DIY Audio Projects Forum

I'm beginning to explore designs for a push-pull power amplifier with two pentodes as the output stage. Having looked at many schematics online, I've noticed a variety of bias strategies ranging from self-bias, mixed fixed / self bias, and fixed bias output stages. I have some questions relating to some of the practical implications of each strategy. More specifically, I'm interested in implementing a self-biasing scheme. My questions are

1) How important is it to have a "matched pair" of tubes for each output channel?

2) If I choose to use a matched pair of tubes, what type of adjustment mechanism do I need in place to ensure that the tubes have the same quiescent bias point? Should I put a trimpot in series with the cathode resistor for each tube so that I can adjust the tubes to have the same quiescent cathode voltage?

3) In the contest of a stereo amplifier (as opposed to a guitar amp), is there a hard logic to using fixed-bias v self-bias?

A final unrelated question: what do black glass tubes look like when they're illuminated? I've found some cheaper Russian tubes which feature black-glass envelopes.

As long as the valves are within 10% no more than 20% diffrent in the idle current inside amp.
Then its good enough.

Diminishing returns for being more matched, you can go as far as you want with matching but it gets stupidly expensive when your already fairly matched.
And over time the mathcing drifts.

Hi, Generally I agree, matching is usually not required. For most purposes 10% is fine. It somewhat depends on what class of amplification you choose. If it is AB then 10% even 20% is good as the tubes are usually running at about only 30-40% of their dissipation at idle and are almost never running at anything near the max spec. If you are doing class A then you need to be careful not to exceed the ratings as they are often at 75-85% of max dissipation. A 20% error can result in either tube failure or short tube life.

Some general considerations: It is helpful to be able to test the idle current in each tube. A one ohm resistor in each cathode will measure 1 milli volt / milli ampere of current. If you use fixed bias that is adjustable for each tube separately then you can very accurately set the idle condition accurately. This does not guarantee it will match at higher power levels though. You will have to check the current from time to time. About every 6 months seems typical. I consider it quite important to be able to adjust the idle current in class A push-pull amps to protect the tubes and get the maximum output before one or the other runs out of current (horrible distortion is the result of this). These too should be checked at regular intervals. IMO an auto bias arrangement is excess complication. Using a shared cathode resistor is quite fine for nearly any PP class AB amp. As an initial project I would recommend this arrangement. Next best IMO would be fixed bias class AB. My personal opinion is that really class A push-pull has the best sound and all my designs are of this type. There are some negatives though. The largest is that the maximum output power is significantly lower. Additionally tubes run hot and overall power consumption as a ratio to output is low. To my ears all this doesn't matter as much as I like the sound class A amps can deliver. My designs all use an unconventional balance method and do not have "bias" in the normal sense. They also need to be checked about every 6 months. It is a simple task and takes only minutes. Look at a few of the projects I posted to see the details.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project