833A (GU-48) Amp

[IslandPink]

Topical subject -
http://hifiheroin.blogspot.com/2019/11/ ... l-etf.html

[Nick]

A) The input direct from preamp ?

Yes, but you will need to add a adjustable bias to the input, whatever DC voltage is on the input will be on the output, thats how it works as a A2 driver. Cap coupler your preamp input to that.

B) The output to 833a grid ?

Yes

And, what would be a good way to configure a +450v / -400v power supply ?

Create a 400v and a 450 power supply, then stack one on top of the other and call the middle ground.

[pre65]

Can you (Nick) remember what bias voltage range you used on your version ? If not would 40v - 90v be a good place to start ? I assume it would be a positive voltage ?

The RCA data sheet has no mention of class A, or A2 operation.

[Nick]

I think it was negative, what bias are you running at the moment?

[pre65]

I think it was negative, what bias are you running at the moment?

None.

I was using my KT66 (as a triode) single ended monoblocks and instead of the usual output transformer through a step down transformer (1-6 ?) to the 833a grid.

[Nick]

And was the cathode connected directly to ground?

[pre65]

And was the cathode connected directly to ground?

Yes.(with 1R for measuring current)

Just checked on the transformer and it is 5K - 200 ohm 30vA. (there is also a 600 ohm tap)

[Nick]

Ok, so you are running it with 0v bias then, so you could do that again.

[pre65]

Ok, so you are running it with 0v bias then, so you could do that again.

I've never had the "red spot" on the anode on my version, would some form of adjustable bias make that achievable ?

Also, have you any experience of the "Powerdrive" circuits which was another method I was thinking about.

[Mike H]

If memory serves, they got the red spots by shoving the anodes up > 1,000V — 1,200 ? 1,400 ?

[Nick]

Yep, the red spot is power dissipated, voltage * current, if one is low the other needs to be high.

I thought the "Power Driver" was just a mosfet source follower.

[pre65]

I thought the "Power Driver" was just a mosfet source follower.

Yes, but I have read glowing reports on one driving 845 and also 833.

[Mike H]

There's a pun there somewhere

[pre65]

I found this article most interesting. Am I on the right lines ?

http://www.tubelab.com/powerdrive.htm

The writer has used it on 833a, he's the guitar playing nutter with the hairy chest.

Looks like I've been thinking about powerdrive for some time now.

Are there any disadvantages of this mosfet follower design ?

[Nick]

Are there any disadvantages of this mosfet follower design

Tempted to say transistor ear but...

As the linked article says, the changing input capacitance may matter, but can be reduced by having a large d to s voltage across the mosfet. Also, theer will be distortion introduced by the source follower in the same way as a cathode follower would. The ACF adds feedback to reduce that.

As to do any of the above matter, hard to say. My 211 uses a el84 in a similar way to driver the 211 grid.