Identifying source of 18W 'cut-out' issue

[stomp944]

Hey, all. Seeking to troubleshoot an intermittent drop in volume to near nothing, associated with a low hum. The build is 18W TMB mostly based on the Richie schematic. 3x 12AX7, 2x EL84, 1x EZ81. Hammond OT, Classictone PT. I did a search and did not find references or solutions similar to this.
The amp has worked flawlessly for over a year since build, run on average once or twice a week for 2-3 hours each time. Very quiet - typically no hum or buzz except with very high gain/vol. I typically run it through a 50W power attenuator between the amp and speaker (12" GB128). All channels up to now working normally.
Issue first came up after 2 hours of rehearsal through TMB channel on moderate gain - it cut out gradually but quickly, with a scratchy choking sound trying to play anything. After 2-3 min on standby, it returned to normal for a few minutes and then cut out again. Again 2-3 min on standby it worked again for a few minutes before I switched to another amp. Few days later at home it worked for about 15-20 minutes before a low hum started and it cut out (no scratch or choke, just stopped). The hi/lo channels seem to work fine. A few times the TMB channel would work but at roughly half the volume of the hi/lo. Changing guitars, cables and bypassing the power attenuator did not correct the issue. Issue is reproduceable in that it cuts out and returns again after standby for anywhere from 2-5 minutes.
The tubes generate a fair amount of heat, but does not seem hotter than before. The power tubes are glowing orange - seems orange to me but not red. I cannot see any glow from preamp tubes in current lighting (and chassis not out of cabinet yet). Do these symptoms point to anything typical?
Great resource and input from members during my build (late 2018) - thanks!

[JMPGuitars]

It's just the TMB channel not working? Have you tried swapping tubes?

[stomp944]

hi. I mostly use the TMB, so it was late in my trial and error to try the hi/lo channels. so cannot confirm with 100% certainty that it only cuts out on the TMB, but before I turned it off last time, the TMB was putting out sound a roughly half volume compared to the hi/lo channel volume level (at half knob volume, un-attenuated). In this instance, the hi/lo to my ear being 'normal' or 'typical' volume level.

chassis not pulled from the cabinet yet. not to sound too obtuse here, but swap between existing tubes or swap in new tubes? power or preamp? I don't have any additional tubes at this point, so was hoping to figure out first if that might be the/a source of the problem.

[JMPGuitars]

chassis not pulled from the cabinet yet. not to sound too obtuse here, but swap between existing tubes or swap in new tubes? power or preamp? I don't have any additional tubes at this point, so was hoping to figure out first if that might be the/a source of the problem.

Well, if it's ONLY the TMB channel, you could switch around preamp tubes and see what happens. If not, then I would start with swapping out the power tubes with new tubes. It would be a good idea to get new tubes for the whole amp to keep as spares.

Once you have the amp out, you can test your voltages and that might clue us in to what's happening. Voltage charts are here: viewtopic.php?f=25&t=24418

Thanks,
Josh

[stomp944]

Ah, yeah, got it. Ok, something to look into, and see the value of additional new tubes.
Got a gig coming up shortly and planned to use this amp... hope I can narrow it down soon.
Thanks, Josh.

[stomp944]

Ok, so I pulled the chassis and did a voltage chart. Again, Hammond 1750PA OT, Weber 025130 PT (sorry, correction from earlier PT identification). Wall 120.5 VAC. Findings attached - a number of outliers are evident.

Testing the amp again, initially no hum, just the hollow feel that the speaker has when first turned on. Hi/lo channels played normally and at normal volume. TMB channel played at (what seems to be) half volume for a few minutes before a hum started and the TMB channel stopped working. The hi/lo channels also stopped working.
I did look more closely at V5, the second power tube. It is glowing hotter than V4, and maybe getting towards a reddish color rather than orange. No blue bits or white plating.
Also did some chopsticking - no clicks or zaps or crackles and nothing seems loose or disconnected.

[Daviedawg]

Something is seriously wrong at your power valves. The plate voltage on V5 is lower than the screen voltage. Both plate voltages V4 and V5 should be identical and quite close to your "B" voltage. And the cathode voltages are way way too high. 12v target 12.5 max on pins 3. I would not switch on again for more than a few moments until you find the issue. But this explains your symptoms. As the valves start to overheat the signal will drop and die.

Writing this I cannot recall if you have done a valve swap. But that would be my first thought a bad EL84 (or two).

Dd

[geoff 1965]

yes that pin3 voltage is very high!
i had a problem a while ago where my el84 cathode voltage was getting higher and higher and the main suspect for this was dc voltage getting onto the grids,i spent 2 days swapping coupling caps and scratching my head and it turned out to be the tubes! the leakage was internal,they were marshall stamped JJ's.

[JMPGuitars]

Your voltages are all over the place. I think you need to keep the amp off until you've done a full highlighter test (or two) verifying the schematic AND layout. Highlight each component only after verifying the value and connections are correct.

Then take your voltages again and make sure you're on the correct meter settings. For the EZ81, pin 1 and 7 are VAC, and pin 3 is VDC. If you're reading low mV on pin 1 or 7, you're probably in the wrong setting.

Thanks,
Josh

[stomp944]

Um, yeah. I think I need a few minutes in the corner and my multimeter privileges revoked.
I did screw up at least one of the EZ81 readings (DC), and hope it essentially comes down to bad power tubes. It has worked without any issue from day one over a year ago until now (that's the denial talking).
Appreciate the input from each of you - thank you!! Going back to retrace values and connections, and grabbing a few new EL84's to swap in....... and looks like I might be renting an amp for a while.

[stomp944]

Update. As some readings I'd measured earlier were fairly wonky I checked several points again, but this time with amp 'cold', before anything got hot or the speaker cut out. It is reading closer to or within spec under this condition. For example,
EZ81 pin 7 285V
V4 & V5 pin 3 at 10.58 and 10.57 respectively
V4 & V5 pin 7 at 319 and 320 respectively
V4 & V5 pin 9 at 295 and 305 respectively

When heated up (and acting up), its a different story.
I reviewed schematic and layout and did a cursory component check but yet to do it in highlighter detail. There are a matched pair of new power tubes on the way in the meantime.

[stomp944]

Hi, all. Matched EL84 power tubes arrived today.
Installed tubes and completed a voltage chart ('old' line representing original EL84's before replacement and 'new' line after new installed). The amp seems to put out sound of a quality and volume as it did originally, without cutting out - though I have not left it on for more than 15 minutes yet. Will get playing time in tomorrow. In the meantime, numbers are looking ok, if a little high on V3.
Sean

[JMPGuitars]

Your voltages look much better now for the most part. Your B+ is a little low, along with your cathode voltage. Do you have any dropping resistors bringing down the B+?

[stomp944]

No dropping resistors. Only a 220K bleeder resistor at the 32/32 cap before the OT.
I figured the lower voltage was essentially a function of the PT (Weber) and OT (Hammond 1750PA) I chose for the build...

[JMPGuitars]

No dropping resistors. Only a 220K bleeder resistor at the 32/32 cap before the OT.
I figured the lower voltage was essentially a function of the PT (Weber) and OT (Hammond 1750PA) I chose for the build...

Oh, okay. Your first post said ClassicTone PT, and the ClassicTone PT should supply standard voltages.

[stomp944]

Yes, I realized I wrote in the wrong PT in first post; sorry, corrected in a later post.

OUTCOME:
Was able to operate the amp for roughly 90 minutes today. Happy to report all channels performing well and nothing cutting out with heat, time or otherwise. Power valve on the way out was the cause. Nice to have the 18W up and blasting again! New pedal testing time.......
Appreciate support from all,
Sean