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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: Pairing up tubes for SE usePosted by floobydust on: 5/22/2009

Romy, et al,

I got rid of my tube tester a few decades ago... yet I still need to match sets of tubes for equipment. When you consider many SE designs with no feedback loops, you need to match "sets" of tubes to get matched performance between amplifiers and/or channels. My approach is dirt simple but it works. I use a breadboard amplifier which allows quick and easy voltage measurements as well as scoping of any part of the circuit.

What I test for on the output tubes is:
- gain
- bias
- distortion
- power output
- hum level balance

The goal is to get a very close matched pair with the above. It's also critical in my view that the internal construction of the tubes is identical. The 45 and 2A3 DHTs were available from many manufacturers and each had numerous internal revision changes over time, some where actual improvements in the design while others were to reduce manufacturing costs.

I match input/driver tubes the same way (sans power output) but you also need to measure output noise as a component of filament coupling to the cathode (IDHT). From here, I then matched the pairs into sets for a stereo amplifier to ensure that the performance of each amplifier pair (mono blocks) is tightly matched for gain, power output, distortion and signal-to-noise.

The plus for this method is obvious... you can get exceptional performance from your designs... the down side is also obvious, it takes a lot of time and your results are better if you have a larger pool of tubes to choose from. I currently have a collection of more than 150 45s and 2A3s... which makes this much simpler. The same applies to input/driver tubes as well as rectifiers, etc.

As for obtaining NOS and/or NIB tubes... usually impossible to verify this. In some cases, I've managed to buy NOS/NIB which are still sealed. Many brands like Sylvania, Zenith and Philco had packaging which was completely sealed or riveted so it was impossible to remove the tube from the packaging without breaking the seals. Of course you still have an old stock tube and you're now at the mercy of Father Time and how well the vacuum seal has kept plus the soldered connections to the base. So testing as noted above still applies.

Of course... once you have your preferred sets... the question becomes how to dispose of the remaning tubes which you don't want ;-(

Regards, KM

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