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In Reply to: RE: Me Again (Ping Mike... Anyone?) posted by Jonesy on April 08, 2014 at 19:13:11
Jonesy
I think you are worried too much about this..Bias is always set under DC static conditions.If the bias varies a tiny bit with the signal tubes installed,that is not a big deal.Short the inputs as Steve suggested and then set the bias.If it varies a lot,you could be picking up noise or a oscillation somewhere but let us know how much it changes when you put the signal tubes in.
A coupling cap can cause that issue as well..Did you change your output couplers?
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/08/14 04/09/14Follow Ups:
If so, you've found the stage with issues. If the bias is still stable --- keep replacing signals tubes one-at-a-time from the PI back to the input stage. Until you get the drifting bias. This will, hopefully, narrow the issue to one stage. Then, you can concentrate on lead dressing and components within that stage.
Good hunting and good luck!
Edits: 04/09/14
Hi Fender Lover,
Hoping you get an email saying "you've got mail"... :)
I thought I had this problem licked after replacing some trim pots, but it is back again so suspecting something as simple as a cold solder joint.
When you say to put signal tubes back in from "PI back to the input stage", I'm not sure what order you mean. Or what PI means?
Also, can I put in one tube at a time while the amp is on?
Cheers!
Ed
"I know just enough to get into trouble. But not enough to get out of it."
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Thanks for all the advice. I've attached some pics. Am I reading the advice correctly?
Mike, I haven't tried the coupling caps. Which would they be? Top or bottom of amp? (See pics.)
Steve, Mike, I will try the shorting caps and report back.
Fender Lover, Bias is steady when only the output, power, phase inverter and one small tube (12ax7) which I believe is part of the power supply, are in. Put the driver and signal tubes into one channel will affect that channel only (drifting).
So the 4 across at the front, which are the 2 matched pair output tubes (6550C), the power tube (6550C) at the back center, and the aforementioned 12ax7 just to the right of the power tube. The phase inverter (6DJ8)is near the back just to the left of the power tube.
Then I have a row of 3 tubes per channel, 6DJ8, which I understand the first two are driver tubes, and the very last one at the back is the signal tube.
Are you suggested having both channels of driver and signal out, then putting in a driver tube one at a time, check bias, another driver, then eventually signal tube. Starting with one channel then the other?
Thank you everyone for your help!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
The coupling caps are the Wonder caps but you have a bunch of caps on both sides so it's hard to tell from the photos but I can look at the schematic.The reason why a coupling can cause a bias drift issue is because they can leak a positive DC voltage from the plate or cathode of the previous stage to the output tube control grid..It only has to be one or two volts to cause problems.We will wait until after you short the inputs and recheck the bias.Also,I would change out those bias pots for some better quality multi-turn type.Mouser has them.
Honest amplification is better than excessive 2nd order distortion anytime.
Thanks Mike.
I'll try the plugs tonight.
Hope it's not a cap. Then I'll have to review all the cap shootouts! Sigh... I thought I was getting passed this... :)
Appreciate the tip on the bias pots!
Sincerely,
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
No, you don't have to review all cap shootouts because 90% of them are bull shit.In your amp,I would use Vitaman Q or K40y paper in oil caps..ARC products tend to lead a bit on the sterile side so putting paper in oil caps on the output tubes helps them a lot because it gives the amp a more spacious sound with more body.The other caps I would use the white solen film and tin foil.They are a bit pricier but only because you can't buy them on Ebay or the surplus markets like you can the Russian k40s or even the Russian Teflons..Lets see what shorting the input does first.
Honest amplification is better than excessive 2nd order distortion anytime.
bias on all output tubes still unsteady.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
...because things now become more complicated. I went back and read your original posts of a week or so ago and what you describe sounds like a low freq instability has developed when the overall feedback loop is closed. Taking out front end tubes opens the loop and promotes stability under otherwise marginal conditions. Based on your descriptions I'd be suspicious of power suppy/stage decoupling caps. They're usually electrolytics and have a finite life. Do you know if any of them have been replaced or are they original? I don't have a schematic so I can't be any more specific but maybe Mike has one handy.
I'll go through all my paperwork and check what's been replaced.
I know I have a schematic of either the amp or preamp somewhere. Maybe I'll be lucky and it'll be the amp.
"I know just enough to get into trouble. But not enough to get out of it."
When you put the shorting plugs in,are you hearing any noise hum or static when you put your ear up to your loudspeakers?
Honest amplification is better than excessive 2nd order distortion anytime.
No noise hum crackling.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
I want you to desolder the 1uf coupling caps C9 and C10 on the side that goes to the output tubes control grid and the 1k resistor..There are four of these caps and I only see C9 and C10 in the schematic as they are showing one channel..Anyway,the side that you lift up on each,plug the amp back in and measure the DC voltage on the desoldered side of each cap to ground..If it reads more than a half volt,change the cap as its leaky.
Honest amplification is better than excessive 2nd order distortion anytime.
Are the two resistors per channel just before the 6550 tube sockets, look like position R37 and R40?
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Well, I can't find any resistors 4 or 5 color banded that equal 1k in my amp. Schematic is MKII. Mine is MKI. I don't know if that makes a difference.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Don't worry so much about that,just do the caps that go to pin 5 of the output tubes...Drop me an email with your phone number.
Honest amplification is better than excessive 2nd order distortion anytime.
Contact info emailed.
Pin 5 it will be!
"I know just enough to get into trouble. But not enough to get out of it."
To try this. Argh. Patience.
Cant wait to get home.
Thanks Mike!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
That's why I love being a college instructor..You have summers off along with weekends from Thur thru Sun in my case and now I'm off on spring break.You have a fair share of meetings but that goes with the territory.
Honest amplification is better than excessive 2nd order distortion anytime.
I had a small business with one employee. Flex hours and freedom was great for about 10 years. Business went in the deep end. Back in the workforce now almost 2 years.
What happens happens. No regrets.
I'm sure you're a good instructor.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
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