ST-70 circuit questions

[Shannon Parks]

I have been listening to some brand-name 300B SET amps for years. However, I wanted to build a project. Will I be disappointed with an amp which uses negative feedback?

Hi labjr,

Not at all. The tube magic will still be there, along with better frequency response, lower distortion and improved dampening (bass). But SETs and PP amps may be more of an apple and orange discussion other than which will sound better due to feedback. Even with my horns, I can really enjoy a high power PP amp.

That said, many amps over the years have tried covering a bad design with copius amounts of negative feedback. Also, PP power amp design really requires careful application of the negative feedback or else you'll end up with an oscillator or otherwise unstable amp - not fun!

also, It was suggested to me in another forum that I should use separate bias pots for each tube, because of matching problems with newer stubes. I would think R29 and R30 would do the trick to balance the tubes.

Yes, balanced DC currents in the output transformers really improve the primary inductance performance for better bass.

Shannon

[dhuebert]

My latest amp has a set of "off the rack" Sovtek 6L6GBs which are very close, I think. At idle they run: 34, 35, 35 and 37 mA. This is fixed bias with a single pot. Originally I was going to use E-H 6V6s but the set I had was all as close as this, except one, which was running 10mA higher than the rest. It glowed red @ full throttle, but due to some other problems it took a good hour to diagnose the problem (poor balance from the phase splitter and an intermitant short in a coax line). When you say matched, how matched do you mean?

Don