pop/crackle in one channel

After getting my amp running again with a variac to keep the AC in the 117-118 range, I seem to have found an new problem: occasional pops/crackles in the right channel. The pops/crackles can be heard during quiet passages with music playing, with the amp at idle, and with the inputs shorted.

The problem remains in the right channel after swapping output tubes left-right, trying another set of output tubes, and after swapping the outer pair on the VTA driver board left-right. I also swapped the speakers, and the noise could be heard only in whichever speaker is connected to the right channel.

Are the relevant sections in the troubleshooting sticky the following?

6. Problem - Hiss, noise, crackling sounds on idle
Areas to check - Noisy driver tubes, noisy resistor on driver board (especially older carbon composition resistors), bad solder joint or bad solder trace on the driver board.

7. Problem - Popping or snapping sound
Areas to check - Usually means that high voltage is jumping to chassis ground somewhere. Could be anywhere even might be inside a tube

I also found this suggestion for the ST-35: "Try powering up with the pre amp tube(s) removed for that channel, leaving only the EL84's in to see if the noise stops." (see Re: rushing with crackle sound coming from my ST-35. Which, I'm guessing, helps to determine if the problem is in the front end of the amp. Is this applicable as a trouble-shooting step for the VTA ST-70?

What else should I be checking: solder joints on the the right output tubes, connections between the right output tubes and the driver board, ...?

Bob Latino wrote:If you have a VTA ST-70, try changing out the CENTER driver tube. The center driver tube has TWO triode sections - one for each channel. The triode section for the right channel in this tube could be going bad ...

Bob

Same situation happened to me recently when I swapped the center driver. In my case, it was merely a matter of carefully cleaning the tube pins and the socket with DeoxIt product. Won't hurt to try. If you've stored the amp for any amount of time, this is likely to be the culprit.

Thanks for the suggestions.

Swapped the center and left 12AU7s on the VTA driver board and the crackling moved to the left channel, but quite a bit fainter. I needed to be right next to the speaker to hear the noise. No audible crackles in the right channel.

There is a little bit of green corrosion on some of the pins, right where the pins exit the envelope. I cleaned the spots I could reach with the brush I have on hand, but there is still some faint noise in the channel with the suspect 12AU7. I'll need to get an inter-dental brush to get at those spots, and probably some DeoxIt.

Sounds like I should order one or two 12AU7 or ECC82 as well.

sendwaves wrote:Thanks for the suggestions.

Swapped the center and left 12AU7s on the VTA driver board and the crackling moved to the left channel, but quite a bit fainter. I needed to be right next to the speaker to hear the noise. No audible crackles in the right channel.

There is a little bit of green corrosion on some of the pins, right where the pins exit the envelope. I cleaned the spots I could reach with the brush I have on hand, but there is still some faint noise in the channel with the suspect 12AU7. I'll need to get an inter-dental brush to get at those spots, and probably some DeoxIt.

Sounds like I should order one or two 12AU7 or ECC82 as well.

You might want to try a 12BH7 in that center driver position. Many of us have found it gives a more rounded sound.
Always clean your pins and sockets when switching tubes. Retentioning the sockets is also a must. Also, work the new tube in and out of the socket several times. All this info is available in detail elsewhere in this forum. Good luck!

DeoxIT arrived today. Net is that after cleaning the pins and sockets the noise is significantly reduced and a little different in character: instead of abrupt pops, it's more of a soft crackle. In the configuration with the 12AU7 the has one suspect triode section installed in the left position on the driver board instead of the center, the noise is now barely audible with my ear within an inch or two of the speaker grille.

I'll probably order some spares and consider getting a 12BH7 to try in the center position.

I would check for loose (unsoldered) wires. If it's not that then consider replacing the sockets. You may want to pick up some spare tubes just in case it's a bad tube(s).

sendwaves wrote:Thanks for the suggestions.

Swapped the center and left 12AU7s on the VTA driver board and the crackling moved to the left channel, but quite a bit fainter. I needed to be right next to the speaker to hear the noise. No audible crackles in the right channel.

There is a little bit of green corrosion on some of the pins, right where the pins exit the envelope. I cleaned the spots I could reach with the brush I have on hand, but there is still some faint noise in the channel with the suspect 12AU7. I'll need to get an inter-dental brush to get at those spots, and probably some DeoxIt.

Sounds like I should order one or two 12AU7 or ECC82 as well.

I think that you just confirmed that at least one ecc82 is bad. I'd suggest you buy
some more ( and throw away the left ecc82/12au7 tube when the repacement has arrived)
Do nothing more until you have replaced the tube.