how to bias my tubes on a ST70 without any biasing points?

Hi Guys,

I'm new to this forum. Recently got myself a rebuilt ST70 with the older VTA driver board (non CCS) and TriodeElectronics Power Supply board. I need to bias my EL34 tubes, however I'm facing a problem.
My ST70 does not come with the dual front biasing socket. Where am I suppose probe my positive pin of my multimeter to measure the DC voltage?


Many thanks in advance!

I did read up a few diagrams and instructions, from my understanding, am I suppose to probe on pin #8 of each output tube?

triotary wrote:I did read up a few diagrams and instructions, from my understanding, am I suppose to probe on pin #8 of each output tube?

Yes. But you will have to figure out q few things :
- is each channel using a common catode resistor ?
- how large is that resistor

Depending on your findings you will have to calculate the voltage across the resistor.
Dynaco stock used a 15.6 ohm resistor common to both tubes in each channel, the
recommended voltage is 1.56V which will give 50mA / tube.

More modern gear might have a different value, and the current wizdom
is to use 40mA / tube as a startingpoint.

Finally: if you don't have any measuring connections outside i would recomment that you create one.

Bob Latino wrote:
The original poster mentioned that this amp had a VTA driver board. I can check with Roy but AFAIK the VTA driver board always used a fixed (adjustable) bias system with a 10 ohm bias resistor on each output tube.
 
Bob

the VTA driver boaard has no 10ohm resistor, they are mounted on the tube sockets.

gee guys, let Triotary figure it out.   resistance from pin 8 on each output tube to ground.   If it's a VTA mod, it's 10 ohms.
You want 40ma of bias.   Ohm's Law, anybody heard of that?   0.40vdc across each resistor

oooooOOOOOooooooohhhhhhmmmmMMMmmm ...

Sounds like time for a circuit mod. OP's gotta get inside anyway, right?

Easy enough to add bias test points - I found some nice ones at Weber's online store when I went in to get my Copper Cap. Anybody got the exact step by step for that specific model to make it happen? Make for a nice sticky too ...

Weber's website is kinda hard to navigate ... just stumbled across these myself, so here's a link:
https://taweber.powweb.com/store/plugord.htm

They've got all sorts of neat stuff buried here and there ... bit of a treasure hunt though.

instructions for tip jacks - connect the center hot lead to pin 8 (one for each output tube)
connect the outer ground lead to chassis ground
there 'ya go, step-by-step

I was hoping for something simpler ... got any pictures? 

Thank you all! Really appreciated! Yes the Amp comes with an older version of VTA board without the CCS. It did fired up well and managed to drive a pair of 15" tannoy berkeley very well, with the help of a Marantz 7C. Now I will go ahead and bias the tubes from pin 8 of each EL34 output tubes.

Yes, i was also hoping that I could bias the tubes while probing some points of the VTA board ^_^"