DCAudioDIY.com

Hi folks. I used to be pretty active on here before 2010, when Uncle Sam sent me out to Los Angeles.
Anyhow, I’m starting to design a GM70 (Soviet era transmitting tube very similar to the 845) monoblock. I have a decent design for an 845 class A push-pull output that I think I could apply, with minor changes. To the GM70.
Can people share their acoustic impressions of class A push-pull vs traditional SE? While not hi-fi, I have a Vox guitar amp that has push-pull EL84 tubes biased fully into Class A and it sounds great. Anyhow, if you happen to have the bandwidth to share your thoughts, I’d greatly appreciate it.

Big question for you.

Have you worked with a transmitter tube before, either PP or SE? I think the practical considerations of dealing with a 1000V B+ power supply, filament supplies (you will need DC 20V/3A per tube for GM70), output transformers selection and cost, etc., are really things you need to understand if you are approaching this without significant build experience. It is much more difficult going from schematic to finished amplifier with a large DHT build, regardless of it being PP or SE.

I think the world is full of opinions on which sounds better, but I think it really depends on implementation, build quality, and system synergy more than anything else.

I have built GM70 and 211 SE amps, 2A3/300B PP and SE amps, plus several PP Class AB using power pentode/tetrodes.

David

Continuing...

The big question is whether you need more than 20W of power that justifies a PP GM70 (or other big DHT) build.

If you can get by with less, a PP 300B is a simpler build, with reasonable voltages, wider selection of output transformers (5K to 6.6K output transformers for PP pentode/tetrode amps), less power/current hungry filament demands, etc. Drive (voltage swing) requirements are the same as a GM70. Start with inexpensive 300Bs to prototype, move up to better once you are confident the filament supplies are working well in ramping up and maintaining the filament voltage. (recommend Rod Coleman filament regulators). It will sound good, many feel this is better sonically than SE300B. That being said, some DIYer's prefer GM70SE amps having lived with PP 300B amps for many years (Kevin Kennedy). For years I used JJ and Chinese 40W 2A3 PP with 2.5V AC filaments, which cancelled out the filament hum in the OT. Same power as 300B PP.



David

Continuing my thoughts on PP GM70

Did some quick loadline modeling of a PP GM70 output stage in Class A. This is with the loadline calculator at the following location:

https://www.vtadiy.com/loadline-calcula ... alculator/

For example, a 60W Class A output is possible with a 12K primary impedance PP output transformer with 1000V on the plate and plate current of 100mA per tube. Grid bias voltage of -95V. This is less swing than you would need driving PP 845s, which makes the driver stage a little easier.

Finding a 12K PP OT is going to be the first thing to look into to scope out practicality. 100W rated. Likely a custom unit, though Lundahl might have something in their catalog. Otherwise, look around. If you are driving 8 ohm speakers, you can obviously use a 6K transformer and connect the speakers to the 4 ohm tap.

Anyway, power supplies, including high current filament supply and 1000V B+, as well as biasing arrangement (fixed bias since unmatched GM70s will need adjustment), are going to be the major work in designing and building such an amplifier. Safety is also a primary consideration at the voltages required.

David

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Walt

Or, just use 833As . Good luck finding OPTs though... I have a pair of 833As in boxes, and a pair of sockets and the necessary 10V 10A filament transformers if you want to prove ot everyone how insane you are...

Roscoe

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I can explain it to you, but I can’t understand it for you.

Walt,

The Cary scheme is for driving the 845 into A2. It is the transformer that permits that, the grid will draw DC through the secondary of the interstage. A direct coupled connection from a cathode/pentode/MOSFET follower to the grid is best for A2, that is what I do with my 211 A2 SET amps. I agree that an A2 SET amp works well, and do not see an advantage of a PP DHT transmitting tube amp with all the overhead needed to support the extra tube. Actually easier to find an SE output transformer at the impedance needed than a PP OT.

I guess my main point was to make sure you actually need the power AND want a PP DHT topology for sonic reasons. Basically, if you can be happy with 20W, then all avenues are open, both reasonable PP DHT at a safer working voltage (using 300B) and SE designs with a transmitting tube (a GM70 SET will do 20W Class A and even more in A2 if transformer coupled), above that, there is much more complication. Class A2 is probably the least complicated way of getting more power, it is more a driver stage thing to get there rather than adding another output tube.

That being said, a parallel SE may be easier from an output transformer standpoint over a PP SE, particularily if you can get it wound with two primaries

Ideas, ideas!

David

Monolith Magnetics have a 833A output transformer. You do not want to ask the price….

Or, just use 833As . Good luck finding OPTs though... I have a pair of 833As in boxes, and a pair of sockets and the necessary 10V 10A filament transformers if you want to prove ot everyone how insane you are...

Roscoe[/quote]

Monolith Magnetics have a 833A output transformer. You do not want to ask the price….[/quote]

833As are almost as big as my amps.! YIKES!

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Walt

I've built SE 845 amps, many here have heard them. David McGown has also built fine SE 211 amps. Personally I have shifted to Class A PP Williamson amplifiers. You can build 20-40 wpc modern monoblock versions for less than $1200 the pair. They are hearty, meat-on-the-bones amps with terrific bandwidth and a wonderful tonal veracity. Far better bandwidth at much lower cost. I had PCBs made for mine. They use excellent copies of well-known Peerless output transformers. SE transmitting tubes have their charms and I woukd discourage you from building something around them, but for my money the PP Williamson offers better sound at less cost.