SII preamp tube question.

Karl5150

Karl5150

Member
Hello everyone, it's been a minute. Anyway, my SII has been sounding great for 10 years but recently developed a noticeable channel imbalance. With ALL the bells and whistles installed, there is no longer a balance control. Cable and speaker lead swaps isolated it to the amp. Swapping the power tubes did nothing, but swapping the preamp tubes brought everything back into line. The pins looked clean. (They had originally been cleaned and treated with a silver-based contact improver.)
Since the volume issue was resolved, not switched between channels, I'm assuming the amp uses different halves of the triode in the original build. If I am correct in this assumption, am I now feeding one channel's shunt regulator suboptimum signal?
Thanks as always, Karl.
 
It's possible. Have you swapped the tubes back to confirm it wasn't just dirty pins?

First check the both triodes in each tube are glowing. If it's still a problem, then voltages on the driver/regulator sockets will tell. Test pins 1-2-3-6-7-8 on each channel. Be careful - those terminals are hard to get to without inadvertent slippage!
 
Both sets glow in both tubes. Switching back puts Tom singing from stage right again.
I haven't checked voltages yet, but just knowing something is amiss will have me ordering a new pair. (They are the NOS pair that came with the kit)
Recommendations for something sweet that won't break the bank?
Thanks Paul and Paul,
Karl
 
I'd just swap them so the image is centered, then let them run. The shunt regulator is not super picky about such things compared to the grounded cathode stage that drives the 2A3s.

You'll find that on the smaller green C4S board that either OA or OB will be higher than it calls for in the manual, which would indicate lowered emission from that particular triode. In the shunt regulator position, this will cause one of the Kreg voltages to drop a little bit, but as long as it's 3+V, that will work OK.
 
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