Non microphonic el84 noise at attenuated wisper vol.

[RockinRocket]

Ive been tinkering with a Jca 20h. It has 370 volts on the el84 plates biased at 70%. I use this amp as my lo vol amp attenuated through my rockcrusher down to quiet wisper to tv vol. With the master past 6 the el84s make a lot of noise when signal is applied and you guessed it its very annoying. I aslo took the el84s out and put them in a amp with 290volts on the plates biased at 70% and anttenuatted the same way and the noise is not very audioable. The noise isnt being amplified but when the amp is working hard under loads of attenuation the noise is to much to deal with. Is the overload of plate volts the reason for this?

[pdf64]

Maybe they are vibrating and making more acoustic noise because they are being working way harder at 370V than 290? Power being related to voltage squared (other variables may have a significant affect).

[Stevem]

The combination of that plate excessive plate voltage and the resistive load from your attenuator would seem to me to be the root of your problem!

[RockinRocket]

The combination of that plate excessive plate voltage and the resistive load from your attenuator would seem to me to be the root of your problem!

The attenuator is a reactive load. I guess power tubes under load do make an audioable noise. Very annoying under heavy attenuation to say the least!

[Stevem]

You might want to get a power scaling kit from London amps and knock down the SPL level that way?

[M Fowler]

I've had to install new and high quality bias trimmers in these amps even new ones.

Mike Soldano and I discussed this and my solution to noise problems was replacing the bias trimmer. Mike was a great guy and had the distributor send me a new pilot light jewel that went missing in action on a used JC head at no cost showing they stand behind their product and customer service.

[RockinRocket]

Interesting Mark I replaced almost every thing on the board except the trimer lol. My amp wasnt that noisey to begin with. This is cheep and ill see if it helps with noise

[RockinRocket]

You might want to get a power scaling kit from London amps and knock down the SPL level that way?

I dont really want to put that kind of money into it to be honest. I was thinking of dropping 40-70 volts with a zenner/transformer. I got like 65 bucks into it and 160 in the amp.

[Stevem]

Than get yourself a big ass bolt down type NTE 5282AK zenner and that will drop you out the 70 volts.