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Old 8th January 2009, 11:06 AM   #11
zman3 is offline zman3  United States
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BTW - at the risk of sounding like a complete goober - what is the purpose of the two 470k resistors that lead from the plates of the phase inverter to the 1k output grid resistors (these shunt the 0.25 uf caps)? I'm not sure I've seen this topology before.

Thanks yet again...
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Old 8th January 2009, 05:43 PM   #12
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Originally posted by zman3
BTW - at the risk of sounding like a complete goober - what is the purpose of the two 470k resistors that lead from the plates of the phase inverter to the 1k output grid resistors (these shunt the 0.25 uf caps)? I'm not sure I've seen this topology before.
Looks like a DC step network to reduce the high DC that would otherwise appear at the grids of the finals if it were a pure DC coupling.
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Old 8th January 2009, 07:04 PM   #13
zman3 is offline zman3  United States
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Removing the NFB loop worked! I immediately noticed a difference upon power-up.

That nagging high plate voltage remains a problem, however. My main idea that it's running a bit hot comes from another version of the same schematic, this one from the Acrosound 1955 catalog:

Click the image to open in full size.

Note the 450 volts entering the OPT. Those shunting resistors I spoke of earlier have increased to 1 megohm, and the balance pot is omitted entirely (they must have used one a well-matched pair of tubes!).

On my setup with ~500 volts Va, my plates begin to exhibit the red glow o' death with the bias set to -66 V, which is a far cry from the -48 V called for on the original schematic. Would a 400-ohm power resistor in series with the 4H choke be a good way to achieve that voltage drop?
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Old 8th January 2009, 07:10 PM   #14
zman3 is offline zman3  United States
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...Oooops - Here's the schematic I meant to post!

Click the image to open in full size.
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Old 8th January 2009, 08:17 PM   #15
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Go to MOSFET Follies and scroll down to the "B+ Reducer", that'll be better than a series dropping resistor, which will cause a tremendous degradation of voltage regulation.
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Old 8th January 2009, 09:31 PM   #16
zman3 is offline zman3  United States
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Thanks, Miles!

I didn't bookmark that page - I saved it! That is an excellent little tutorial on how to use such solid-state devices for hollow-state purposes.

I've seen schematics on other websites that exhibit strange hybrid topologies. This page gave me a really good insight into just what may be going on in some of those.

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Old 8th January 2009, 10:41 PM   #17
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Couldn't you just reduce the value of the input cap in your PSU to back the B+ off a little?
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Old 9th January 2009, 02:24 AM   #18
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Couldn't you just reduce the value of the input cap in your PSU to back the B+ off a little?
Yes, that's the way I'd prefer to do it, too.

Quote:
Looks like a DC step network to reduce the high DC that would otherwise appear at the grids of the finals if it were a pure DC coupling.
This is a way of getting the benefits of DC coupling without having to cope with the crazy voltages that can result. The coupling cap itself is not actually eliminated but the effect is almost the same as if it had been. This can tame a Williamson, or any other amp design with 'too many' coupling caps for global NFB.
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Old 9th January 2009, 12:22 PM   #19
zman3 is offline zman3  United States
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Ok, so now I've got a couple of options on B+ reduction.
  • Dropping the value of the input cap: My filter input cap consists of two 22 uf 450V caps in series, with 100k 1W equalizing/bleeder resistors. How far down can I go before my filter essentially becomes a choke-input filter? Not that I'm worried, mind you; My choke is 4H, which is within the ratings of the 5V4 I'm using.
  • MOSFET/zener reduction: If I go this route, what specifics do I need to look for other than a high gate-source breakdown voltage? Mouser has these for 2.95 each:
http://www.toshiba.com/taec/components2/Datasheet_Sync//77/2982.pdf

IIRC these are rated for 1000V breakdown at 90W dissipation. Can I mount one of these to the chassis for heat dissipation, or will I have to use an isolated heatsink?

My apologies for the endless barrage of questions. With tubes I know just enough to be dangerous; when it comes to solid-state, I'm a veritable babe-in-the-woods.

Thanks again...
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Old 10th January 2009, 03:46 AM   #20
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My filter input cap consists of two 22 uf 450V caps in series, with 100k 1W equalizing/bleeder resistors. How far down can I go before my filter essentially becomes a choke-input filter? Not that I'm worried, mind you; My choke is 4H, which is within the ratings of the 5V4 I'm using.
I wouldn't go much lower than that. If it hasn't been mentioned already you should download PSUD2 http://www.duncanamps.com/psud2/index.html and get to know it well.

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IIRC these are rated for 1000V breakdown at 90W dissipation. Can I mount one of these to the chassis for heat dissipation, or will I have to use an isolated heatsink?
The size of the heatsink depends on how much power is being dissipated by the FET. Look for something in a TO-220FP package. Its electrically isolated and can be chassis mounted with no worries. A quick search turned up this http://www.st.com/stonline/books/pdf/docs/9701.pdf Solid state isn't my strong point so maybe someone else could recommend something more tried and true.
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