Some valve pron

[lurcher]

Yep, single ended, so always A. They were running at about -7v on the grid when idling, so the challenge is to find a driver stage that will handle the 80v P/P signal, but also the grid current of 50ma or so as the grid goes positive (A2) for half the signal and negative (A1) for the other half.

It was just a lash up for the day, so no protection built into it specifically, but a lot of monitoring at startup. Paul built the output stage and heater supply (10A @ 10V), I built the driver stage, ECC40 to augmented cathode follower (12AX7, 12BZ7, 6BX7). The two parts had never met each other before the day (we just agreed on the plug and socket to connect the two. The power supply and driver is aimed at a big SE project, but not 833. There was a 5A fuse in the 1500v power supply, does that count? In fact, it was a expensive gold plated one, it was the only 5A 20ma fuse I could put my hands on at the time :-). Oh, and I did put a old perspex turntable cover over the HT supply to protect from fingers on the cap terminals.

As for efficiency, yes thats 220W per side, + 100W for the heater. As you say max efficiency for SE is 25%, but thats only true for A1 operation. As A2 can get you closer to the diode line, max efficiency is closer to 40%. Just dependent on how far the driver can push towards the diode line. I dont think the is a solid state equivalent of 2 (because of course you can have B2 and AB2 push pull amps). Thats why I can get 32W out of my 211 Set, though thats a much easier valve to drive into A2.

[SPS]

Nick and Paul are talking about running them at 3kV next time!

don't post that over on audiotalk Ali... you may have phil may be planning to go even higher...

we didn't just have this 833...another member brought one too which also sounded very good...

i was very impressed

most of us stood well back... just to be on the safe side.

not the sort of amp for the lounge (nick may not agree ?)... but would look just right on the set of an old horror film

[RochaCullen]

Glad to see you were making use of our latest discovery the Belkin PureAV interconnects.

There was some scary looking stuff at that show and a heck of a lot of chip board.

And I did spot some very fancy isolation being used for your amp, a tea cup.

But the thing I loved most was the sign stating, "Do Not Touch. Not only will this kill you, but it will hurt the whole time you are dying"

Genius.

[lurcher]

They were specially shaped ceramic stand off insulators. They may have looked to the untrained eye like tea cups :-)

The heater supply inside the chassis overheated without the extra air flow.

[Ali Tait]

Yes, the warning signs were great!

[shane]

There was a 5A fuse in the 1500v power supply

That's relief. For a minute I thought it might be a bit dangerous.

[lurcher]

It was on the 240v side though :-)

[Reid Malenfant]

Actually i was talking cobblers the other day Theoretical maximum power efficiency of SE class A is 50% if it's choke loaded rather than constant current load. It'd also be nearly 200% voltage efficient.

I do like the look of these monster valve amps though, i'm just not sure i like the voltages involved. Low voltage DC isn't too bad, 240V AC can give a nasty shock, but 1500V DC will lock you onto it due to muscle contraction

Very sensible putting up warning signs

[WOStantonCS100]

You guys are flippin' crazy! I luv that! But, my life insurance policy clearly states I'm not to go near or even think about building such a beast or the family gets nothing should I be discharged into another life!

[Ali Tait]

Ah, but you should hear how it sounds...