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I was just starting to get the hang of leaving feedback to fellow inmates and dang... now I have issues!I was doing some tube rolling in my Audio Research D70 driver positions V2 and V3, adjusting all bias positions and tweaking the output tube positions as well accordingly.
All going well, but when I put my original bugle boys back in V2 and V3, I now could not bring one of them down below 100 (where 90 v is the norm). (Unfortunately I did not check the bias readings before the swapping to see if it was out of whack already.)
I swapped V2 and V3 positions and the tube was still unable to come down in it's new position. The other trimmed well.
Okay, so maybe the tube was passed it's prime. But here's the deal. I had a spare set of bugle boys, one of those I could not bias down, and of the new dozen of (cheapies) 6922 Sovteks and 6DJ8 Sylvanias I keep around for the other spots as spares, 7 out of 12 I can't bring down below the mid 90 v's.
This took a long time to do, swapping positions, allowing settle in time, reversing positions, etc!
Is it just luck of the draw on the tubes, or is my bias circuitry probably getting out of controllable range?
Last night at 3:00 am I just about had with tubes! Luckily this morning my D70 just looked so good sitting there I'm giving it another chance!
Save me from solid state!
Follow Ups:
I can't find a schematic on line for the D70 but I don't understand why the front end tubes should make a difference to the output tube bias. This could only be be true it the driver tubes are direct coupled to the output tubes. (I think I have a paper copy of the schematic, I'll look for it later today and let you know)You may have a coupling cap between the driver tube and the output that is leaking.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks so much for helping me with this Tre'.I tried cleaning all of the power and driver tube trim pots to see if that would make a difference in the bias "range" dialing of the driver tubes. Sure made biasing a lot quicker with almost zero fluctuation once set. But in the end none of the suspect New or NOS tubes could be dialed in any lower, still over 100.
I should also mention that none of the dialed trim positions have ever really waivered much over the years, and they all are pretty much mirrored from left to right channels. This must be good.
On the asylum about a year ago that I posted that I couldn't dial in some new sovtek 6922's. One inmate replied they were probably out of spec. That's when I first tried some really old tubes I had kicking around for the first time. I found I could easily dial them in, was blown away by the sound, and got into the whole NOS use, at least in the critical positions, ie sections V1, V2, V3. Eg Phillips, RCA, Bugle Boy, etc.
But it just doesn't seem right that if I buy 7 new sovteks to have on hand for back-up emergency situations (eg. while bidding on NOS, ha, I'm hooked!). Likewise 4 new sylvanias that I bought in the 80's and never used, and can only dial in 4 out of the whole combined batch of 11.
Now one of the Bugle Boys that worked fine until my rolling. And one(1) of the bugle boys that I had ordered as a spare matched set a year ago but never tested at the time. Serves me right if it turns out to just be bad and out of spec tubes in the end!
Sorry for reiterating my previous post by laying out all the facts again, but it's therapeutic!
Also, I can't quite figure out going through the archives if a tube is bad/going bad if the bias can't be dialed up/down or either?
Whew, thanks for reading all this!
Sorry, the schematic I have is for the D76. It's not direct coupled and the driver tubes should not have anything to do with the output bias adjustment.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre'Thanks for your trouble. Will keeping pluggin' along here.
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