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PostPosted: July 8th, 2020, 11:37 am 
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So a few years ago my friend Jim bought a used Sofia Electric PP EL34 amp, only to learn after receiving it that the previous owner had "modified" it for 6L6's. Whatever he did, the thing never worked and kept blowing tubes and fuses. Jim took it over to Richard at Sofia to see if he could "fix" it, but Richard said it was such a mess he couldn't do anything. It sat in a closet for a few years, then when Jim and his partner retired and moved to Lewis, he sent it to me to see if I could do anything with it.

It arrived last month. What a beast--must weigh 80 pounds--and what a mess. Stuff glued to the inside, wires everywhere--just a mess. I'm a messy builder but this made me look like Engineer of the Year.

Anyway, I thought, well, decent transformers, lots of room, what can I do? Fortunately Sofia uses the same chassis for all their stereo amps, so there were a number of extra holes and stuff, plus an easily drilled sub-board.

I figured out what tubes I could add where, then hunted around for a circuit that might be fun. In my stash I came across that old PP 300B circuit from Sound Practices, by Al Bryant, "Homebrewer of the Month," who built his amps on cake pans (anyone remember that?). It was a perfect match for the chassis. I don't know of anyone ever actually building it, that I heard of. So here we go:

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PostPosted: July 8th, 2020, 11:40 am 
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Stripped everything out of the chassis and off the sub-board (had to use a chisel for some things he glued in there) and reconfigured the sub-board:

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PostPosted: July 8th, 2020, 11:44 am 
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I've never been a big fan of fixed bias, but for 300Bs in that circuit you don't have enough PS voltage to use cathode bias. Of course, for 2A3s you've got too much B+ to not use cathode bias ;)

Roscoe

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PostPosted: July 8th, 2020, 11:47 am 
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The only thing I changed was using individual AC supplies on the 300Bs so I could more easily bias the tubes, and AC on input stages.

The 300Bs are running at about 420Vp and 60mA each. It puts out 22wpc, even power response from 30Hz to 20kHz. And it sounds darn good:

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PostPosted: July 8th, 2020, 11:53 am 
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Old Al used a common DC supply for the 300Bs, but I don't like the idea of balancing the tubes off the plates--any opinions on that?

I can't get 32wpc as he claims, but that may be 1) power supply limitation, or 2) modern 300Bs don't quite have the power of the originals, or 3) I need to tweak it a little more and do some more tests. I'm also thinking of replacing the Sofia outputs with a pair of Dynaco A470's I have on hand. The Sofias are 5.5K p-p and rated for 25 watts output. Al used what Ithink is a Triad 4.5K p-p, 50W OPT, so that might help, and they'll drop right in. But even as is this is a nice amp. Compared to my Williamsons it has that nice direct-heated honesty and lack of distancing from the music.


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PostPosted: July 8th, 2020, 1:16 pm 
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Roscoe Primrose wrote:
I've never been a big fan of fixed bias, but for 300Bs in that circuit you don't have enough PS voltage to use cathode bias. Of course, for 2A3s you've got too much B+ to not use cathode bias ;)

Roscoe


Same here, and I tried cathode bias but it sucked in this instance. I assume the objective was to build something comprable to a Dynakit or Heathkit with some "real-world" power. He probably had a crate of 300Bs sitting around when they were cheap and plentiful. And it pretty much works as advertised, though I'd like to squeeze a bit more power out of it.


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PostPosted: July 8th, 2020, 1:27 pm 
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That's interesting in that he did not realize that with fixed bias he could have just dropped in the 6L6s, adjust the bias and it was a done deal -- as long as the B+ did not exceed the tube's ratings.

Another drop-in choice could have been KT-88s, 6550s, or even KT-66s without screwing up the amplifier.

Overcoming a mess is messy (pardon the pun). A buddy of mine came to me for a repair of a ST-70 that a Washington, D.C. hi-fi shop 'hot-rodded." I took one look at the mess (it was horrible!) so i told my friend it's not going to happen.

Then he proceeded to yank out of storage two more ST-70s -- one a kit-built piece and the other a factory job. I took the factory job and gutted it. I kept the steel and the transformers.

Using a full differential driver board from a now defunct company in New Jersey, nice mil-spec parts that serve the music, and as an added feature by installing a bias meter that fit a pre-cut now un-used hole hole with a 3-way switch to adjust left or right cathode current using a pot installed on top of the chassis -- when out of the circuit the tubes were run with grounded cathodes that much improved dynamics. I had all of the steel polished and chrome-plated and it has been in service for over 40 years now driving either Spendor speakers used as recording monitors -or- Quad Electrostatics for just plain listening pleasure. :thumbup:

A note about the 300Bs. My Carys have 300Bs with fixed bias used as drivers for the 845s that have cathode bias. Having diddled with bias levels from 45ma to 60ma I found a sweet spot at 48ma that opened up the sound. Worth a try.


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PostPosted: July 8th, 2020, 1:35 pm 
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Grover Gardner wrote:
Old Al used a common DC supply for the 300Bs, but I don't like the idea of balancing the tubes off the plates--any opinions on that?

I can't get 32wpc as he claims, but that may be 1) power supply limitation, or 2) modern 300Bs don't quite have the power of the originals, or 3) I need to tweak it a little more and do some more tests. I'm also thinking of replacing the Sofia outputs with a pair of Dynaco A470's I have on hand. The Sofias are 5.5K p-p and rated for 25 watts output. Al used what Ithink is a Triad 4.5K p-p, 50W OPT, so that might help, and they'll drop right in. But even as is this is a nice amp. Compared to my Williamsons it has that nice direct-heated honesty and lack of distancing from the music.

If your speakers are efficient you may not need power in terms of current. Play with transformer taps. My Altecs have a nominal impedance of 8-ohms but just love the 16-ohm amplifier taps. Paul Elliot's Magneplanars were rated at 4-ohms and opened up when switched to the 8-ohm taps of Paul's amp. The sound in both cases (mine a Paul's) got "relaxed" with much less "strain" (for lack of a batter word).

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PostPosted: July 8th, 2020, 1:38 pm 
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Walt just used the words 'efficient' and 'Magneplanars' in the same breath.......
Whaaaaaatt...????
:confusion-confused:

Just kidding.
I agree with you, lots of speakers that you think need lots of current don't necessarily sound better on the 4 ohm tap.....


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PostPosted: July 8th, 2020, 1:47 pm 
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FerdinandII wrote:
Walt just used the words 'efficient' and 'Magneplanars' in the same breath.......
Whaaaaaatt...????
:confusion-confused:

Just kidding.
I agree with you, lots of speakers that you think need lots of current don't necessarily sound better on the 4 ohm tap.....

That was kind of an Oxymoron. Tee-hee! :crazy:

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