833A (GU-48) Amp

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IslandPink
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#1156 Re: 833A (GU-48) Amp

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Nick
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#1157 Re: 833A (GU-48) Amp

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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.
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#1158 Re: 833A (GU-48) Amp

Post by 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.
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#1159 Re: 833A (GU-48) Amp

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I think it was negative, what bias are you running at the moment?
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#1160 Re: 833A (GU-48) Amp

Post by pre65 »

Nick wrote: Tue Nov 12, 2019 9:02 am 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.
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#1161 Re: 833A (GU-48) Amp

Post by Nick »

And was the cathode connected directly to ground?
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#1162 Re: 833A (GU-48) Amp

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Nick wrote: Tue Nov 12, 2019 11:01 am 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)
Last edited by pre65 on Tue Nov 12, 2019 1:04 pm, edited 1 time in total.
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#1163 Re: 833A (GU-48) Amp

Post by Nick »

Ok, so you are running it with 0v bias then, so you could do that again.
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#1164 Re: 833A (GU-48) Amp

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Nick wrote: Tue Nov 12, 2019 1:04 pm 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.
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#1165 Re: 833A (GU-48) Amp

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If memory serves, they got the red spots by shoving the anodes up > 1,000V — 1,200 ? 1,400 ?
 
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#1166 Re: 833A (GU-48) Amp

Post by 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.
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#1167 Re: 833A (GU-48) Amp

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Nick wrote: Tue Nov 12, 2019 2:59 pm
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.
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#1168 Re: 833A (GU-48) Amp

Post by Mike H »

There's a pun there somewhere :lol:
 
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#1169 Re:

Post by pre65 »

pre65 wrote: Fri Jul 08, 2011 2:46 pm I found this article most interesting. Am I on the right lines ? :wink:

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 ?
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#1170 Re: 833A (GU-48) Amp

Post by 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.
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