hmmm, guess I didn't look all that hard at Edcor. Gonna go monoblock!! I would want 12 pin sockets if I were gonna use damper diodes (in the future), right? If I plan on that I don't need to worry about a transformer with a 5V tap.
....OR, if I do go with 5V rectifier tubes I could just add a cheap transformer just for the heaters (e.g. Antek AN-0005 -- which I think has a typo stating output of 50ma on html page -- pdf spec is 500ma).
The opus bias adjustment kinda confuses me a bit. Can I think of it this way: The voltage is applied to the connection between PI and KT88 and then some is drained off by the adjustment pot? Whereas the voltage is drained off prior to reaching the wire from PI to output tube in the soloist?
The powerdrive circuit doesn't look that hard to implement and I haven't found any negative comments about it (but haven't looked extensively). One could use the CCS configuration without the MOSFET for the bias voltage, right?
....OR, if I do go with 5V rectifier tubes I could just add a cheap transformer just for the heaters (e.g. Antek AN-0005 -- which I think has a typo stating output of 50ma on html page -- pdf spec is 500ma).
The opus bias adjustment kinda confuses me a bit. Can I think of it this way: The voltage is applied to the connection between PI and KT88 and then some is drained off by the adjustment pot? Whereas the voltage is drained off prior to reaching the wire from PI to output tube in the soloist?
The powerdrive circuit doesn't look that hard to implement and I haven't found any negative comments about it (but haven't looked extensively). One could use the CCS configuration without the MOSFET for the bias voltage, right?
I almost built this one......
A few links talking about the PP1-C (if you haven't found them already)
http://www.diyaudio.com/forums/tubes-valves/180273-rundmaus-work-pp1c-without-sand.html
http://www.diyaudio.com/forums/tubes-valves/133755-wanted-simple-el34-push-pull-design.html
The PP1-C is set up for class A operation, so it will be in the high single digits as far as watts go IIRC. I wouldn't hesitate to build this if your speakers are sensitive enough. I have a hard copy of the schematic filed away somewhere with some notes on it, I think it was 6 or 7 W or so.
Similar to Poinz' 6GK5MM; 2 stage PP triode no feedback with a CCS in the tail of the LTP.
A few links talking about the PP1-C (if you haven't found them already)
http://www.diyaudio.com/forums/tubes-valves/180273-rundmaus-work-pp1c-without-sand.html
http://www.diyaudio.com/forums/tubes-valves/133755-wanted-simple-el34-push-pull-design.html
The PP1-C is set up for class A operation, so it will be in the high single digits as far as watts go IIRC. I wouldn't hesitate to build this if your speakers are sensitive enough. I have a hard copy of the schematic filed away somewhere with some notes on it, I think it was 6 or 7 W or so.
Similar to Poinz' 6GK5MM; 2 stage PP triode no feedback with a CCS in the tail of the LTP.
I'm going to mull it over for a while. With the circuit I've been working on I am getting 10W per channel and staying class A all the time as well (according to the PP calc). So, more or less I'm shooting for the same thing but am not as likely to do it as well as the PP1-C. 6922 are cheap as well...
OK, I actually got around to measuring the turn ratio on the outputs I intend to use for this project. Turns out that it is even lower than what is suggested by the schematic and BOM I have from the donor amp. They are coming out 21:1 which translates to 3.5Kohm. This is a much better match for triode strapped KT-88 than EL-34, right? If so I'm considering biting the bullet and shelling out for new tubes. It just seems silly to put all this effort into something, trying to shove non-optimal parts together just because I happen to have them, when a large part of my goal is to have a good sounding amp. The other options would be flip the iron and get something more appropriate for EL-34. I suppose that I could also use a tube that likes 7K if I shifted which taps the speakers were connected to.
Another question: The OPTs I have were used with 7355 which are rated up to about 30Wpc. When determining whether this iron can handle PP KT88 in triode I need only to worry about the output power of the tube? or the total heat generated by the tube? if the latter then I don't think it would be a go.
Another question: The OPTs I have were used with 7355 which are rated up to about 30Wpc. When determining whether this iron can handle PP KT88 in triode I need only to worry about the output power of the tube? or the total heat generated by the tube? if the latter then I don't think it would be a go.
Two options, both good. Yes, there are lots of others. Two pair of EL34 per channel or one pair of KT88/6550. The Genalex reissues are really good, Ive heard several versions of them and all performed well and are reliable. I use the KT88. I bought them from Jim McShane, the Citation guy. He has fair prices and tests/matches the tubes.
Unless you'll be doing a Class A amplifier and driving the crap out of them (high bias current) or unless you are doing maximum power tests, I wouldn't worry about it. When you have the amp together and you're testing low frequency power, you'll see visible distortion on the scope when you reach the limits of the iron. So, if you're running EL34s, 30-50mA each (200mA per channel) bias current should be fine in the transformer. KT88's 50-70mA each, 280mA total. As always, breadboard it and monitor the heat of the transformer over an extended period of time. Use a little IR thermal scanner and see that the temperature levels off and doesn't get too hot.
Stuart
thanks!!
one question though -- for KT88 that figure would be 140mA/channel, right? we're just talking about a single pair as opposed to PPP for EL34.
I actually have seven of the winged C EL34 on hand but there are good circuits for KT88 going and I'm really tempted to just go with one of those at this point. I've got other iron to play with after this set and I'm sure I'll keep learning if I keep at this -- which was half the point of trying to just use what I already have.
one question though -- for KT88 that figure would be 140mA/channel, right? we're just talking about a single pair as opposed to PPP for EL34.
I actually have seven of the winged C EL34 on hand but there are good circuits for KT88 going and I'm really tempted to just go with one of those at this point. I've got other iron to play with after this set and I'm sure I'll keep learning if I keep at this -- which was half the point of trying to just use what I already have.
Oops, yup, my bad, 140mA it is.
Please be very careful with Russian (and maybe Chinese) output tubes. Pins don't always fit into sockets tightly. A number of failures have occurred because of intermittent bias voltage to the grid pins. I've fixed a dozen or so for a friend (with many vintage amps)
Played with 7K in the PP calc. Probably will stick with EL34 for now and just shift the secondaries. If I do that and then do put a 4 ohm load on the 4 ohm tap it still looks fine. I intend to use this with an 8 ohm load for the time being anyway. Maybe work it out for KT88 down the road some.
Well, an update. Very close to changing my mind about the triode strapping thing. I've decided to keep a pair of speakers as my mains that clock in at 86dB sensitivity as opposed to the 92-93dB pair I was earlier set on. That might mean that I use this iron with EL34 running in pentode mode almost all the way into class A. I'm really not sure...Right now I'm running those speakers off of a Maggie 9303 (el84 pp). I tried my friends Pilot SA-260 (EL34 pp ul) with them and the difference was much to big to ignore -- the music came alive and the speakers went away. I know it isn't totally a question of power. I'm going to tweak the Maggie first just to see what kind of improvements are possible with it. I'm worried that if I stick with the original plan I could end up with a great amp that is underpowered for any speakers I might attach to it in the near term. Maybe just shelve the iron for now...? Dunno...
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