Page 1 of 2
#1
Hey good folks of GB&C,

I'm doing a radical overhaul of my Harley Benton GA-5 (a Valve Junior clone). I thought you guys might be interested.

I've basically gutted a different (crappy SS) amp and am going to use its bigger, more roomy chassis to put the VJ goods in. I'll also be adding an extra preamp tube and an excessive amount of controls. Here's the schematic:



If anyone has a nice idea of something more to add: suggestions are very welcome and will probably be implemented if possible with the parts I have on hand.

Also welcome are corrections on the schematic, although I hope that won't be necessary.

Cheers!
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#2
Put R7 before R2. The way you have it now you are cutting signal before it even reaches the amp and gets amplified. The resistors are set up as a voltage divider now.
#3
Quote by kurtlives91
Put R7 before R2. The way you have it now you are cutting signal before it even reaches the amp and gets amplified. The resistors are set up as a voltage divider now.
Nice catch, thanks! I'll probably wire that 68k resistor right on the socket.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#5
I like the designation of C11 as "not much uF."

Have you thought about bigger filtering caps? I had positive results using 47uF (450V) instead of 22uF. Made the bass seem more "clear" to me and less farty.
#6
Quote by cedricsmods
I like the designation of C11 as "not much uF."
Hehe, it's this little yellow cap in the VJ without any capacitance markings, but I'm pretty sure it doesn't have much of it.
Have you thought about bigger filtering caps? I had positive results using 47uF (450V) instead of 22uF. Made the bass seem more "clear" to me and less farty.
I have, but I'm trying to reuse a much as possible from the VJ due to budget reasons.
I did upgrade the reservoir cap (a bit). It should be pretty easy to replace them later on, though.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#7
is this going to be on an eyelet/turret board of some sort? or just using the PCB?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#8
Quote by ECistheBest
is this going to be on an eyelet/turret board of some sort? or just using the PCB?
Eyelet board. I've done some modding before on the VJ, but the PCB is just not mod-friendly. Every time you want to swap a component you have to take the whole thing out and traces started lifting after a while.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#9
cool, a few things. hopefully you haven't fabricated the board yet...

R15 (powertube's grid stopper) should be on the tube socket's pin. then R5 (grid leak resistor) should be on the other side of R15. for now, u got it set up as a voltage divider. it wouldn't lose much output tho.

if i were you, i would up the power rating of R10 (power supply) to 5w. i think it's logical. that resistor gets all the current to the plates of each of the tubes.

by raising the value of the cathode bypass cap on the power tube (C5), you'll have a tighter, more pronounced bass, and you'd want to bypass most of the frequencies rather than leave some unbypassed. at least for the powertube, since you have the OT's primary set at 5.2k, you don't want the OT to see the same impedance coming out of the tube.

also, the plate resistors on each triode could be something like 100k, or even 82k, 68k. i dont think you need much voltage gain from each tube. you'd actually want less from each tube. after getting all this clipping, you're gonna end up with just a square wave... a fuzz can do that better.

from this, i would recommend going back and forth, between hot and cold, for each tubes' bias. quite a few high gain amplifiers do this, including the highly sought after Soldano SLO100. I made a "higher" gain amplifier that goes between hot and cold, and it does have a better sound, than just having symmetrical clipping from each section (4 gain stages is a lot). this is done by adjusting the cathode resistors, so say from stage 1 to stage 4, 1k2, 2k7, 1k5, 3k3 or something similar. you have a switch on the third gain stage, i'd put it on the last stage to make it even "colder" (as in more resistance).


i have more haha. the cathode bypass capacitor is all .47uF. of course, you don't need much bass response through this, but to get a tight, chunky bass response, i would play around with these values. first stage i'd use 1uf~10uf, i had success with 10uF and 2uF. well, when you get to play around with it, it'll be more fun haha. .47uF won't have enough "body" i think.


sorry long post. haha have fun! and be safe with lethal voltages.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#10
Holy crap, that's a lot of suggestions! Thanks!

I'm not using a board with traces, actually. Just two rows of strips, so it's still open to mods at this point. I gotta go now, but I'll give your suggestions a good read later on.

Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#11
well if it's an "overhaul", then you want a lot of crap, and the crap done right.

so you got terminal strips (or something like that) like the AC30s. nice. but keep in mind, you use a lot of those terminals fast if you don't plan everything out right.


i think i have few more things to add, but i guess that can come later. you're probably dying to make this thing already haha.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#13
Alright, got some time to answer you properly now.
Quote by ECistheBest
R15 (powertube's grid stopper) should be on the tube socket's pin. then R5 (grid leak resistor) should be on the other side of R15. for now, u got it set up as a voltage divider. it wouldn't lose much output tho.
Corrected! That's the second time I made that mistake...
if i were you, i would up the power rating of R10 (power supply) to 5w. i think it's logical. that resistor gets all the current to the plates of each of the tubes.
I actually can't find the original resistor that's supposed to be there. Right now, I've got only one 5 watter available, a 330 Ohm one. I'm thinking that might be a little low, but it's all I have atm. I'll might get a higher one later.
by raising the value of the cathode bypass cap on the power tube (C5), you'll have a tighter, more pronounced bass, and you'd want to bypass most of the frequencies rather than leave some unbypassed. at least for the powertube, since you have the OT's primary set at 5.2k, you don't want the OT to see the same impedance coming out of the tube.
I really don't want more bass, honestly. I've actually thought about making a high-pass filter at the power tube grid.
Not sure what you mean with the impedance (goes slightly over my head). Although, this is the stock value, so it should be safe.
also, the plate resistors on each triode could be something like 100k, or even 82k, 68k. i dont think you need much voltage gain from each tube. you'd actually want less from each tube. after getting all this clipping, you're gonna end up with just a square wave... a fuzz can do that better.
I'll definitely experiment with those when it's up and running. I'll be able to hear the difference, then.
from this, i would recommend going back and forth, between hot and cold, for each tubes' bias. quite a few high gain amplifiers do this, including the highly sought after Soldano SLO100. I made a "higher" gain amplifier that goes between hot and cold, and it does have a better sound, than just having symmetrical clipping from each section (4 gain stages is a lot). this is done by adjusting the cathode resistors, so say from stage 1 to stage 4, 1k2, 2k7, 1k5, 3k3 or something similar. you have a switch on the third gain stage, i'd put it on the last stage to make it even "colder" (as in more resistance).
That is a good idea, but I thought it would be good to have a somewhat neutral clipping stage at the end to kind of smooth things out. I guess this is also something to experiment with.
i have more haha. the cathode bypass capacitor is all .47uF. of course, you don't need much bass response through this, but to get a tight, chunky bass response, i would play around with these values. first stage i'd use 1uf~10uf, i had success with 10uF and 2uF. well, when you get to play around with it, it'll be more fun haha. .47uF won't have enough "body" i think.
Like I said, I wanna keep bass response low. I am under the impression that using a smaller bypass cap leads to less bass and thus more tightness. Is that not correct, then?
sorry long post. haha have fun! and be safe with lethal voltages.
I will. Have never shocked myself while modding, so far.

*knocks on wood*
Quote by ECistheBest
well if it's an "overhaul", then you want a lot of crap, and the crap done right.

so you got terminal strips (or something like that) like the AC30s. nice. but keep in mind, you use a lot of those terminals fast if you don't plan everything out right.
Yeah, I noticed! But it turns out I do have enough terminals. Barely:



Reservoir cap should probably be separate anyway, so it's for the best I don't get to put that on the terminal strips. Now I get to use some of my leftover crappy perfboard instead:



If you're wondering why that big cap seems to have a smaller cap strapped to its back, I read this by Merlin Blencowe:
Quote by The Valve Wizard
The reservoir capacitor needs to have a low reactance at frequencies up to 40kHz, because the HT at this point will contain high frequency harmonics caused by rectification. Its performance at these frequencies can be greatly improved by adding a capacitor in parallel with it, ten times smaller in value. (Another can also be added, ten times smaller than the second, but this would be usually be considered an extravogance.)
And I had this spare cap laying around that was about 10 times smaller in value, so I thought "Why not? Sounds good to me". I'll also probably be strapping those loose high wattage resistors to the chassis later on.
i think i have few more things to add, but i guess that can come later. you're probably dying to make this thing already haha.
Hehe, I was.
Quote by kurtlives91
SE tend to sound nasty in the bass imo. I'd keep the bypass caps low like your doing. Use poly over electrolytic.
Yeah, exactly. VJs are pretty farty when you crank 'em.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
Last edited by Kanthras at Sep 1, 2010,
#14
A little update:







Some pretty ghetto solutions in some places, but things are coming along quite nicely, even if I do say so myself.
Although I can't find my drill for the holes for the tube sockets, so I gotta go to the store tomorrow. Also need a new soldering iron, mine is almost completely busted. It was possible to work with before, but now that things are getting more fiddly, I really need a fresh iron.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#15
Well, I got nearly all the wiring done today. Drilled holes for the tubes. Slightly too big holes actually. I need a new socket for the power tube because it's too small.

In other news, I had a potentially interesting idea. I have this nice toroidal transformer left from the amp that donated the chassis:



I am loath to leave such a nice piece of iron collecting dust. I was thinking, maybe I could use the primary windings as a choke? There's sure to be plenty of Henries in there... Or will it just die a fiery death or something?
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#16
I've been very busy, but I finally got the time to put it all together for testing.

As expected, it doesn't work.
After some debugging, I noticed I forgot to connect the EL84's screen grid (socket pin 9, right?) to the HV supply, so I fixed that.

When I fired it up, R10 (the first resistor after the reservoir cap) started getting REALLY hot. I thought that's really odd, since it's higher value than standard (330R, 5W vs 220R, 3W standard). Any ideas what might be wrong? After removing the connection to the screen grid, it doesn't get hot anymore. Maybe an internal short in the power tube? I don't have any extra on hand atm, unfortunately.

Also interesting, with the connection to the screen grid on, the caps drain properly. With the connection open, I have to drain them manually (yay, sparks!).

edit: Just checked, my multimeter cannot detect any internal shorts on the power tube.

edit2: Some more info: heaters work correctly, B+ voltages are good.

edit3: Got some pics too. Maybe you guys can see the fatal flaw I'm missing:







You'll have to ignore the way the power tube is mounted... It'll have to do for now.

moar edit: there is a faint hum coming out of the speaker, so at least the power tube isn't (completely) shot. Anyone have any idea what's going on? Help would be really appreciated, since I could really use this amp next week.

moar moar edit: Is there really nobody interested? Anyway, I found out part of the problem. The power tube's cathode was shorted to the ground. That's probably pretty bad and fixing it made no difference to the operation of the amp, however. Anyone have any ideas? Is there someone out there at all?
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
Last edited by Kanthras at Sep 17, 2010,
#17
the thing is, using 330R instead of the 220R for R10 might have been tough for the resistor. R10 beasts ALL the current coming out of the rectifier out to the power tubes' plates. so that might be one reason it's getting hot.

however, those power resistors DO get quite hot in every amp. so they should be raised off the board so it doesnt heat the board.

i don't know how you're doing the powertube from the schematic. you didn't finish the cathode.

on the board, what's with the lugs left with no connections? i hope they're all getting connected to ground or something on the underside.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#18
Quote by ECistheBest
the thing is, using 330R instead of the 220R for R10 might have been tough for the resistor. R10 beasts ALL the current coming out of the rectifier out to the power tubes' plates. so that might be one reason it's getting hot.
Now the cathode isn't connected to ground anymore, that resistor remains quite cool. I should've mentioned that.
I think the power tube was drawing a lot of current because the bias was kinda screwed up.
however, those power resistors DO get quite hot in every amp. so they should be raised off the board so it doesnt heat the board.
I'll do that. Although they don't get noticeably warm anymore it's probably a good idea anyway in case something goes wrong and the board gets torched...
i don't know how you're doing the powertube from the schematic. you didn't finish the cathode.
Aside from that bypass cap, everything is stock (http://www.rawbw.com/~emiller/Euthymia/DIY/VJstock.jpg).
on the board, what's with the lugs left with no connections? i hope they're all getting connected to ground or something on the underside.
No, they aren't. Will they cause interference/hum/noise?
I think at this point it would be easier to just take the lugs out.

Anyway, thanks for the suggestions!
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#19
Quote by Kanthras
Now the cathode isn't connected to ground anymore, that resistor remains quite cool. I should've mentioned that.

yea, sounds good. however, for more headroom (when you want it) that resistor can be jumpered to increase B+ voltage. however, this might red plate a tube or two im not sure. but putting in bigger resistors here won't give you much sag because this is a single ended design.

I think the power tube was drawing a lot of current because the bias was kinda screwed up.

yes. when the cathode and grid are both at the same potential, a lot of current will flow. massive loads. i'm not sure how your B+ values were ok though. it should've been dropped significantly by the 330R resistor.

I'll do that. Although they don't get noticeably warm anymore it's probably a good idea anyway in case something goes wrong and the board gets torched...

you can. most modern designs seem to do that. however, with 5w there and 330R, you're probably safe.

Aside from that bypass cap, everything is stock (http://www.rawbw.com/~emiller/Euthymia/DIY/VJstock.jpg).

ok just wondering. need to make sure there is a resistor there. missing a resistor is worse than missing the capacitor on that position.

No, they aren't. Will they cause interference/hum/noise?
I think at this point it would be easier to just take the lugs out.

i meant the lugs with components on it, but no wires visible. if you have a lug that's open and touching nothing, leave it there it won't do much harm.

Anyway, thanks for the suggestions!

Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#20
Quote by ECistheBest
yea, sounds good. however, for more headroom (when you want it) that resistor can be jumpered to increase B+ voltage. however, this might red plate a tube or two im not sure. but putting in bigger resistors here won't give you much sag because this is a single ended design.
The reason I put that 330R resistor there, is because... I lost one of the 220R resistors.
I figured it would be better to use one of my 330Rs in the power supply (at least temporarily) than at the power tube cathode.

yes. when the cathode and grid are both at the same potential, a lot of current will flow. massive loads. i'm not sure how your B+ values were ok though. it should've been dropped significantly by the 330R resistor.
They were OK in the sense that nothing crazy was going on.
Didn't really write anything down at that point, unfortunately.

I'm gonna check again in an hour or two and I'll take exact measurements with and without tubes.

i meant the lugs with components on it, but no wires visible. if you have a lug that's open and touching nothing, leave it there it won't do much harm.
Ah, yes. Those are wired on the underside.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#21
I just measured the voltages:

With tubes:

Heater: 6,32
EL84 anode: 308
V1a anode: 159
V1b anode: 215
V2a anode: 262
V2b anode: 232

Without tubes:

Heater: 8,88
EL84 anode: 383
V1a anode: 360
V1b anode: 347
V2a anode: 346
V2b anode: 346

I'm not sure what to make of this, although the first triode stage seems quite low.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#22
what i assume here, is that your V1A anode voltage is low because of two reasons.
1) the anode resistor is a higher value (220k?)
2) the cathode resistor is a lower value (1k~1k5?)

it should be around there. however.

when you're measuring voltages, it would be nice if you can measure from the other leg of the plate resistors, and the cathode voltage. other side of the resistor coming from pin 1 and 6 of each 12AX7 (anode), and the voltage (should be around 1v~3v) right at pin 3 and 8 (cathode).

mainly... what you're lookin for when the tubes are in
-plate supply voltage (top of the resistor on each anode)
-voltage right on the anode pin (what you measured)
-from those two sets of voltages, you can figure out the current through the triode.
-grid voltage should be 0v, unless you have a cathode follower (not for you).
-cathode voltage should be between 1v and 3v (maybe a little lower/higher, but for most amps, this is normal. 1.5v or so.)
-between pin 4+5 and pin 9 should be around 6.3vAC.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#23
I found out another part of the problem. Pretty stupid mistake, I connected the coupling caps to the wrong side of the anode resistor. I feel really stupid now.

After fixing that and turning it on, it made a humongous humming sound. I decided this is probably bad and turned it off. While it turned off, the humming faded out in about a second or two, but oddly also gradually increased in frequency.

Some crazy parasitics? This amp is proving to be quite the PITA.
Quote by ECistheBest
what i assume here, is that your V1A anode voltage is low because of two reasons.
1) the anode resistor is a higher value (220k?)
2) the cathode resistor is a lower value (1k~1k5?)
Yikes, there might be something wrong then because the anode resistor is 61K.

when you're measuring voltages, it would be nice if you can measure from the other leg of the plate resistors, and the cathode voltage. other side of the resistor coming from pin 1 and 6 of each 12AX7 (anode), and the voltage (should be around 1v~3v) right at pin 3 and 8 (cathode).

mainly... what you're lookin for when the tubes are in
-plate supply voltage (top of the resistor on each anode)
-voltage right on the anode pin (what you measured)
-from those two sets of voltages, you can figure out the current through the triode.
-grid voltage should be 0v, unless you have a cathode follower (not for you).
-cathode voltage should be between 1v and 3v (maybe a little lower/higher, but for most amps, this is normal. 1.5v or so.)
-between pin 4+5 and pin 9 should be around 6.3vAC.
I would gladly do that, but I'm a bit afraid of turning it on, atm.

Gonna give the circuit another check to see if I made any other stupid mistakes.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#24
well, if the cathode on V1A was shorted to ground, then that would give you more current going through, thus more voltage drop on the anode resistor.

every triode/pentode stage in your amp should look like the example. a tube in the middle, with plate, control grid, and a cathode.

1) anode should have an anode resistor on it, connecting that to the power supply. coupling cap should be right on the anode pin (pin 1, 6).
2) on the grid, there should be a grid stopper (wired on pin 2, 7) and then a grid leak resistor around 220k~1M, or a volume knob on the other side of the grid stopper resistor.
3) on the cathode, there has to be one thing, a resistor leading to ground. capacitor in parallel here is optional.

ya.


your hum could just be your input jack unplugged. i don't know why it would fade out and increase frequency, but i guess the power supply cap is providing the hum?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
Last edited by ECistheBest at Sep 18, 2010,
#25
Tried firing it up again it with all the gain controls down, and it's definitely coming from the preamp. Fiddling with the controls tells me that a lot of the noise/hum is coming from the first stage. Maybe it's related to the low plate voltage?
Quote by ECistheBest
well, if the cathode on V1A was shorted to ground, then that would give you more current going through, thus more voltage drop on the anode resistor.
It was the EL84 that had the cathode shorted.
every triode/pentode stage in your amp should look like the example. a tube in the middle, with plate, control grid, and a cathode.

1) anode should have an anode resistor on it, connecting that to the power supply. coupling cap should be right on the anode pin (pin 1, 6).
2) on the grid, there should be a grid stopper (wired on pin 2, 7) and then a grid leak resistor around 220k~1M, or a volume knob on the other side of the grid stopper resistor.
3) on the cathode, there has to be one thing, a resistor leading to ground. capacitor in parallel here is optional.

ya.
Gonna check this and measure voltages, thanks.
your hum could just be your input jack unplugged. i don't know why it would fade out and increase frequency, but i guess the power supply cap is providing the hum?
Checked that, it's not the input jack. There's definitely a grounding issue here, though. Touching a metal part of the chassis produces a slight pop.

This amp must be cursed.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#26
Alright, these are the voltages:

V1a
Plate: 210 V
Grid: 47 mV
Cathode: 1,62 V

V1b
Plate: 217 V
Grid: 0,5 mV
Cathode: 1,9V

V2a
Plate: 281 V
Grid: 1,6 mV
Cathode: 3,54 V / 1,83 V

V2b
Plate: 231 V
Grid: ??? mV
Cathode: 2,28 V

EL84
Plate: 306 V
Grid: 13 mV
Cathode: 10,7 V

Okay, so that doesn't look good...

The reason why I don't know V2b's grid value, is because it was pretty volatile. Coming near it with my multimeter probe actually caused hum. Touching it caused MASSIVE hum.

Grid voltages seem pretty ****ed up in general, and the power tube's cathode looks pretty high too.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#27
power tube's cathode should be around 10v for an el84 with screen voltage around 250~280. the grid is not properly 0v. either your coupling caps are leaking DC voltage, or something is ****ed up.

i hope the plates are measured at the pins again... those don't mean much as the voltage at the filter caps tho!
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#28
Something is definitely ****ed up, the coupling caps are not leaky. Besides, there's no coupling cap at the input stage, and that's got the most ****ed up grid voltage.

The plates were measured at the pins. Here's the filter caps:

Reservoir: 347
Filter A (to OT): 329
Filter B (screen grid): 326
Filter C (V2): 321
Filter D (V1): 323

How could the input stage's grid collect so much voltage, anyway? That should bleed off through the grid-leaker, right?

edit: input grid is actually hovering around 88 mV now.

[insert -stack here]
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
Last edited by Kanthras at Sep 18, 2010,
#29
do you have a grid leak resistor on the input stage? most commonly, 1M? does ur guitar have DC?
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#30
Quote by ECistheBest
do you have a grid leak resistor on the input stage? most commonly, 1M?
I do, a 1M resistor. Total resistance from the grid's pin to the ground at the reservoir is 1063 Ohm.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#31
ground at the reservoir? all ground should be at the same potential... (hopefully 0v.)
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#32
How would I determine the reservoir ground's potential?
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#33
your chassis should be at 0v potential, because you have the EARTH lug bolted onto your chassis. measuring form that point, all "ground connections" should be 0v from there.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#34
Quote by ECistheBest
your chassis should be at 0v potential, because you have the EARTH lug bolted onto your chassis. measuring form that point, all "ground connections" should be 0v from there.
Reservoir is at 0V.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#35
hmmm, weird. your grid voltage isn't at ground potential? sounds like... bad wiring/mistake, or shot tubes...
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#36
Quote by ECistheBest
hmmm, weird. your grid voltage isn't at ground potential? sounds like... bad wiring/mistake, or shot tubes...
I think I might know what's causing this retarded behaviour.. It appears the board I'm using for the reservoir cap is slightly conductive. The resistance between the AC leads of the PT and the ground is about 700 Ohms. Does that sound like something that could be bad?

I've also tried swapping preamp tubes, makes no difference.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#37
if your HT supply's center tap is hooked to ground, 700 ohms is a bit too much me thinks. should be less. haha.. but for now, i think its fine. if you got the DC voltage after the first reservoir cap correct, nothing needs to be changed. unless your PT catches on fire. haha. but i would get some kind of wire clamp thing, and put star ground on the chassis instead. ground everything there. which is the method of choice for many amp builders out there.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#38
Quote by ECistheBest
if your HT supply's center tap is hooked to ground, 700 ohms is a bit too much me thinks. should be less. haha.. but for now, i think its fine. if you got the DC voltage after the first reservoir cap correct, nothing needs to be changed. unless your PT catches on fire. haha. but i would get some kind of wire clamp thing, and put star ground on the chassis instead. ground everything there. which is the method of choice for many amp builders out there.
While we're on the subject of grounding, where should the heater be grounded? Also the chassis?
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#39
oh right, you have DC heaters. the ground should be going to "ground"... where it's 0v potential aka the chassis' star ground. the filament should measure 6.3v between pin 9 and pin 4+5.
Call me "Shot".

ShotRod Guitar Works

Custom Hand-wired Amplifiers and Effect Pedals.

Est. 2007


Source to everything I say about Guitars, Pedals, and Amplifiers: I make them.


UG's Best DIY PedalBoard
#40
Quote by ECistheBest
oh right, you have DC heaters. the ground should be going to "ground"... where it's 0v potential aka the chassis' star ground. the filament should measure 6.3v between pin 9 and pin 4+5.
Okay, done. The amp seems to get a little better every step. There may yet be hope for this little bugger...

I can now run pots 2 and 3 (those before stage 3 and 4) completely open without the amp going batshit. There's just a lot of white noise and a bit of hum. With just pot 3 open, the rest closed, the amp is nearly silent. Gonna try swapping preamp tubes again.

Tanks for the help so far, I really appreciate it!
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
Last edited by Kanthras at Sep 18, 2010,
Page 1 of 2