Studio Pre problem/tube Q's

[jaredpc]

Hey all,

I recently acquired a Studio Pre (words cannot express my love for this unit) but have run into an infrequent re-occurring problem. Every so often (and it's rather intermittent at this point) the volume starts fading in an out by itself ,an issue that I'm assuming has something to do with a bad tube. Question is, what tubes would be likely suspects?

This happens occurs on both the Lead and Rhythm channels, so I'm assuming V4 is not a contender (V4 controls only lead, right?).

I'm curious as to whether the tube on the reverb circuit could be a culprit. There's a Mesa SPax7 in there although the circuit board is marked 12at7. A) Does the tube type even matter (ax v. at)? and b) would that make a difference in volume?

Opened up the Pre today and all tubes were glowing, tapped them individually and didn't hear any catastrophic changes in the sound. All were seated properly and the unit looks really good for its age (Nov. '91) V1-4 are JJ's and look newish.

I have 2 extra 12ax7's around the house so I'm starting off swapping V1 and working my way forward, but like I said the problem is very intermittent (hadn't done this is about a week and today I was playing for roughly 40mins without complication), so it could be awhile before the mystery is solved.

BTW: I've ruled out bad cables, issue happened today while recording: output A was through the speakers and output B was to the board. It isn't like a cut or short anyway, it's definitely a signal fade.

Any ideas?

 

[joe web]

first of all congrats to an awesome preamp. i love mine as well.

V4 is responsable for the output signal, change this tube and give it a try.
V5 (the reverb tube) should be a 12AT7 (ECC81) and not a AX7 (ECC83)! the AT7 can handle more voltage as the AX7, so maybe this could be your problem as well, but i didn´t think so.

btw. try V1,V2 and V3 matched JJ ECC83S (highgain) tubes for more gain...if the output signal is now to high, change V4 to an 12AT7 (ECC81) to lower the output signalrate.

you could try the send out as well. for me it sounds better as the normal outputs.

 

[goodwill559]

Here's what I did when the same problem showed up in mine.

I started with V5 since it's for reverb, so pull it and put reverb on zero.

Problem solved? Congrats. Get a new AT7.

Not solved? Continue:

V4 is for the main and recording outputs, so pull it and use the loop out.

Problem solved? Congrats. Get a new AX7

Not solved? Continue:

Swap out v1, v2, and v3 one at a time with a tube you know to be good until you find the culprit.

 

[killerburst]

It sounds like a tired FX return jack. Either use the Send jack as your output, or connect a short cable from send to return. Or replace the FX return jack.

 

[Dead Moon Rising]

could be a dying cap. Mine had a dead lead ch and weak clean when i got. turns out a cap was bad letting dc go where it shouldn't.

 

[jaredpc]

I ended up doing a full re-tube on the unit and unfortunately I'm still having the same issue.

I'm not using the FX return jack, only recording outputs A and B...I'm not sure if that would still matter or not.

 

[jaredpc]

Also,

This problem shows up only after the Pre has been on awhile, say 20-40 minutes.

I don't know if that helps or serves to confuse you further (as it does me). =(

 

[Dead Moon Rising]

i think it's time to take it to a Mesa tech.

 

[francm]

I'm not using the FX return jack, only recording outputs A and B...I'm not sure if that would still matter or not.

Yes, it does matter. Plug a short patch cable from the effects send to effects return and see if the problem persists