833A (GU-48) Amp

[pre65]

I've downloaded U19 data sheets (Osram and Marconi) and they suggest Choke input (min 10H) or Cap input (max 4uf), and R source of between 500R - 700R.

They speak of "Bi phase half wave circuit (2 valves).
Is that what would be commonly called Full wave ?

And "Single phase full wave circuit (4 valves).
Is that what would be commonly called a bridge ?

Lastly, from your experience do the filaments need to be AC, or DC ? (4v @ 3.3A) And does each U19 need it's own filament transformer ?

[Nick]

I don't feel comfortable here giving advice. Start with RDH and understand what you are doing. It's not safe to busk this stuff.

[pre65]

I don't feel comfortable here giving advice. Start with RDH and understand what you are doing. It's not safe to busk this stuff.

Fair enough.

[Nick]

Is your output tx safe at 3kv? If you are doing this to get them to glow then p=i.v, leave the voltage where it is and increase the current

[pre65]

3000V ?

The old version was running about 950v HT, and I had thought of going up to 1500V HT, but investigations are ongoing.

The data sheet does not give a max working voltage for the OPT (Hammond 1624SE) but the RMS hipot test is at
3500V, and hipot primary-secondary at 4250v DC for 1 second.

With regard to increasing current, I'm not sure how if one filament terminal is grounded.

PS If you can just point me in the general direction I'd be happy, this is a learning experience.

[pre65]

OK, so playing with PSUD-2 I'm back in a similar ball park to last time.

Around 1000v with a 6D22s hybrid bridge, so the 10H choke will be OK.

The 5uf cap is rated at 1200v and 64A (1680v max) and the 130uf cap is rated at 1900v dc and 40A.

[Nick]

To increase current through the valve (not the filament) increase the voltage on the grid and run in A2.

[pre65]

Just to remind, the 833a (GU48) grid is driven by my KT66 (trioded) SET amp using a 5K - 200R interstage.

The interstage also has a 600R tap and I cant remember why I ended up using the 200R one.

[pre65]

To increase current through the valve (not the filament) increase the voltage on the grid and run in A2.

That may happen in the future when I get round to building the buffer that James D designed for me.

Input to a 112A triode with Hammond 126C interstage then buffer to grid. It will then run in A2.

[Nick]

3000V

It was a bit of a trick question to try and gently expose unknown areas. The output transformer primary can swing up close to twice the b+ voltage, so 1k5v can get close to 3kv.

[Nick]

You can get into A2 with the transformer coupling you have. It's not perfect, but A2 often isn't. What current do you get with 950v on the anode and the grid at 0v via the transformer from the set?

[Mike H]

Just to mention, when Danbury Electronics was operating, the transformer formers he used were rated at 2,500V max., that is, windings to stack. Most if not all of his parts came from Transteel (I think? Yes, just Googled), so that was an industry standard kind of thing. This was for both OPT's and mains. Don't know what Hammond's max V rating would be, but is something that needs careful consideration.


I seem to remember once we were playing with an idea to drive the 833 with an EL34 or someting? But didn't work out...

[pre65]

Going by data sheet graph it should be 200ma.

The OPT should max out at 300ma.

[Nick]

If it is 200ma, then if the B+ is 950v then the valve is dissipating 190W. Should be enough for most purposes. If you want to get closer to 300W, try adding some nicad batteries between the interstage secondary and ground to push the grid positive for display purposes, assuming the power supply is up to it. I would add a 1R resistor under the valve to check the actual current.

[pre65]

If it is 200ma, then if the B+ is 950v then the valve is dissipating 190W. Should be enough for most purposes. If you want to get closer to 300W, try adding some nicad batteries between the interstage secondary and ground to push the grid positive for display purposes, assuming the power supply is up to it. I would add a 1R resistor under the valve to check the actual current.

Thanks Nick, I'll get it running first then check the current.

In actual fact, I've just checked and there is a 1R resistor under the valve. I'd forgotten about that.

I will be giving them a thorough check over before use.