PPP monoblocks w/EL84's?

[barry robin]

not long ago i was talking to Roy Mottram about him making me some PPP monoblocks using the ST-35 board, and the idea stuck. so i'll throw it out to you guys: what's the best way to do this? one board bridged or two seperate boards bridged?

obviously, the power supply would have to be the size of a mack truck...

[EWBrown]

One possible method would be to use one ST35 board per channel, and use one channel of the VA and PI to drive both pairs of EL84s, into an OPT with a 4K primary, to the appropriate secondary impedance value (4, 5, 6, 8, 10 ohms, as required). THis would use the adjustable cathode biasing, and would be a relatively easy approach.

One PA774 and the PSU components would power a PPP monoblock channel with 4 EL84s. There is a 240VAC/50Hz primary version of PA774, I don't recall the model number (from Triode Electronics).

I have been thinking along building up PPP 35WPC monoblocks (or dual mono) along similar lines, but using a slightly modified "Poseidon" VA / LTPI board for each channel, driving 4 PPP EL84s, in fixed bias mode. I already have two very nice 4K:8 ohm "1773" OPTs, and two PA774s.
This would be electrically very similar to the design of the Manley "Stingray" orVTL TT25s, but with a lot less exotic mechanical design demands.

All I need is time and a good shot of motivation

I could go with everything on one very large chassis, with separate dual mono channels ( this would be a heavy beastie) or go with two smaller (8X12X3 inch steel Hammond chassis) which would be easier to handle and apply.

HTH

And now for something completely different...

A somewhat "loony" alternate to this design idea, of which I have worked out the details of the necessary circuitry, is a UL PSET using four EL84s in parallel, driving a Hammond 1627 or One Electron UBT-1, and using a 6CG7 SRPP driver stage. This would have lower output power - about 8 WPC in triode strapped mode, and 12 watts in UL, with a very low 2HD and 3HD figure, and which could conceivably give a 300B SET a good run for its money, at a much lower cost. I have all the necessary parts for this approach, too

/ed B

[barry robin]

an interesting project there, EB. you can do it-i know you can!

on my monoblocks: what kind WPC do you think i'll end up with, either with one bridged board or two? and if i used two bridged boards as one channel i'd wind up using 8 EL84's! what would that be, about 70watts? yikes!

-b

[EWBrown]

A bridged or PPP channel should deliver about 30 to 35 Watts RMS , with 4 EL84s. Using a greater number 8 in PPP) , there are dimininishing returns, and the total plate current for the 8 EL84s would be about 280 mA at no-signal idle, and increase as the power level increases ( with Class AB1 operation). At that point, it would be better to use EL34s or 6550s.

/ed B

[barry robin]

good point! i think 280 mA at idle is just a tad silly. might as well just keep the stock set i have. roy also suggested a compromise of a PPP six-pack, and to simply bridge them for about 50 wpc. whatdy'a think?

[EWBrown]

Bridging always works, this was a built in feature of the Dynaco ST70, the little sterero/mono slide switch on the front, next to the input jacks, and the OPT secondaries could be parallel connected, to make them into 70 watt monoblocks. I presume this was done, before the Dynaco MKIIIs and MK VIs were produced

The ST35 and all its derivations could also be similarly treated.

/ed B

[barry robin]

thanks EB; so how do you put up with people like me?

don't get me wrong-i love my ST-35; i love the smooth sound of the EL84 tubes. i just want more of it...about 50-70 WPC more. like, say, a monoblock version of the manley EL84 stingray or a AES/Cary Sixpack with EL84's...or tranendent audio's t-16.

oh: my speakers are old stacked quad 57's, so my choice of amp is pretty much limited to a push-pull design. SET amps tend to turn old electrostatics into a 4th of july fireworks show.

-b