HT Stage 60 - Bias procedure

Discussion - HT Venue amplifiers. Inspiration from Studio to Stage.
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Posts: 3
Joined: Wed Mar 11, 2015 2:54 pm
Location: Falkirk

Wed Mar 11, 2015 3:15 pm

I recently purchased upgraded valves, and had them installed by a valve specialist who also set the bias.
This cost!! I would like to know if anyone can give me details of a) how to replace the valves and b) set the bias plus the tools I would need?

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Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Thu Mar 12, 2015 12:44 am

Disclaimer: Setting bias requires working inside a live amp containing high voltages and lethal currents. If do not understand the information stated below and/or you are uncomfortable doing this, you should continue to have the amp serviced by a professional. I will not be held responsible if you kill or maim yourself. I will also not be held responsible of you fry your amp or otherwise turn it into a cinder block.

First of all, a lot of people are going to tell you "Just set bias to 'xyz'". These people do not understand what they are doing. Proper bias setting is going to depend on the specific tubes in the specific amp. Every amp is a little different. Every tube is a little bit different. The only proper way is to measure, calculate, adjust and repeat until it is dialed in.

More specifically, proper bias depends on the voltage measured at the plate (pentode) of each tube in a push-pull pair. Bias is the current running through the cathode as the tube is at idle. Since current is flowing from the plate to the cathode, as you adjust bias the plate voltage will change. This is why it takes time to dial in the proper bias and why there is no "one size fits all".

It's not necessary, but if you do not understand how a tube works and how it amplifies signal (single ended or push-pull), I would suggest you read up on that so that you better understand what you are doing.

-----------MEASURE & ADJUST----------------
There are two basic methods for reading and setting bias current for a push-pull tube configuration. The first is the cathode resistor method. The second is the OT shunt method. A lot of people will say that the OT shunt method is more accurate....but that's only because it requires very accurate (and thus expensive) Digital Multi Meters (DMM's) to do it. The same meters using the cathode resistor method would be just as accurate.

Because not everyone has a pair of DMM's that cost $200 each and the fact that it's a little easier, the cathode resistor method is detailed below:

The basic procedure is:
  • 1) Remove the chassis from amp
    2) Remove old power tubes
    3) Insert new power tubes
    4) Connect load (speaker) and power on
    5) Measure plate voltage (each power tube)
    6) Calculate bias based on desired dissipation rate
    7) Measure bias for each tube across a 1 ohm resistor
    8) Set bias for each tube using the bias and balance pots
    9) Repeat steps 5-8 until dialed in
    10) Install chassis back in amp
The calculation itself is fairly straight forward.
  • (Plate dissipation /Plate vdc) x 0.7 = Bias per valve in amps
    • - Plate dissipation for an EL34 tube is 25W.
      - Plate vdc is measured
      - 0.7 is assuming you want to run the tube at 70% of the tubes maximum dissipation rate
So let's say you measured 496V at the plate. The math would be (25W/496V) x 0.7 = 0.0353A. Take that and multiply it by 1000 to convert it to 35.3 mA per tube.

The actual measurement is taken by inserting a 1 ohm resistor between the cathode and ground (hence the name cathode resistor method), thus allowing it to be measured in mV (Ohms law). You would then measure bias at the cathode of each tube. The BIAS adjust pot will bring the both values up or down. The BAL (balance) pot will "pan" the value between the tubes.

In the above case, you would fiddle with both pots until you get 35.3mV on each tube. Then you would measure plate voltage again...if you adjusted the pots even a little, it will have changed. So you'll have to do the math and adjust again. Rinse and repeat until it's dialed in.

You could measure the voltages and currents directly at the pins of the tubes. All you need are DMM's then...but there are much safer and easier options. The easiest way to do all these measurements is with some type of bias probe.

One like this is fairly cheap and used with a DMM. Ideally you would want two so you can measure both sides of a push pull pair simultaneously. They will cost you around $25 each plus the cost of 2 DMM's. Assuming you pick up a couple of inexpensive ones, you are looking at around $100-$150 total.

Another option is something like the Compu-Bias. It's purpose built just for measuring tube bias. It will however set you back over $200.

There are other options out there as well. If you do some Google searches you'll find them.
Last edited by thephantum on Thu Mar 12, 2015 6:56 pm, edited 1 time in total.

Posts: 3
Joined: Wed Mar 11, 2015 2:54 pm
Location: Falkirk

Thu Mar 12, 2015 2:31 pm

Many thanks that's a great help. I'll read up on it more and decide if I will be able to do this.

Posts: 45
Joined: Wed Aug 01, 2012 12:00 am
Location: Illinois, USA

Sat Mar 14, 2015 7:28 pm

Phantum, that was an excellent, easy to understand synopsis of biasing procedure.

In the past, I have used bias probes (purchased from Eurotubes) with 2 multimeters, so I can measure each pair simultaneously.

That being said, I recently purchased a CompuBias, used on Ebay, and find it to be accurate and easy to use.
I decided that the cost of the CompuBias was more than worth the risk of being shocked or electrocuted by carelessness.

If I may also add, Bias current should be measured at amp idle, and with a load attached, therefore:

1) ALWAYS have a speaker loaded to the amp output, or amp damage may occur.
2) Recommended to turn volume and master volume to zero to achieve idle current.

I have read many of your posts, and appreciate the effort you put into this forum.

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