Something wrong with the triple rect

I have problem with my triple rectifier head, the 3 channel non multi-watt model (year 2008). While playing, after for about 10 to 15 minutes it suddenly has severe volume drop, like cut off. From the cab just came out faint/little hum, when this symptom happen the guitar sound become very weak for all 3 channels, hard to hear. I already check the position of send pot, guitar & cable all good, cab (Mesa 4x12) and the spkr cable in working order too. And this will back to sound normal again just by switching off the standby switch and switching it back to on after some seconds or sometimes it need to completely switch off the amp and switch to on again. Intermittent issue? Or relays or JFET failure? I'm not so sure. Previously i suspected it was tubes, or more specific the tubes positioned in the cathode follower, but replacing with the new tubes (Mesa's or JJ's or Shuguang's) the problem persistent, and so to the p.i. position tube. I notice with (all) new tubes replacement its only take 3 or 4 or 5 circles of playing and the problem will appear again and again. If i tap the signal from the send jack to other power amp, it just sound normal no matter for how many hours i play the head. If i feed the guitar or other pre-amp signal direct to the one end of 0.02 yellow cap, the one that go to the grid of V5b (power amp section in), it just sound normal for hours of playing with no volume drop or cut off. What really happen with the head ? I need advice from this great forum, please. If i just want to use the pre-amp section, is it ok/safe if playing without all the power tubes installed?
Thanks,
dux

Ouch, hope it's not the output transformer. You'll need to check signal to the OT when it occurs. Be careful messing around in a powered up amp, take it to a tech if you do not have experience with this.

If you have bias socket probe, you can meter a tube and see if you are losing B+ when it happens, without too much risk of shocking yourself to death.

Thanks so much for the advice. Pointed to rectify the cause of the problem to the OT make me lil' discourage. I remember back in the day when i first play the amp, it was brand new, all features of the amp, visibly, functioned properly: channel switching, foot switch, loop send & return, all pots, etc. But after a couple minutes of playing the issue happened, it was when switched from clean channel to high gain channel, suddenly loss in volume, just whoosshing sound like in the beach and distorted faint noise came out from the speaker. I managed to contact to Mesa and they suggested that the cause of the problem most likely tubes failure. So i replaced the tubes in all position and keep playing the amp day after day but that unwanted symptom still came and go, it has been 4 or 5 years. If i lucky i can continuously play the amp at gig volume level without any problem in one occasion but the next 2 or 3 the same problem appeared, it funny if i turned the head in upside down position the problem seems like disappear. That turned me into acquiring another heads; Marshall JVM 410 H & HJS, Peavey 6505, 6505+, Laney, Randall and others high gain amp, to recently, a multi-watt 3Rect model (now its become my main tone generator) and always want back to the 'brutality' of 3Rect sound. I've checked all the resistors: plates, cathodes and all in the PI and bias circuit. All values measured exactly like the particular bands/color code in each. And so to the internal operating DC voltage include the B+. Today i hardwired the power amp input section to a female jack and feed one the live output signal of my Rectifier Recording pre-amp into it. It just sound good, tight loud and clear. I have been playing with this configuration since 4 or 5 hours a go and no problem whatsoever. Well, still make me wonder what really the culprit is? Cold solder joint, PT, OT, diodes?? I play it in bold, ss rectification and all the power tubes installed now are Sovtek 5881's, the pre amp all Mesa re-branded (JJ?) 12AX7A and the speaker cabinet is 8 Ohm Mesa 4x12 straight cab. When i switch back to use the head as normal: guitar to input in the front panel to cab its also no problem for now, within hours of continuous play. Have no idea yet.. :?:

I notice the strange thing happened before. When playing the amp and suddenly loss volume and it just like crackle noise, i switched the stand-by switch to off and on, couple times until the sound back to normal. When this problem came and switch the amp to off, i can hear crackle noise or something like 'whooshing' sound followed after the stand-by switch turn to off before it completely silent. Or when i play the amp for some time and turn it off and hear the crackle noise or 'whoosshhh' sound after the stand-by switch off, most likely in the next cycle to turn on the amp, the problem of loss volume and crackle noise/whosshhing sound will occur. I know from the 3 other Mesa head (Single Rect, Roadking II, and multi-watt 3Rect) that working properly, every time i switch the stand-by switch to off position after playing, it just quite, there is no crackle noise or 'whooshh' sound to be heard. For now after hard-wiring the power input jack which connected directly to the 0.02 cap before V5/PI tube, the amp just sound normal. Whether i play with external pre-amp, connected to that 'added' power input jack (the switches at the back panel: loop select to bypass and channel select to particular 1 or 2 or 3, in this set up the selected channel master volume become the power amp master volume), or i play normally through the input of front panel, it sound normal too. But i still in observation now, turning the amp in the playing mode, continuously hour after hour, play it loud, switch the rectification back and forth from tube to ss, switch the channels, turn the amp on, and play, and turn off and turn it on again... No conclusion of what is the cause of the previous problem to be made yet... perhaps just cold solder joint, or components failure, or other cause (??)

Continue to observe what will happen next, when i just leave the amp on in playing mode, the chain is: guitar--cable-- to input (front panel)--speaker from just quite silent (in channel 1 mode, master at noon, gain, channel master and eq all at noon, send pot at noon) for about 2 hours, suddenly i hear a noticeable hum sound, and when i turn the loop switch at back panel to bypass position the hum gone and i can play the amp normally. I turn it back to active loop and the amp can sound normally too. It make me confuse, what i understand is, if (a) parts or components inside the amp about to fail or already failed why it still come to function like the amp back to sound good again. But never know for sure if it will fail again or not for next. Or any other cause for this unreliability issue? The system switching failure? Transformers? Choke? Caps? If relays, JFET's or Tubes are about to fail, are they act like this in the amp, go bad and come good again? It does not make sense to me. Have any idea?

Now i almost come into the conclusion to blame it on the cold solder joint. At the spot of power section input cap, right before the PI. That the only place i touched with the tip of soldering pencil when i connected a female jack for another external pre-amp input to the power section, beside the other end of 2M2 resistor that connect to ground. I turn it on and play continuously for more than 8 hours, with the same set of pre-amp and power amp tubes that previously suspected as faulty tubes, they were installed when the amp suffered with the issue mentioned earlier. So far the amp just work as normal, no volume loss, no hum, no whoosshhh, no crackle noise when turn the stand by switch to off just dead silent no other popping noise like before, no intermittent behaviour. And this already for 4 consecutive days on observation.

Based on the experiences and expertise that you can share, afu, masterotono and others, what can you suggest on the described issue i have? Thank you so very much.

I'm sorry that I didn't reply before. I'm injured and feeling a bit overwhelmed by it. I also can't sit up for long at my PC.

First, to be honest, I'm having a hard time reading the big blocks of text. Perhaps you could bullet point the things that haven't worked and then bullet point the things that do work, if this post doesn't help.

That it's intermittent makes me think something is dirty with corrosion or the cold solder joint is an issue. It isn't going to hurt anything to resolder that joint. The other thing is that bypassing the loop and wiring directly into the input to the PI seemed to solve your problem. Be sure that the tube in V4 is rated for more than 200 V on the cathode. If that is fine, try cleaning the tube sockets with Deoxit or another solvent made for electronics while the amp is unplugged and after the caps have a few minutes to bleed off. While you're at it, clean the jacks, too. May as well.

It isn't a good idea to flip the standby on and off. That can damage the cathodes to your tubes. However, it does show that something is wanting to fail or needs to reestablish its connection. That again brings corrosion or the cold solder joint to mind.

Good luck. If this doesn't work, let me know.

That it's intermittent makes me think something is dirty with corrosion or the cold solder joint is an issue. It isn't going to hurt anything to resolder that joint. The other thing is that bypassing the loop and wiring directly into the input to the PI seemed to solve your problem. Be sure that the tube in V4 is rated for more than 200 V on the cathode. If that is fine, try cleaning the tube sockets with Deoxit or another solvent made for electronics while the amp is unplugged and after the caps have a few minutes to bleed off. While you're at it, clean the jacks, too. May as well.

It isn't a good idea to flip the standby on and off. That can damage the cathodes to your tubes. However, it does show that something is wanting to fail or needs to reestablish its connection. That again brings corrosion or the cold solder joint to mind. [/quote]

Thank you so much for the advice and suggestion, now i will trace all over the tracks in the PCB and the solder joints, especially from the PI to power tubes, check all the grid resistors and rectifiers joints. Perhaps i should take out all the PCB and re solder the joints especially to ones which looks already change in color or corroded, and not to forget to check the rotary switches at the back panel. I've checked all the J-FET (J175's) which functioned to mute the signal to ground. I just have no idea how to examine the relays. I prepare contact cleaner to spray all the tubes sockets too. I'll report what the findings and the next observation.
Thanks again.
dux

After almost a couple of months observation, including some re-soldering work to some points, mostly at the vicinity of V4 & V5 circuit, i should come to the conclusion that it is, most likely, those pre-amp tubes (it might be the CT or PI or V2, i wish i could test with the tube tester device like Orange VT-1000). If there other failed parts like OT or something in the PT taps, i am not really sure. But the thing happened like this: after meticulously inspected and measured all the resistors to their right values and checked all the caps, switches, J-FETs, relays, diodes, re soldering, and reassembled again with all the old tubes reinstalled, the issue still persisted. Until i replaced all the pre-amp tubes with a set of 12AX7's i found in my stash, and its functioned normally, no issue till now. Now i set to spongy and tube rectification. I find a sweet spot with this setting. Previously i made some modifications to the circuitry, and after a period of trying i reversed it back to the original. For some reason i also found it sound different in certain way, especially in the distortion sound, that i like, comparing to the multi-watt version of Triple.

Yikes, that's a lot of work. I put a new resistor in one time. Solved everything. PETE

I would like to give credit to Afu for the insight and valuable info on recto circuit. At last, i solved the problem. It's the coupling capacitor located before the V5 b (PI). It has been the culprit from the day one and all efforts to solve misguided to blame it to 'faulty' tubes. New yellow .02 Sprague replacement and voila!

You're welcome. Faultytubes are a common issue for what you described, as is dirty contacts, so checking those first was a good idea. I'm happy for you that you were able to find and fix the issue.