Little sound from left channel

I could use a little troubleshooting help with my ST120, which is about three years old. This design uses 12AU7 tubes.

I am getting virtually no sound out of the left channel (just some light crackling sounds related to the music playing). The right is fine. I checked all four bias voltages and they were OK. I have since changed out all of the tubes with no change. I also swapped the input channels and separately switched the speaker connections. In both cases, the failure stayed with the left channel of the amp. This all leads me to think that something other than a tube has failed, affecting the left channel only.

I am an Engineer, but amplifiers are not something I'm familiar with. I bought this amp assembled.

Any troubleshooting suggestions would be welcome. Thank you.

One common issue is the LM334 located at the cathodes of the LTP, it's easily damaged
by tube rolling.

Could be loss of signal, RCA's are notorious for losing contact. Check if plug feels loose, possibly try different cable.

I already swapped cables around to confirm that neither they or any upstream equipment were the cause.

I like the the LM334 idea. I'm going to try swapping out the one on the left side.

Thanks for the help.

Hello, Peter. Is there an easy way to test the LM334 in circuit? Dale

Dale Stevens wrote:Hello, Peter.  Is there an easy way to test the LM334 in circuit?  Dale

Not as one know of ( except from comparing the voltages from the other side)
But they are easy to replace and reasonable cheap , thus one could replace to test.