Volume Issue in Tweed mode only -- bad capacitor?

[swbo101]

I have a Mark IV A long head which I've had since '91. In that time I have only needed to change tubes when needed. Lately I've noticed that in Tweed power mode only.. the volume will slowly drop until it's gone completely in all channels. If I then switch to Full power OR go back to standby for a while and then power up in Tweed mode it's fine and I can continue playing with no issue. This does not happen at all in full power mode...So bad capacitors? and why doesn't it affect the full power channel?

Thanks!

 

[JOEY B.]

There were a few posts a while back about certain Russian preamp tubes that didn't like the lower voltage of the "Tweed" mode, but worked fine with "Full Power". Which preamp tubes are you using?

Copied and pasted:

"Mesa Memo

SOUND “DROP-OUTS” / INTERMITTENT SIGNAL

One Possible Cause



Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower” tube
stages in their preamps. Tube selection is CRITICAL in these stages.



Specifically, in a 12AX7 tube used as a “cathode follower”, the voltage difference
between that present at the cathode, as compared with the heater voltage, can be
withstood or tolerated by certain types of tubes, whereas other tubes will fail. The
failure of a “cathode follower“ tube will cause sound dropouts or signal loss.



For the past few years, Mesa has been using two types of 12AX7 tubes: ones
originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek)
tube is NOT reliable as a cathode follower. Of the tubes we are using today (March
2008), ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.



In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp,
suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked
as “CHINESE” (silk-screened on the tube itself).



Below is a partial list of Mesa amps and cathode follower tube locations:

GUITAR AMPS BASS AMPS

Lone Star & LS Special: V3
M-Pulse: V2
Stiletto: V3 & V4
Venture: V2
Road King I: V3 & V4
Big Block 750: V4
Road King II: V3 & V5
Titan: V4
Roadster: V3 & V5
M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4
Bass 400+: V2
Tremoverb: V3 & V4

ANOTHER POSSIBLE SCENARIO may occur in the “SPONGY” (or on some models,
“TWEED”) power setting: the reduced filament voltage may cause very low output from
a RUSSIAN preamp tube. Again, the recommended fix is to replace the “sagging” tube
with the CHINESE type of Mesa 12AX7 - which are more immune to this type of failure."

 

[swbo101]

My pre-amp tubes were purchesed through MESA directly, they are 12AX7 (except for the reverb pre-amp which is 12AT7). They are marked as 'RUSSIAN 2'. Could that be it? Should I consider swapping them out with the Chinese pre-amps?

Thanks

 

[zebpedersen]

If you're going to buy a preamp tube for diagnosis, maybe consider buying a SPAX7 and swap it into all the positions individually to isolate the fault?

 

[swbo101]

Do you know in which tube position the cathode follower would be for the Mark IV?

 

[JOEY B.]

Do you know in which tube position the cathode follower would be for the Mark IV?

The Mark IV does not use a cathode follower design, but the last part of the "Memo" still applies. Any MESA that has the reduced voltage of "spongy", "tweed", or "variac" power will have the issue with the Russian 2 preamp tubes. I think the current 12AX7 from is a re-label JJ ECC83S. These tubes will not have the same issue with the lower plate voltage, but I would suggest looking into other brands such as the Penta Labs 12AX7 available from Doug's Tubes. The NOS route is the best, but can get a bit expensive. :wink:

 

[swbo101]

Thanks!! much appreciated! Will look into those tubes.