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trying to decide on my next amp. the -35 sounds awesome with the el84m's but looking for more power. anyone tried PPP EL84'swith the poseidon or st35 driver???
Tube Amp Newbie
Yellow_Light_Colorz_PDT_09

Hi - Welcome to the board.

I've built a bunch of Shannon's DIYTube amps ... the ST-35, then an ST-70 clone with 7591's and most recently a pair of Poseidon MK-III clones with all new parts. I also have a big azz Velleman EL-34 PPP amp, and my conclusion is that power isn't all that important. With good speakers, the low power amps are just as good as the big ones. For a simple build, with no metalwork necessary - a pair of Poseidons using the eBay MK-III chassis is a simple build - and they sound great.

... tom

http://mcnally.cc/amps.htm <<< pix of my stuff

I built an amp a few years ago using my own driver PCBs to run six EL84s in PPP, kind of a cheap Cary SixPac knockoff. It worked really well and sounded good, but I still prefer my modded ST70 amp.

zippy1540 wrote:trying to decide on my next amp. the -35 sounds awesome with the el84m's but looking for more power. anyone tried PPP EL84'swith the poseidon or st35 driver???
Tube Amp Newbie
Yellow_Light_Colorz_PDT_09

I've done it with an ST70 driver.
Here's what I learned:
1. Keep the B+ reasonable. It IS possible to run EL84 PPP ultralinear
at 425V, if you handpick the tubes... using a B+ of 350 or less
will avoid a lot of potential problems.
(BTW, you can get 50W out of PPP EL84 at 425V Yellow_Light_Colorz_PDT_09
2. Keep the grid circuit resistance down. The DIYtube ST35 has
470K, which is way too much. You really want 150K or less,
the DIYtube ST70 board has that stock, the Poseidon 100K.
3. You may have to experiment with screen resistors and/or
small pf caps betwixt plate & screen to avoid oscillation,
keeping the B+ reasonable reduces the likelyhood of that
kind of problem. So does using cathode bias, although that
cuts your output power down a bit.

1. Keep the B+ reasonable. It IS possible to run EL84 PPP ultralinear
at 425V, if you handpick the tubes... using a B+ of 350 or less
will avoid a lot of potential problems.

This brings up a question that I've wondered about for some time. I've got an old pair of EL84 PPP amplifiers that use a B+ of 480V. Any thoughts as to how they can do that?

Duncan

DuncanM wrote:This brings up a question that I've wondered about for some time. I've got an old pair of EL84 PPP amplifiers that use a B+ of 480V. Any thoughts as to how they can do that?

Duncan

What's the screen voltage? If you keep the screen voltage at a reasonable
value, the plate voltage can be very high without hurting the tube, actually,
the limitation is usually the socket rather than the tube. The dilectric strength
of a vacuum is pretty good.

Just out of curiousity, what model of amp is that, anyway?

What's the screen voltage? If you keep the screen voltage at a reasonable
value, the plate voltage can be very high without hurting the tube, actually,
the limitation is usually the socket rather than the tube. The dilectric strength
of a vacuum is pretty good.

Just out of curiousity, what model of amp is that, anyway?

It's an old VTL "Tiny Triode." As the name implies, the EL84s are running as triodes with no separate screen supply.

IIRC those VTL tiny triodes can be run 25WPC triode mode or 45WPC pentode (or UL?) mode.

My (currently vaporware) PPP EL84 with 1773s or FT2020s for OPTs is basically "borrowing" from that idea, if I ever get around to completing it.

No, I won't be beating the EL84s at 480VDC, more like 350-380V, and with fixed bias rather than cathode bias , so that effectively adds another 14VDC plate to cathode potential difference.

Stay tuned, winter's coming Yellow_Light_Colorz_PDT_21

/ed B in (soon to be frozen wastelands of) NH