2.8v on the bias left bias #8

Hi all, looking for some debugging pointers. My VT120 has been running for a few years, and I love it. It's pushing a set of very efficient Klipsch CF-4's. The other day I was listening to it in my shop and pop, all went dead. It has Genalex Gold Lion KT88's and a JJ tube rectifier. Turned out the 5A slow blow fuse popped. So given about 3 years of operation and questionable lifespan on the JJ's from what I've read, I figured it was high time to get a Weber WZ68, which I did.

I know I need to rebias the amp. So I pulled all the tubes but front left, set the switches back, plugged in teh WZ68, and a new 5A slow blow fuse. Set the meter on pin #8, and fired her up. Once the delay ran out, the voltage slowly climbed to 2.8V with the bias turned all the way down (counter clockwise). Strange. So I proceeded to install each additional tube which came in low with their respective bias knobs turned down. I adjusted each to .500V. For grins I turned the amp off, then started it again. Same thing. I swapped tubes at front left and of course same thing.

So I'm wondering what went south? Thoughts?

Thanks,

Filippo

If the 10 ohm resistor is "blown" the cause is a flash-over in this tube. This killed the gz34 (probably)
and the 5A fuse.

Replacing the GZ34 won't be enough, you should consider a new set of power tubes.
A tube that has flashed will most likley do it again.

Thanks Bob and Peter. I've gone ahead and ordered up another set of tubes since I have a fair bit of time on these (and the remaining match set gives me a spare pair). I'll flip this guy and check the bias resistor. I'll report back for posterity sake once finished.

Thanks again!

Filippo

Hi Guys, ok I have more information, and hopefully enough to debug. Hopefully Bob sees this post. I'll remind this is an ST120. A recap is that 3 power tubes bias fine, and one does not, holding steady at 2.8v at the test point. It was suggested that the 10 ohm bias resistor may have been shot when the KT88 flashed.

Here's what I have:
- the 10 ohm resistor checks out fine on the multimeter
- pin 5 on the octal socket reads 20v, adjusting the bias pot changes nothing
- checking the other 3 power tube sockets, I can swing 20v to 58v

soooo .... thoughts? Would it be a bad bias pot?

Thanks, folks!

Filippo

Actually I may have figured it out. Looks like a solder joint went bad on the PCB board, for the left front bias pot. The joint had a hollow in the middle of it. I'm now getting the full sweep on pin 5. I'll assemble and test the complete unit tomorrow and report back.

Filippo

p.s. Oops posted and then saw Bob recommend I check the pot solder joints. Yep ... and thanks, yes, negative voltages.

Alright, all is back together and running, with a new match set of KT88's from Genalex, which gives me a spare set of tubes. The Weber WZ68 is in, and all biased to .500v and holding nice and steady when warmed up. I learned a few things I want to pass along to others - those will little experience, as they will be "duh" for anyone with a clue:
- My amplifier had become a bit noisy. That was a sign of something starting to happen. Now all together with a few joints reflowed, it is dead, whisper quiet.
- I did not frequently check the bias. Dumb in hindsight. I would have caught this issue, most likely. I'll check it monthly from here on out.

This amp has been a joy these past three or so years. Through my Klipsch CF-4's it fills my luthiery shop with joy.

Thanks, Bob, for the support.

Filippo

As the chain of events seem to be a faulty bias pot(which is fixed now) chances are that your tube
did not be faulty at all. You should save these set for future use.

The bias pot the was not faulty (I have not replaced it), but rather a solder joint was faulty at the bias pot. I'm pretty sure that tube got cooked. Of the four tubes, it's the only one whose silk screen is now beige (the rest are red). Before fixing the solder joint, I couldn't get less than 2.8v at the bias point. I think the safe bet is to send that tube to the dumpster in the sky. I'll keep the rest as spares.

Filippo

The lead-free solder is to blame for lots of these problems. Its more difficult to solder, it
heats the components much more ( most important for surface-mount ) , and it degrades
in a way that 60/40 tin never did.