Setting Bias Without a Manual

[BenjaminWebber]

Hi, I hope I my understanding is sufficient.  I am posting here just in case I am off the rails, which could well be the case.  I want to set my bias on my mono blocks which have 3 tubes each. The output tube is a 45 tube. Some years ago, a friend made them for me. He planned to tweak them much more, but passed away before that could happen.  I have listened to them for several years now, and absolutely love them.

I never knew what to set the bias to, and my friend was dead. Each mono-block has a bias pot. I recently bought a couple tubes that tested stronger and ended up messing with the bias again. I have ordered a bias tool from china that will allow me to plug the "tool" into my 45 tube socket and insert the two leads from it directly into my meter. While I wait for the tool to arrive, I want to make sure that I know what to do with it. I asked the seller, but he responded in Chinese.

I have looked up the Plate Current for several brands of 45 tubes and the three numbers that keep coming up are 31, 34 & 36. So, my question is, do I just want to have say 34mA come up on my meter, which is the middle number for plate current? Or, is it more complicated?

If having 34mA come up on my meter gets me into the ball park, I plan to do the following.

Set meter to milliamps 
insert tool into socket, and tube into tool
attach leads appropriate for measuring current
adjust bias pot to lowest bias
turn amp on, and let it warm up good
slowly turn bias pot up until reaching 34mA

Please tell me if I have any part of this wrong.  Please help me get the best sound I can out of this amp WITHOUT killing tubes which I can not afford to replace very often.

Sincerely,

Benjamin Webber

[Doc B.]

Do you know what the intended operating point is - plate voltage, bias voltage, plate current? Do you know the primary impedance of the output transformer? Intended power output? Are these 45 tubes cap coupled to the driver stage? Do you know the raw B+ voltage? Can you measure what the DC bias voltage is on the 45 grid right now? Typically you chose a current based upon the transformer primary impedance and intended power output, then look up what the grid bias voltage needs to be to get the current you want at the given plate voltage.

[BenjaminWebber]

Thanks Doc, but I don't really know any of what you have mentioned. Is there a place for me to start? I turned the pots all the way down (never listened this way before) and it sounds different, but not bad. I have had the pots more in the middle for the past few years. In the very beginning I had them so hot the plates would glow, but I had tube problems and put the pots more in the middle. What I can say is that the output transformes which came off from a dead amp, had multiple wires. I think we at least meant to connect to the 8 ohm taps. My friend suggested adding a coupling cap (I have the place it would go marked) but I never did, since it sounded good to me as it was.

[Paul Birkeland]

Benjamin,

If you are heading over the pass to the Seattle area in the near future, I could help you sort this out.

-PB

[BenjaminWebber]

Thank you Paul!!!