True direct-coupled PP amp

[JesseG]

Hi All:
I was reading through an old Radio-Craft book (from 1947) and came across this Direct-Coupled 10 P-P amp design using 6L6s. And, it is fully balanced as well.

All 6 pages of the article are attached.

I have read it through and it is truly direct-coupled - NO caps in the signal path, only the PS.

I have attached the whole article. Maybe one of you design gurus could re-calculate the values to build this with better (read quieter) tubes than the 6L6. How 'bout EL34s or 6550s? Also, the input tubes are 6SJ7 - maybe change them to 6DJ8 or 6H30?

I am pretty well decided to build one of these to see how it sounds, but would love any input from the tube experts out there.

Jess

[mwiebe]

Jess,
It should work fine, but note it’s going differential, or splitting phase, off the input jack with 1meg between legs. This would probably be hard to drive straight off a CD player and may sound funny so keep that in mind. I’m not sure the feedback scheme will work as drawn unless you are using the output transformer spec’ed in the article.
On a lark I did this direct connect P-P and it sounds better than I expected. It should be clear I had lots of Edcor bits around.
Matt

[JesseG]

This would probably be hard to drive straight off a CD player

Thanks for the comments, Matt

I was thinking of driving it with a balanced preamp (that has SE inputs). To some extent I can tweak the preamp output so that this amp's input sees what it needs.

and may sound funny

Won't be the first bit of gear I've built that does that...

Jess

[smoking-amp]

Looks to me like this scheme wastes a lot of power. (requires class A output mode even, besides the voltage dropping resistors.) The high value resistors in the output tube cathodes are going to make the output Z rather high too. I don't think that global feedback "option" is going to be optional.

Why not just use the standard approach of two high value series resistors from each of the input stage plates, connected to a B- rail (or use CCS's on them to a - 30V rail). Center connection of the series resistors goes to the output tube grids. Causes a 2X loss of gain (or no loss of gain with the -30V rail CCS's). Could put CCS's in place of the input tube plate resistors as well to B+ for more gain and less distortion yet.

Don

[mwiebe]

Maybe I’m reading the schematic wrong, but I only see 1.1K in the 6L6 cathodes, even without a feedback scheme, I don’t think output Z would be unreasonable. I think the hard part of implementing that output feedback as drawn is finding an output transformer with those tap options. Using a different transformer would require recalculating feedback, and would move the plate voltage of the 6SJ7 around a bit.

One cannot leave the feedback out because the gain of the 6SJ7 would be higher than the 70 to 90dB the article says it will provide, which is already too much for a modern amp. It’s not easy to swap tubes or gain profiles with a direct-coupled design, there are too many balls in the air. It’s kind of build it exactly as someone worked it out or do your own design from scratch.
Matt

[JesseG]

Looks to me like this scheme wastes a lot of power. (requires class A output mode even, besides the voltage dropping resistors.)

Saving electrons was not a high priority in 1947
Still, the total power consumption will not be much more than 200 watts, no?

It’s not easy to swap tubes or gain profiles with a direct-coupled design, there are too many balls in the air.

I certainly agree with that observation - and I don't have enough design smarts to tackle one from scratch. I guess I'll just have to build it and see

As for the 500 ohm tap used for feedback - will this provide a particular ratio of power for the feedback that cannot be derived from a 16 ohm tap, adjusted with different R & C values?

Jess

[moonbird]

Originally Posted by mwiebe

Jess,On a lark I did this direct connect P-P and it sounds better than I expected. It should be clear I had lots of Edcor bits around.
Matt

I have a boatload of 6aq5s - which make me interested in the schematic you posted. I have many questions - may I start a new thread? thx.

[fg031956]

Very good reports from mr Brault, french ingeener (circa 1950)

[Triode Kingdom]

These aren't amplifiers, they're IMD generators. If you build a pentode without NFB, you had better be planning to use it with a guitar.

[mwiebe]

Jess,
Sure you can recalculate feedback for the transformer you use, but it still may move the plate/grid voltage around. As long as you have about three millimeters clip leaded in your circuit and kids calling out values as you fire it up you should be fine. Tubes have a sense of humor that allows for slight mistakes as you get things working.

The French amplifier posted looks simpler then the one at the start of this thread. And its much lower gain is more in line with modern amplifiers, so it should be easier to work with. But you do have to have the tubes.

Moonbird,
There are lots of designs using 6005s, 6AQ5, EL90, and etc. so I’m sure if you start a new thread there will be lots to think about. What I learned with that design is to use ceramic sockets under the 6005 for reliability, those little tubes can pass enough current to burn up sockets. I have no experience with the tube beyond the schematic I provided.
Matt
Matt