I wish to build a stereo vacuum tube amp using 6V6 output tubes. The schematic calls for a transformer at 700 volts center tapped. I have an 800 volt CT transformer. Can I use this transformer with some modification?
Please post the schematic you have in mind. I want to see what the PSU looks like.
You should be able to do just fine for 6V6 "finals", with the 400-0-400 power trafo you already have. If you SS rectify and choke I/P filter the B+, the rail voltage will come at about 360 V. That rates to be fine for self biased 6V6s.
Hmmm....
I don't think "a cool red glow on the 6V6 plates" is such a good idea.
Seems I read somewhere that this indicated one was exceeding the recommended plate dissipation and would lead to shortened tube life.
But, I probably read it on the internet so ignore me.....
I don't think "a cool red glow on the 6V6 plates" is such a good idea.
Seems I read somewhere that this indicated one was exceeding the recommended plate dissipation and would lead to shortened tube life.
But, I probably read it on the internet so ignore me.....
The OP's reference schematic is here.
That's a mono amp. The remark about red plates being bad is CORRECT. The 6V6 should be biased at about 23 V. and power O/P is about 15 W.
The circuit shown follows the Scott/Dyna pattern of pentode voltage amplifier and "concertina" phase splitter. Notice the "Cheap Charlie" bias supply.
Bob, per my 1st post, you can use the power trafo you have. Switching from "fixed" to self bias is in order. Is a stereoblock build part of the plan?
That's a mono amp. The remark about red plates being bad is CORRECT. The 6V6 should be biased at about 23 V. and power O/P is about 15 W.
The circuit shown follows the Scott/Dyna pattern of pentode voltage amplifier and "concertina" phase splitter. Notice the "Cheap Charlie" bias supply.
Bob, per my 1st post, you can use the power trafo you have. Switching from "fixed" to self bias is in order. Is a stereoblock build part of the plan?
What is the power capability of the PTX you have? Voltage is fine, but if you haven't got current, you haven't got squat...
Dude,
Don't forget that the full VA capability of the rectifier winding will be available, if choke I/P filtration is employed. It would definitely help, if the OP provides us either make and model of the PTX or its spec's.
The remark about forward drop in 5R4s being LARGE is correct. It's a whopping 67 V. at a 250 mA. draw. Choke I/P filtering in combination with 5R4 rectification should prove highly satisfactory. "Fixed" bias might very well be feasible.
yep Eli - plenty of ways to skin the cat but easier if you know if its a domestic moggy or a mountain lion! 🙂
That's my amp. The transformer was on hand, which happened to supply 700VCT or whatever. A regular Hammond 300-0-300 will do an excellent job here.
The bias supply is required to keep the tubes in class AB1, since 400V is a wee bit too much for 6V6s in class A. (BTW, some 6V6s have this tab cut out of the plate that tends to get a little hotter than the rest -- I think some of these may glow even at supposedly rated power. Go figure. The bias shown should be within the limits of any 6V6.)
Funny you say that -- in fact, since the screen supply is unregulated, the grid supply must be unregulated! 'Course, the screen supply varies a hell of a lot anyway, because of that big fat resistor and being the thing's in class AB. But I don't really care, it's a tube amp, it's supposed to have these stupid things, right?...
If I were building it from scratch today, I think I would put in more gain (it's just barely got enough gain as shown, and it's supplied by a preamp), and I'd use a more suitable power transformer (lower voltage, closer to 220VAC), with SS rectification, to get an operating point closer to textbook values (300V plate and screen, 20mA per tube). The SS supply and lower B+ will keep the screen voltage much more stable. No need for regulation, it's only a simple 6V6 amp.
Tim
The bias supply is required to keep the tubes in class AB1, since 400V is a wee bit too much for 6V6s in class A. (BTW, some 6V6s have this tab cut out of the plate that tends to get a little hotter than the rest -- I think some of these may glow even at supposedly rated power. Go figure. The bias shown should be within the limits of any 6V6.)
Funny you say that -- in fact, since the screen supply is unregulated, the grid supply must be unregulated! 'Course, the screen supply varies a hell of a lot anyway, because of that big fat resistor and being the thing's in class AB. But I don't really care, it's a tube amp, it's supposed to have these stupid things, right?...
If I were building it from scratch today, I think I would put in more gain (it's just barely got enough gain as shown, and it's supplied by a preamp), and I'd use a more suitable power transformer (lower voltage, closer to 220VAC), with SS rectification, to get an operating point closer to textbook values (300V plate and screen, 20mA per tube). The SS supply and lower B+ will keep the screen voltage much more stable. No need for regulation, it's only a simple 6V6 amp.
Tim
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