Metropoulos Forum

Hi guys,

After 5 months of use I thougt it's time for a little voltage and bias check (CC resistors )
When I fired my amp up in June, I set the bias to ca 36mA (490V-500V depending on wall voltage, measured with TAD Bias Master).
But a few days ago when I checked all this stuff I measured on one tube about 50mA!!

I never thought, the values drift soooo far apart.
First I swapped the bias resistor by a metal film resistor and set the bias back to a cooler setting. (34.5mA on the hottest tube, 29mA the coolest).

Then I did a little measurement on the voltages.
Here are the odd ones (in brackets the 'should be value':
V1: Pin6 254V (240V)
V2: Pin1 219V (205V), Pin6 367V (360V), Pin7 219V (205V), Pin8 221V (200V)
V3: Pin1 305V (270V), Pin2 26V (30V), Pin3 44V (48V), Pin6 283V (260V), Pin8 44V (48V)

Any concerns?

Cheers,
Dominik

those voltages are almost spot on! try a 10k voltage drop resistor instead of the 8.2k if you want a tad lower voltages

you can try using CF resistors to bridge your Power Tubes rather than CC (1K5w bridging pin's 5 - I believe recalling from memory)... they tend to be more reliable for that application and do not have an effect on the tone...