My Dual rectifier head keeps losing gain/cutting in and out!

[JPaiz11505]

So, the other night I was playing a show, and my head just stopped working. When I brought it home where it was actually the only thing that I hear, I noticed that on every channel it cuts in and out. And on my gain channels it goes back and forth from the normal gain and Les gain, and then a clean sound. The rectifier tubes and power tubes have less than 30 hours of play time but I have no idea how old the pre-amp tubes are. Could it be the pre-amp tubes?

 

[screamingdaisy]

Did you just retube the whole amp? Or only the power/rectifier tubes?

Is the gain/volume fluctuating on the clean channel, or only on the high gain channels?

What brand OD preamp tubes are in it?

 

[JPaiz11505]

It does the same thing on the clean channel, cuts in and out. And I just changed out the power and rectifier tubes three months ago. One of the pre-amp tubes is a mesa 12ax7, one is an electro haemonix 12ax7EH, one is a sovtek 12ax7LP, and the other two aren't marked. I've had this head for three months and it just started giving me issues.

 

[screamingdaisy]

Sometime back pre-2008 Sovtek changed their design specs and their preamp tubes started having issues in Rectifiers... They can't handle the voltages present in certain positions (I think it's V3 and V4) and the symptom is volume/gain fluctuations.

The EHX tube is also made by Sovtek, as is the Mesa if it says "Russian" or "Russian 2" on the label.

The issue can be solved by swapping a JJ or Chinese tube into the V3 and V4 positions. Current issue Mesa 12AX7s are JJ.

 

[vick1000]

Make sure the tubes in V3 and V4 (cathode follower) are not spiral filiment tubes.

Chinese Sino/Shuguang variants, and JJ ECC83S variants are durable enough for cathode follower positions.

http://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025/jj-ecc83.html

http://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025/ruby-12ax7-ac5-high-grade.html

 

[domct203]

As already mentioned, the cathode follower positions (V3 & V4) in you amp need strong robust tubes.

http://forum.grailtone.com/viewtopic.php?f=4&t=25102&start=19

Dom

 

[JPaiz11505]

So, I replaced the V3 and v4 position tubes with a Chinese made 12ax7 and it immediately blew the Mesa tube that was in the V1 spot. So I put the sovtek in their and it's doing the same thing as it was before...

 

[vick1000]

It's not the same Sovtek that was in V3/V4 is it?

 

[domct203]

So, I replaced the V3 and v4 position tubes with a Chinese made 12ax7 and it immediately blew the Mesa tube that was in the V1 spot. So I put the sovtek in their and it's doing the same thing as it was before...

What do you mean by "immediately blew"? (The amp is OFF when you are swapping tubes, correct?)

Is the tube all white inside where it was once silver?

Dom

 

[JPaiz11505]

Yes, the amp was off when I changed the tube. And yes, it was all white where it was once silver, and it also cracked it.

 

[JPaiz11505]

Yes, the amp was off when I changed the tube. And yes, it was all white where it was once silver, and it also cracked it.

 

[domct203]

Yes, the amp was off when I changed the tube. And yes, it was all white where it was once silver, and it also cracked it.

You probably cracked the tube pulling or inserting it into the socket. I've done it a couple of times myself, and one of those tubes was a favorite NOS that I had, a mid '60's RFT 12AX7. The reason the getter flash (silver part) turned white is from the loss of vacuum (and gain of oxygen). It's pretty rare for a tube that was working to "just blow".

Are you troubleshooting with the FX Loop in HARD BYPASS? Just Guitar-->Amp-->Speaker, right? Just unplugging cables and turning the FX Loop off with the footswitch is not the same, and any fault in the FX Loop circuit will still be there.

What did you put in for power tubes recently?

Dom

 

[vick1000]

If the EH or Sovtek were in V3 or V4, they are damaged. If the no name tubes were in either V3 or V4, they need to be eliminated from the equation, so leave them out.

What happens when you put a spiral filament tube into a cathode follower position, is it dmagaes the tube. So for troubleshooting purposes, make sure whatever tubes were in V3 and V4 are replaced with non-spiral tubes.

Also, disconnect any footswitches or FX, and as said, bypass the loop. Just guitar to amp, and try other guitars and cables if you have them.

Please also include info for us on your cabinet/s and impedance, and make sure you are using a speaker cable to the cab in the proper impedance output.