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Old 11th September 2007, 03:33 AM   #1
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Default new 6V6 amp

considering the following implementation. Power shouldn't be amazingly over 10W but sound could hopefully be good. I went with the 6N7S as the LTP driver simply because it's somewhat similar to the 6N1P in gain but in a nice octal bottle.

Any thoughts? Think it'll work?

Click the image to open in full size.

I'm thinking that I'll use a 400-0-400V 200mA PT with various chokes in the power supply. Hopefully the input cap won't have to be quite so big in order to get the B+ of around 400-410V.
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Old 11th September 2007, 04:57 AM   #2
mach1 is offline mach1  Australia
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I don't think the 6V6 will last very long with a Vp-k of 410.
315V would be more appropriate.
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Old 11th September 2007, 05:28 AM   #3
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the thing is, at the 40mA operating point, the tube's plate to cathode voltage probably isn't that high. however with audio signal applied, the swing along the load-line would approach the B+ during transients. is that still unacceptable?

also, apparently some new 6V6's like the 6V6EH are advertised as having high voltage tolerances, of 450V or more.. what do you make of this?
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Old 11th September 2007, 05:43 AM   #4
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Hi,

regarding the schematic, I would suggest to return the LM334Z CCS to a negative voltage of, say, at least -6V, or better -12V. Ignoring this and operating that chip too close to its absolute minimum working voltage you will risk stalling or even most dangerous oscillations.

From own experience I can confirm what Morgan Jones wrote about this chip in his book: "You have been warned."

Tom
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Old 11th September 2007, 05:48 AM   #5
Salas is offline Salas  Greece
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Quote:
Originally posted by sorenj07
the thing is, at the 40mA operating point, the tube's plate to cathode voltage probably isn't that high. however with audio signal applied, the swing along the load-line would approach the B+ during transients. is that still unacceptable?

also, apparently some new 6V6's like the 6V6EH are advertised as having high voltage tolerances, of 450V or more.. what do you make of this?

The impedance of the OT primary will not eat B+ apart from dropping on the maybe 200 Ohm DCR of each half. So you must expect around 400 VDC across each tube when biased. If 6V6EH or even better JJ 6V6S can take is another thing. If they actually do for long time without problems, one negative thing I see is that you will not be able roll NOS. Me, I would not burn 16W as in your design on any 6V6GT. Their envelope is small. Even if the modern ones are taking the voltage, I see a 14W limiting value in pentode mode for the JJ. They won't last.
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Old 11th September 2007, 06:57 AM   #6
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ok, I'm glad that's settled for me - I hadn't factored the B+ voltage into my calculations for dissipation, rather was using each voltage*current point along the operating curve. This clears up quite a few things! B+ should be around 300V, and current should be around 40mA, for a dissipation of 12W, or around 86% of maximum. Those numbers shouldn't be too objectionable, right?

I hope that a 350V transformer rectified with a 5AR4 with choke-input should net around the right amount. The little Y-shaped cap assembly, of .22uF 1KV from each end of the choke to each other then to the star ground point seems to have thrown off the voltage values for my monoblock amplifiers, a trend which isn't accounted for by any simulation I can see. Given that mains voltage is higher than 115V, often around 122V, that makes the HV secondary around 370V. 370V*.9 for choke input nets me around 335V, and minus 25V, 310 should be acceptable. I'm considering using an improved Dynaco PA060 as the PT for plenty of headroom that 50V winding would also be great to rectify and filter down for the -18V or whatever.

A PA060 for power, and some Z565's for output, with 6V6's - I'd call it the Dynenstein or something. Or do you think the James transformers - PT, OPT and everything - are worth it?

p.s. I have nowhere near the kind of money I'd need for James transformers so don't worry about me building prematurely
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Old 11th September 2007, 01:22 PM   #7
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In my own experience, NOS 6V6GT run fine and last a long time with as much as 450V on both the plate and screen at 14W Pd (~30mA.) I have no experience with new production 6V6 as I haven't played with these tubes for years. But, since new ones are cheap I don't see any reason to coddle them if they sound better running at higher voltages. For that matter, NOS 5V6GT are cheap as dirt if you can arrange a 5V heater supply (dropping resistors from a 6.3V supply?)

Having said that, you might like the sound better at something like 350V, either pentode or triode. I have no experience with UL, but in pentode mode I suggest a lowish load like 5k to 6.6k. No higher than that. In triode mode higher is better. 10k seems about right. Again, I can't comment on UL.
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Old 11th September 2007, 02:09 PM   #8
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the datasheet that I saw described an output power of 14W into an 8K a-a load at around 285V of B+ so I don't know what I was thinking with my 400V B+. i'm looking at an interesting article here

http://www.tubecad.com/2005/May/blog0046.htm

and will probably want to implement it in some way to keep my tubes happy and correctly biased into eternity. b+ will be around 300-320V before the voltage drop due to this biasing scheme.
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