Dynaco ST70 el34 bias

Hey everyone,

First time posting, also a first time dynaco user. I am a noob from a tech/electrical standpoint for transparency haha. This is my first dynaco st 70 which was built brand new by Kenny Russell from Funtastic Vintronics. It's a beautiful piece. Anyway, I have been running the amp and it has been performing beautifully since I received it back in April of this year. I decided to do some tube rolling and installed a matched quad of mullard reissue el34's purchased from upscale audio. When I went to bias I noticed the reading was quite low at .40v and was told by the builder that it should be adjusted to between 1.4-1.56 vdc for each side but the bias pot maxes out at 1.2 vdc both sides. I take it thar this is not normal? I understand some tubes have different biases but kinda at a loss of understanding tbh. Hopefully some more experienced minds can shed some insights?

Thanks in advance

Justin

Justin,

Hi there. I love Kenny! I have two of his amps. Reach out to him and he will help you solve your bias issue. You are correct that something isn't right. You should easily find that 1.4v somewhere in the middle.

Reset your multimeter to the 2 volts range (not 20V as it is in the photo) and try again.

Yes. Mijohn is right. .4 is actually 4v in that position. Move the multi selector down a notch to the 2 volt range. DO NOT run your amp biased that high. Thats 100 mA per tube more or less. Should be closer to 50.
1. Turn the bias selectors fully counter clockwise
2. Turn the power on with an 8 ohm load or speaker connected
3. Allow to warm up for a few mins.
4. with the multi-meter in the 2v position, turn the pots clockwise until you hit 1.4-1.5v.

Here is a formula that can help you better understand and dial in your bias voltage.

(CR)/500 = V C-current in millamps, R = resistance in ohms (this comes from the 15.6 ohm bias resistors installed in your Kenny amp), V = volts on meter

Here are three examples, assuming 50 mA per output tube max... from 40-50 mA cool to hot...

So if you want 40mA per tube….
(40*15.5)/500 = 1.24v Bias to 1.24 v DC on meter

If you want 45mA per tube….
(45*15.5)/500 = 1.v Bias to 1.40 v DC on meter

If you want 50mA per tube….
(50*15.5)/500 = 1.55v Bias to 1.55 v DC on meter