helppp ''fixed biased tubes'' RH 50

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Re: helppp ''fixed biased tubes'' RH 50

Post by kell » Fri Oct 11, 2013 11:38 pm

Sorry for my post before.

I owned a RH30 two years ago and this was "self biased". That's why I wrote plug and play which is correct.

A fixed biased tube amp must be biased after changing the output tubes for proper operation.
Running the tubes too hot wears them out quickly or leads to red plating and failure. An idle current of about 60% will work pretty well for most stuff.
Biased too cold does not damage anything but your sound.

A matched pair is the way to go, as tubes change their values over time and they will become different.
To a certain amount this is good for the sound, but too much difference will give the opposite result.

Normally there is a trim potentiometer on the board inside the amp or on the rear, which must be adjusted.
First measure the plate voltage of the output tubes directly at the socket, write it down.
If there is a 1Ω resistor from cathode to ground (each output tube) you can measure the voltage across it with a DMM set to mVs.
As R = U/I or 1Ω = U/I, the mVs is equal to the mAs flowing through the resistor. This is your idle current or bias current.

max. dissipation of PA tube (W) * 600 / plate voltage = bias current (mA);
Adjust the trimmer until the measured value meets this. That's it.

Don't get me wrong, but if you don't know what your doing, you can kill yourself as the device is running with high voltage!
Keep yourself alive!

A tech will do this save for you at minimal costs.


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