Long Tail Pair

[Bandersnatch]

Originally Posted by aroS3

my calculations show that with -75 at G1 you are bias them around 27 mA ...which is far too low.... a better approach could be running them around 55 mA which is around -65 Volts at G1, but be careful not to exceed the max tube dissipation

A 6550 or KT88 with 550V on its g2 will need a substantial amount of bias. The question should be, what current is the -75V delivering? I do agree that a 55-60 mA/tube idle current is desired...I don't think any of the 6550 or KT88 data sheets give enough information to determine bias voltage that accurately. Since you think so, do plese enlighten me as to your methods; share the wealth if you'd be so kind.
cheers,
Douglas

[mr2racer]

Bandersnatch,
This is what I was referencing on Duncan's page;

AB1 U/L 40% 553 -75.0 2x50 4500 100 2.0

I didn't mean -75 volts bias. This spec is for voltage on grid 1. If I'm reading Morgan correctly the - sign is for inverting not negstivr voltage. I thought that was the spec for the input voltage. Sorry

[Bandersnatch]

No, you've had it right the first time. 50 mA/tube idle with g2 and plate at 553 volts. 100 watts out at 2% distortion with a -75V bias on g1.

On LTP gain it is about mu/2 each plate to ground, it is mu plate to plate( this is one grid driven, the other grounded).
cheers,
Douglas

[mr2racer]

Hey Guys,
Is the drive voltage simply a function of gain? Or does a greater voltage swing give better quality sound? Like more dynamic range?