suggestions for non UL stereo tube amp idea

[imaradiostar]

I have a pair of largish 50 watt output tf's, around 4k:2/4/8 ohms. They're new sensor NSC018343 transformers from a project I built long ago. They're larger and heavier than a twin reverb output transformer so I thought they might make a nice stereo amp for monitor speakers in the garage or even running a wedge if needed. I was thinking two 6550's or EL34's per side should make at least 60 watts per channel without having terrible bass response. I would use a toroidal power tf from antekinc.com...amazing prices for some stout power!

Here's the kicker- I don't know the first thing about tube hifi amps and all the schematics I look at tend to favor Ultralinear output sections. I was thinking I could build something with a triode/pentode switch for hi-fi and sound reinforcement usage respectively.

Any recommendations for a circuit to try?

jamie

[Steve Conner]

Hi Jamie

I believe hi-fi amps are much the same as instrument amps, you can just bolt the output stage of one onto the driver bits of another.

There were basically three hi-fi driver circuits: the Williamson, the Dynaco Stereo 70 and the Mullard 5-20. So just pick your favourite. (If you have trouble deciding, mine is the Williamson.)

The only trouble you might come across when mixing and matching is instability, since hi-fi amps used quite a lot of negative feedback, and your output stage and OT will have different gain and phase shift than the one the driver was designed for: maybe different enough to upset the feedback loop. In particular a pentode or tetrode output stage will have more gain and more LF phase shift than a UL or triode.

You can't just get rid of the NFB, because any speakers you buy nowadays are designed around solid-state amps with high damping factor, and a tube amp with no NFB has a lousy damping factor that will give you a different bass response than the speaker designer intended. (Usually different as in terrible.)