Marshall 2204

[cobretti]

I am currently using EL34s in this amp and want to try 6550 just for fun. How critical is the bias RG resistor value? The schematic shows 2 variants. One variant is for 6550 that uses 150k RG resistors, and the other one is with 220k RG resistors for EL34. My bias resistors are gone, I am using master pot of 2x250k (with 1.8M safety resistors) instead. Besides adjusting the right bias current for 6550s what else should I be aware of? I am pretty sure it will work just fine if I set the proper bias current and the value of 250k pot shouldn't be a big deal. Or not?

[Printer2]

Just bias them up and make noise.

[BJosephs]

The grid resistors are lower for 6550 because that is what the data sheet says is appropriate for stabile bias. Higher then necessary grid resistors allow charge buildup that can cause problems. In truth, lots of amps violate this so you’ll probably be ok getting away with it. I’m going off memory of a PPIMV but the tube grid only sees the full 250k when the master is maxed out. At, say, 3 on the master dial the grid actually sees less than 25k.

[cobretti]

Originally Posted by BJosephs

I’m going off memory of a PPIMV but the tube grid only sees the full 250k when the master is maxed out. At, say, 3 on the master dial the grid actually sees less than 25k.

Yes, that is true. I plugged new 6550s in and they are just fine. They need more negative bias voltage than EL34s. I have to replace one resistor in bias section because I am maxed out on the trimpot running them currently at around 60mA at 410V, but I still have 10W headroom. They get hot. I had EL34s biassed to only 35mA before.