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I recently rebuilt a 1963 Stereo 70 using a DIYTube driver board and have noticed that in the left channel, the bias is very touchy and tends to drift at the slightest provocation. The end result is that either of the two tubes starts to get a little toasty (visibly) and I jump up from my chair with the mini screwdriver and DMM and rebias that channel. The other channel, however, is steady as bedrock. I have not (as of yet) tried swapping the tubes to see if is the tubes or channel because biasing is a PITA, but that's probably the next step.

One additional clue(?) is that the left channel gives a thump on turn-off whereas the right channel is silent upon turn-off. Could one of the coupling caps be passing a small amount of DC and causing the bias to be twitchy (as well as causing the turn-off thump)? Caps are 0.047uF K40Y and 0.1uF Sprague 715P (and are under the board).

Thanks,

-D

p.s. Here's the amp (at an apparently stable moment):

Swapping the left channel and right channel tubes offers a slight improvement (e.g. the former right channel tubes balance fine, the former left channel tubes balance with a slight amount of drift, but it could add up after a long listening session). The thump on shutdown has moved to the right channel, so evidently it is (somehow) related to the tubes.

Any ideas?

-D

My money is on a buggered pair of tubes

Measure the voltage drops across the 150K grid resistors (R23-R26). If just one of the pair is dodgy, you'll know quick. Post your measurements if you can. Possibly we could lower the 150K grid resistors to help a leaky pair out, but 150K seems plenty low enough for EL34s. Also remember that due to the criss-cross drive lines that a front resistor is for the back EL34 and vise-versa.

Shannon

Geek wrote:My money is on a buggered pair of tubes

Yeah...unfortunately that's what I'm thinking as well, given that the behavior follows the tubes. The tubes are AFAIK original to the amps (RCA/Mullard) and test OK, but fail at higher heater voltages so they are getting somewhat close to the end of the dusty trail. I was hoping I could eke a bit more time from them, but if I can find a good pair of Dynaco badged Mullards to match the other two tubes in the amp, that'll be just fine.

For the record, the sound is really, really good. Not quite as good as my P2P / EFB Stereo 35, but pretty damn close. Guess it stays while one of the other amps gets sold...

-D

I finally was able to get the two RCA/Mullard tubes to stabilize. They'll now drift back and forth a bit between +/- 8mV difference between the two once warmed up. Not ideal, but good enough. The Dynaco/Mullard tubes, on the other hand, just lock into their bias point and camp there. Here's to good-quality old-stock tubes!

-D

Spoke too soon. Started drifting again last night. Looks like I need to do some tube shopping.

Dammit.

-D

I all ways liked the sound of 6L6GC's or 5881's in my ST 70 over EL34's .

battradio wrote:I all ways liked the sound of 6L6GC's or 5881's in my ST 70 over EL34's .

GASP!

*has a coronary*

KT-66's put on a show...

20to20 wrote:

9.jpg

KT-66's put on a show...

Pretty...

However, I'm still sticking (for now) with the Mullard EL34s...especially if I can find another pair of Dynaco/Mullards that test well and are well matched.

-D

Got 'em. Did the trick. Stable bias and this thing sounds great!

-D