Dynaco ST-70 Clone

[slovell]

I just bought my first tube amp, a new Dynaco ST-70 clone. I'm trying to figure out how to use the biasing jacks next to each output tube. I can see the adjustment screw in the bottom of the jack, but how do I hook up the voltmeter to it? Does the positive lead from the meter plug into the jack? I know the negative lead will ground to the chassis. I'm a bit stumped. I'd really appreciate it if someone could explain this to me. The jacks are the thingies with the white plastic collar next to each output tube.
Regards, Sam

 

[gouldglen]

I just bought my first tube amp, a new Dynaco ST-70 clone. I'm trying to figure out how to use the biasing jacks next to each output tube. I can see the adjustment screw in the bottom of the jack, but how do I hook up the voltmeter to it? Does the meter plug into the jack? I'm a bit stumped.

You should contact member "rustnrot." It his his amplifier i believe.

 

[nerdorama]

It's not certain the negative would got to the chassis. Signal ground isn't always the same as chassis ground. My suspicion from the pictures is that the negative lead would connect at the center jack in front of the power transformer. Looks like a VTA board with a separate positive jack between each tube and the corresponding bias pot.

John

 

[derekva]

Check to see if you have continuity between that center jack and chassis ground. If so, then the black lead goes there, and the red lead goes to each tube jack. A quick and dirty trick is to get one channel in the ballpark then run red lead to one tube and black lead to the other tube and try to get 0V offset.

-D

 

[slovell]

Thanks for the help, much appreciated. It makes sense to me now.
Regards, Sam

 

[rustnrot]

We got in touch over at diyaudio. I'll post a labelled pic here for completeness.

 

[nj pheonix]

Looks like the tubes4hifi vta board. Instead of using octal sockets in front it has test jacks,. It probably makes it slightly easier. It should sound very good when its set up. Rolling the three driver tubes gives you an opportunity to tailor the sound to your ears. Just out of curiosity, Do you know what kind of coupling caps that has?

 

[rustnrot]

Panasonic Metallized Polypropylene Film Capacitors Type ECWF(A)

 

[slovell]

Any suggestions as to different driver (signal) tubes?

 

[nj pheonix]

I like Telefunken and Mullard (Holland or Great Britain) 12AU7s in the middle position. I like RCA long black plates in the outside positions (12BH7)