Bias question on a 50W 6550 amp

[Bernardduur]

Hey!

I just wanted to know; I need to add 6550 to a 50W amp as the owner wants it. I usually bias through the 1ohm resistor version and now I was wondering

Should I bias the amp up like it was carrying EL34's (25W) or should I keep the wattage of the 6550 as major key and bias it up to 42W?? I'm just lost; I know the amp won't give me the 100W output as the voltage on the poweramps is lowish (approx 420V), still, do I need to bias em up hot or just low and heavy?

My first guess would be to keep the amp at 70% of the 42W of the 6550's

[Gtr_tech]

It really depends on the pwr supply. I would think you'd wanna treat them like EL34s biased on the hot side. The HT winding only has so much current avilable no matter what tube is in the output section. And...at 420 Va, that really ain't enough to "wake up" a 6550. Tone won't be the greatest. 6550s don't really get hap'nin' until at least 550 Va. One of the engineers who helped in the design of the GE 6550a recommends 600+ for best tone.

My advise? Talk him into a 6L6 type instead. They'll sound similar to a 6550, but sound better at your available plate supply. JJs or Ruby 6L6GCMSTR is a good choice.

[Wilder Amplification]

The valve's dissipation rating doesn't determine the recommended bias point. The surrounding circuitry does.

Sure, you don't want to exceed 70% of the valve's dissipation rating for idle bias. But this doesn't mean that every valve type MUST be set to 70%. EL34s need to be close to that to sound good in a 425-450V circuit, but 6550s don't need to be biased anywhere near 70% of their rating. In the same circuit, you would bias them the same as you'd bias EL34s.

In a high voltage circuit where you have the screen voltage at 50-60% of the main B+ you would have to bias them around 70% of their rating. But a standard 425-450V circuit does not warrant the need to bias them that hot.

[Bernardduur]

Ah, very clear!!!

Thanks both! I get the point

I've tried to make him understand the proper dis-use of a 6550 in this amp but he is (was) very stubborn...... guess all that older amps with these tubes make em interested......BUT yes, I've talked him out of it and added a sweet set of 6L6's; unit sound really sweet so I guess he won't want to go back to the 6550 idea!