Tube amp noise question

I’ve got a Zesto Bia 120 on trial from a dealer and they included two sets of KT88 tubes they said have very low hours, maybe not even broken it yet. The amp also is supposed to have very low hours and it looks like new FWIW.

No matter how I rotate the tubes (amp is auto bias) I eventually end up hearing some degree of noise typical of bad tubes in the V1-V2 pairing. Crackling, “frying pan” type noises. When the tubes that are noisy in V1-V2 are rotated to the V3-V4 sockets they are quiet. One set of tubes (Gold Lion Kt88) is very noisy, you can easily hear the noises at idle from the listening position 10 feet away. The original (JJ Kt-88) tubes are less noisy. I found one pair of JJs that was completely silent for 10-12 hours and then started making small amounts of abnormal noises on warmup and has otherwise been normal after the amp has warmed up (so far.) The amp sounds fantastic except for the noisy crackling noises at idle.

The dealer is perfectly willing to take the amp back and investigate and do what’s right. I just don’t want to subject the amp to 4000 miles of round trip FedEx abuse again unless necessary. My question is: how likely is it that this could be an issue only with the tubes?

I suspect the tubes.

If V1-V2 has the noise, I would clean the sockets and make sure the connection is good on those sockets. Tubes can be finicky on their connections, not all tube brands have the same diameter of pins.


Easiest experiment is to start with a fresh batch of tubes.

1 Like

I cleaned the tube sockets with a socket brush and contact cleaner and that did affect the noise level a bit for the better. New tubes are on order.

1 Like

Have you retention the sockets? You can take a small jewelers screwdriver and gently push the female pin towards the center for a tighter connection. Is the static noise only in one channel?

No I didn’t re-tension the tube sockets, never knew about that trick. Yes, the intermittent noise is only in the left channel. The Gold Lions are noisier than the JJs and in addition to the “frying pan” noises they also make noises when they’re warming up that very faintly sound like someone’s in the basement banging on the water pipes. You have to put your ear right up to the midrange driver to hear the “water pipe” noise it’s so faint.

I’ve had to clean older nos tube pins before. Used my Dremel, low speed with various buffing pads with good results

May want to try cleaning the pins (by whatever method you see fit) while waiting for replacement tubes if the pins show any tarnish

Sounds like tubes to me as well…


Your noise issue most likely is coming from a 12AU7 tube if you can Swap one out at a time with a new one.

I agree… we tend to focus on the big power tubes, but I can’t remember having noisy power tubes. It was always a 12AU7 or another little guy … but only the classic NOS expensive ones. Newly made tubes… well I never had a problem like noise… only from the expensive, old classic ones.

Bruce in Philly

1 Like

I thought it was likely the 12au7s as well and first tried two new sets of them but that had no effect on the noise. Prior to the noise issue developing I did have one of the JJ 12au7s die within a few hours of firing up the amp. Completely kaput, no sound at all.

I have had noise in NOS tubes and new production small tubes and the noise is just as he described. My power tubes have always just failed without warning.

I wonder if the biasing circuits are malfunctioning. Something is not right, obviously. Maybe the gear needs a thorough going over.

I once had a problem with a VTL monobock amp… called VTL and got a “it shouldn’t do that” and they wanted me to ship these monsters to CA. I called Conrad Johnson as I thought the problem may have been my CJ preamp… the engineer knew exactly what the problem was… it was the VTL amps not handling voltage sags at my wall. No need to ship, but I installed a power system that kept voltage constant and fixed the issue. In short, you need to talk to an engineer… someone who knows tube equipment.

Bruce in Philly

1 Like

I have seen the source be a problem like this, have you swapped the left and right inputs to see if the problem changes channel or stay with the left? @Photon46

The noise occurs with or without an input signal so i didn’t see the need to swap inputs.

1 Like

Bingo… that was it. You don’t need an input to hear noise from these tubes.

Bruce in Philly

Even after installing a brand new set of matched KT88’s the amp had intermittent noise issues (at a lower db. level) so I returned it to the selling dealer. A pity because it was truly a wonderful sounding amplifier. It did generate a tremendous amount of heat though.

Hopefully a better opportunity for a used tube amp in good condition shows up.