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compact Dowdy/6SN7 direct-coupled to PP2A3

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Posted on May 8, 2004 at 19:46:26
keto
Audiophile

Posts: 1006
Joined: September 29, 2001
What's that venerable name doing in the title??

V2nd was kind enough to safety-check my direct-coupled PP2A3 amp schematic and also included a suggestion for a PP input instead of the SE one I'd been using. Lacking parts for the negative rail, I reached for the Dowdy Choke on hand for current sink duty on the 6SN7 shared cathodes, a perfect 16mA CCS for that 5V cathode bias (an 8mA C4S current source on each plate). 100K pot on one 6SN7 grid, and the other grounded.

With a 12uF cap input (wish I had a 4uF on hand) and what looked like "arcing" in the 6BY5, I decided to return to choke input and be satisfied with just a couple watts of output.

One of the 6SN7's I'd collected balanced up (via C4S w/ trimmer) nicely, and the amp sounds pretty sweet. Thanks V2nd, for the PP input suggestion.

--keto

 

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another 6SN7 variant, posted on May 11, 2004 at 14:18:43
keto
Audiophile

Posts: 1006
Joined: September 29, 2001
Buenas tardes, all...

The idea above generated some sort of resonating humm, which I couldn't figure out, so I changed things around to a single-ended 6SN7 direct-coupled to the "compact" PP2A3, a simple circuit type that's also worked well with the 6N1P and with a pair of 6C45s. A second, identical (but grounded-gid) triode sets the grid potential of the second 2A3, and by means of a trimmer on the C4S, humm is nulled to 1mV or less. I know the measure of humm isn't the only consideration, but it does correspond with things sounding good.

With the 6SN7 installed, I dared to remove my trusty 100K Sweet Whispers attenuator, and to install instead a Cobalt B7, using its primary as a grid choke. Well, it was a little too loud, but by far my favorite sounding circuit to date. Now the B7 is functioning as a step-down, which provides a pretty appropriate listening level. When I get the chance, I'll try a pair of 27s with the B7 primary as grid choke.

In the direct-coupled "compact", the input capacitance would seem to be increased with respect to a single-ended circuit, and asymmetric between the two output triodes. That would seem to be its major compromise. In reality, the 6C45 has been the best sounding driver, compared now to 6N1P and 6SN7.

That grid choke idea of Mike's is evidently a great one, but my use of the 6C45 will necessitate some form of attenuation. What are the chances you would add a few taps to that grid choke, Mike? It probably uses that easy, 48-gauge wire, right? And would that even attenuate the signal? I'd go for one that had two additional taps, for off, soft, medium and loud.

My 6BY5 was sputtering and blowing fuses, so I unstalled 1N4007 diodes, which work just fine. Later I'll import some UF4007s and also install the Bottlehead RRSF trick. With a BFFC choke input, B+ is about 375V, which I assume means that not only does 18 ohms DCR minimize the voltage drop, but it changes the very nature of the filter. Bottom line is that with a single LC (470uF), I can shoehorn a direct-coupled, PP2A3 out of a pair of PGP8.1s.

That's the latest... --keto

 

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