Preamp? Thoughts on my system? IMF speakers.

[Paul Birkeland]

You may just have one dirty tube pin.

[Paul Joppa]

I agree with PB as to the likeliest cause. But an alternative (less likely but not impossible) explanation is possible:

There are two triodes in each envelope; one is the driver stage and the other is the shunt regulator. The two channels switch which triode serves which purpose. Since shuntreg operation is dominated by the 431 chip, it may work OK with a marginal triode when that marginal triode is unsatisfactory for audio.

[jbrahen]

Thank you both so much for helping me troubleshoot.
I didn't see any obvious dirt on any of the pins, but still cleaned everything with some deox-it. Sprayed into the connectors as well. On retry, the same issue was present with the same tube only on the left side. So, no change.

I had some other 12AT7s, so put those in place. I checked the voltages and now the low KREG B voltage is back - 2.85 with the new 12AT7s, unchanged if I switch the tubes around too. I connected some speakers and alas, the crackling sound is back in the left channel when I move the left tube!

Im interpreting all this to mean there has got to be a loose connection somewhere on the left channel. I quadruple checked all the pin connections and they are SO solid but maybe I should just reflow everything.

I will keep hunting and reflowing until it works, unless you guys have any other ideas. Thank you so much for being so supportive!

[Paul Birkeland]

It could the be the non socket end of something connected to the socket.  One I would definitely recommend looking at is the small resistor on that socket that will either go to pin 2 or pin 7 (can't remember which for the left side) to the center post of the socket.  That is something we put in for the initial build that doesn't actually get used until you put in the SR upgrade, so a problem with that connection could go unnoticed until now.

[jbrahen]

I checked and reflowed every joint on the driver sockets and all non-socket ends. I tested all those resistors as well, they all check out. Both KREGs on the shunt regulator board then tested good! Then I mounted the shunt regulator and the small PCB back on their standoffs and KREG B tested bad again after that. I went crazy reflowing all those connections again. Still tested bad. I then took a breath and went to the bathroom.

With a clear head, I decided to just take out the shunt regulator. It can go back in later with a fresh start.

Now amp is back to it's glorious non-upgraded version. It sounds glorious, but i'm not sure if I heard what the shunt regulator can add (is it significant?)

All measurements checked out except two I want to mention to you:
Terminal 7 reads 240 VDC
Terminal 14 reads 258 VDC

I want to make sure 14 isn't concerning, seems outside of the 10% tolerances (supposed to read 170-230)

[jbrahen]

oh, also want to include that I burned the tip of my finger with the soldering iron. Not sure if that affects the sound of the amp, but wanted to mention. It doesn't hurt that bad, since I'm really strong.

[Doc B.]

It doesn't affect the sound, but it smells like steak hitting the grill.

[Paul Joppa]

T7 and T14 are the 12AT7 driver plates, and should be within the 170-230v range. The higher voltages you measured indicate weak emission from the cathodes, especially T14. This is a probable reason it sounds off-kilter.

Many times an old tube will recover emission with 50-100 hours of run time re "re-form" the cathode surface. No signal is needed, just power.