Whistle from KEL84

[hooger]

Hello again,
I built my KEL84 a couple of weeks ago and I notice now that there is a high pitched whistle coming through the loudspeakers whilst the amp is warming up. Unfortunately I no longer have a scope to measure it with. It seems to disappear after a few hours of warming up. Is this cause for concern?
Hooger.

[NealG]

It could be HF instability, Greg had a similar problem with his 2A3 PSE after applying feedback and using high capacitive twisted twins CAT5 'speaker cables.

You could try different cables just to see or try a Zobel network across the 'speaker terminals at the amp end. A 2W 10R in series with a .1uF cap would do to start with.

Of course it may not be this at all!

[Ianm2]

well either valve or resistor noise is a kind of rushing noise I think, but it goes quite quickly.

[acorn]

Hi Hooger

you may have the primary windings of the output transformer the wrong way around, ie out of phase, do you get this effect on both channels or one and does the whistle increase the longer you have it on.
just a long shot.

Regards Acorn

[hooger]

Thank you for all your suggestions. It seems that it is only the one channel and may be a hangover from a previous problem. Previously (see wadproblems-> another KEL84 Problem) when I had the 220R's instead of the 22R in R8/9 above a certain volume the left channel (being the one of interest here too) went unstable. Replacing the 220Rs with 22R appeared to solve the problem. Is it possible that this has simply shifted the instability up in frequency? It does indeed get louder the longer it is on, but sometimes disappears altogether. I will certainly check the polarity of the primary windings and hopefully get hold of a scope to see what is going on. Thanks again.
Regards Hooger.

[David Mccallum]

Hi Hooger,i've got a similar type of thing happening to my kel84.It seems to whine at quite a high frequency when it's been on a while.Its faint but there nonetheless,it seems in mine to affect the right channel.It comes and goes at irregular intervals starts quite faint,builds up then fades out.Seems that the right hand ecf80 resonates if i tap it with my fingernail it stops.I've tried changing both of them to mullards but to no avail.The whine at its hight can be heard faintly through the speaker (right hand) but is unaffected by altering the volume control setting. It's a shame because otherwise it sounds terrific,no hum to speakof even at full volume with nothing playing.If i crack it i,ll post it,meanwhile if anyone has any sugestions? i'd be glad to hear them,please dont be too scathing(first build atempt)!