Peavey 6505+ 112 combo, all preamp tubes gone microphonic, possible?

[AkselBT]

Hi,
I've had some trouble with a constant stream of noise and crackles coming from my amp, especially at louder volumes. I've had all the stock tubes out of their original position, cleaned them and their sockets(with contact cleaner), and switched their places (also the power tubes), regardless, when tapping the tubes, all the preamp tubes are still microphonic.
The amp is about 1,5 years old, and it's still running on the stock tubes.
As stated in the title, is it possible that all the preamp tubes in my peavey 6505+ has gone microphonic or is something else the culprit?
Thanks in Advance,
Aksel

[Chuck H]

All tubes are microphonic. This is especially apparent in high gainers that have cascade stages (like your amp). Even vintage amps are very sensitive at the input tube. A ping when tapped is normal. If the amp breaks into squealing at higher gain settings you may have a tube that is too microphonic. Even if all the preamp tubes seem to ping or thump when tapped it's possible that it's still mostly vibration transfere to the input tube. Try pulling the input tube and then tap the remaining tubes. I think you'll find the noise greatly reduced. Though pobably not gone completely.

Unless your amp is breaking into oscillation (that isn't due to guitar pickups) you don't need to worry about it. A preamp input tube that didn't make a sound when tapped would be more unusual.

Edit: Failed to address the noise and crackling issue at high volumes.

Do pull V1 and tap the other tubes. Now take out the second tube in the chain and plug the tube you pulled into that socket and tap it. If any of them make a noise other than a flat thunk or a ping, like a rattling noise, you should probably replace it. Swapping the preamp tubes around in your amp isn't a good idea because with high gainers the MFG often uses a particular tube or a selected tube for the input. It won't hurt to have it in another position but it's very possible that any other preamp tube in the amp is too microphonic for the input tube.

If you do have a tube that seems especially noisy, replace it with a low noise/low microphonics selected tube and use that new tube for the input position.

If it's not the tubes then it could well be an intermittant fault in the wiring or on the board or a noisy component as Enzo suggested below.

PS: Notice I didn't use the term V1, Enzo? See... I'm learning. The input may be V1 but I'm not looking at a schem so I stuck with "input tube" instead

[Enzo]

ALl tubes are microphonic to a point, but it usually doesn't show up except for the first stage.

However it is VERY unlikely that ALL your tubes went overboard together.

When you tap on one tube, the vibration travels through the board to the others. Like when you tap on your mic stand, the mic picks it up. Usually then, one tube will be much more sensitive than the rest, and it will be the culprit. Less common, but does happen, is some component on the circuit board becomes microphonic, and tapping the tube also vvibrates that part.

Isolate the problem. Ball up your fist and whack the top of the amp. If that triggers the noise, then something is loose somewhere, perhaps inside a tube, perhaps a cracked solder connection or failing ribbon connector. That test was for mechanical sensitivity. Now electrically isolate things. DO ANY of the controls affect the noise in any way? For example do the EQ controls change the tone of the noise? And if you zero the post control is the noise gone? Or the pre control? Does it matter which channel has been selected? Have the footswitch? DOes turning the FX loop off kill the noise?

[AkselBT]

Reply

Thanks for your replies,
Tried some different stuff with the amp.
- It doesnt appear on the rythm channel
- (lead channel) with post at zero theres zero noise
- Knocking it didnt do anything
- it's not the effects loop
- it's not eq related

Swapped all the tubes around into first socket of
the chain both with and without an input tube.
When using all tubes, no matter which tube comes first,
the amp builds up a ringing scream(on the lead channel).
It's worse with more gain.
With no tube in the V1, no noise appears regardless tubes.

Where I too rough when handling the tubes?

I'm not rich but wouldnt it be easier to just
buy 5 new tubes?
Best regards,
Aksel

[Chuck H]

You just need a low microphonic tube for the lead channel input tube in that amp. And every once in awhile that tube will go microphonic and need to be replaced, BUT, don't throw away the tube because it's probably still OK to use in the other preamp positions. As mentioned, all tubes are microphonic. It's just a question of how much. Some amps. like yours, need a selected tube for the high gain input.

One of the amps I build has similar issues because it's high gain, but only when cranked. So there's the combination of high gain sensitivity and high volume. I absolutely DO need to select a tube that behaves for the input tube or the amps will squeal. For the amps "I" make I select from my stash of NOS tubes. But for the production design I spec'd a Sovtek 12ax7WA for the first tube. The Sovtek 12ax7WA isn't as bright and shiny sounding as some other tubes, but it's ultra low in microphonics. I recently had a few that were hummy right out of the box though. So make sure whatever you get, you can return it if it doesn't perform as advertised.

[AkselBT]

Do you think I smashed the tubes since it got worse?
I think I'll buy two just in case

[Chuck H]

Do you think I smashed the tubes since it got worse?
I think I'll buy two just in case

No... I'm just saying that tubes that are too microphonic for the first position in high gainers are sold for everything else everyday new in the box. So just because a tube is too microphonic to use in that application doesn't mean it's a bad tube. The Sovtek 12ax7lps may be the best sounding current production preamp tube. But it's far too microphonic for the first tube in a cascade preamp. That's why I went with the 12ax7WA. Doesn't sound as good (to my ears) but also doesn't squeal in the first position. I spec'd the LPS for every other preamp socket in my amps. Good sounding and well behaved tubes are too often the exception, not the rule. As designers we work around this as a fact of life.

You'll spend a good bit more, but IMHO the RCA short gray plate 12ax7a tubes with the halo getter sound as good as any and are often very low in microphonics. I buy them as I find them and sift out the best ones for the first preamp position. If your only going to get one or two you'd need to spend extra to have them selected for low microphonics. But it's one of the best tubes I know of for getting good sounding and well behaved performance.

[AkselBT]

Thank you bert much for helping me out
Best regards,
Aksel

[Chuck H]

Your welcome...

And don't call me bert.

[AkselBT]

Haha sorry about that!
I meant "very"

[Enzo]

There you go, CHuck. Surely he didn;t mean it.

[Gtr_tech]

And don't call him Shirley....

[Chuck H]

To be frank, I donk mind.