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Having read Aspen Pittman's Tube Amp Book, he talks about fixed bias vs. variable bias and wonders why manufacturers don't use variable bias circuits as standard. According to him, when using different tube types - which is one of my hopes for building the Tramp - the bias is going to vary somewhat (not to mention between different tubes of the same type with different tolerances). I have seen a few extreme differences in my own amps from power tube swaps, although I've also swapped tubes with no noticeable change.

If I've read things correctly so far (in the forum), the Tramp is a fixed-bias amp, right? I'm wondering about the thoughts behind this design choice, and how difficult it would be to set the amp up with a variable bias circuit instead? Is it more of an issue with a PP design than an SE one? Would it be more complicated than it's worth, or would it really be an aid in setting the amp up correctly with different tubes?

The Tramp is Cathode biased, and is very easy to adjust (we do it to accommodate 6V6 and the bigger 6L6 etc.. You could put more resistors in there, and a special switch to vary the bias, but I dont think it would make a big difference.

Cathode bias is very easy to set up, maintain and doesnt require any special effort to change tubes, as in a fixed bias amp (like the Triwatt). Plus it's reliable.

In a Single ended amp, like the Tramp, it's the only way to go.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I've not read that book myself, but it's not much respected among experienced amp builders and designers. Which is one main reason I haven't bought a copy myself. So take anything you read in that book with a pinch or two of salt....

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Great tones come in small glass jars!

Thanks - reading up some more on cathode biasing. Interesting to hear about the Tube Amp Book - I did know that some of the stuff in there was but I didn't know that it was not really that highly regarded. (Still great as a schematics resource, though!)