I just snagged a pair of new Quicksilver single-ended transformers on E-pay. They look as though they were intended for use with the Quicksilver SET Mono amp,9W/channel, using a KT88 and 6H30 tube complement. Since the transformer impedance is 3.3k, I suspect that the KT88 was used in triode mode. Anyway, I have no intention of just trying to copy whatever Quicksilver did, but want to go my own way. I am open to some suggestions as food for thought, as it'll be a while before I get around to wrapping an amp around this iron. How would you use these?
Hello Wrenchone.
Do the transformers have UL taps?
What idle current will they use?
DT
All just for fun!
No UL tap, and they are supposedly good for 120mA or so. One notion I had was to use a triode-connected 6550 or super 6BG6GA and a pentode front end, with Schade-style partial feedback to lower the output impedance of the output triode even more. This approach would be suited for the transformer impedance/turns ratio.
Lets see here. Well the first thought is a KT88 cranked to about 100 mA in triode. But that would be too much like "copying what Quicksilver did." I call that a Simple SE and it is an amp worth building, but far closer to mainstream than unusual. You don't strike me as a 300B kind of person, but I had to toss that out there, been there done that, even like it. Maybe a big fat sweep tube in Schaded pentode, but haven't you done that one already? Could do it again with a bigger tube (35LR6).
Do the OPT's have multiple secondary taps to allow 6K operation? I assume that they are gapped for DC current. That would make it a waste to use them for something like a circlotron.
What about something totally different. Maybe a DHP with a dose of Schade?
Do the OPT's have multiple secondary taps to allow 6K operation? I assume that they are gapped for DC current. That would make it a waste to use them for something like a circlotron.
What about something totally different. Maybe a DHP with a dose of Schade?
Gottum plenty 6CB5 at the 2 dollar price. Got heap bigger sweep too...
I'd like to get 10W or so per channel. This means I'd need 250V swing available at the plate. If you conservatively estimate the triode to need 100V of headroom to make that possible (probably less if A2 operation is permitted), that means plate B+ of 350V, or more if doing cathode bias. I may do a "No Light" cathode bias circuit just to be perverse (also for performance reasons). The tube in question would need a little under 90ma bias. Dissipation would be hovering around 30W. That's enough of a back of the envelope sketch to get me thinking about ways and means.
In reply to Tubelab, I was talking to Doug Tuthill (dtut) yesterday at Burning Amp about various projects I had in the works, and he just out and asked me "you haven't done a plain, straightforward amp yet, have you (paraphrase)". I thought about it a bit, and had to laugh and and say I hadn't.
I'd like to get 10W or so per channel. This means I'd need 250V swing available at the plate. If you conservatively estimate the triode to need 100V of headroom to make that possible (probably less if A2 operation is permitted), that means plate B+ of 350V, or more if doing cathode bias. I may do a "No Light" cathode bias circuit just to be perverse (also for performance reasons). The tube in question would need a little under 90ma bias. Dissipation would be hovering around 30W. That's enough of a back of the envelope sketch to get me thinking about ways and means.
In reply to Tubelab, I was talking to Doug Tuthill (dtut) yesterday at Burning Amp about various projects I had in the works, and he just out and asked me "you haven't done a plain, straightforward amp yet, have you (paraphrase)". I thought about it a bit, and had to laugh and and say I hadn't.
I think my intentions for this iron have crystallized overnight. I'll use a pentode front end, most likely a 12HG7, with a fet-assisted bias scheme I've been wanting to try. The output will be a triode connected 6550. I'll start by using the Sovteks I have around. I may sling a follower between the input and output stages to allow A2 operation on peaks. I may use "No Light" cathode bias on the 6550. I'll use partial feedback to the input to further lower the impedance of the output triode, and experiment with closing the loop around the transformer. The Quicksilver iron may be good enough that I can forego the global feedback. I'm up in the air as to whether I should do these as monoblocks or full stereo. I'll probably have a better idea once I see the iron.
In my applications the 6550 in triode needs no GNFB. In fact I generally use no feedback at all. When I really want to thump some big speakers around I use local feedback from the speaker terminals to the cathode of the output tube. Cathode feedback can make a cheap transformer sound better and cleans up the bass a bit when driving 15 inch OB speakers using the Transcendar OPT's. The Electro Harmonix 6550 has no problem running at 400 volts and 100 mA. The higher current lowers the damping factor a bit.