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In Reply to: Re: Looking for a GOOD 211 / VT4-C Schematic posted by mrfixit on July 18, 2003 at 04:03:36:
Thanks for your link to your website. I see on your website that you have 2 versions (Cathode Bias & Parafeed). Which particular circuit did you use? I've already purchases a pair of 10Kohm OPT, 10H 500ma chokes and would like to make use of these parts.
Follow Ups:
I used the "final amp" schematic with fixed bias. I did try cathode bias briefly while I was building the bias supply, but it just didn't have the punch I needed. Paul's comments on parafeed are legit even more so since you have decent output iron; I may play with parafeed since my output iron is both budget and 5k, and I have some nice PP vintage iron.
Also based on your query above, yes the original solid state power supply that I based this amp on is a PITA; look at mine (similar to Gary Kaufman's, who also helped me finish this beast). Mine is loosely based on an Altec design, and much simpler. The only change not reflected in the web site version is that I went with 3@ in series of a higher rated diode since I managed to nail my 2 (x2) diode set up twice over the last couple months.
Also, as someone mentioned to me - during the design/testing/early break in period, use fuses on the plate to output transformer connection. You do not want to toast your iron which is possible even using a variac. The +1000v range is a whole different level of design and saftey issue, please be careful!
Good start on parts collection.Just read you other responses and would like to just encourage you. Though the modern Chinease 845 is a good beast plenty of us really like the nos 211 (which is mostly available as vt4c). I wouldn't say it was time to change direction entirely and start again with the 845. Enjoy the 211 project it's a good idea. I see plenty of ideas I wouldn't carry out, this isn't one of them.
Parafeed may be impractical because of the problem finding a suitable cap. Secondly if your OT's are wound for SE they won't do parafeed justice. The parafeed boys maximise inductance of the OPT for best effect.
The 6c45pi circuits are certainly an option, there may be a better way for you. The thing is to start with the curves and determine an operating point for the 211. It is a very valid point someone made about the cost/availability of high voltage caps. Your parameters are min. 10 watts from 2 volts, 10k load, only A1, preference for high voltage.
I'll go and doodle on the 211 curves and come back.
IN brief the circuits available some mentioned here are a 6c45pi to 211 in various forms, the FI Primer style (which includes Ongaku) multiple stage then direct coupled cf driver, which I think we want to try and avoid as it isn't necessary for us. The Sukuma like 211 IT 211 style fo which there is a truly indulgant version in Sound Practices. The Camille version in Sound Practices, this is very complexed and the shunt regulation which is so integral to the design is complexed (requiring a second article). However I rather fancy that to use the valve topology of that design without the shunt regulation may become an option. That is to say a single voltage amp/driver made of parallel 6SN7 (either in srpp or Kimmel MU) maybe direct coupled to your 211. It all depends what power we can achieve from 2v after it's been through the 6sn7 stage. Do you mean 2v peak or rms? Peak would yield about 28v peak drive to the 211. I think that would be enough for at least 10 watts, but I'll have to go away and look at the curves. I'm rusty on them because I'm working with the 212 these days instead so none of this is part of my current mind set.
Paul,I really appreciate your help. At the moment i'm trying to order the CD archive 'Sound Practices' to educate me more on DIY hifi.
Preamp drives 2v RMS. Actually, it is Nelson Pass's SS "Balanced Line Stage" pre-amp i've built 2 years ago. My friend uses the same preamp to drive his 300B amp and we couldn't believe the improvement over a regular passive pot. But to go back, I made this preamp mainly because it was fully balanced to match my power amp (with XLR inputs). However, this particular pre-amp also serves as a converter from XLR to RCA or vice-versa. This is another area I wouldn't mine looking at - why... almost all tube circuits i've seen are not balanced? The response I get is "fewer parts in a tube amp... the better the sound". But what if the tube amp could benefit from reduced noise by CMRR in balanced circuits?
If you can spare some time for me in getting a schematic for me - I would be greatly in debt to you. I'm still not experienced enough to make from scratch, a la my-own-design.
Balanced? Main benefit is in low level stages, once you've amplified signal to 2vrms maybe the benefits do nopt outweight the added complexity. That said Steve Bench has an 845 circuit which is push pull up to the 845 and then single ended. This could be adapted to retain your balanced topology right up to the 211, but it would be a lot easier to stay single ended.2vrms is quite good, 2.828v peak.
I was wrong about power out. It takes greater voltage swing than I thought. That's working with the 212 for you, it's clouded my judgement. Take the standard high voltage operating point of 1250v 60 mA into your 10k load. You can develop 13 watts from a grid drive of 60v peak (theoretical maximum just under 20 watts from 75v peak drive)
To develop 60 volts from 2.828v requires a voltage gain of 21. It's no use saying good the 6sn7 idea will get us there, because you will necver develop a voltage gain equal to the mu of the valve, and not many 6sn7's you test will actually have a mu of 20. I think you must select a valve that has a little more gain in the first place.
It's now becoming clear why the 6c45pe is populare for this application but it's not the only option, and it isn't pressed into service without some care in application (parasitic oscilation around the 6c45pe can make the resultant amp sound quite solid state like).
That said the circuits already exist and hand it to you on a plate. Buiuld it on a bread board and see how you like the sound, then try something else, to see if it can be improved upon.
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