833A (GU-48) Amp

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pre65
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#1141 Re: 833A (GU-48) Amp

Post by pre65 »

I can't see at the moment, but I think I use 2 x 100W R1 (.1R ?) resistors and capacitors.

So, they are in the filament positive lead, and the caps are not across the filament.

Is that "bad" ?
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#1142 Re: 833A (GU-48) Amp

Post by Mike H »

I would say, no. :D

That's what I was thinking of.

So I'm not sure it needs improving.
 
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#1143 Re: 833A (GU-48) Amp

Post by Mike H »

How did Paul / Nick heat theirs? (Open question)
 
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#1144 Re: 833A (GU-48) Amp

Post by Paul Barker »

Switched mode, I put a tube current sink (crude shunt reg) at end of power supply on the 5v rail of the filament power supply which prevented it from cutting out when faced with the cold 833a filaments.
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#1145 Re: 833A (GU-48) Amp

Post by Paul Barker »

If I hadn’t done the above I would have used ac heating and injected the hum into the grid per S Bench which is how my last amp I brought to Owston was heated.

My next amp will be very simple and low powered, probably a 10 driven by a 5687 cf volt amp would probably be a cascade of 76’s or 27’s or similar. All ac heated and hum corrected with S Bench.
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#1146 Re: 833A (GU-48) Amp

Post by pre65 »

I came across two filament chokes on Ebay, they are 2.5mH @ 12.5A.

I was "watching" them for a few days, then got a message saying the seller had reduced the price by 40%, so I purchased them. :D

They will go into the box of bits for the big rebuild.

Message for Nick. Some time ago I was chatting to you about augmented cathode follower drive for the 833a, and you thought you might still have a couple of PCBs for the circuit. Did you ever find them ? If so I'll send you some beer vouchers. :)
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#1147 Re: 833A (GU-48) Amp

Post by Nick »

I will have a look.
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#1148 Re: 833A (GU-48) Amp

Post by pre65 »

Nick wrote: Wed Oct 30, 2019 10:11 amI will have a look.
Cheers Nick.
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#1149 Re: 833A (GU-48) Amp

Post by pre65 »

The filament chokes were delivered this morning, not a bad service from Latvia.

I'm going to rethink the way this amp is constructed as the total weight is likely to be backbreaking. :(
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#1150 Re: 833A (GU-48) Amp

Post by Mike H »

Separate modules?
 
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#1151 Re: 833A (GU-48) Amp

Post by andrew Ivimey »

He already uses separate output transformers...it'll be just like giant Lego.
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#1152 Re: 833A (GU-48) Amp

Post by pre65 »

andrew Ivimey wrote: Mon Nov 11, 2019 3:02 pm He already uses separate output transformers...it'll be just like giant Lego.
Modules, that sounds good. :D

Are you still travelling round the world Andrew ?

Been in the "shed" lately ? :?:
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#1153 Re: 833A (GU-48) Amp

Post by andrew Ivimey »

The shed is seriously damaged.

I've just booked a flight to Beirut!
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#1154 Re: 833A (GU-48) Amp

Post by Nick »

Nick wrote: Wed Oct 30, 2019 10:11 amI will have a look.
I think back in the day when I made them I hadn't got my head around eagle PCB software so used a package that didn;t produce gerbers. As such I don;t think I got any spares made. However it would be simple enough to copy the circuit on the paper about augmented cathode followers. Not as simple as populating a PCB, but not that hard. I used ACF-2 in the following doc. You could use a depletion mosfet in the tail of the LTP.

https://www.desmith.net/NMdS/Data/955_A ... lowers.pdf
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#1155 Re: 833A (GU-48) Amp

Post by pre65 »

Thanks Nick.

On that ACF2 circuit is

A) The input direct from preamp ?

B) The output to 833a grid ?

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