a very flat response compared to something like a Celestion V30. It's going to sound very bright and fizzy, and an open backed cabinet is only going to make the amp sound even more terrible. If you're running a cabinet like that, you're going to have a very hard time convincing anyone that the 6505+ lacks low end. I don't intend that to sound mean-spirited, but such a cabinet is simply not fit for purpose.

In this context, arguing that its cost prohibitive to buy a cabinet that's actually fit for purpose for this amp is a bit like buying expensive new pickups for your guitar to improve the tone of your Marshall MG. It just doesn't make much sense to go through the effort of tweaking the amp when a much more significant issue remains unresolved.

If you want to get a solidly built V30-loaded closed back cab economically, get a Harley Benton cab. They're insane bang for the buck, and certainly not as expensive as you might think.
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Last edited by T00DEEPBLUE at May 24, 2016,
#21
I like the mods you're doing and I'm following because I want to see if I can replicate any of this in Peavey Revalver 4 which lets me mod the schematic.

I just think that you may be barking up the wrong tree as far as low end goes. An open cab with Seventy 80s is not gonna do the low end thing you want no matter how much you mod the amp. Closed with V30s or K100s? That will.
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#22
Quote by oneblackened
I like the mods you're doing and I'm following because I want to see if I can replicate any of this in Peavey Revalver 4 which lets me mod the schematic.

I just think that you may be barking up the wrong tree as far as low end goes. An open cab with Seventy 80s is not gonna do the low end thing you want no matter how much you mod the amp. Closed with V30s or K100s? That will.

I get what you're saying. Which is why I'm picking up some MDF and black tolex within the week to close the cab up. And the speakers definitely have to go; however, I want to see how far modding my amp gets me. I've always loved the Recto tone, when dialed in right, so I wanted to see if I can somewhat cop that vibe with this amp, regardless.

What bugs me about this amp is that right now, with the current mods I've made, the amp has plenty of low end on the clean/crunch of the rhythm channel, but when I jump on the lead channel, it's all mids and highs, unless I max out my bass pot. Part of that is the removal of the 22k resistor at R13. That will really boost the bejesus out of the low end on the rhythm channel, mainly in clean mode. I'm thinking about putting a 33k in there, because it'll be subtly more low end than the stock value, without throwing the balance of all the channels out of whack.

Back to speakers: I was thinking for so long about modding my amp into a head, but now I'm having second thoughts. I may just keep it as-is, and replace the speaker. I'm gonna do G12T-75s in the Blackstar cab, because I like the sound of them, and then a V30 in the combo to complement the T-75s. But that is down the road.
#23
I think I'm going to try more of these mods along with you. My father is an electrical engineer and hobbyist, so many of the components needed for these mods are ones I already have.

There's still a few parts I'm missing, but I've ordered them. They're all dirt cheap.

The change of R12 to a 39K resistor is something I may not bother with though. The stock 37.4k means changing it is not likely to make a difference worth mentioning.
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Last edited by T00DEEPBLUE at May 25, 2016,
#24
Ok so I did all the modding this weekend. This only affects the tubes and tonestack in the lead channel. I have something else brewing for the rhythm channel, so stay tuned for that. There were a lot of things I tried that I actually ended up abandoning because I just didn't like the tone, but YMMV.

It turns out, I never modded C5. It was the stock 470pF cap. Which explains a lot, so the first order of business is that. This is what I did:

--The 5150/SLO spec is a 2.2nF capacitor and a 470KΩ resistor in parallel. The Mesa spec is the 2.2nF cap and 680KΩ resistor. I tried the 680KΩ and didn't like it. To me, it takes away too much cut. Some will like this. I don't, so I went with the 470KΩ.
--Replace the 1nF capacitor at C26 with a .022µF capacitor. This is the same spec for the 5150, SLO, and Dual Rectifier. The stock value really takes a bit of low end away.
--Replace the 2.2KΩ resistor at R51 with a 2KΩ resistor, this biases the EQ driver a little hotter.
--Replace 1uF capacitors at C43 and C46 with a 1uF capacitor and 47KΩ resistor in series at each. This is the Mesa spec.
--Replace 470pF capacitor at C1 with a 680pF cap. This makes the treble control like that of the Red Channel in Modern mode on the Dual Recto. Using the 680pF cap shifts the center frequency of the treble band downward, and makes mids and highs interactive to a degree.
--Bypass the 10KΩ resistor at R9, the 39pF capacitor at C22, the 100nF capacitor at C19, and the 68KΩ at R2. This increases dynamics, and picking response. MAKE SURE TO REPLACE R9, C19, AND R2 WITH JUMPERS, OTHERWISE YOUR AMP WON'T WORK. You want to keep the 1MΩ resistor at R55. This makes the amp more dynamic and more organic with better feel.
--Solder a 10pF cap between pins 2 and 3 of V4. This actually takes away some fizz, and brings out some clarity.

I also replaced the 47KΩ resistor at R48 with a 22KΩ, thinking it'd shift the mid frequency downward. What it really does is scoop the mids a bit more. Some may like this (and it definitely lends to the Mesa vibe), but I honestly couidn't tell much difference, so I went back to 47KΩ.

THE VERDICT: I liked this tone for about a day. This actually lowers gain and increases midrange (which makes no sense). If you like this, with even more mids around 800-1000Hz, go for it. I hated it, so did T00DEEPBLUE (see below). As far as making it a Recto...buy a Recto. It's not gonna happen.

Apologies for not taking pictures. Lighting was not great where I was, nor did I have a decent camera on hand.

I found these items immensely helpful:

http://www.duncanamps.com/tsc/">Duncan's Tone Stack Calculator is a great tool for seeing how components within the tone stack interact with one another.
6505+ 112 Schematic. Also includes PCB layouts, which makes this whole thing easy.
Original 5150 Schematic
5150 II Schematic
Soldano SLO100 Schematic
Dual Rectifier Rev. F Schematic

Again, stay tuned, because I have something I'm wanting to try for the rhythm channel. That will be next week when I get to Fry's for some resistors.
Last edited by dkennedy88 at Jun 13, 2016,
#25
I've finally got around to doing all the mods you've listed.

The clean channel is MUCH better than it was, is actually pretty nice and very usable for pedals.

But I have a few issues.

Replaced 470K at R91 with a 1meg, 6.8K at R69 with a 10K, and 68nf at C25 with a .047uf to counteract volume loss from the above C36 mod.

I don't understand why, but doing this doesn't seem to correct any of my volume losses.
Replace the 2.2KΩ resistor at R51 with a 2KΩ resistor, this biases the EQ driver a little hotter.
--Replace 1uF capacitors at C43 and C46 with a 1uF capacitor and 47KΩ resistor in series at each. This is the Mesa spec.
--Replace 470pF capacitor at C1 with a 680pF cap. This makes the treble control like that of the Red Channel in Modern mode on the Dual Recto. Using the 680pF cap shifts the center frequency of the treble band downward, and makes mids and highs interactive to a degree.

I don't understand why either, but my lead channel sounds more like a crunch channel in light of doing some of these mods mentioned above. There's tons less gain and the midrange is really honky. I hate it.

Is there something I have done wrong or is it supposed to sound that way?
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#27
^

I haven't done anything that would risk my safety.
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#28
Anyone?
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#29
I noticed the same thing with the mids on the Lead Channel. I as well hated it so I aborted. One thing that can help with the mids is reverting back to a 470pf capacitor at C1. It shifts the inherent dip in the mids upward from about 500Hz to around 700-800Hz.

Also, Peavey really screwed the pooch when they went with a 50K pot for mids. The 47K resistor at R48 doesn't do anything but fuck with the taper of the pot. So I'd suggest replacing it with a 20-25K linear pot (AES sells replacement pots for the Marshall JCM 2000 and these should drop in).

As for the gain, the 47K resistors in series with the cathode caps make a world of difference. It wasn't enough for me either. So I cut them out and soldered the other leg of the 1uf back in. I do, however, only set the gain to 11:30-noon now. The volume losses on the new cleans I've found can be (somewhat) counteracted by turning up your Pre gain. For some reason the gain jumps up around noon-1:00ish and around this point is where the levels seem to even out between clean and crunch. This gain jump manifests as a volume increase on the clean and an increased saturation/distortion on the crunch mode. Especially with a 12AY7 or 5751 in V1 the cleans will be substantially better than stock, but will be more Bassman than Twin. I actually set my pre gain a little past noon and it evens it out, even if there is some breakup.

Sorry it took me so long to reply. I got a promotion at my job and thus I've been busy. In truth I HATED these mods...actually I think hate is a nice word to describe it...and have since modded again to 5150 Block Letter spec...at least on the Lead Channel!!! I am receiving 20K pots for the mid control on both channels this week, and this should provide a more scooped tone than your average 5150. As it stands now, I have to turn my mids down to 8:00-8:30 to get enough chunk out of the amp. On a 20K pot, that would be closer to noon.

I've done further mods to the clean channel, which will further drive home the point of the cleans tending to be more like a Bassman or JTM, which isn't a bad thing unless you HAVE to have pristine cleans. I'll explain further when I get home.
Last edited by dkennedy88 at Jun 13, 2016,
#30
Ok so you're gonna hate me, but restore everything at C22, C19, and R2 to stock. Solder a 22K resistor at R9. This is the 5150 spec. Restore C1 to 470pF, and remove the 47K resistors from C43 and C46. This should undo the loss in gain and honky mids.

Bypass C27, then solder a .01uF cap in series with R46, opposite the ground. This is how the original 5150 is wired. Now your lead channel's signal path is identical to that of the original 5150.

One thing I did do is solder a 20pF cap from the leg of R36 going to V2 to ground. This helps tame some harsh frequencies and helps take care of the inherent 5150 noise. However, if you do this it'd be a good idea to remove the 10pF cap at V4.

I'm gonna try removing R48 and replacing VR8 with a 20K pot. If that doesn't help with the Peavey's inherent mid emphasis, then I'm gonna call it a day and rely on just the EQ. At this current state, I can get a dirty tone I've been happy with for the past week.

NOW FOR THE RHYTHM CHANNEL:
Go ahead and replace C48 with a 10uF cap. This will lower the cutoff frequency at the cathode and will provide a deeper bass (not really a bass boost, but more of an extension of the lower frequencies).
Now the tone stack you can do a few different ways. Keep in mind that it's basically a Marshall 1959SLP tone stack with a 10K mid pot instead of 22-25K, and a 100pF cap in parallel with the 470pF treble cap. If you like this, great, but if you want more of a Fenderish sound, do this:

Option 1 (Twin tone stack)
100K slope resistor at R26.
220pF capacitor at C4.
33pF capacitor at C47 (this is in parallel with C4 and will give you 253pF total for the treble control).
.1uF capacitor at C41.
.047uF capacitor at C42.
10K pot at VR3 (linear is stock but the '65 Twin Reissues use a 10K audio. You can swap VR13 with VR3 and achieve this AND have a more usable presence control on the rhythm channel).

Option 2 ('59 Bassman Reissue tone stack)
100K slope resistor at R26.
220pF capacitor at C4.
33pF capacitor at C47 (this is in parallel with C4 and will give you 253pF total for the treble control).
.1uF capacitor at C41.
.022uF capacitor at C42 (this is the stock value anyway).
20-25K linear pot at VR3.

Option 3 (Actual 5f6a Bassman/Marshall JTM45 tone stack)
56K slope resistor at R26.
220pF capacitor at C4.
33pF capacitor at C47 (this is in parallel with C4 and will give you 253pF total for the treble control).
.022uF capacitor at C41 (this is the stock value anyway).
.022uF capacitor at C42 (stock value).
20-25K linear pot at VR3.

I have a curiosity about what R59 does for the tone stack. It appears to be a 1Meg resistor in parallel with VR10 (Lead Post Gain), with the intent to lower the resistance of said pot to 500K or close to it. If this is true, then this should in theory increase gain across the Lead channel and possibly offset the volume drop between Rhythm and Lead Channels (at the same respective levels on VR5 and VR10). I noticed that VR5 doesn't have a parallel resistor, which may explain why the Rhythm Channel is louder than the Lead Channel naturally.

The final gripe I have to address is the presence control. It's useless below 3:00. This is due to the taper of the 10K pot they used. It should've been a linear pot, but they went with audio. Swapping with a 10K linear should give you a more useful range. You can use the 10K linear pot at VR3 (Rhythm Mid control) or at VR11 (Reverb Level) to swap with the audio/log pots at VR13 and VR15. These pots (which are selected by relay) are both connected to C28, which is a .22uF cap. I'm gonna try to lower this to .1uF (which should keep the presence control out of the high mids).

A final note, I've found that tube selection plays a huge part in making the cleans cleaner and overall sounding better. I'm currently using a JJ 5751 in V1, followed by a Sovtek 12AX7LPS in V2, then JJ ECC83S in both V3 and V4, and a JJ ECC832 in V5. I'm gonna play with moving the 5751 to V5, and using a 12AY7 in V1 (much like the Tweed Bassmans of the late '50s). This should help lower gain at input stage and result in less breakup, as well as less gain at the phase inverter.

These will be my final mods for this amp. I can't see anything else that I would want to try. This has been, to say the least, a learning experience for myself, not only in learning how tube amp circuits work, but also learning my own amp in detail I'd never deemed imaginable. I hope I have helped others here.

If anyone needs to replace pots for this amp (whether because they're bad, or you like to tinker like me) I have found these sites carry pots that will fit this amp:

Studio Sound Electronics -- Peavey Amp Parts
Antique Electronic Supply
Last edited by dkennedy88 at Jun 13, 2016,
#31
You bet I'm pretty mad.

Sure it really annoys me when I accidentally destroyed caps that I do not have spares, so now I have to buy new ones. But that's okay, it'll be put right eventually.

I think you should make a proper and comprehensive blog on your findings. This sort of information would be very useful to anyone who is willing to put effort into getting the most out of this amp. And the sort of mods you're doing are the sort of things I've not seen anyone else try before. Even if these Mesa mods didn't really work (I would agree) it would very much be worth documenting what you (and I) found in said blog.

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Last edited by T00DEEPBLUE at Jun 14, 2016,
#32
Definitely doing a comprehensive blog (and crediting you). This whole ordeal blows my mind. My 20K pots should be here sometime Thursday and Friday. Plan to mod late Friday night (which going from my Texas time would be early Saturday morning your time). And I should be done on Saturday. Fortunately I'm only doing 5 capacitors (470pF at C1, 220pF at C4, 33pF at C47, .1uF at C41, and .1uf at C28) 1 resistor (100K at R26), and 5 pots (10K linear pots at VR3 and VR11 are being swapped with 10K log pots at VR13 and VR15; 20K linear pots going into VR3 and VR8).

I'm sorry my prior hypotheses were a complete and utter fail. I plan to remove R59 to see if it will make the Lead volume loud enough to match up with the Rhythm volume, but if not it'll be simple enough to fix (I have plenty extra. I bought in bulk at the start of this project). I'll get back to you with my findings. I'm predicting that like on a guitar pot, removing R59 should increase volume and brightness.

Right now, here are my lead settings on my amp:
Pre Gain: about 11:30
Low: 1:00-1:30
Mid: 9:00-9:30 (bear in mind this is a very middy amp)
High: 1:00
Resonance: 2:00
Presence: 4:00 (with log pot, will be significantly lower)

And I pick very hard.

This kinda gives me a Load/Reloadish tone with my PRS, which isn't DAS BROOTZ but covers A LOT of bases.
Last edited by dkennedy88 at Jun 14, 2016,
#33
It's funny you mention it having a Load/ReLoad vibe that because I thought just the same thing when I tried these mods too.

That's certainly not a bad thing either; I think the guitars in that record sound badass.

Let me know how your mods go.

Your settings are not dissimilar to mine, but I scoop the midrange out even further than you (so I'm even more keen to get these 20k pots in) because I have a lot of my midrange coming from a TS9 I run in front of the amp. I like the lower mid raunchiness of the TS9 and how the gain seems to tighten up the bass end a little. But some of the upper mids are a bit hard to control.
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#34
TS9 is so much win.

Just got the pots today. Gonna start with the mods either tonight or tomorrow. Probably starting with the caps/resistors today then pots tomorrow. Using the 470pF treble cap vs. the 680pF that I suggested earlier actually moves the mid frequency closer to the 700-800Hz range, which is a nasty range. Before you're looking at 500-600Hz, which left some of the nasally and honky high mids in there. And I'd rather cut those and boost the low mids, especially to glue the bass and guitars together so that when played tightly, they lock in better.
#35
MODS ARE DONE!!!!!

And I can finally say I'm done with it and can move on. The lead channel is pretty much exact original 5150 spec, with the exception of two things: a 20K? Mid pot and the removal of two resistors. The rhythm channel is a combination of a few Atomium Amplification mods and a tone stack derived from a Fender Reissue '59 Bassman. The intended result is a much more serviceable clean tone and a more classic-sounding crunch tone from the rhythm channel, and more low end with less mid emphasis on the lead channel.

So, in order, these are the modded specs I have in my amp vs. a stock 6505+ 112 combo.

INPUT STAGE:
--R9 is a 22K? resistor (10K? stock).

RHYTHM CHANNEL:
--R13 is a 33K? resistor (22K? stock).
--R69 is a 10K? resistor (6.8K? stock).
--C25 is a .047uF capacitor (.068uF stock).
--C36 is a 100pF capacitor (6.8nF stock).
--C48 is a 10uF capacitor (2.2uF stock).
--C47 is a 27pF capacitor (100pF stock).
--C4 is a 220pF capacitor (470pF stock).
--C41 is a .1uF capacitor (.022uF stock).
--R36 is a 100K? resistor (33K? stock).
--VR3 is a 20K? linear potentiometer (10K? linear stock).

LEAD CHANNEL:
--C5 is a 2.2nF capacitor and 470K? resistor in parallel (only a 470pF cap stock, no resistor in parallel).
--There is a 20pF capacitor soldered between pin 2 of V2 and ground (nothing here stock).
--R32 is a 470K? resistor (82K? stock).
--R46 is a 10nF capacitor and 47K? resistor in series (just a 47K? resistor stock).
--C27 has been bypassed (10nF capacitor stock).
--VR8 is a 20K? linear potentiometer (50K? stock).
--R48 has been removed (47K? resistor stock).
--R59 has been removed (1meg? resistor stock).

RESONANCE/PRESENCE:
--C28 is a 100nF capactior (220nF stock).

I planned on replacing the presence pots with 10K? linear pots (they're 10K? audio stock), but after how much of a pain in the ass it was to replace the mid pots, I elected against it. It's just a matter of adjusting settings to account for taper.

THE END RESULT:
The clean mode of the rhythm channel is a night-and-day departure from stock. With the pre gain at 1:00, I still get a clean tone that breaks up only if hitting the strings hard. The breakup that does occur, is much more musical and thus something I am totally ok with. This is great for modulation effects such as chorus, flanger, phaser, etc. It's not just a workable clean, I actually like it.
Going to crunch mode, since the whole amp itself is very Marshall-esque, and the tone stack now being somewhat similar to the early JTM tone stacks, it's very easy to get that basic AC/DC crunch tone. This works great for me, because I use a lot of semi-dirty tones mixed in with my clean and heavy tones. It's a bit different than the stock crunch tone, but this is far less metal and allows for more versatility.
The lead channel still has the ubergain that the stock amp has, but that's ok. Modding C5 and adding the 20pF cap from V2 to ground cuts the right amount of fizz and provides the added low end punch that was missing from this amp. It is still a bright amp with some fizz, but it is much more manageable. Going back to the stock value 1uF cathode caps (without any resistance in series) brings the monstrous gain back to the amp that was lost in the earlier experiment, setting this apart from the crunch channel (thank God). Switching to a 20K? mid cap and removing the parallel resistor at R48 cut the mids. Typically you don't want to do this to an amp, but the entire 5150/6505 line is known for being excessively mid-heavy. This evens it out, and makes finding heavy tones easier to dial in now. And by lowering the cap value at the presence control (C28), I can turn up the presence knob without introducing unwanted upper mids.

I honestly didn't find much difference in removing R59, but I'm just too lazy to put it back in to prove that. As for the rhythm vs. lead volume mismatch: I'm just glad each channel has a master volume so you can match volumes. I can't find anything that will even that out without just adjusting the knobs. The clean and crunch on the same channel match up better in volume the higher the pre gain is (I like it at 1:00).

I know we were disappointed with the first mod idea, but this should make up for it. It does for me.

I'll be posting a blog on my Tumblr and on the Peavey.com forum detailing the above info from the whole experiment.

If you try the mods, T00DEEPBLUE, let me know how it goes for you. Thank you for your help bud.
#36
I'm waiting for more parts to come in at the moment. I like the idea of having a psuedo bassman amp in the clean channel. I'm still waiting for the pots to come in as well. It might take me another week but I might not have time to get the amp back together. I've got a business trip in a fortnight.
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#37
I've just received some more parts, including the new 20K pots for the mid controls.

I've just had a thought; what if the 50K pot that was used for the lead mid control was used for the reverb control? Perhaps that would increase the amount of reverb the tank produces. Assuming of course that the 10K pot is a bottleneck which wouldn't surprise me given all the wacky pot values this amp has. Kind of difficult to know how much of an effect that would have on the amount of reverb, or it'll just mean that all the reverb will appear in the first 20% of the pot's travel and the rest of the travel does nothing. Might be worth a try since the amount of reverb this amp has is a bit anemic stock.

Either way I'm not fussed; I get my reverb from my pedalboard. Right now I 've just swapped the presence 10K log pot with the reverb 10K lin pot around.
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#38
Did the 10K linear help with controlling the presence for you?

Peavey used some RETARDED pot and resistor values on this amp. I mean, it helps with making it a metal monster, but just not a good overall amp. Maybe a 50K would help with reverb. I'm sure it's a bit different, but when I was dicking around with the tone stack calculator I found that increasing the pot value of one of those bands actually gave you higher output in that band. Makes sense because when you mod a guitar's pots and go from 250K pots to 500K pots it gives the effect of higher output, but also adds brightness. This is why many people who own Norlin-era or later Les Pauls tend to swap out the stock 300K pots with 500K. So, in theory, a higher resistance pot could get you there. Worst case is as you said: you get to 9:00 and that's the max reverb.

I actually don't even use the reverb. I actually entertained taking the pot out and wiring in a DPDT switch to flip back and forth between a 47K resistor or 33K resistor (the old JCM800 value) at R47, but decided against it. I, like you, use reverb through other units. I'm using an old PODxt right now, but I'm about to switch to an old Boss GT-3 I found super cheap, because I can do some sort of bastardized 4-cable method with it AND control it in MIDI. Because of the modded cleans, I don't even use amp modeling anymore.

As for the 20K mid pots, one thing I can say is that it's not gonna make a world of difference in tone--in fact, you'll be able to get a lot of the same tones as before, you'll just have to tweak EQ settings. The real benefit of swapping these pots is more in being able to fine-tune the lower end of that range of resistance than stock. So, for example, if you leave R48 in there, what was noon stock is the same as 3:30 after adding the 20k pot. Since I removed R48, mids set to noon stock would be the same as 1:00 after the mod, which is much more subtle, but is just enough for me to control the mids better. Regardless of the pot value, you're still getting nothing at the 7:00 (zero) position, thus limiting the effective range of the pot, but increasing the control over that limited range.
An example: the 47K resistor still in R48 and a 20K pot maxed is the same as 1:00 with the stock 50K pot and R48 in parallel, which means you have about 67% more "turn" on the pot's taper to increase about 60% of the resistance.
Last edited by dkennedy88 at Jun 24, 2016,
#40
I've finished the mods at last. But I have a problem. There is a huge volume loss on the post gain of the lead channel and there's this very strange harmonic squealing that occurs when you turn the pregain up to 6-7 but disappears after 7 and before 6. The amp sounds pretty great everywhere else except for these strange problems. I'm chalking this up to accidentally overheating the pots and damaging them. But I am not sure.

Oh and changing the presence controls to 10K linear pots really does help make the sweep of the pots a lot more controllable, so I recommend it.
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Last edited by T00DEEPBLUE at Jun 24, 2016,
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