Setting Bias for Ampeg SVT 6550

[Doug]

It's an early to mid 70's SVT, it's my first time setting bias on a cathode bias amp (is that right). Which pins am I to connect with resistor and which short. Is that on all tubes? Then take the voltage reading from the bias points out the back of the amp? Any help greatly appreciated.
Thanks
Doug

[Dave Curtis, dB AudioTech]

It's an early to mid 70's SVT, it's my first time setting bias on a cathode bias amp (is that right). Which pins am I to connect with resistor and which short. Is that on all tubes? Then take the voltage reading from the bias points out the back of the amp? Any help greatly appreciated.
Thanks
Doug

It's fixed (but adjustable) bias, and THIS amp isn't one you should be cutting your teeth on. Cathode biased amps are much easier...

That said:
This amp already has 1 ohm bias sense resistors so measure mV from K1 to ground. The schematic states .072 volts, so use the bias pot (VR1) to set it there or a bit lower. Then measure from K1 to K2 and adjust VR2 until you see 0 volts.

Schematic:
http://www.freeinfosociety.com/elect...iew.php?id=830

See #2 under the "Calibration Adjustment" note.

Hope this helps,
-DC

[Doug]

It's that step 1 I'm unsure about...

Thanks for the help Dave, I think I see what I was confused about, the 4 ohm resistor note is regarding p3, the amp output. I just assumed they were talking about the power tube pins, I'll pay better attention. So a 4 ohm speaker load? And then measure accordingly?
Thanks again

[Enzo]

Yes Doug, you ought to have test points on the front of the chassis.

SInce bias is measured without signal, it doesn't matter what load you apply, 4 ohms is fine, 2 ohms too. But you do need a load.

[soundmasterg]

Yeah, be very careful with an SVT. They have quite high voltage for a consumer amp, and the output section will self-destruct itself pretty quickly if you have the bias grossly out of adjustment. They're a very cool amp, but you do need to pay attention when working on one!

Greg