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From another thread, a post by D. Berning:

Hi All,



One of the most beautiful sounding amps I've ever heard was one by Richard Sears. We listened to it at David McGown's home a few years ago. I was in another room, and the natural, flowing sound drew me in. We'd listened to a number of creations, but there was something about this little PP Class A 6AS7 amplifier which uses a cheap power toroidal transformer as an output transformer. the best comparison I can make to explain it's natural sound is to compare it to a 15 IPS two-track analog tape. It has THAT sound.

I looked up the information about it on Richard's site, and it is really an amazing little piece. I wonder how hard it would be to scale up it's 4 WPC power output to what would be, for me, a usable range, say 20-40 WPC.

Richard's site appears to be malfunctioning, so the whole description is no longer available. However, I've included a link and attached Richard's schematic.

Gee, I'd be interested in a listening session that included one of these!!! Beat it and you REALLY have impressed me!

http://www.triodeguy.com/6as7_pp.htm

Stuart

There is another builder from Ireland who also has built pp amps with a similar topology and toroid transformers.

Here is a link to his 2nd version.

http://www.diyaudio.com/forums/tubes-valves/160059-dc7-mkii-new-schade-6080-amp.html

Here is a link to his 3rd version.

http://www.diyaudio.com/forums/tubes-valves/228774-dc7-mk3-up-running.html

I have a bunch of 6080's and am contemplating building one of them later in the year.

ray

Thanks Ray.

I'd actually seen Shoog's postings in other DIY threads, but not this one. Another very interesting design.

Gotta build one. But like "The Captain" I need more power, MR. Scott!

I also have a class A 6AS7 P-P 5-watt integrated that I use for winter listening.

David

Hey Dave,

Do you mind sharing your chosen operating points for the output tube? Power and distortion out?

Stuart

Found the schematic for the amp. On the 6AS7, the voltages are:
Plate 484 V, cathode 300 V, Grid 220 V. (plate to cathode 184 V, dc coupled topology thus the elevated cathode voltage)
Plate load 3.6 k P-P with an eight-ohm speaker.
Test results:
3 ohm measured output impedance, no feedback used
output power @ 8 ohms 6 W; 7 W @ 6 ohms; and 8 W @4 ohms.
This is a ZOTL.
Other: There are two additional stages of gain with balanced 12AU7s having bipolar transistors for the cathodes current sinks. There is cap coupling between the first and second stage only, with dc servo feedback to null the dc at the output speaker.

David

Forgot one other parameter. The idle current is 83 mA per pair of triodes, for a (plate) power dissipation in the tube of 15.3 watts in the bottle. Did not measure the distortion, or at least I didn't note it, but the power I mentioned is taken at the point just before visible clipping on the scope.

Davdi

6AS7s can indeed sound very good for a little bit of money. The do need a lot of drive and, as suggested above, they run hot as hades but it is still one of the more common surplus tubes.

I had an Atma-Sphere OLT with eight (maybe more, I forget) 6AS7 tubes for review while I was in Texas and literally put it in the fireplace to try vent the heat. Definitely a wintertime, Northern climate piece. It was like having a good space heater on in the room, full blast at all times. Atma-sphere is in Minnesota, I think.

Another dual triode regulator tube I had a lot of success with is the 5998. They get hot too and they are no longer so easy and cheap to procure but a great sounding tube.

I suppose once could parallel a few tubes for more power and run it with a really strong driver but you won't win any points with that environmentalist crowd! And you would need some big custom power and output transformers for a conventional implementation. Starts looking cumbersome to approach 20W, unfortunately. But nothing is free in this world, especially 20W of Class A triode power.

I haven't tried it but some report good results using 6AS7 type tubes as line amps. Appropriately low gain and reasonably low output Z from a plate connected output.

If anybody is building, I'll donate whatever is left of my collection of 6AS7s and 6080s.

Joe, you have a PM.

Ah, didn't realize we were discussing a ZOTL. Never mind! Thanks anyway, though.

I'd like to try simple and see where that goes.

Stuart