Loud squeal from speakers - KEL84

[hunterwd]

Hi Folks

Hope you can help

I have a kel84

I used this about a 2 weeks ago and all was well but decided to connect my other amp up instead.

Went to try the amp again yesterday and when i turned it on the red light didn't change to yellow and a horrible squeal came from the speakers.

Very worrying. I just tried it again using different speakers as i don't want to damage my decent ones and its still happening.

Any idea whats causing this?

Thanks
D

[Richard]

Sounds like a feedback problem or faulty valve.
Check for accidental plugging or selecting a wrong input.
Is it both channels?
I'd remove, clean and refit the valves and try other ECF80s if you have any.
Try it without the ECF80s fitted to see if the output stges are ok.
If you have load resistors try it without speakers to see if the light changes.
Have a look inside for any wires (feedback wire etc?) that may have come off with moving it around. Hopefully something simple if it was fine before.

[hunterwd]

Hi Richard

Thank you for your reply

All connections were checked and ok.

I do have some spare ecf80 valves that i can try tomorrow.

I know nothing about electronics - sorry

So it is entirely safe for me to connect my speakers without the ecf80's plugged in and turn the amp on? I don't have any load resistors - or for that matter know what they are

Am i right in thinking that the ecf80's are the preamp side of the amp?

Thanks again for your help and sorry for having to deal with a complete ameteur

Darran

[Richard]

Hi Darran, yes the ECF80 each have 2 valves inside the glass envelope. The first is a high gain pentode section to increase the signal coming in and the second is a triode section arranged as a concertina phase splitter and driver which feeds the EL84 output stage.

Connect everything as usual using your old speakers, remove both ECF80s (or just one if only one channel is noisy) and see if it still squeals. If it stops the noise the problem is probably not the EL84 OP stage.

It could be the ECF80 valve(s) themselves or it could be that removing them has disconnected the feedback circuit (the feedback wires run from the speaker terminals back to the ECF80 pentode cathode). Hopefully a bad valve but if the noise persists with new valves you may want to let somone have a look at it

[hunterwd]

Hi Richard

Just wanted to let you know.

I replaced the ecf80 valves and all is well.

Had it on all day on Wednesday and powered it down and back on again 5 or 6 times with no problem.

Thank you so much for your help

Much appreciated

Darran

[Richard]

Hi Darran, that's great news, thanks for letting us know

The ECF80 first stage is a high gain pentode which is capable of very high frequency amplification and can be susceptible to oscillation. The majority of KEL84 amps have no problem but it can be down to an individual valve or different make etc. Valves also change as they age and may also be influenced by outside RF interference.

The very high frequency response is lessened by R4 and R5 grid stopper resistors. If it should trouble you again have a good tech change R4 and R5 to 10K and position them close to the valve pin solder pad as in the attached diagram. If it is still a problem increase them further to 15K or 20K. (This will not affect the sound and I had to do similar with KIT88 using 15K on the input when I first built it.) If it should still not settle go back to 10K and add a 100pF capacitor from the same number 2 valve pin to the nearest ground solder pad. All we are doing is lessening the very high frequency response of the valve without encroaching into the audio band.