Dynaco 70 Bias

Tubes Asylum

Questions about tubes and gear that glows. FAQ

210.254.151.98

I need a bit of help regarding the output tube bias setup on the Dynaco ST 70.
When I set the bias on the bias potentiometer on one channel, which tube am I actually biasing? I can see on the schematic that pin 8 is tied to pin 1 and the 15.6 ohm cathode resistor goes to ground.
On my amp if I switch the position of the aft tube on the left channel to the forward position the amount of travel I must use on the bias pot changes drastically. It takes a lot more travel on the pot to achieve bias voltage of 1.56v. The tubes are a new matched quad of Svetlana's. Might my "matched" quad be not so matched?
What gives? I don't have a tube checker available. With no power applied the left bias point to chassis ground reads 15.6 ohms. Dead solid on the money.
Is this just a tube thing? Am I biasing the tubes in parallel or just the aft sockets? I'm looking at the schematic and it seems it would have made more sense to provide separate bias adjustments for each tube, not each set.
John

This post is made possible by the generous support of people like you and our sponsors:

Amplified Parts

Topic - Dynaco 70 Bias - John H 04:11:56 07/21/02 (5)

Re: Dynaco 70 Bias - Jim Carlon 08:43:22 07/21/02 (0)

Re: Dynaco 70 Bias - Joe Curcio 07:14:44 07/21/02 (3)

Re: Dynaco 70 Bias and PAS 3 - John H 13:30:58 07/21/02 (2)

Re: Dynaco 70 Bias and PAS 3 - Joe Curcio 09:12:35 07/22/02 (0)

Bad Tubes - John H 15:51:29 07/21/02 (0)

You can not post to an archived thread.