The 4 20K Bias Pot Mod That Could NEVER WORK!

Dynaco-Doctor Forum

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Yup, it's me again...
I've verified that ONE of my EH 6CA7 Fatbottles is indeed DEAD. However, as Russ had suggested - I could get NO SUCH READING of ANY OHMS to Ground no matter WHICH tube I tried - and from any combination of Pins 1 and 8 (together or singly). So whatever.
I just put the tubes back in (swapping their positions) and noted that ONE of them was dead (I don't remember not seeing the filament glowing - and this one tube wasn't hot like the others when I pulled it out - so perhaps it wasn't getting any plate voltage). Anyhow, I put another quad of EL34's in and at least INITIALLY, ALL four readings were near target! And I was filled with glee:-D
So here's what's frustrating the hell out of me:
With regard to the Bias READINGS, SOMETIMES I can get readings of NEAR .5 Volts (the tartgeted voltage) and SOMETIMES I can NOT! My voltmeter switches back and forth between .XXX volts and XXX millivolts, which I SUPPOSE are the same thing - but EITHER WAY, it doesn't seem like the 20K pots have ANY effect WHATSOEVER upon these figures - - in ANY WAY!!! I can swing them back and forth all day and get virtually no change - so what's happening here???
I will go and take some voltage readings internally and see what could be going on at Pins 5, etc... but for the time being I'm getting variations of .9 Volts on some (like maybe one of the) tubes and 373 millivolts on others - and .7 Volts on others - - and again, no matter HOW the pots are dialed in, it makes virtually NO difference on the voltages I can measure! It's almost like they're just not even in the circuit at all - so again, what's happening here???
Are they WIRED incorrectly? I doubt it at THIS point, since I've verified the CURRENT scheme from a number of different sources.
Furthermore, there is a slight crackling sound coming from the Left Channel... OH YEAH - by the way, the MUSIC sounds pretty good:-D I hooked up the amp to my preamp and cd player and it DOES INDEED function as an power amplifier now, so THAT's the GOOD news!
Unfortunately, I'm getting a massive hum when the PREAMP is connected - and I DID install a switch to connect or disconnect the Ground Prong IN THE ST70, but it makes little difference here, since it seems the hum is coming from the preamp (although it NEVER DID before and was DEADLY silent).
So to reiterate, the ST70 "works" and you can now play music through it - but the pots have virtually NO INFLUENCE on the voltage readings and the readings are nowhere NEAR .5 volts anymore... They seem to appear this way when the amp is first turned on - but with time they are simply "on their own" and I can have no effect upon them whatsoever!
No explosions, no burning smells, no smoke, etc. - but I am still unable to get it to do what I wanted - - and I am frustrated as hell!
Yes, with regards to the 4 Pot Mod - "DON'T DO IT" is right!!! But once again, it has nothing to do with the sound (although I will NEVER be able to verify THIS, since I haven't HEARD my amp play MUSIC in almost 2 YEARS!). "DON'T DO IT" because it DOESN'T WORK!
Now obviously I'm kind of pissed off right now... and maybe I will figure out how to MAKE this thing work with more help. In any event, this mod (and NOT the PDMPS Mod, which I'm pretty sure I've corrected at this point) has been nothing short of a NIGHTMARE for me since I went about it almost two years ago!
In fact, for what it's worth - I really WAS able to use this amplifier WITH the two power supplies hooked up as they were in my previous scheme. I listened to and enjoyed music for SEVERAL HOURS this way in fact - and I had even USED a separate 5V Filament Transformer for the SECOND GZ34 at one point - and so really DID essentially have two separate power supplies. The only REAL problem I think I had with the former scheme was that those voltage dropping resistors were incorrect... But I'm rambling again. All I will say is that I did not simply IMAGINE hearing music played through this amp for SEVERAL HOURS, sounding DAMN GOOD! It was REAL (believe it or not). But alas, maybe that session (in and of itself) "KILLED" my Dynaco - so who knows?
I am currently using 4 "matched" JJ E34L's from 2001. They were probably used for less than 6 months of totally intermittent service, so still have very LOW hours on them. I LOVE these tubes and really don't want to kill them too!
I have done SO MANY things to this amp over the years, but NOTHING has been more upsetting and nightmarish than my multitudinal attempts at making this 4 pot mod WORK! I don't know WHY it APPEARS to "work" at first and then doesn't - and I REALLY don't know why the pots have virtually NO EFFECT upon the voltage readings... If I can get some help HERE, perhaps this nightmare will finally cease to be - and I will be able to move on with my life - but for now it continues - and I am just so sick of it... I'm just really sick and tired of dealing with it.

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Kimber Kable

Topic - The 4 20K Bias Pot Mod That Could NEVER WORK! - BrianM23@msn.com 15:46:55 07/14/06 (4)

More Information (from conversations OFF the web): - BrianM23@msn.com 00:39:43 07/15/06 (0)

For Russ - Some Important Corrections. . . - BrianM23@Msn.com 23:59:36 07/14/06 (0)

Ever consider decaff? ;) (nt) - Marshall W 19:35:53 07/14/06 (0)

More on Bias Voltages. . . - BrianM23@msn.com 18:45:22 07/14/06 (0)

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