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In Reply to: RE: Best sounding amp I own - I Second that posted by Jim Doyle on October 12, 2011 at 11:19:23
Hi Jim:
I'd be tempted to use a bit larger of a plate choke on the anode of the triode connected 6V6--- and use an inductor that has been designed as a plate choke.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Follow Ups:
The 6V6 as a triode driver is a 20ma affair. I believe that Tango had an IT part in their catalog that was precisely built for 6V6 - everyone was using it about 15 years ago. It was either the NC-16 or NC-20... Nonetheless, the 6V6 is a very sweet and clean sounding tube.
Direct coupled amps are really only for people who know what they are doing... Because one tube sets bias conditions for it's neighbor - it's very easy to blow things up when stuff goes wrong (tube out of socket, dead filament, anything). Too easy to destroy something with that topology.. Ontop of that, you are always dealing with a 700-900V supply due to one tubes plate being tied to anothers grid.. So even the PSU becomes hairy and expensive... The Welborne DRD amps use this topology - I'd be anxious to hear about in-field reliability... :)
Nonetheless, direct coupling the first two stages is safe and inexpensive.. LC coupling the Komuru driver sounds every bit as good
as IT coupling on my 845... Cheaper too - but I'd already bought my interstage transformers but I did listen and audition out an LC coupled amp on that driver and it was awesome!
I'd also lower the total gain of the Komuro stage.. It has 20*8 mu = 160.. You could sub out the 6J5 (mu 20) for something
around 10 - using a 76, 6P5 - which is what I do. Omitting the cathode bypass cap on the first will also reduce the net gain... In my implementation, the driver gain is 64 - which is perfect for a 300B or
LV 845 and a 2V input. You might find other ways to modify the net gain - for instance using perhaps the 71A rather than the 6V6. One of the very nice thing about the 6V6 / 76 combination is that the resistor complement needed to set bias is very straight forward - 47K, 470 ohms, 4.7K ohms.. :)
I have been following this post with interest to build also. My question is what would you use in place of the UTC power transformer and the Stancore 30H choke
The UTC transformer is rated 150mA, but only supplies 61mA according to the schematic. That opens up a lot of other transformers, such as the PGP8.1 that Mike makes. The only question is whether the HV supply can be raised 300 volts.
A BCP-15 gapped for 40mA would handle the 27.5mA of the 6V6 easily, and give 50 henries for better bass.
If the PGP8 can take the voltage offset, you could do the whole power supply with the one transformer, by using 1200-v Schottky diodes instead of the tube rectifier. It would no longer be the real Komuru design, but would be easier to build and probably sound very similar.
Just another idea...
Please correct me if I'm wrong... it has been known to happen!!
Cheers
FM
Haha! Well, I don't imagine Mike has lost the drawings ... :^)
Mike has some really nice power transformers in his archives, but his time is limited, and I for one feel that time is best spent on iron that handles audio signals. It just makes more difference there. Lots of people make decent transformers, and a decent power transformer is good enough to make beautiful music. Damn few make excellent audio transformers, and excellent is audibly better then pretty darn good - which is not all that common either. Excellent beats the crap out of decent in audio iron.
Just my opinion, of course.
thanks for the response. It gives me something to think about.
PJ wrote:
"A BCP-15 gapped for 40mA would handle the 27.5mA of the 6V6 easily, and give 50 henries for better bass."
And I could airgap it for 30 madc and get > 50 henries of L.
MSL
Builder of MagneQuest & Peerless transformers since 1989
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