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Dave, Grover,

I modeled up the power supply in Power Supply Designer (PSUD2) just for kicks. It is a full wave rectifier (not bridge) with doubled up plates on the 5U4, and from what I can tell the power transformer taps are 250V-120V-0-120V-250V (note the schematic lists voltage between taps, not the voltage at each tap), Assuming the plate current on each 300Bs of around 85 mA each, plus 15mA for the driver section, I get only around 225V B+ on the plates.

That seems to be a really low operating point, so maybe I am misreading the taps on the transformer. Any case, the current through the 5U4s stays well below limits at the initial charging of filter caps. Now if the voltage is actually higher, (say if the voltage between the first tap and second is 230V, therefore a 350-120-0-120-350 or 700VCT transformer), which is more like I would expect for a 300B amp, the dual rectifier is at the current limit at turn-on for a single cycle. That is probably fine, since the datasheet max transient current is actually much higher than the value used in PSUD (4.6A per plate vs 1.6A in PSUD). So I think the Manley rectifier setup works into the large value caps as designed.

David

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Walt

12 watts rated output from a pair of 300Bs in parallel seems low, so maybe the plate voltages are that low...? Seems like a very unusual approach, but there are a lot of unusual things about this amp.
We don't know the resistance of the secondary windings on the power transformer, so that's an estimate in the PSUD model.

I used the default 33 ohm power transformer resistance in PSUD2 , but I did play around with dropping the resistance to 25 ohms, which increased the voltage by a couple of volts. The 100 ohm resistor between the PT CT and ground basically adds additional primary resistance, much more than the transformer primary and reflected secondary resistance itself.

I wonder if Manley originally designed this amp for 2A3s, except of course for the filament winding (a minor change). In fact. for not much more complication, one could have a convertible 2A3/300B filament arrangement using a dual secondary filament transformer. Two 2.5V/1.25A supplies could be connected in series for a 300B and in parallel for a 2A3.

David

Cary had a SET monoblock that you could just swap 300Bs and 2A3s, 7.5vct with a 1ohm resistor in each leg…

Roscoe

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

Like I mentioned, I'm not a tech and most of what you guys discuss is above my head.
But keep in mind that the power transformer is a special one designed/wound in house at Manley so that it can operate in both single ended and push-pull.
(Unless I mixed up the power and the output transformers.)

The amps do sound very nice and so far I'm preferring them single ended with no feedback, but that's whatI'm most familiar with.
My speakers are Cornwall IVs by the way.

Their web site says it's a full-wave bridge. Maybe it's not clear on the schematic (I don't know where that comes from or or exactly what version it is) but they also claim they're not running the 300Bs terribly in order hard to extend their life, so a plate voltage of 350 or less seems plausibe. It's also interesting that they use an output transformer with very large inductance to make up for the traditional gap in SE outputs.

I'm sure they're lovely. Sounds like they've been a good investment for you!

I'm too old to have investments

Welcome, Dave!

I'll try to keep you and David straight in the coming months!

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- Guy