Preamp issue?

Ether,

It could easily be aging and or bad tubes,I would switch them side to side and see if the problem follows.

Looks like you ruled out the other obvious potential problems.
Tubes don't last forever and have to be changed occasionally.

Best of luck to you,

Kenny.

Every issue I have had with tube preamps has been the result of faulty tubes.  Noisey, popping sound like pop corn, microphonic howl.  But never what you describe.  But I suppose it is possible.  Defintely swap the tubes - that is the first step in the process of elimination.

Are you sure it is your preamp?  Hate to state the obvious but you always need a spare set for this reason and for when they eventually crap out.

Good luck!

I'm voting for tube issues too. I've had power tubes go bad very quickly, leading to popping, crackling and the like.

The good thing about my pre coming with crappy Chinese tubes was that I replaced them immediately, leaving me a spare set (of inferior quality..) to diagnose problems with as to use while I wait for good replacements.

The first troubleshooting step should be to swap the interconnect cables at the amplifier input end and see if the issue moves to the other side. This will show that the issue is in the pre-amp (or possibly source) and not in the amplifier. If you determine that the issue is in the pre-amp or source, you can forget about troubleshooting the amp and speakers. You can swap other cables to isolate the pre-amp or source as the cause of the issue. Once you know the problem is in a certain component, then troubleshoot that component. If it's the pre-amp, swap the tubes side to side to see if the problem moves.

Of course, turn off the power to everything when you're swapping cables and especially tubes......

^^ What reubent said. Swapping the interconnect cables left for right is your first step, otherwise you don't know if its the preamp or the amps.

tubes dont last forever......it might be time for a new ones.

Thanks for the feedback... will swipe the tubes next.
I have replaced tubes in my amps in the past so this isn't anything that I am afraid of doing...I just didn't think that this "symptom" is characteristic of dying tubes.
Kind of weird for the pre-amp to "lose" some power in one channel.
Was more thinking of issues with resistors or other parts.

BTW - I have verified for sure it is the preamp and no other part of the system, including the interconnects etc...

Will get back to you guys tomorrow after testing...

Thanks!

Confirmed it is the tubes.....
Changing the tubes from L to R - made now R channel lower in volume than the L channel.

Question.... The original tubes are electro-harmonix 12AU7A.
I do have relatively new signal tubes Sovtek 6H30

Can I use them in my pre-amp?

Ether,

I'm glad you found the problem,you can't use the 6h30 in place of the 12au7's.

The 6h30 need to be used in a circuit designed for them,don't try it.

Best of luck to you,

Kenny.

I've had tubes on one channel lose gain or volume before but they usually get noisy and grainy in the sound first.

Please don't put the 6h30 in your preamp!  There are plenty of NOS and new 12au7 options you can try and they are quite cheap.  You can do a lot better than EH.