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Old 23rd January 2017, 12:02 PM   #1
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Default Dual supply for plate current, grounding?

Hi,

I'm planning on rebuilding an old SS project (op amp preamp into a Hypex UcD180 class D module). I still want the Hypex power amp stage, but I'd like to build a tube preamp section.

I'd like it to be as compact (and cheap) as possible, so I'm trying to find a way to use the existing dual supply toroid (160VA +/-30VAC) powering the tube preamp.

My assumption is, that as the trafo gives 60VAC rail-to-rail, it should be possible to build a tripling/quadrupling rectifier to get a suitable plate voltage for the tube stage.

The part where I get a b it confused, is how to solve the grounding of an amp like this. The power amp module wants to be grounded at 0V center of the dual supply (which is also signal gound), but can I do that, and still use the full rail-to-rail output for the plate voltage?

Is this even possible, or should I just bite the bullet and get another trafo for the tube section (and a bigger enclosure)?

To clarify, the Hypex is connected like so:

Click the image to open in full size.
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Old 23rd January 2017, 12:26 PM   #2
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You can feed a Greinacher/Villard cascade multiplier from one half of your secondary winding only. So you'll need about eight cascade stages, or opt for an preamplifier operating at a lower voltage.

Instead of a Villard cascade you can choose a full wave arrangement. The number of the stages will remain the same, but component count will almost double:
https://en.wikipedia.org/wiki/File:F...rd_cascade.svg
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Old 23rd January 2017, 12:39 PM   #3
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An ECC83 will run happily on 65Volts. 40 on the anode. 30 V AC through a tripler will be adequate.
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Old 23rd January 2017, 12:48 PM   #4
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Quote:
Originally Posted by Kay Pirinha View Post
You can feed a Greinacher/Villard cascade multiplier from one half of your secondary winding only. So you'll need about eight cascade stages, or opt for an preamplifier operating at a lower voltage.

Instead of a Villard cascade you can choose a full wave arrangement. The number of the stages will remain the same, but component count will almost double:
https://en.wikipedia.org/wiki/File:F...rd_cascade.svg
Thanks! I thought about that, but how about supply noise? Will eight cascade stages need more filtering?
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Old 23rd January 2017, 01:03 PM   #5
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Quote:
Originally Posted by JonSnell Electronic View Post
An ECC83 will run happily on 65Volts. 40 on the anode. 30 V AC through a tripler will be adequate.
Thanks! That might work, but I was a bit worried about headroom. I would like to be able to use the full headroom of the power amp without cranking the preamp stage into (too much) distortion. And this depends on the plate voltage, right? Most schematics have 200-300 volts for the ECC83 stages.

Maybe I'll just have to do some simple prototype and see if it works to satisfaction.

EDIT: A rough calculation using 3*(30*1.41) (is this correct for a tripler?) gives 126V, which should be enough, yes?

Last edited by Fredde79; 23rd January 2017 at 01:09 PM.
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Old 23rd January 2017, 01:30 PM   #6
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A half wave tripler gives 3 * 0.707 * ac voltage.
There are a number of effects units on the market with an ECC83 running happily on a PP3 9Volt battery, even for the 12.6Volt heaters! Not a lot of headroom and no cathode resistor with a 10M grid resistor.
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Old 23rd January 2017, 02:05 PM   #7
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Quote:
Originally Posted by Fredde79 View Post
Thanks! I thought about that, but how about supply noise? Will eight cascade stages need more filtering?
Not more than usual, as you have to expect 'common' 50 Hz hum from the half wave version, and 100 Hz from the full wave, respectively. You'll need big enough electrolytics in the multiplier, so look for 1000F at least.

In your case, I'd prefer a 6922/ECC88/6DJ8 valve over the ECC83. They once were dedicated for cascode use, thus can be operated at lowish plate voltages easily. And, due to their frame grid construction, they're low noise and high gm, too!

Best regards!
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Old 23rd January 2017, 02:09 PM   #8
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Quote:
Originally Posted by JonSnell Electronic View Post
A half wave tripler gives 3 * 0.707 * ac voltage.
Sure? In a Greinacher cascade, the first capacitor charges to Vp, the second one and all the following caps to Vpp.

Best regards!
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Old 23rd January 2017, 02:36 PM   #9
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Thanks for your replies!

I also thought about the more expensive but simpler solution to just order a custom toroid with secondary windings for something like:
2x30V, 200V, 6.3V

That way I would also get the heater voltage without any additional circuitry. But I'll have to get a quote for this, It might be pretty expensive.
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Old 23rd January 2017, 03:40 PM   #10
MelB is offline MelB  Canada
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Hammond makes a nice little transformer for what you want to do. If you have room. Put a doubler on that and you will have a very acceptable voltage.
See attachment.
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File Type: pdf 262A12.pdf (131.8 KB, 11 views)
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