Tube amp 'whistle' problem

N

notCardio

I walk the line
This might be a common problem, but in my 45 years of playing plugged in, I've never heard this before.

My Blues Jr has developed a 'whistle'. Some might call it a squeal, and if you turn it up loud enough I'd be inclined to agree, but I'm calling it a whistle for now.

It does it without anything being plugged in, but if you plug something in it's worse. If you turn the master down to about 1 1/2-2, and then turn up the volume, the whistle starts and gets louder. Logical.

If you set the volume at about 6 (of 12), all 3 tone controls to 6, reverb to 0, you'll get the whistle. As you turn the Treble up, the pitch will rise. Turn it down, it will lower in pitch and volume until you can hardly hear it. If you turn the bass all the way up, you'll get low feedback.

I'm assuming it is in the preamp stage, but does this sound like a tube going bad?

I seriously hope that's all it is, otherwise this falls under Getuhgrip's heading of 'I really don't need this shit right now'.
 
Almost certainly a pre amp valve gone "microphonic" . You should be able to track it down with a pencil!

If you can run the amp out of the cab safely so you can get at the valves, turn the controls to about mid way and gently tap each valve in turn. The culprit should announce itself very obviously!

Note, you can often swap a miccy valve for the one in the PI position which is usually next to the output valves. The PI is not critical in this respect unless the fault is very bad. NB! Only swap over types for types! Fender are one of the makers that did NOT fit adequately rated anode (aka plate) load resistors in some amps and the wrong valve can burn them out. Similarly the reverb driver might be a different type.

Output valves can also suffer from microphony these days. Not something I ever saw in my yoof!

A good quality, good value ECC83 replacement is the 7025HG.

Dave.
 
First off, I'm no expert, but I know that much amp weirdness comes from tubes going spazzo.

Gently flick the tubes as the noise is happening. See if it changes.

In 45 years of playing, surely you've amassed a collection of tubes, right? Swap in a known good tube in each position and see what happens.
 
It may very well be a tube as already stated ^^^ but that's easy enough to check with a simple tube swap.
If you still get it...I'm wondering if it might be a cap that needs replacing...?
 
sounds like a microphonic tube ...... just tap on them and you'll hear it momentarily quit or change the way it sounds when you get the right one.
 
Thanks guys. I had wondered if this was what was referred to as tubes going microphonic. I've heard the term for years, just never experienced it myself. You've gotta remember that it's only been me in my bedroom for 30 years. So, no Greg, I haven't amassed any pile of tubes. In fact, I'll bet that my 65 Vibro-Champ has at least one tube that's still in it from when I bought it in 1973. OK, so I'll probably just have to go down to GC tomorrow and get a 12AX7 and take turns swapping them out. What are the chances that more than one went at the same time? If I end up having to replace one, should I replace both of them (I think it's got 2 12AX7s, I'll have to check to make sure)?

I'll try the tapping thing first, though.

Is it possible that this is something that would happen if one or more just came loose? I didn't think so, but I thought I'd ask anyway. I've already tried reseating them, but I might try again.

And I should have pointed out that this just showed itself after transport. I hadn't tried it at home first right before I took it, so I can't be sure it wasn't already that way.

Thanks again for the help. I'll let you know tomorrow how it turns out.
 
And it's always nice when you first learn about a problem with your equipment as it's being tested at the pawn shop.
 
I don't know if I should post this as a separate thread, but I'm sure it's already been done to death before (so why isn't there a sticky on this topic, hmmm?) so,

Assuming I have any choices at all considering it has to be something in stock, what are the recommendations for pre tubes? I just read something on another board whre the dude didn't like Groove Tubes, or Rubys, barely tolerated Sovteks, I think his go to was Mullard.

What say ye, gents?
 
Yeah, it doesn't do it if you just turn the Master up, only when you turn the Volume, so I figure that pretty much pins it down to a problem in the preamp.
 
notCardio said:
I don't know if I should post this as a separate thread, but I'm sure it's already been done to death before (so why isn't there a sticky on this topic, hmmm?) so,

Assuming I have any choices at all considering it has to be something in stock, what are the recommendations for pre tubes? I just read something on another board whre the dude didn't like Groove Tubes, or Rubys, barely tolerated Sovteks, I think his go to was Mullard.

What say ye, gents?
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I say don't worry about it. The internet is full of bullshit. Just go to GC, buy a Groove Tubes 12ax7, stick it in, and play. Then when you're ready for a full retube, get a set of JJ, or Tung Sol, or Electro Harmonix, or even Mullard if you wanna spend the money. Something relatively inexpensive and plentiful is fine. Tube snobs are weirdos.
 
I agree with Greg ....a Mullard's expensive and unless you want to listen for subtleties it won't be worth it .... plus .... if you're gonna pawn it may as well go cheap.
 
What is good in the valve market changes constantly.

These guys are pretty much on top of it...View Discussions and Categories on The Fretboard

The 7025 I mentioned is a TAD and is still a good valve AFAIK for around £10. They also make a pretty bombproof GZ34 (tho' I would just fit silicon!) way better than Sovtek.

The best, i.e most reliable valves at the moment seem to be JJs. The above bods are not enamoured of GC.

Mullards? Well if you can afford them and they truly ARE original Old Stock as yet unused fine, lot of re-marked crap about.

Like death and taxes, valve failure will always be with us and good ones are only going to get rarer and therefore more expensive. There will never be a better time to stash away a spare set..Or two!

And! I don't know if it was an isolated (x4!) incident but I had a set of new Ruby KT88s that did not last a day on an amp life test about a year ago!

Dave.
 
OK, I tapped the tubes and all of the preamp tubes sounded like you were banging on something metal, pretty much like the sound Craig Ferguson uses for 'It's a great day for America'. One tube sounded a little louder and had a slightly different pitch. So, does this mean that's the bad tube, or does it mean they're all bad? Or are none of them bad because that's the way they're supposed to sound? I thought they difference in pitch might be becuase a 3 tubes have a different function.

So, replace one, replace them all, or replace none because that's not the problem?
 
arcaxis said:
Somewhat common with the Blues Jr's as a few comments on it found.......

https://www.google.com/search?num=2....7.0....0...1c.1.52.serp..3.4.472.Hsh_wPtLW5I

I kinda sort of remember something about the shielding or similar around the input jack and self oscillation when the gain is cranked, but it's been a few years and the memory cells aren't retaining information like they used to. Tube could be microphonic and maybe a change might help.
Click to expand...

Thanks, that led me to the billm mods site, which I had forgotten about. I was gonna have an amp tech do the mods, but in my current financial situation that's out of the question for a while.
 
Until you swap tube by tube....you'll never know for sure if it is a tube and which one is causing it...or that it's someting else.

Tapping won't always reveal that. Most times they all make noises when you tap on them.
I've swapped tubes based on "tapping"....only to find that it was a different tube that had some kind of issues.
 
miroslav said:
Until you swap tube by tube....you'll never know for sure if it is a tube and which one is causing it...or that it's someting else.

Tapping won't always reveal that. Most times they all make noises when you tap on them.
I've swapped tubes based on "tapping"....only to find that it was a different tube that had some kind of issues.
Click to expand...

That's what I was going to try, getting one new tube and going down the line with it. But I thought I'd ask the question first in case someone said that it's obviously not a tube.
 
The answer points to it being a bad tube....but it's hard to say 100% just based on a the description of hearing a "whistle" noise.
I mean, there is a possibility it's something else in the circuit....?

Most of the tubes I've had to replace because they went microphonic, had more of feedback/squeal type noises, often random or only on some notes, etc....
I've never had a bad tube situation where the amp just whistles like that steadily. *shrug*
 
notCardio said:
OK, I tapped the tubes and all of the preamp tubes sounded like you were banging on something metal, pretty much like the sound Craig Ferguson uses for 'It's a great day for America'. One tube sounded a little louder and had a slightly different pitch. So, does this mean that's the bad tube, or does it mean they're all bad? Or are none of them bad because that's the way they're supposed to sound? I thought they difference in pitch might be becuase a 3 tubes have a different function.

So, replace one, replace them all, or replace none because that's not the problem?
Click to expand...

You don't need a new valve or set of valves at this point. Just swap them around (number them with a Sharpie first) assuming they are all the same type number e.g. ECC83 aka 12AX7.

Dave.
 
ecc83 said:
You don't need a new valve or set of valves at this point. Just swap them around (number them with a Sharpie first) assuming they are all the same type number e.g. ECC83 aka 12AX7.

Dave.
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OK, I don't understand. I mean, I understand what you're saying to do, but I don't understand why. I don't see what that will tell me, except maybe that, yeah, I've got a bad tube. How will that tell me which one is the bad one?
 
notCardio said:
OK, I don't understand. I mean, I understand what you're saying to do, but I don't understand why. I don't see what that will tell me, except maybe that, yeah, I've got a bad tube. How will that tell me which one is the bad one?
Click to expand...

Ah! My apologies. You perhaps don't understand how the valves are arranged in a guitar (and most other) amps?

They are for the most part in "series" aka "cascade" that is, one feeds the next and that feeds another and so on through to the tone stack>PI and ultimately output valves.

It follows then that a valve early on in that chain will have any imperfections amplified by later stages. A valve can be hissy, hummy or, as in this case, microphonic in a front end but be perfectly fine for driving a tone stack or as a PI.

Yes, you should replace the miccy valve when you identify it (by a process of elimination, hence the Sharpie!) but in truth it will probably last for ages in a less critical position.

Dave.
 
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