ok to use 1 ohm res from pin 8 for bias chk JCM MK2

[pup tentacle]

Just wondering if it's acceptable practice to install 1 ohm resistors between cathode and ground on the 6550's in a 1974 JCM 50 watt amp. Not all that familiar with the Marshall amps and these tubes. On this same amp someone had replaced the first pre amp tube bypass cap with 1uf and 2uf electrolytic caps in series. Any idea why this mod?

[martin manning]

1uF in series with 2uF results in 0.66uF; another way to get close to the standard 0.68uF value. Re adding 1R resistors, no reason not to.

[pup tentacle]

The prints I have seen show a 320uf with an 820 ohm resistor across it. It seems a pretty radical change to go to .67uf.

[martin manning]

What I meant was standard for later Marshall's.

[Stevem]

The 1 ohm resistor needs to be 1% tolerance and I use 3 watters just to be massivly on the safe side in case the tube melts itself down.
It gets super hot around the chassis near the output tubes and you want the 1 ohm to stay at 1 ohm and not drift.
If you can only get 2 watt resistors, leave as much of its lead lenght on as you can to act as a heat sink..
Sometimes I coil each lead around a pen or such very close to the resistors body.