high pitched noise after amp service - any ideas?

[worrasf]

So I got my amps back from Glenn last week and am really pleased with the SQ. However, they/it have started to produce a high pitched "whine" after being switched on for an hour or so. Obviously, I have discussed with Glenn and it is down to the S4s which was recapped and uprated to KT120's. I've "swapped" out all the valves including the 85A2's (but excluding the KT120's as I dont have spares) but no joy.

It makes no difference if the amp is run without any connections (other than speakers ;)) and as I say starts after an hour or so when I assume things are nice and hot - but there is no evidence of any overheating etc.

The noise comes through both speakers (tweeters) equally and is really piercing. It's not noticeable when music is playing but is audible when no signal going through from a few feet away. Otherewise sound is perfect, no distortion etc. The noise is not coming from the amp but through the speakers.

:scratch:

I am taking the amp back to Glenn next week for a look over but wondered if anyone had any suggestions as to a possible cause that might save me a round trip to Scarborough?

Steve

[Firebottle]

My guess, for what it's worth, is the KT120's are going into oscillation.
I don't know Glenn's circuitry but I would either fit or increase the value of grid stopper resistors on the KT120's.

[worrasf]

Thanks Alan
I'd heard of this phenomenon on one of the "self-build" amp forums.
Guess it's a trip to Scarborough then :eyebrows:

Other option is to revert to 6550's or KT88's but having heard the KT120's (and paid for a new quad) I guess I'm loath to do that.

Regards
Steve

[anthonyTD]

It does sound like some sort of borderline stability issue, ie, either to do with the over-all feedback loop, or instability issues within the output stage since the output valve change.
I am sure Glenn will get to the bottom of it, just give him some time to get his head around whats going on.:)
Anthony,TD...

[worrasf]

Yep - that's what I'll do next week - In the meantime I've ordered a quad of Svet 6550c's which will be useful spares and also should nail down if its a "KT120 issue"
Steve

[worrasf]

Reduced bias to 55mV and much better so guess is a KT120 "instability issue" - going to try a set of Svet 6550c's as I just get the feeling the KT120's are "a valve to far" for the S4s - it was built in 1987 after all
Steve

[worrasf]

It does sound like some sort of borderline stability issue, ie, either to do with the over-all feedback loop, or instability issues within the output stage since the output valve change.
I am sure Glenn will get to the bottom of it, just give him some time to get his head around whats going on.:)
Anthony,TD...

News Flash!
Have now modded the S4s according to Glenn's instructions which involved adding a couple of resistors to the power valve pins and all is now well - better than ever in fact as the KT120's are working properly! Huge sound stage - massive bass - great PRaT and no squealing!
Steve

[The Black Adder]

One thing I did on my Crofted Quad II's was to clean the pins of the valves with a glass fiber pen (Marco suggested this). I also had the high pitch whine, sounded like an old CRT telly. Once I'd cleaned the pins the sound vanished.

It did come back later on but only very very slightly. Anyway, the amps also sounded much nicer too probably due to using NOS valves and therefore 40+ years of dirt/surface oxidizing The pens cost a couple of quid and clean pins can't hurt.

Nice and cheap little tweek! :)

[worrasf]

clean pins can't hurt.



Agreed.
I use deoxit and deoxit gold on cotton wool buds to clean all valve pins. A tad tedious but only needs doing once and it's surprising how much dirt comes off.
In my case the pins were already clean and the KT120's were brand new. The new mods have worked a treat though.

Steve

[Firebottle]

Excellent Steve, what value of resistor did you fit?

[worrasf]

Excellent Steve, what value of resistor did you fit?

From memory (am at work and Glenn's "back of beer mat" circuit diagram is at home.

1. Moved output transformer wire from pin 4 to pin 6
2. fitted 150ohm resistor between pins 4 & 6
3. Increased resistor from pin 5 to 4K7

So not only does it now sound great I can say with some degree of honesty that I personally upgraded a Croft amp :lol:

Steve

[The Black Adder]

Hi Stephen.

I also asked Glenn about using Deoxit and he really didn't recommend it so maybe ask him about that. I asked because of this issue on the KT88's

[worrasf]

Hi Stephen.

I also asked Glenn about using Deoxit and he really didn't recommend it so maybe ask him about that. I asked because of this issue on the KT88's

Hi Joe - interesting.

I'll run it by him - probably wait a while as I suspect he's fed up with my nattering him lately ;)

Steve

[The Black Adder]

No probs matey.. I think he said it was something to do with the high impedance loading on the output valves... Can't remember entirely so please correct me when you have a chat with him.

Glad the mods worked though. I'm looking in to an S4 too but need to source one.

[worrasf]

I'm looking in to an S4 too but need to source one.

Good luck with the hunt Joe - if you do find one I've got loads of "spare" valves. Mine is 1987 vintage and the caps were about to pop so well worth shipping to Scarborough for an MOT and service.

Steve