Studio 22+ rehab (weak lead)

[GottaBoogie81]

Hello all, long time visitor, first time poster. I recently acquired a Studio 22+ with EQ section. I know it has been mentioned by other owners that it is known to run on the hot side of things. I have yet to replace the tubes, as the clean channel sounds great for the jazz I bought it for. The issue I'm having is this: when first powered up, the lead channel is everything it should be: creamy/crunchy saturated Boogie greatness. However, after about five minutes or so, kicking into the lead channel gives a wimpy (for lack of a better term) gain response, with little to no distortion. I'm assuming replacement of the V2 12AX7 might remedy this, and indeed all of the tubes should be replaced, but I'm wondering if this is a known issue related to temperature, and not the tube itself. I know this amp has been called finicky by others, but it is so much more portable, useful and musical than my DC-2 combo was before I sold it, that I really want to put things right. Incidentally, I am a CAD artist as well, and intended to use this amp to draw a more useful schematic than the difficult-to-read pdf that is floating around the web. Unfortunately, both the Studio 22 I purchased first and this 22+ are of the inverted main board variant, so work is progressing much more slowly on that front than I'd hoped. For the record, everything else works well, including the switching itself, and I have cleaned and lubed all internal components and sockets using recommended methods (Deoxit F5 and CRC Contact Cleaner where appropriate), so my instinct is that this is just a simple temp/tube issue. Thanks in advance for any responses, and thanks also for the great info that you guys have shared about this gem of an amp previously.

 

[Bugeyed Earl]

I was tempted to produce a more readable schematic myself, but was forced to admit that I'm lacking the graphics skills. If it would help your effort, I took some photos of the component side of the board of my non-eq 22+ when I had it apart last.

I'd try popping the reverb tube into the V2 position, it could be useful as a quick diagnostic to see if you're on the right track with a marginal V2 tube. You might have seen my comments elsewhere in this forum on the switching issues I had with my amp. I'd be curious to know what the voltage is on the lead/rhythm footswitch circuit, and whether it changes to coincide with the the gain drop.

 

[GottaBoogie81]

Thanks for the reply. In preparation for swapping the V2 and V4 tubes, I spent the last hour doing more testing for baseline purposes. To begin, I switched to lead mode on power up and played for a solid 20 minutes to see if there was any noticeable tapering off of gain saturation as the amp warmed up. No problems there, sweet and chuggy. Then I started switching back and forth between the clean and lead modes every 30 seconds or so. Finally, the issue surfaced on about the tenth switch to lead, giving a thin, out-of-phase-ish, saturation-absent tone. I played it in lead mode for about a minute, and the goodness started to creep back in, but only a little. Switching back and forth from clean to lead again produced the same thin tone, but this time, after playing for several minutes, nothing nice for the tone.

I then repositioned the amp so I could sit down in front of it, and as I did, it produced a slight crackle and gain returned in earnest. Repeated channel switching over the next 30 minutes failed to reproduce the thin tone. I'm letting the tubes cool now to perform the swap, but won't be able to test thoroughly again until this evening. Could it be a cold solder joint for the V2 socket (just thinking out loud)? As an aside, I wish the sockets in these amps were chassis mount, as opposed to board... Alas.

If it would help your effort, I took some photos of the component side of the board of my non-eq 22+ when I had it apart last.

It all depends on being able to view resistors and capacitors closely to sus out their values. I'm happy to try. If you get a chance, post those photos here so we can all enjoy a gander! The .22s are super sweet when all is well. The bass response and clean tone really put my Hot Rod Deluxe to shame, and the distortion leaves nothing to be desired. I never use the lead channel on the Hot Rod, as the solid-state rectifier sound it produces disgusts me. We deserve a legible schematic for this baby, no?

 

[Don]

Did you clean the effects loop jacks? My .22+ cut out while I was trying it at the previous owner's house. Being the nice guy that I am, I repaired it by plugging a cable in and out of the jack a few times then paid his asking price for the amp.

I found the schematic for the .22+ that's available on the net to be very clear, more so than the .22 or other older Mesa schematics. I can send you my copy in case I just happened to get lucky and found a good one.

Also, if you want to know more about how the amp functions, check out Mesa's patent for the .22 circuit (4701957). It's very clear and explains the amp in great detail.
http://www.pat2pdf.org/patents/pat4701957.pdf

 

[McBarry]

From your description and symptoms, I'll bet last weeks' wages that yours is a thermal problem..
My money is on a dry joint, and although these are usually near the power tubes, I have seen suspicious joints over on the preamp side.
Here's a pic of a screen resistor joint in a Studio 22 - see the way the solder has mover AWAY from the resistor leg.. The resistor was actually loose in the joint..
And notice the browning of the area -kinda like a slow cooking over a couple of decades....
Go over your board visually, maybe even with a magnifying glass.. If all looks well, then move on in your troubleshooting..

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[GottaBoogie81]

Thank you all for your kind responses. I'm currently out of town, and posting from my phone. I can report however, that after swapping some tubes around, (V2 & V4) I was unable to get the problem to resurface. When I have the funds, I intend to get a replacement set from Eurotubes. I am not ruling out the possibility of a solder joint, as the unit does run very hot, compared to other tube combos, and has done for almost thirty years now. I'm not willing to call it solved, but until I am home and can get the chassis out, you all have my thanks! In the meantime, has anyone had success adding a cooling fan to this unit? If so, any advice would be welcome.