65 Super Reverb Reissue Plate Voltage/biasing

[old guy]

Greetings my old friends, I've come to pick your brains again ('cause I lost mine playing clubs for too long).
I just picked up my last Super Reverb - a brand new 2017. It's the fourth one in 4 decades ... kinda like 'em
Taking it out of the box, I stuck my bias probes in 'er and set about kickin' 'er up to where I need her to be ... 40 - 42 mA. My starting factory plate voltage was 457dcv and bias came out at 35-36.

My question is; "what is of most importance when biasing this amp - the plate voltage or the bias #???? As you know, dropping the PV raises the bias setting. To achieve my 'perfect PV of 450, it cranks the bias to 46 - 47 which, is too distorted for my musical tastes - great for ACDC covers but not my Larry Carlton stuff.

What, do you think is a good, happy medium setting that won't hinder performance or hurt anything?

[pdf64]

Adjusting bias to achieve a particular plate voltage seems like looking down the wrong end of a telescope to me.
The plate voltage will vary according to the particular mains voltage at any given instant.
Just set the bias to achieve the minimum level of idle current that sounds good to you.
If you think the plate voltage is too high then consider using a saggier rectifier, eg 5U4.

[Enzo]

Yeah.^^^

The hotter the bias the more the plate voltage will drop. Hotter bias meaning a lower bias voltage on the grid and resulting higher idle current through the tube.

Dropping the plate voltage doesn;t raise bias, it is the other way around. The bias is what is adjustable, not the high voltage supply.

Plate voltage (or more correctly the voltage from plate to cathode) times tube current yields dissipation at idle. 42ma or 38ma by themselves doesn;t mean a lot. Two different amps might have plate supplies of 480v and 360v. The same 42ma on one is a lot hotter than on the other.

There is nothing magical or electronically"correct" about the 70% rule of thumb, but many guys use that as the target. 6L6 is a 30 watt tube, and 70% of that is 21 watts. So the target becomes the product of plate voltage and tube current to total 21 watts. I am perfectly happy at lower dissipations. The Peavey 5150 stock runs at a very cool 15ma or so with B+ of 485-500v.

Bias setting is extremely flexible. If the bias voltage is too low, the tube will red plate - the plate gets red hot - from excess current. Go the other way with to high a bias voltage, the tube current drops and the amp can have excess crossover distortion. Any point in between is electrically fine. So like pdf said, adjust it to the least current that sounds good. Within the two extremes of course.

[Enzo]

Yeah.^^^

The hotter the bias the more the plate voltage will drop. Hotter bias meaning a lower bias voltage on the grid and resulting higher idle current through the tube.

Dropping the plate voltage doesn;t raise bias, it is the other way around. The bias is what is adjustable, not the high voltage supply.

Plate voltage (or more correctly the voltage from plate to cathode) times tube current yields dissipation at idle. 42ma or 38ma by themselves doesn;t mean a lot. Two different amps might have plate supplies of 480v and 360v. The same 42ma on one is a lot hotter than on the other.

There is nothing magical or electronically"correct" about the 70% rule of thumb, but many guys use that as the target. 6L6 is a 30 watt tube, and 70% of that is 21 watts. So the target becomes the product of plate voltage and tube current to total 21 watts. I am perfectly happy at lower dissipations. The Peavey 5150 stock runs at a very cool 15ma or so with B+ of 485-500v.

Bias setting is extremely flexible. If the bias voltage is too low, the tube will red plate - the plate gets red hot - from excess current. Go the other way with to high a bias voltage, the tube current drops and the amp can have excess crossover distortion. Any point in between is electrically fine. So like pdf said, adjust it to the least current that sounds good. Within the two extremes of course.