Reasons for tonal changes as amp warms up

[daz]

Can you guys list some of these for me? I'm not sure where to look, but the first thing that comes to mind is output tubes and thier need for a fan. But then i'm only playing at low volume and the tubes aren't biased hot at all. The deal is, after about 20-30 minutes of play the amp often will start sounding a bit fizzy and thin. It seems like too much of a change to be my ears, but i don't know what to look at besides the el34's, and i don't think thats it since i tried another pair once the amp got how and it started and it sounded about the same. What else could cause this? Amp is my by now infamous homebrew jcm 800 style amp with a more gain and variable NFB.

[tedmich]

something is heating up likely! You could try some cooling spray on components (not tubes! ) and see if the good sound returns, then replace with more stable one. IME ceramic capacitors are among the most thermally sensitive passive components, thats why I drop the big bucks for C0G /NP0 grade ones.

[The Dude]

I would start by checking your bias current after that 20-30 minutes of playing to see what, if anything has changed.

[daz]

I have checked bias, and it's about 36-37 ma when the amp is fully hot. I must admit i never checked it right when it started doing that, but again, i tried another set when it was doing that and it didn't seem to matter much. And I biased them after i put them in.

As for caps, no ceramics, just micas and mallory 150's. Can PSU electros cause this? I have a 100/100uf main and the preamp nodes all use a 20/20/20/40uf JJ multicap. What caps would be likely suspects? I seem to recall the same sort of thing happening when i was playing out regularly and using jcm 800 combos, tho the tonnal change was a bit different. They'd get mushy sounding.

[Enzo]

Start by isolating the problem. Does the amp have an FX loop? If so, first try patching across it with a cord. If that changes nothing, then input a signalo to the return, does that have the fuzzy and thin, or strong and clean? And run a cord from FX send to some other amp. Does it sound OK over there ir still fuzzy and thin.

[g1]

I think Dude was asking if the bias changes from cold to hot. Checking it hot is only half the picture.

[eschertron]

As ear fatigue sets in, there's the temptation to turn something up, which might in turn be affecting the tone of the amp. Are you changing any panel or guitar controls over this time period? If the answer is no, please disregard.

[daz]

As ear fatigue sets in, there's the temptation to turn something up, which might in turn be affecting the tone of the amp. Are you changing any panel or guitar controls over this time period? If the answer is no, please disregard.

Not turning it up at all, and it's low to begin with. Think loudish TV volume.

Enzo:no loop

G-one: It does change, but i have it pretty cool to begin with and it changes very little. About 1-2 ma. Buy the way, the bias i mentioned is actually less because thats what it reads via the 1 ohm cathode resistor method and not taking screens into account. So it's probably more like 32 ma per tube at 440v plates.

By the way, this seems to happen very abruptly. Like one secvond it's ok then within seconds it's fizzy, thin and weak.

[R.G.]

Electrolytic caps are prone to changing value and ESR as they heat. The insides of tubes are generally running hot anyway. Coupling caps, even film ones, can get leaky as they heat. Solder flux and board contamination change conductivity and that does matter on the grids of tubes. Bias drift is another one. Both the tubes and the rectifier/filter cap/divider pots that set bias are another. Defective aging resistors are another. Almost failing solder joints are another.

The one universally applicable shotgun trick is to remelt all your solder joints. It never hurts unless you're really bad with a soldering iron.

[g1]

If it's that abrupt, then it is not some component "drifting", it is a fault or intermittent connection. Did you try the loop stuff Enzo mentioned?

[daz]

If it's that abrupt, then it is not some component "drifting", it is a fault or intermittent connection. Did you try the loop stuff Enzo mentioned?

No, like i said it has no loop.

RG: thanks, great list of possibilities, and i think the solder joint idea may be right on because i have a number of sloppy ones since i was experimenting with it recently. However, i tried something that may prove it's something else. I left it on for about 45 minutes then played it. Sure enough it had that fizzy top. I had played it when i got home and it was great. So i put a small house fan on it and within about 3-4 minutes it was back to normal. I don't think that would suggest a solder joint tho i will indeed redo them all cleanly to be safe. But the bias was fine and i'm just wondering if when tubes get hot they can do that. I would think not since most amps don't have a fan and don't have this issue. But that fan seemed to cool something off because the difference was maybe not dramatic, but it was easily noticeable. I hate fizzy top end and when it was gone it was very obvious.

I think what i will do is bias it real cool and clean up those joints. One other thought tho....I've mentioned it here b 4 but no one seemed to think it's anything to worry about. But i have a hammond PT and the heater winding runs about 7.1v, and yes, thats under load.

[The Dude]

You may want to try, as tedmich suggested earlier in the thread, using some freeze spray to narrow it down to a particular component. You've proven with your house fan that it is likely a heat issue, so let's figure out which component has a problem with heat.

[daz]

You may want to try, as tedmich suggested earlier in the thread, using some freeze spray to narrow it down to a particular component. You've proven with your house fan that it is likely a heat issue, so let's figure out which component has a problem with heat.

I did. Upside down duster. Tried it on plate resistors in the preamp, coupling caps, screen resistors. Wasn't sure what else to try. I asked earlier but got no answer so i just tried what seemed likely.

[The Dude]

Have you tried swapping out any preamp tubes? I've witnessed preamp tubes turn flaky on my tube tester if I leave them on the tester long enough to get good and hot.

[daz]

Yeah. Every tube in the amp got swapped. At first thats what i suspected even tho normally i'd suspect the outputs because i have those tung sol reissues (except in the CF) and one did a very similar thing before.