High-Pitched Squealing at High Volumes

[harry]

I have a late 60s/early 70s Carlsbro cS100TC.

It's a 4xEL34 twin channel amp; each channel uses both sides of a 12AX7 and there's one for the phase inverter.

I recently re-capped the little beast and hum from the power stage is now minimal.

However, if I turn the volume up (no instrument plugged in) past 7 or so, I hear a high pitched whine/squeal coming from the amp itself and a little glowing spot (blue/grey I think) appears on the tubes.

The point at which this squealing occurs changes with different preamp tubes. Some don't do it until 9 on the volume dial, some do it on 6.

I haven't tried doing this with an instrument plugged in, but I imagine it would result in a bad sound.

Also, I'm not sure if I'm imagining it, but one of the (new) tubes might have a little more of a blue haze than it used to.


What is this high-pitched squealing? What would cause it? And why is it affected by the preamp volume?

Any help/comments would be much appreciated,

Thanks,

Harry

[harry]

oh - schematic is here: http://www.chambonino.com/carlsbro/carlcs100tc.html

[txstrat]

I would assume the (microphonic) squealing is caused by the tubes themselves since it differs with different tubes.
The preamp tubes run at 160v (lower voltages let the tubes be driven into compression more easily). Combined with a 1k cathode resistor this makes up for a relatively high gain preamp. (the lower the cathode resistor the higher the gain - all others being equal).
You might try selected (V1) tubes known with less microphonics.

[harry]

Sorry, I wasn't very clear, the high-pitched squealing is very quiet and comes from the circuitry rather than the speakers.

I've heard something like it before, when I turned a tube amp on without a speaker connected.

[txstrat]

Did you watch for the right polarity and voltage rating on the filter caps? The squealing might be stressed caps. Or output transformer - I hope you always have a speaker connected when turning the amp on..

[harry]

Ah, I can see that it could be stressed caps.

I replaced 6x 50uF 450V caps with 100uF 450V caps. The polarity is right, I would imagine something much worse would happen if not. The amp sounds great, it's just that I used to be able to turn it up to 9 with no weird effects and now I'm presuming I can't.

I'm not in a position to be able to test it at high volume with an instrument, but I'll be on a 10-week tour soon and I'll need to turn it up to 5 or 6 every night.

I do have a speaker attached when testing. In fact, I first noticed this problem when I was trying to see how much noise I'm getting from my preamp (quite a lot, old carbon comps to blame maybe..)

[harry]

Thanks for your help so far txstrat, I appreciate your suggestions.

Looking again at the schematic, the filter caps are in series and I think someone has modified my amp with a resistor to spread the current evenly between the two capacitors.

So the two 50uF 450V caps that were in there have essentially been doubled up to give 25uF 900V caps... in which case surely these are over-rated anyway?

[txstrat]

It should look like in the attached pic.
The resistors are necessary to spread the stress over both the caps.
The two 100uF/450V make up for one 50uF/900V. Well, actually two 350V caps would be OK. Even 250V ones seem alright but I wouldn't go down that close to the B+ voltage.

[harry]

B+ I believe was 520V with no tubes... quite high so I'm going to look into that.

Actually, I don't think those divider resistors were wired correctly on the amp. Even though they're not in the original schematic, they should definitely be there, shouldn't they? After all, that's why the 3 x banks of filter capacitors have 2 x 50uF caps in series, to increase the voltage rating.

Could it be that the resistors are wired incorrectly, aren't spreading the stress over both the caps, and therefore one of the pair is getting stressed?

[txstrat]

520V with no tubes is OK.
The voltage will drop with tubes in the sockets.

Could it be that the resistors are wired incorrectly, aren't spreading the stress over both the caps, and therefore one of the pair is getting stressed?

Yes, that's right.
When you use two caps for a higher voltage rating you MUST use those resistors. (for every pair - look at the next pic).

[oc disorder]

I suspect you are getting unwanted internal positive feedback now you have the amp running as it should (well nearly ! )
This could be just a simple matter of a poor ground at the input.
Try grounding the inputs even temporarily running a ground wire from both
(maybe one at a time) 68k junctions and see if it goes away.
Has the lead dress changed ie have any wires shifted at the output that could induce back to the input ?
You could try removing the input jacks and fitting a star washer after
rubbing with emery paper so that there is a good ground.
Plugging in a shorting jack may help prove this or even an instrument with the volume turned down.
Other than that new 68k and 1 meg resistors may be the order of the day.
This is what alerted me
"the high-pitched squealing is very quiet and comes from the circuitry rather than the speakers."
It could be oscillating at a higher frequency and you are only hearing the lower harmonics. The grey/blue glow is just the impurities fluorescing within the tube and will do it in sympathy with any audio signal as I understand it.
Does the case have foil shielding and are your test leads to the speaker
away from the input ?
If the amp is upside down can you lay a sheet of metal to shield the internal chassis ?
The stray capacitance effect may shut it up !
Just my tuppence worth :-)

[harry]

Wow, there's a lot of suggestions there, thank you.

I am just using the normal speaker output on the back of the amp. The inputs are on the front, so they're quite a distance from each other. The case isn't foil shielded though. I will try this, as I am always looking for ways to reduce noise.

The input jacks do have shorting inputs but they're not in use. Does this involve just wiring the tip of the jack to ground on the side that gets opened when a jack is inserted?

I am going to change out all the resistors in the preamp section for metal film. I understand this will reduce thermal noise; also, I understood that carbon comps don't meant to give much of that "pleasing" second-order harmonic distortion with low voltage swings.

I will try both of your suggestions, txstrat and oc disorder. Thank you for your help.

[oc disorder]

"Does this involve just wiring the tip of the jack to ground on the side that gets opened when a jack is inserted?"

Well just for now ground them and see if it stops oscillating.
If they are open ie the tip is not grounded via the normally closed switching action of the jack thats all it is.

Are you saying the "squeal" is coming from the amp not the speaker ? ...

Well may be above the response of the speaker.

[harry]

OK, I'll try it with an instrument with the volume turned down.

The jacks that are in there have four pins, and two are linked to contacts that open when an input jack is put in. I'm guessing a wire from the tip of the always-closed pins, to the ground of the jack-dependent pins.

[txstrat]

Well just for now ground them and see if it stops oscillating.
If they are open ie the tip is not grounded via the normally closed switching action of the jack thats all it is.

+ that would explain, why the squealing was different with different tubes.