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Are Stax valve amplifiers self-biasing, or is occasional adjustment required? (and when replacing valves). If so, is it an easy procedure - I've had valve speaker amps in the past that were tedious to bias.
With 1 tube per channel, adjustment of the balance between the tube sections is needed. With 2 tubes per channel, adjustment of the balance between the tubes is needed (sections are paralleled). The measurements can be made on the outside of the amp in the Stax socket between the + and - lead of the channel. Adjust for 0V DC. This can be hard because the balance pot is a single turn type and even a tiny rotation makes a huge difference. Clever design indeed.
Also, DC offset has to be adjusted (offset pot). This is measured between the + lead and the ground clamp on the back of the amp.
An occasional check (can be done from the outside) wont hurt, valves tend to drift, especialy new ones.
Keep in mind that the power supply of these amps is not regulated, so fluctiations in mains voltage will effect your adjustments. For that reason, I feed these amps from a power regenerator which regenerates a true and clean sine of 230V (Europe) out of DC.
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The main issue with the 727 is that it simply isn't linear and clearly coloring the sound. This design would have made a lot more sense with much higher bias running through the output stage but as Stax are so keen to point out, that means more weight, more components and thus much higher cost. The simplest solution is to just extend the feedback line all around the amp and it behaves like a high end amp should.
There were some noise issues that cropped up. These are an issue with all of the Stax amps given the crappy PCB's they use so a change in the compensation caps would have been needed. I never had the time to fully look into that though.
I haven't noticed noise issues so far. Under what circumstances did (or will) they occur?
Am I putting anything at risk if I keep using the amp with the gain modded?
Which compensation caps do you mean (I'm not a tech expert, but i'm willing to investigate as I really like the sound of the modded amp)?
Does anyone have the schematic for the 727? That would help.