Rebel 20 - valve noise, microphonics and hum?

[simon_d]

Hi

Had my Rebel 20 for a week or so, and I like it a lot but it has developed some noise (crackly noise typical of a bad preamp valve) and a bit of a farty sound on low notes. So I swapped out the12AX7s for a few others I have, but no matter what valve I tried in V2 it was always very microphonic and the farty sound didn't really improve much.

Also the amp hums when I touch the Bright and Deep switches. Is it broke?

 

[Steedee]

Have you swapped out V1 with a known good tube?
What about the tube mix, does it do it on both 84's and 6V6's?
It sounds like a preamp tube, but you cant rule any of them out.

 

[simon_d]

Hi, I've swapped it with new and 'known good' from other amps as I don't have measuring equipment for tubes - maybe the Rebel circuit makes special demands on tubes, dunno? It's V2 not V1 that's microphonic BTW.

I tried 4 different tubes including a new Sovtek 'WA' like the Egnater branded tubes and that was the worst, it almost started to feedback it was ringing so bad. Never liked Sovteks much.

The noise and microphonics is definitely preamp tubes (you can tell by gently tapping around the tubes), the fartiness maybe power tubes, I can't be sure without more testing. Bah! Makes troubleshooting a pain when there are multiple variables.

Can someone tell me if the hum when touching the Bright and Deep switches is normal or not? Its not subtle.

Damn it was sounding so good for a bit.

 

[bruce egnater]

Well....we have recently been having trouble with microphonics tubes......again!!! Even the sovtek WA, which has been the most "non-microphonic" 12AX7 for a very long time has been turning bad. Once again, we gathered dozens of samples from various manufacturers to try to find the best one at this time in history. We found the TungSol 12AX7 to be the least microphonic of the lot. Even still, we found a substanital percentage of those to be too microphonic for high gain amps. If your amp is that new, contact nate@egnateramps.com and ask him to send you a replacement tube. You should not be buying them when your amp is under warranty. Also, the hum is normal and harmless when touching the toggle switches. Just touch your guitar strings when touching the switches and they won't do that. FYI, the Rebel does not really put "special demands" on the preamp tubes but.............being a high gain preamp does make a somewhat microphonic tube much worse.

 

[simon_d]

Hi Bruce, thanks for the reply. It must be frustrating for you to have to hunt for good tubes like that. I'll drop nate an email, though I'm in the UK so might be better to ask at the shop I bought it from first.

Yes, I noticed the touched switches didn't hum while I touched my guitar strings. Glad I can eliminate that now. I don't notice it when the master is set lots lower - I cranked it while checking the tubes and noticed the hum then. Truth be told, I'll probably never play it loud enough for the microphonic feedback to be a big issue, but the noise/crackling is a problem for recording.

But, crackly tube aside its a very good amp. Love the wattage control and the tone controls. Nice to have an amp EQ that works usefully over the whole range for a change!