Peavey Classic VT - Noise help

[Barnsleyboy]

Hi everyone.

Just registered, so big helllo all round!

Would anybody be able to assist with a problem that I have with an early eighties Peavey Classic VT amp which I've had for many years. When it's switched on with no guitar connected, all volume controls set to zero, I have excessive noise which can be heard here:

https://soundcloud.com/user-676608456

I took it to a local guy to sort out who I have used before and is very good. He has serviced the amp and replaced the valves. When I tested it before I brought it home I pointed out that it was still noisy. He suggested that it is due to the age of the amp and general electrical noise introduced by the components into the signal chain. Having tried it in my quieter home environment, I'm just not happy about how noisy it is. I'm going to take it back to him, but thought I'd see if anybody here could offer any thoughts on this.

I believe that the amp still has the original capacitors - could it be that they are the source of the white noise? Why is there noise even when no instrument is connected? The level of the noise level steps up for a few seconds, then cuts back down again, could this point to a specific component that has failed or is on its last legs?

The amp has a solid state pre-amp and tube power amp (2 x 6L6GC). The noise appeared some time back, but as I had another amp to use, I didn't bother getting it sorted out. I've now fallen in love with the amp again, and would love to re-instate it to it's former glory.

Any help would be much appreciated!

Cheers,

Paul.

[xtian]

If you plug in a guitar, but turn the guitars volume all the way down, is the amp still noisy?

[Barnsleyboy]

If you plug in a guitar, but turn the guitars volume all the way down, is the amp still noisy?

Yeah, it's there all the time.

[g1]

Does turning the master down get rid of the noise?
How about if you plug the guitar into the "power amp in" jack, is the noise still there?

[Barnsleyboy]

No. with master down, and both gain volumes down it's still there.

Plugging into the power amp makes no difference either! Any ideas?

[Chuck H]

I vote bad electrolytic capacitors. Could also be very old and noisy power tubes.

[g1]

In my experience, electrolytic caps will not give hiss and noise. But I've been wrong before.
I've recently had a pair of JJ EL34's that gave this type of noise, and I'd never heard power tubes do that before, never mind both of a pair.
Suggest you try another pair of power tubes, or try these in another amp and see if they hiss.
The driver stage is solid state, so that is another potential source of noise. The 2 transistors IQ1 and IQ2 or the diodes CR12 & CR13 (shown by PA In jack) could be culprits.
Freeze spray would help locate defective semi-conductors.

[Chuck H]

I didn't look at he schem before so I wasn't aware there were silicooties involved. But, indeed, the most common place I've heard this noise was associated with bad op amps.

I HAVE heard this noise in connection with bad filters too. But it was literally just before they popped!

[g1]

I HAVE heard this noise in connection with bad filters too. But it was literally just before they popped!

That I can understand.
Same with power tubes until I ran into that JJ pair. Sometimes they'll crackle and pop on the way out and if you tap on them they'll make a big pop or you'll see sparks or red-plating. But this pair just ran happily along with some low level crackle and hiss. Any time I had ever heard that before was always preamp tubes. Not this time.

[Barnsleyboy]

It sounds like a bit of a minefield! I don't think it is the valves, as they are brand new, and the sound was there before the amp was worked on.
Hence, the reason it needed fixing. I've posted this on other forums, and have also had a suggestion that could relate to failing carbon comp
resistors, or IC problems. I suppose the worry is that it will cost a lot of man hours and cash to get it fixed, but I've got a soft spot for the
amp!!!

Thanks for your replies, much appreciated!

[g1]

The fact that you still hear the noise when you plug into the "PA In" jack eliminates everything in the circuit before that jack (including IC's).
That only leaves the section after the PA In jack, which is not a lot, but you will need someone who is familiar with both tube and solid-state circuits.
Hope that helps, good luck!

[Barnsleyboy]

The fact that you still hear the noise when you plug into the "PA In" jack eliminates everything in the circuit before that jack (including IC's).
That only leaves the section after the PA In jack, which is not a lot

... now that, is a very good point!

Thank-you for taking the time to reply!!!!

[Enzo]

I don't think it is the valves, as they are brand new,

No, actually even new tubes can be faulty. ANY tube can fail at ANY time.

[Barnsleyboy]

Because the amp has the same white noise that it had before the valve replacement, I'm guessing that it's likely to be
another component that's gone south.

I couldn't be that unlucky could I? Getting a dodgy set of valves to replace the original dodgy set?
Oh the joys of pre-transistor technology... that's why we love them so much!!!

[Chuck H]

Well, the probability of getting bad new tubes to replace the bad old tubes has increased enormously in the last decade. It happens often enough that it's a common stop in the check list for troubleshooting now among the initiated. In fact, when I'm doing a new build I buy two sets of tubes (both preamp and power amp) in the hope that I will have enough good tubes on hand when the building part is done. Usually there is at least a couple of preamp tubes that are questionable for either noise, microphonics or both. On my last build I had to buy four sets of power tubes and also used another pair I had on hand before getting a pair that was acceptable. Two pair shorted one of the tubes soon after installation. Another pair had slowly fading filaments on one of the tubes until there was nothing left. The other two pair were microphonic. I ended up using the least microphonic pair and employing vibration reduction measures. So, in five pairs of new tubes I never did get a "good" pair. That's probably an extreme case. Though it tainted my opinion of new tubes irreparably. YMMV

Still... Let's go with the idea that it probably isn't the new tubes for now since the fault was pre existing. It's not really a can of worms or a mine field. It's just an amp. You've isolated the noise, somewhat, by bypassing the preamp with g1's suggested tests. It could be a noisy SS amp. If the piece of a schematic I looked at on line was correct that amp has SS devices in the phase inverter. If you open the amp and set it up to make the noise then you can blast a shot of freeze spray on any chips inside and see of that quiets the noise. I'm betting it will. Replace any chip that makes the amp quiet when hit with the spray. If the spray doesn't quiet the amp you can still check voltages for the low and high supplies to at least be sure they're correct (you'll need a schematic for this). And I still think that electrolytic caps could be replaced. The amp is old enough that it wouldn't hurt to do it anyway.