Dicky Twin Reverb.

[Sea Chief]

Hi chaps- I rebuilt my Twin Reverb (poor 1978 UL with conductive board > rebuilt to near-as-I-can BF circuit, on new board) last year & has had infrequent use since, but last 2x Ive used it I get severe intermittant noise/ distortion. Happens with or without pedal (vib/ rev) pedal engaged.

Strange as its only been used a few times since the comprehensive rebuild, & all well tested & quiet etc, new 6L6's.

This noise kicks in it seems only after ~45mins of playing & then pretty constant/ its so loud & intrusive its impossible to play over it, prior to this its normal quiet hum. I was wondering if these were typical symptoms of something (hopefully minor), or rang a bell with anyone.

Thanks, Sea Chief.

[vintagekiki]

This noise kicks in it seems only after ~45mins of playing & then pretty constant/ its so loud & intrusive its impossible to play over it, prior to this its normal quiet hum.

Question
Does the constant loud noise that occurs after after ~ 45mins exist when all volume potentiometers are set to min (0)

It happens that when the tubes are warm enough up comes to difference in the emission of previously matched tubes (dynamic non-matched).
If there is noise the cause is probably a failure of some of the 6L6. It is best to place a 1 Ohm resistor in the cathode 6L6 (pin 8) and then measure the cathode current through each tube.

Do any of the 6L6 glows (red-hot) when noise appears.

[Chuck H]

It sounds like an intermittent connection. It could be a little oxide on a tube socket hole or tube pin or perhaps a cold solder joint. That it happens after some warm up time probably means it's the amp, but you shouldn't rule out any possibilities for other gear in use (guitar, cord, any pedals, etc.). You can try gently rocking the preamp tubes when the problem happens and if that changes anything or fixes it then it's probably dirty socket contact. Often you can determine if it's a cold solder joint by giving the amp a good whack. If that causes a grunt or pop. Or if giving the amp a whack after the problem starts changes anything. If it seems to be a cold solder joint then it will be necessary to open the amp back up and start poking components with a chop stick. If poking a component causes obvious noise then re solder those eyelets.

[Helmholtz]

A better description of the "noise" like hum (50Hz or 100Hz), hiss, static, motorboating or else could help to narrow things down. Or even better post a recording.

[Sea Chief]

It sounds like an intermittent connection. It could be a little oxide on a tube socket hole or tube pin or perhaps a cold solder joint. That it happens after some warm up time probably means it's the amp, but you shouldn't rule out any possibilities for other gear in use (guitar, cord, any pedals, etc.). You can try gently rocking the preamp tubes when the problem happens and if that changes anything or fixes it then it's probably dirty socket contact. Often you can determine if it's a cold solder joint by giving the amp a good whack. If that causes a grunt or pop. Or if giving the amp a whack after the problem starts changes anything. If it seems to be a cold solder joint then it will be necessary to open the amp back up and start poking components with a chop stick. If poking a component causes obvious noise then re solder those eyelets.

Ive ruled the problem to the amp: this much I know for definite, before making the thread.

If the amp was tested (by Rift, who also designed the new bias area for the re-config'd circuit) & was working fine for the year+ since it was rebuilt.. does that sort of rule out the likelihood of a cold solder joint-? I mean such a new one doesn't deteriorate in this short timeframe surely. I must say this is something Ive never had, once, in the few amps Ive made & it seems to me like looking for a needle in the hay if this was so, for me to find it. So I'm really putting this as last-thing-to-try, due to so many solders in this large circuit: really hoping to find the cause before I go down this avenue.

So I'll certainly 1st look at tube socket contacts. I do recall though Rift when 1st checking it out (the conductive board meant it went OTT on some heat related avenue, leading to the old power tubes dying) with the new set of 4x matched JJ 6L6 I sent, that at first he couldn't sort the OTT problem out & as a consequence the tubes red plated.. but obviously he powered down just in time (but was -one- JJ was perhaps damaged as a result-?) as when all was well/ problem solved by way of me rebuilding the whole amp/ I noticed one JJ measured ~ 16mA & all the others measured the 'correct' 22-24mA (I may have exact figs wrong but the disparity of this one vs the others is accurate). Whether this 'rogue' one is a problem, damaged slightly, I've no idea tbh.. & it might well be ok. Rift said ok. But if I bought them matched, & one checked out different from the others after Rift used them to check out the amp in its poorly state before being stripped apart & rebuilt (I guess my new JJ's had to be used to test, in order to find out what the amp's innitial major OTT problem was was. I guess).. is that cause for any concern?

SC

[Sea Chief]

A better description of the "noise" like hum (50Hz or 100Hz), hiss, static, motorboating or else could help to narrow things down. Or even better post a recording.

Good idea.. but how the nuts Id do this is beyond me. I do have an ipad tho (just got one..).

No its not a hum (there's a normal hum when its on). Nor hiss, nor static, nor motorboating (I get a bit of this but not enough to call it an issue).

Its a sudden very prominent crackly distortion, also when I dig into the strings (a TR on vol 1.5) its exagerated. Its loud enough to mean its not poss to play over/ during it, or you go insane. Imagine J Mascis at his dirtiest end of a song or something, a ton of distort/ odd howl & squeal/ a grungey dirge. But not in a good way.

[Helmholtz]

Does the noise persist when you unplug the guitar? Or is it always a signal distortion?

[vintagekiki]

Question
Does the constant loud noise that occurs after after ~ 45mins exist when all volume potentiometers are set to min (0).

Question
- Are the 6L6 new tubes or like new.
- Whether between pin 2 and pin 3 on 6L6 or his sockets exist traces of spark or soot (skipped spark between anode and heater)

[Sea Chief]

A better description of the "noise" like hum (50Hz or 100Hz), hiss, static, motorboating or else could help to narrow things down. Or even better post a recording.

Just had another hour on it, vol 1.3 or so (even that's loud enough on a TR in a sittingroom).

Ok prominent noise was there on turn-on this time, & it sounded like a classroom of kids tearing up large thick sheets of paper for 15mins, then it calmed down in fact, to mild background 'interference' like a radio. Then nice & quiet but with occasional loud 'POP' (making me swear & flinch).. then some soft general squealing, just like a pig with a thick coat on, being prodded to get up for breakfast but it doesn't want to.

SC

[Chuck H]

The noise as described sounds like a microphonic tube busting into oscillation. Try tapping the tubes with the amp on (and hopefully stable for the moment). If the amp refuses to be stable it's possible that simply holding a preamp tube will stop the problem temporarily. If you choose to try this with the power tubes you'll, of course, need something like a dry terry rag because of the extra heat.

[SoulFetish]

Ok prominent noise was there on turn-on this time, & it sounded like a classroom of kids tearing up large thick sheets of paper for 15mins, then it calmed down in fact, to mild background 'interference' like a radio.
SC

tearing paper sound,eh? my money's on a noisy carbon comp 100k plate resistor.

Try this:
when you can get the noise so that it's pretty steady, try pulling your preamp tubes one at a time - starting with V1 (closest to the input jack side of the chassis).
Pull each tube until the noise stops (assuming that you can get the noise without having to have your guitar plugged in). Then report back with how it went.

[Sea Chief]

The noise as described sounds like a microphonic tube busting into oscillation. Try tapping the tubes with the amp on (and hopefully stable for the moment). If the amp refuses to be stable it's possible that simply holding a preamp tube will stop the problem temporarily. If you choose to try this with the power tubes you'll, of course, need something like a dry terry rag because of the extra heat.

Hi Chuck H, the problem is the POP noise made me jump so much, that possibly making the amp do this again by my own hand fiddling with a tube or whatever.. I can't face it. I'm actually pretty scared turning on the amp now its done this as it makes you jump out your skin. Damn tube amps- this is why I play my cheap acoustic 95% of the time.

[SoulFetish]

Hi Chuck H, the problem is the POP noise made me jump so much, that possibly making the amp do this again by my own hand fiddling with a tube or whatever.. I can't face it. I'm actually pretty scared turning on the amp now its done this as it makes you jump out your skin. Damn tube amps- this is why I play my cheap acoustic 95% of the time.

loud noises still make me a bit jumpy too. I'm going to amend my suspicion to a bad tube.
Have you tried pulling out the each tube, 1 by 1 yet to see if the noise stops? You can do it like this - Put the amp in standby, then pull the first tube, and power it back up. Listen. Smack, or hit the top of your amp with your hand. If the noise is still there, put the amp back in standby and pull the second tube. Power it back up and repeat the exercise until the noise goes away.

[vintagekiki]

Testing whether the tube is correct or bad, test with new tubes, not "as new".
For amplifier test is required 12AX7 x 1; 12AT7 x 1; and 6L6 (EL34) x 1 duet. Use the test tubes as a tool.
When determining which tube is bad, test selection is from the output (power amp) to the input (preamp).
For FTwin (preamp) it is enough to check V1 (normal ch); V4 (vibrato + reverb); V2 (vibrato ch)

[Sea Chief]

loud noises still make me a bit jumpy too. I'm going to amend my suspicion to a bad tube.
Have you tried pulling out the each tube, 1 by 1 yet to see if the noise stops? You can do it like this - Put the amp in standby, then pull the first tube, and power it back up. Listen. Smack, or hit the top of your amp with your hand. If the noise is still there, put the amp back in standby and pull the second tube. Power it back up and repeat the exercise until the noise goes away.

Hi SF.. my suspicion is a bad tube too, but I thought tubes don't just deteriorate being left idle as the amp's been most of time since I built it, so I don't understand how this could be the cause of the problem if it wasn't like this the last time I used it.

I can never work out this 'standby' thing. What does 'put the amp in standby' mean? Ive no idea if it means put the switch up or down (up or down is useful here, with a TR, as my last one was the same: with both switches DOWN pointing to floor.. the amp is off. With both switches in UP position.. the amp is on. That is all I know as a reference point).

When I went to gigs waiting for Trumans Water or somesuch magnificent band to come out the wings, the (conincidentally) TR amp lit, they come onstage go behind & flip a switch so the amp is on properly: I know only that the amp now is in fully-on bc I hear it being played. So, what was it whilst I waited? I can only surmise it was in "standby" mode, the word perfectly matching what me, the band, & the amp (afaict) was actually doing at this time. I also know there is a 'standby' switch.

So I can only determine then that 'put the amp in standby' means, like my waiting example, the amp is not on yet. That makes sense. Afaict. But if you're suggesting I do tests with amp in standby, how can I determine anything if its in waiting mode, & not producing any noise-?

Your advice above tends to suggest doing the tests with this standby switch is both positions, unless I'm mistaken-?