Help. High freq noise

[Danplifier]

Hi everyone, first time poster, just hoping someone can help me with my valve amp.

It's a VHT Special 6. It sounds great at low volumes, but when I turn the volume past halfway it emits a really high frequency hum (I'm guessing about 18,000Hz or so). I've never needed it that loud so I've not noticed the problem before, but I'm thinking about using it for gigging with when I get my band sorted out. I can't take it back to the shop because I've had it over a year (just about).

Anyone know what it could be? I'm still using the cheap tubes that came with it (probably Chinese), so do you think if I swapped them out for a better brand this might cure the disease?

Any other ideas?

Thanks in advance

Dan.

EDIT: It's definitely not feedback before someone suggests that. Cheers.

[Chuck H]

It's definitely not feedback before someone suggests that. Cheers.

Oh, contraire... It could be a microphonic tube. And that IS feedback. But I can appreciate that you understand the difference between squealing pickups and a misbehaved amp. It boggles my mind and even irritates me that some players don't!

Do the treble and presence controls affect the problem? If you tap on any of the preamp tubes do you hear more than just a small "tink" or "thud"? If you hold the first preamp tube (with a cloth) while turning the amp up, does this affect the problem?

[Danplifier]

Oh, contraire... It could be a microphonic tube. And that IS feedback. But I can appreciate that you understand the difference between squealing pickups and a misbehaved amp. It boggles my mind and even irritates me that some players don't!

Ah, OK thanks for the info. But yes, I knew it wasn't the "good" type of feedback. I welcome that.

Do the treble and presence controls affect the problem? If you tap on any of the preamp tubes do you hear more than just a small "tink" or "thud"? If you hold the first preamp tube (with a cloth) while turning the amp up, does this affect the problem?

There aren't any treble or presence controls, just a volume and a single tone pot. Really simple amp - which was one of the attractions.
I've not actually opened the back up yet, and didn't know I could touch the tubes while the amp was switched on. Is this safe - if I use a cloth of course? Any chance of electric shocks? I'll give it a try later and get back to you.

Thanks for replying.

[Dave Curtis, dB AudioTech]

Yes, you can touch the tubes while it's on; the cloth is just in case the tubes are very hot. Preamp tubes generally don't get very hot, but the output tubes can burn your fingers. No shocks from just touching the tubes; just don't touch any wiring connections inside the chassis. You might want to get another 12AX7, anyway. But really, a 6 watt amp isn't the best choice for a gigging amp...

FWIW, I've seen stock Mesa preamp tubes wrapped with a strip of duct tape to help quell microphonics.

[J M Fahey]

And please answer the question, does the tone control affect the high frequency oscillation?

[Danplifier]

Only just had time to check, yes the tone does effect the frequency oscillation. It only seems to be audible when the tone pot is between 1 and 3 o'clock, and when the volume knob is pulled for boost (push pull potentiometer), and again only at 1 - 3 o'clock.

Do I remove the metal pre amp valve cover before I touch it?

Anyone any closer to knowing what the problem is?

Also, why do you not recommend a 6W amp for gigging? It's quite loud and will be mic'd up anyway.

Thanks guys.

[Chuck H]

Lots of guys use a small amp with a mic in front (or a speaker driven line out) into the PA.

Do remove the tube cover for the test.

It's interesting that the problem is most pronounced with the tone between 1:00 and 3:00. Usually the brighter/louder you adjust, the worse it gets, all the way up the adjustment ranges. I suppose it could be a more specific resonance then IF it is a microphonic tube. Do pull the tube shield for the tube closest to the input jack, induce the problem and then lightly hold the tube. Use a cloth if it's too hot. Any change?

[Danplifier]

It's interesting that the problem is most pronounced with the tone between 1:00 and 3:00. Usually the brighter/louder you adjust, the worse it gets, all the way up the adjustment ranges. I suppose it could be a more specific resonance then IF it is a microphonic tube.

Well, there's no sound until 1:00 and then it gets higher in pitch the more I turn it, until I get past 3 when the pitch is too high to hear.

And just to make it clear, this only happens when the volume pot is in boost mode and between 1 and 3 as well as the tone pot.

Do pull the tube shield for the tube closest to the input jack, induce the problem and then lightly hold the tube. Use a cloth if it's too hot. Any change?

OK I'll try that in the morning, too late to make a noise now.

[Chuck H]

O.K... It's very telling that the pitch increases and then dissapears into the ultrasonic. It's starting to seem more like a parasitic oscillation than a microphonic tube. That's not good since this is a pcb amp (which means there's not a lot that can be done about lead dress and grounding scheme). It's not a real high gainer. And IMHE modern, moderate gain commercially produced amps don't leave the design stage with any such condition. Are you just noticing this for the first time because you've never turned the amp up before? Did the amp not do this before? Did you purchase the amp used? Might it have the "distortion mod" that I found floating around for that amp model? Or any other modification? And finally... Can you describe your signal chain in detail, please?

[Danplifier]

I've just tried squeezing (lightly) the pre amp tube whilst "the noise" is present. It does effect it but it doesn't stop the noise completely - just seems to dampen in slightly.

It's not PCB it's hand wired. I'm not bad at soldering if it's something quite simple, and I've got some good electronics solder. So if you tell me what I can do to fix the problem I don't mind having a go.

It was brand new from Dawsons Music, which is a reputable nationwide company in England. No mods or anything, and yes I've only recently noticed the problem because I don't usually play at that volume. I could always just set the volume to 12:00, which will be loud enough, but I'd rather eradicate the problem.

[Alex R]

I kow the Pittbulls use an EF86 tube in the preamp - I wonder if the Special 6 does too? These types are very prone to microphonics. If you have one of these (the ones I see in VHTs tend to have been sandblasted or something, they have a frosty finish, oddly) it's odds on that it's the problem, and anyway I would change it for a decent NOS type. The Mullards aren't cheap but I used to have a stock of Phillips Miniwatt EF86 and they were good. The current Russian production EF86 is not good, all the ones I have seen are too microphonic to use in a Pittbull.


eta I checked it out and it doesn't use an EF86! so scrub that. Try tapping the 12AX7 preamp tube that IS in there, with the amp on. Does it make a ringing sound the same sort of frequency as the feedback? If so change it. There are some Chinese 12AX7B around, these are low microphonics.

[Dave Curtis, dB AudioTech]

Definitely make sure it's not the 12AX7. Swap it with a known good one to rule it out. First things first.

And certainly you can gig with a 6 watt amp run to the PA.
That was a personal opinion I should have kept to myself. I don't see lots of guys doing it, but yes, it's done.

[Danplifier]

Hi Alex, thanks for the reply. Yeah, I should have said what what sort of tubes are in it - forgot to mention that.

12AX7A and 6V6GT
The 12AX7A is clear glass, not frosted, and it has some sort of chrome paint coating the bottom of the inside. No brand - just says made in China 2007.
The 6V6 is black painted glass and says made in China 2010

I've tried tapping the pre amp tube but nothing happens.

Chuck - forgot to say, I'm plugging straight into the amp, no effects. I do use effects but not since trying to fix this problem.


Could the volume push/pull pot be the problem since it only happens when that is pulled? It feels a bit cheap compared to the tone pot. The tone pot is nice and smooth at adjusting whereas the volume feels a bit flimsy when you turn it.

[Danplifier]

Definitely make sure it's not the 12AX7. Swap it with a known good one to rule it out. First things first.

And certainly you can gig with a 6 watt amp run to the PA.
That was a personal opinion I should have kept to myself. I don't see lots of guys doing it, but yes, it's done.

OK I'll try that.

Yeah, I've had louder amps in the past but the sound engineer always tells me to turn it down on stage so he can have more control over the mix. I only play in 200-300 capacity venues you see. And then the amp never reaches it's sweet spot (volume wise). I like it on the verge of breaking up as my clean sound, and then use an OD pedal if I want it heavier.

I'll let you know how I get on.

Thanks

[Chuck H]

I'm betting on a parasitic oscillation. The fact the the pitch increases with the brightness setting up into the ultrasonic is the reason. I've NEVER seen a microphonic tube do this.

Is this amp the ordinary Special 6 or the gainier model??? I'm looking up info for it. The first thing I notice in the open chassis shot is that the preamp wires are bundled and too long and the OT is on the input side of the amp.

EDIT: Still looking at the chassis shots... The OT secondary leads are WAYYY too long and run right under the preamp board and input!!! The preamp wire bundle is stuffed under the board on top of the OT!!! The output jacks are located center chassis under the tone control!!! I would be much more surprised if the amp DIDN'T oscillate. Someone with experience could stabilize it, maybe. But it'll take some tweaking and a long effort. Relocating leads, possibly grounds, maybe adding a bleeder cap. Trying to idealize that layout isn't for a novice.