833A (GU-48) Amp

[Greg]

I was going to use a couple of these high voltage multi pin connectors when contemplating a modular build. Hunt around and you’ll find connectors of this type with different numbers of pins, up to about 27 IIRC.

https://www.ebay.co.uk/itm/Military-14- ... xyB0VRxBuM

EDIT. I see on the same page, 65 pin connectors are available

[Paul Barker]

Thanks Izzy,

Phil and me.both will have heavy power supplies so b+ has to travel,

[Mike H]

Just out of interest, I managed to get the completed unit on the bathroom scales.

27KG.

Flippinus Eckus ...

[pre65]

Just out of interest, I managed to get the completed unit on the bathroom scales.

27KG.

Flippinus Eckus ...

My name is Brian, and so is ------------.

[pre65]

I tried an experiment with the filament heater supply, to try and remove the two dropping resistors, big heatsink and two capacitors.

With the SMPS turned down to minimum, and just the filament choke and filament in circuit, the least volts I can get is 10.7v.

One data sheet I looked at said 10v +/- 5% which is 0.5v, so a bit over. So I could parallel the two existing 1R 100w resistors and lose 0.5v which should bring it down to 10.2v. Safe enough ?

[andrew Ivimey]

Safe enough!? The whole amp is a potential lethal disaster area. Dont worry about the tiny percentages. What is point!

[pre65]

I thought my brain was playing up earlier, but just realised the dropper resistors were not 1R, but R1, which makes my empirical experience understandable.

[andrew Ivimey]

And you still want to go ahead....?

[pre65]

And you still want to go ahead....?

Of course dear boy, why ever not ?

It might seem to others that building, and refining such a monster is madness, but it will be the pinnacle of my audio hobby and then I can sit back and say, yes, I did it.

[rowuk]

And you still want to go ahead....?

Of course dear boy, why ever not ?

It might seem to others that building, and refining such a monster is madness, but it will be the pinnacle of my audio hobby and then I can sit back and say, yes, I did it.

Or be laid to rest as one undaunted...

[Boiss]

This is an old picture from years ago when I built the first iteration, but it shows what the GU48 looks like lit up.

Mine mounted on a Teflon Base with movable solid brass sections to allow for expansion, while checking a computer SMPS modded for 10V

833Apowered s.jpg (179.06 KiB) Viewed 14716 times

[Paul Barker]

I very much like the sound of the 833a at low power. First time Darren (a former member, whose status Im not sure about) and I heard it in his sitting room. You can’t go wrong with it doesnt have to be a super power amp. Its good as low powered amp Effortlessy. Costly to run dangerous and too weather too hot.

I still have the last output stage which Nick brought the driver for when we played it at Owston. Tribute transformers I had made for the 4212. I might dust it off one day. The sound of that valve is worthy. Darren and I were that eccited it was the Monday after a weekend in Wales, we rang Island Pink (Mark) “youve got to come and listen to this!” Unfortunately he had to go to work I think, hes a clever person in his field of work, and brings his kind of mind to this field. But his job is very responsible so unlike me and Darren he couldnt drop everything and play for another day. That moment though was a great lasting memory.

No harm in another 833a (or equivalent) amp in our midst.

Are you likely to come to Owston where we meet up once or twice a year?

[pre65]

Well folks, it seems my interest in these 833a amps is starting to gain momentum again.

Both monoblocks are in a state of unfinished alteration, so I need to carefully check what's what and decide how to progress.

I think, for now, I'm going to go back to the last working iteration, with my KT66 SET amp driving the 833 grid through an interstage, but go with the increased HT using the custom toroids I had made.

I did also (some time ago) get some Russian filament chokes for the filaments and got as far as testing the filament voltage, which were fine with a bit of adjustment to the 12v 30A SMPS.(that was Sept 2020, how time flies)

Watch this space.

[pre65]

The 193Q choke I was going to use in the power supply has a max rating of 1000v. On the data sheet there is no mention of what voltage they use to test insulation, so that causes a problem.

Tomorrow (or later today now) I'll fire up PSUD-2 and play with different scenarios of rectification, capacitors and resistors.

For many reasons I'd like to stick with the 6D22s hybrid bridge, but there are other options. And I've still got a new pair of huge Hammond mains transformers and U19 rectifiers to go with if necessary.

[Nick]

U19 are great. But make sure to read ALL the spec. They are intended for choke input of course at those voltages and the time they are from. Mv's are in many ways a better choice.