LXII CH A quieter than CH B when in stereo

Power Amps LXII
1

May I pick a professional’s brain on something?

I bought a used LXII a few months ago. Used it for a couple practices and at a show with no issues that any of us in the band noticed. Typically running a helix rack in stereo to the LXII then out to 2 8 ohm 4x12 cabs with the LXII volume consistently around 65-75%

I recently ordered new tubes and installed them. From Tube Depot I ordered a matched pair of Sovtek 5881/6l6WGC (initially forgot to order 4 instead of 2) and from Ruby Tubes a matched pair of 12AX7AC5s HG+ for the phase inverters. Since I had mistakenly ordered only two power tubes at the time I installed them into CH. A. Upon powering up the next practice the fuse on CH A blew.

I’ve since replaced the fuse and the last set of power tubes. I’m getting sound from both channels now however, CH B is significantly louder and also seems to have more presence.

For trouble shooting I’m running a Helix amp sim in stereo into the LXII set to stereo going to separate 1x12s at 8 Ohms. Because I’m testing this mostly at night in an apartment I’m keeping the LXII volume no higher than about 15-20% as it gets loud quick.

Things I’ve done to troubleshoot:

  1. Swapped speaker cables to rule out both speaker cables and speakers
  2. Swapped the 1/4" instrument cables running from the Helix into the LXII to rule out 1/4" inst cables
  3. Swapped power tubes between the channels to try and replicate issue on CH B.
  4. Swapped phase inverter tubes between channels to try and replicate issue on CH B.
  5. Even though it wouldn’t make sense I swapped the fuses between CH A and CH B.
  6. Put all the previous tubes that came with the unit back in

None of these have fixed the imbalance of volume between both channels. The only other thing that comes to mind is getting up early during the day and cranking the volume a bit to see if at higher levels it evens itself out. One thing I did notice is when I was switching around the power tubes in between playing a few notes on guitar was that the tubes in A seemed hotter to the touch than the ones in B. I didn’t think to turn it on with the cover off and watch the glow on the tubes as initially I figured it was probably safer to keep it on when power was being supplied to the unit.

Looks like I’ll probably have to send it somewhere to get looked at but any advice would be much appreciated. Especially if anything I’ve described raises any flags for you.

2

Hello Jay and welcome to the community!

Thanks for the excellent question, it is really well posed, that helps a lot.

Is good place to start would be to open the unit and take a photo so it fills the frame and high resolution so text on the tubes can be read. It sound also be useful to know the serial number, when and where you purchased it.

I and suspecting the tubes are not matched or if they are their plate current could be a poor match for how the bias is set.

3

Thanks for the response. I’ll check it out next weekend when I have off and get back to you.

4

Hi Jay,

The preamp tubes and power tubes must be fairly closely matched to make sure they are not contributing to a volume difference between channels. No need to be too OCD about this though. Generally if the power tubes are a matched quartet, that will be the most important in balancing the channel outputs.

You can also run one side of your Helix into the LXII A/Mono input, keeping the amp in Stereo mode and out to your two cabs. That will eliminate the possibility that your stereo source is not perfectly in balance.

Keep in mind that at very low volumes, the volume controls don’t track as accurately as a higher setting, like around 12:00. That can contribute to a bit of a channel imbalance.

A workaround for that is to turn the output level of your Helix down and turn the LXII volume up accordingly. That will get the volume control up into better tracking territory.

Dave