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Greetings all,
Over the years I've accumulated several pairs of loose push-pull output tranformers. And with my kids out of diapers, I finally have some time to do something with them. I'd like some input as to what a good project would be for these and which one should I try first?
This is what I have right now.
1 pair potted Dynaco A470(UL) 4.3k 35watts
1 pair Thordarson 22S13 4k(UL) 70 watts (NOS from defunct Melos Audio)
1 pair Conrad Johnson MV75 OPTs 4.3k(UL) 75watts
1 pair Eico ST-70 OPTS 7k (pentode)
3 pairs Eico-HF20 OPTS 6.6k (UL) 30 watts
Does push-pull 300b's sound like a good project?
Thnx.
Follow Ups:
I'd make some KT77-based monoblocks with either the Thordarson or CJ OPT's. A nice 500-550VB+ choke-filtered from a 1200VCT or so PT, plenty of current available for each channel (300mA), with some type of long-tailed totem-pole driver setup. You could have a killer amp. I'd also recommend SS rectification because tube rectification is a HUGE hassle. If you like the triode route, these OPT's are a bit overpowered anyway.
PP 300Bs are a great idea. Any of the output transformers you listed should work fine. The 4K impedance will give you more power, but higher distortion, than the 6- 7K ones. I have built a varaiation of Kevin Kennedy's PP 300B amp - see link below. I added cathode followers between the 6SN7 and 300Bs and substituted a 6N1P for the 6SL7 (better bandwidth). I use regulated +250V and -300V supplies, but the 400V supply is not regulated. By the way, this is a bit of a problem - if the B+ is unregulated, the bias supply should be unregulated as well. This reduces varaiations in idle current with line voltage. I used the One Electron BFT-1B (a great power transformer - highly recommended) in the power supply.
Here is alink to Kevin's web site
http://www.kta-hifi.net/
Have fun.
Hi Mikey,
I have never heard this amplifier, and the designer himself seems to be quite pleased with his work. Did you try the original as per the schematic?
I do have my doubts if the driver is up to the task with a 6SN7 running only a couple of mA. But that's probably why you added the cathode followers :-)
Regarding balance, you could try to raise the cathode resistors of the output stage a little.
"Did you try the original as per the schematic?"
No. I can't just leace a design alone, you know...
I built two versions of the driver stages - 6SL7/6SN7, as in Kevin's design,and a 6N1P/6SN7 vesrion. The later provided much better bandwidth - DC to about 40 KHz for the 6SL7, DC to over 80 KHz for the 6N1P.
You can view the schematics of my version:
http://webpages.charter.net/smburns/amp300bp1.pdf
http://webpages.charter.net/smburns/amp300bp2.pdf
http://webpages.charter.net/smburns/amp300bp3.pdf
"But that's probably why you added the cathode followers"
Correct!
much improved as far as I can tell, should have enough oomph to drive those difficult DHTs, and no cap coupling to the output stage either. Any pics of the amp available?
I would suggest a SETH (a 2a3 design) but perhaps with 300B's. It is one thing I'm still planning to make. But have not made time for. This design would save you on an input transformer and complexity. While I don't think it would sound worse (armchair bit).
(Picture of Kudret's SETH)
To infinity and beyond!!!
PP 300B is nice and the 6.6K OPTs fit the bill. The question is, how are you going to do the phase splitting and what driver tubes will you use?
I can recommend Lynn Olson's Amity but you need some more (good quality) iron to build one. Good, but not cheap. You can cut costs on the CCS (use a resistor) or the tubes, but not on the input/interstage iron. These are critical.
Have fun!
What Fisher and Scott did, you can do too. Decent driver circuitry for "12" W. tubes also works for 7591s. Take the Eico ST-70 trafos and build an "El Cheapo" variant using 7591 "finals". You'll get a nice 15 WPC in triode mode.
Eli D.
Why not use two tubes for a bridge rectifier? Since the plates and cathodes have dedicated pins this should be easy.
I may try this with the many 6AL5's I have. Thes erae "cute" little tubes that will look good on the chassis landscape plus we get a tube bias supply to boot.
You must remember that "El Cheapo" is an exercise in obtaining lots of sound for little money. The hybrid bridge is "noiseless", while holding costs down. UF4007s are approx. 15 cents each, which is LESS expensive than 2 tube and socket sets. Also, you deal with significant forward drop only in the vacuum half of a hybrid bridge.
Eli D.
The EICO HF-20 transformers work very well with PP 300B's.
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