KEL84 noise in left channel - World-Designs-Forum

YummyFuzz · #1 29th November 2019, 06:20 AM

Hello WD forum,

I want to see if anyone had some suggestions on some noise coming from my KEL84 amp if I describe the symptoms.

Setup: KEL84 powered-on with speakers connected, but no audio inputs.

- A moderate volume intermittent popping/rustling comes from the left channel only for about 10 minutes after powering on from cold.
- If left on, the sound does not seem to reoccur when the amp is warm/hot.
- The sound stays at the same level, regardless of the position of the volume knob, or the input/mute selection.
- The sound remains in the left channel even when I swap the tubes one-by-one between left and right (i.e. It doesn't follow a tube).
- It's not the speakers (if I swap the speaker wires L2R and R2L it comes out the right speaker).

If there is an input audio signal playing (at low volume) during this time, the popping rustling can be heard just under the audio. The audio does not cut out or reduced in volume when this noise happens.

So I'm guessing it is one of the electrolytic capacitors on the left channel of the amp, which I can try replacing.
Does this seem right, or are there other tests or components I should try?

(This is the KEL84 that I found at the recycling centre so I'm not sure where or how it may have been stored in the past.)

Ali Tait · #2 29th November 2019, 07:55 AM

First thing to try is swapping valves between channels and see if the noise follows any of them.

bob orbell · #3 29th November 2019, 08:27 AM

His post says that he has done that

. Bob

John Caswell · #4 29th November 2019, 10:00 AM

Hi,
I would doubt it being a noisy capacitor more likely a noisy resistor or possibly a dirty PCB.
I would guess that from your description the nose is there all the time , so remove all inputs, volume at minimum and check if the noise is still there.
If so then suspect noisy anode/screen/cathode resistors on/around the ECF80.
Substitution is the only known check I'm afraid.

John

snowman_al · #5 29th November 2019, 10:14 AM

If you pull the ECF80 out, do you still get the popping noise?
If yes look at the components round the EL84, if no look at the components round the ECF80.
Sounds like a heat related issue, could be a simple poor connection in a valve holder or dry joint on the board or wiring. Give the valves a jiggle - does that make it better or worse?
Equally likely to be a noisy resistor. If removing the ECF80 clears the problem, then suspect the 100k resistor R10/11 first, then the 2 x 27k resistors R12/13/14/15.

Not sure it will tell you anything, but check the voltage measurements round the 'noisy' side and compare to the good side.

Crossed posts...