fried bias resistor

[andrew]

Hello, This the first time for me to try a fixed bias in an amp. I used the Express power and bias supply but the 470 ohm resistor on the bias always fries within a few seconds ( even a 10w! ) Any ideas? Thanks!

[LOUDthud]

The 470 ohm is used when a bias tap (~50V) is available on the power transformer. When the bias is taped from the main winding, a 220K is used. If you used a 470 on the main winding, you probably damaged the bias supply's filter caps. Check for shorts on the caps and diode.

[andrew]

I put in a 5w 220k and changed the cap. Thanks for the info. If I wanted to use 6550s would any change be needed in the bias circuit? Thanks again.

[LOUDthud]

In higher power amps, a 6550 can require that the bias go to -70V. The moderate B+ of an Express won't require that much, but maybe -60V would be a good target. If you need more voltage, decrease the 220K but you will need to increase the wattage.

[andrew]

I decided to stay with 6v6s. When I use the main power with a 47k in the bias circuit it works fine. When I use the 45v tap (WEBER PT) and a 500ohm I get a loud, low frequency hum. I'd like to use the tap but what changes do I need to make? Thanks!