A screaming amp sound

Possibly one of the tubes has become microphonic, and when you crank up your amp, you get feedback.
http://en.wikipedia.org/wiki/Microphonics

Here's a common method of hunting microphonic tubes:
http://www.ehow.com/how_2043383_-locate-microphonic-tube-amplifier.html
Though, my preferred tool is a pencil with a large rubber band wrapped around the end, not just using the eraser itself as a tapper.

that sounds like exactly the problem - thanks very much for that - i will have a look at my amp when i get home from work.

thanks again, Al

thanks again for the help - it worked and i do indeed have a microphonic tube.

The only problem is, i am not sure where to buy replacements, and certainly wont be able to get any for this weeked when i have 2 gigs.

if i remove the faulty tube, and play the gigs, is that likely to cause damage to my amp in any way?
also, when i do replace a tube, do i need to get the same type of tube, or will any old tube do?
finally, will i need to replace all 4, or just the microphonic one?

thanks, AL

Depends...
Tubes should be replaced with the same part number (or one selected from a substitution chart). Manufacturer is a matter of personal preference (or religious fervour for some people :roll: )

According to the manual, your amp should have:
4 x 6L6GC, 3 x 12AX7 or ECC83

In most cases the amp needs it's full compliment of tubes. There are a few specific exceptions, but that is dependant on the design of the amp, and which tube you are talking about.

The 12AX7 tubes are the smaller type, and they are in the pre-amp section.

Removing any one will probably result in no sound at all.

The 6L6GC tubes are the big ones in the power amp section.

Depending on the amp's design, it might be possible to run with only 1 pair of 6L6 tubes, but you would have to be certain you are removing a pair, not one from each pair. If I could find a schematic, I'd have a better idea.

You might be able to move them amongst themselves, and perhaps land the microphonic one in a less critical spot in the signal chain (perhaps a lower gain part of the circuit) and reduce the noise for this weekend.

:!: :!: :!: careful :!: :!: :!:
The tubes can get hot enough to burn the skin off your fingers. Not good for gigging.

*edit*
Now that I re-read your post :oops: I see you mentioned "all 4", which means it's the 6L6.
See if adjusting the texture control on teh back has any effect (though it might change your tone in ways you don't want). It's job is to adjust one pair of the 6L6 out of the system. With luck, it might turn down the noisy tube and it's partner.
If this amp (and thus it's tubes) are a few years old, you should replace the 6L6 tubes as pairs, or as a complete set for the best sounding results. There's no harm in replacing one, it's only about the sound.

just a quick update to let you know what happened.

when i carried out the microphonic tube hunt, it appeared that 2 of the tubes we faulty. both gave an almighty scream when tapped with a pencil. One of the tubes was not working at all (so that left me with only 1 decent tube).

i bought a new set of 4x 6L6GC tubes, and managed to break the first one i opened :cry: :oops:

so now i have 3 new russian tubes, and one stock tube - but NO SCREAMING FEEDBACK :P :P :P

thanks for your help and advice, i am now a happy man again!!

p.s just to clarify, you did say that if i had to mix tubes from different manufacturers (which i have because i broke one of the new ones), that it wouldnt cause damage to the amp??

p.s just to clarify, you did say that if i had to mix tubes from different manufacturers (which i have because i broke one of the new ones), that it wouldnt cause damage to the amp??

Damage: no
Odd tone: possibly (but if you like how it sounds, who cares)
Annoyance to purists: probably

Al you may or may not need to bias the amp now that you have repalced the tubes, more so if they are not a matched set.

Al you may or may not need to bias the amp now that you have repalced the tubes, more so if they are not a matched set.

True, but the bias is fixed (I.E. not adjustable) on most modern Peavey amps.

As long as the pairs of tubes are of similar age and brand there shouldn't be an issue.
Even a slight mismatch won't hurt the amp. it may slightly reduce the lifespan of the tubes, or sound a bit off to some ears.

kent, when I did some checking when I looked up fixed biased amps it says that it's actually a misnomer that in fact fixed biased amps do need to be biased when the tubes are changed.
Fixed bias means that bias voltage is applied from an extenral source, usually a negative bias supply, and remains constant throughout the flow of signal or with different valve samples.
Most Marshalls use this bias technique, with the exception of the 18 watters and a few others.
Fixed bias is usually adjustable, as in you can set the "fixed" voltage by adjusting a pot. Mesa Boogies are one exeption to this. You cannot adjust the bias voltage on a Mesa because they want you to buy their valves which are known to operate well with their fixed bias voltage level.
Remember the term "fixed bias" comes from the fact that the bias voltage remains fixed no matter what valve is there or whether there's signal or not.

Now with that said I recently changed all of the tubes in my Fender Deluxe with tubes I bought from the TubeStore and didn't bias them and they sound OK, not sure if it could sound better if I had. Anyway as Kent mentioned the main issue would be shorter life span of the tubes if they aren't biased.

I have been looking, and the manual for Almann's amp doesn't say anything about bias.
And I can't find a schematic online. That would tell us for sure.

I wouldn't be surprised that some of Peavey's more modern tube amps might have some circuitry that makes them "plug-n-play", but I have no way of verifying my guess.

Bottom line, you won't blow up your amp.

Oh yea you defintely won't blow it up but you may have premature tube failure and some sound deficiencies

The ValveKings are cathode biased - which means they are self-adjusting.

Well there you go your all set