#1
I'm not exactly ready to spend money on a bias probe when I plan to use it once in a very long while. I'm trying to bias my amp and cannot get a decent plate current measurement. I have the plate voltage coming in at 473 volts. This is my 6505+ by the way, runs on 6L6GC's.

To measure the plate current I soldered one end of a ten ohm resistor to the pcb where pin 8 from the tube hooks up. I tried grounding the other end of the resistor directly to the chassis on an unpainted spot as well as soldering it to the chassis ground. Either way I cannot get a voltage drop reading. I have verified that my DMM is functioning.

I can't figure out what I'm doing wrong. It has to be something obvious, but I'm too tired and crabby right now to go troubleshoot it anymore.

Any help

Also, I am aware that it is SOP to use a 1 ohm resistor instead of a 10 ohm, but that's what I had laying around and they are in the same order of magnitude so I figured it would work. I don't think this is the issue.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
#2
By definition a 10 ohm resistor is a whole order of magnitude bigger than a 1 ohm one.

Sure you are using the right voltage scale on the meter?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#3
Problem: Too tired and crabby and using the wrong resistor?

Solution: In the morning, take the amp to a proper technician and let him solve the problem.
#4
Quote by Cathbard
By definition a 10 ohm resistor is a whole order of magnitude bigger than a 1 ohm one.

Sure you are using the right voltage scale on the meter?

Well that's what I get for using technical lingo when I'm tired. Either way ohm's law works the same and I don't think the slight amount of extra resistance will bring in enough error to completely throw off the calculation. I'm trying to set it for 30-40 mA so I should be getting a reading of approximately 0.35v. The problem is I'm not getting a reading at all.

I did double check the meter and tried it on all the voltage scales with no luck. I also confirmed it was functional.

I rechecked the connection and resoldered it a couple of times to no avail. I even swapped power tubes to make sure. I double checked that I was connected to pin 8 as well. I suppose I could have screwed that up, but I checked on the sockets to see where pin 8 was on the board.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
#5
You cut the trace that connected pin 8 to ground didn't you?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#6
You already changed your bias resistor?
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#7
Quote by R45VT
You already changed your bias resistor?

Yeah. I had the pot set really low because without actually measuring I didn't know what kind of current I had in there. Better safe than sorry.

Quote by Cathbard
You cut the trace that connected pin 8 to ground didn't you?


I knew it had to be something obvious. I was going to put the resistor in series with the pcb and ground without cutting it, to make this modification reversible if I ever needed to switch back for some reason. Once I found out I had to remove the power pcb to do this I decided to just be lazy and connect it on top, but forgot to slice the trace.

On a positive note, the bias mod combined with a 5751 in V1 and 12at7 in the PI really did wonders. I'm still deciding on whether or not I want to replace a few caps in the lead channel to give it a bit more raw low end. I figured that combined with the bias and preamp tube selection will sound amazing.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Last edited by Tremolo Bum at Mar 5, 2013,
#8
Alright, I went back to work on it today and stopped the trace from pin 8. First i tried to just cut it out, but it was at an odd angle so I just took everything around it out and drilled it. I checked and there was no continuity over the trace. I soldered in my resistor and wired it to the chassis ground and checked it to make sure it was continuous, which it was. Still no voltage reading across the resistor, which is now a 1 ohm resistor.


I also noticed the two tubes nearer to the center of the amp were significantly hotter than the two near the edge, but all the tubes lit up. I play tested it and it still plays fine. Could there be another underlying issue here like a bad screen resistor? I don't really know the symptoms that would bring about but something seems odd here. I did get the amp secondhand, so I don't know for sure this amps history so anything is possible I guess.


Here's some pictures of how I have the resistor wired up with my crude labeling of the tube pins.


The harsh light really makes the scratches on top look bad but they don't cut into anything and are light surface scratches.



-Edit- It's a bad picture but the resistor leads are vertical with respect to the pcb, it's not laying flat on the board across anything else.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Last edited by Tremolo Bum at Mar 5, 2013,
#9
By hotter are you referring to the bias as being hotter? Or temperature of the tubes?
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#10
The actual temperature of the tubes. I pulled the two on the edge out when I was done and when I went to pull the third one out I almost burned my hand, but the first two I pulled out weren't that hot.

Here's a link to a diagram of the power amp section. I have the full schematic on my computer as well.
https://lh3.googleusercontent.com/-aGjKzEHBXcE/T_SQXbnRI6I/AAAAAAAAEBk/re3FTB-fGz4/s1440/5150%2520II%2520PA.png

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Last edited by Tremolo Bum at Mar 5, 2013,
#11
You only checked the bias of one tube?


Edit: I hate to say this... Would have been worth the 20 bucks for the bias probe.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Mar 5, 2013,
#12
I haven't actually checked the bias on anything because I cannot measure the voltage across that resistor. My plan was to wire the resistor across the bias probe points in the back of the amp and cut the traces on that pcb so I could use the test points to measure my bias current. Yes, that would only allow me to measure one tube, but I want to set my bias conservatively so that it won't be killing tubes, but will sound better than stock.

Right now I'm thinking something else might be wrong because I cannot figure out why this isn't working. I don't want to cut the traces on the other tubes yet if I can't get this one to work, but I may have to to try those. I can always wire them back up, but it will be less convenient.

I am probably going to remove the power pcb and check all the solder connections and really clean out the sockets in case that is causing the issue.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Last edited by Tremolo Bum at Mar 5, 2013,
#13
With it all powered down and discharged measure the resistance across the 1 ohm resistor.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#14
When I did the continuity test I got a resistance reading of 1 ohm. My multimeter cannot read low resistance any more accurately.

Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
#15
As long as it reads the same as it did before you installed it you're sweet. You have to check the resistor you use. I had to test 5 or 6 before I found a suitable resistor for my JTM. When you start getting that low it's not uncommon to get out of tolerance examples, even when using expensive ones.

Check between the resistor and earth. ie. make sure you have a good earth.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#16
Alas, I have found the problem. I'll elaborate for anyone who happens to find this thread via a google search :P. I figured I had screwed up something while installing the resistor, but it turns out the amp had problems all along. I did everything with the resistor correctly.

I found that the screen voltage was 1 to 2v on the pins with the cold tubes, hence why they weren't hot. It turns out the screen resistors for those sockets were not functional. The other two sockets screen resistors were fine. This is also why I could not get a voltage reading across my 1 ohm resistor. There wasn't actually any current flowing there (duh).

Anyone know what may have caused this? Hopefully I can get replacements in town tomorrow. I want to get this project finished up. It sounds amazing even though its half broken, lol. Any suggestions on other resistors other than the stock values for these things (100 ohms, 5 watts)?


Heres a picture. They're the big white ones, the two leftmost are the ones that don't work.


Jackson RR3 Rhoads and DK2M Dinky
Peavey 6505+ w/ Avatar 212 cab
Ibanez TS9, ISP Decimator, MXR 10 Band EQ
-Digitech RP1000
Last edited by Tremolo Bum at Mar 6, 2013,