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Old 14th May 2016, 10:12 PM   #1
ericj is offline ericj  United States
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Default Sanity check? HV supply for 125wpc sweep tube amp

I mentioned a while back that i think i would like to try and build a 125wpc stereo amp based on pete's (almost) universal driver board, some big sweep tubes, and some of the solutions applied to making big power versions of the Big Red Board amp.

I know I need a pair of 300-ish volt supplies, and tightly regulated 150v supply for the screens.

My plan is to etch a pair of power supply boards, one for each channel. At the moment i have a pair of big industrial control transformers which can supply a bunch of 300vac each. I'd be using those plus an antek toroid for the heaters, and a small dual 36v transformer for the bias supply. Bias supply is handled by the universal driver boards, so that isn't reflected in my schematic.

So, shamelessly copied from the big red board, here's what I have come up with so far:

Click the image to open in full size.

Should the secondaries of my 300v trafos be tied together at pins 2 and 3 of P1? I suspect that they should be.

I am also curious if capacitor values should be increased, since the big red board was originally specified for much lower output.

Am i on the right track?

Thanks for any input.

Last edited by ericj; 14th May 2016 at 10:16 PM.
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Old 21st May 2016, 09:36 PM   #2
ericj is offline ericj  United States
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Click the image to open in full size.

Using 35mm footprint for the B+ capacitance so i can fit whatever i need to fit.

I've got a dozen or so 8"x4" sheets of copper clad FR4, thus the formfactor.

Managed to do this with just the one top side jumper.

I'll have to go through this again once or twice before i etch. There's got to be a better way to use wide traces with to220 parts, for example, and I might need to adjust the pads for the big caps, since I'm not sure that footprint really works for a snap-in cap.

I don't really like that long trace at the top. I might ditch the 5 position terminal block and go with a 2 position for B2 and screen supply, and a 3 position for ground, heater reference, and B1. I could delete my 0 ohm resistor at that point, too.

Last edited by ericj; 21st May 2016 at 09:46 PM.
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Old 21st May 2016, 09:52 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by ericj View Post
Should the secondaries of my 300v trafos be tied together at pins 2 and 3 of P1?
The two transformer secondaries must NOT be connected together, because two of the supplies
are stacked in series. Both C1 and C2 will likely need to be several times larger for a 125W amplifier.

It may be better to use three identical circuit blocks side-by-side for this layout,
leaving some extra room for the Zeners in the screen supply block.

The two series supply blocks should be adjacent, with the grounded series supply in the center,
and with the screen supply on its other side, nearer the output common.

Last edited by rayma; 21st May 2016 at 10:21 PM.
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Old 21st May 2016, 11:02 PM   #4
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Why only one screen supply? Are you aiming for individual monoblock type channel separation, or happy to have some interaction?

Have you done a PSUD2 simulation of say the max continuous loading on the basic rectifier/filter to determine the ripple level, and then tried to settle on a fixed output regulated voltage that is suitably lower than the min raw B+ (due to ripple) and including mains voltage tolerance?

Have you then worked out the max power dissipation in your regulator FET when the raw B+ is at max due to mains tolerance, and the loading is max?

That would at least indicate if your FET choice and heatsinking is practical, or as rayma highlights you may want to do some redesign of the capacitance needed (and hence pcb layout).

Last edited by trobbins; 21st May 2016 at 11:06 PM.
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Old 21st May 2016, 11:11 PM   #5
ericj is offline ericj  United States
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Quote:
Originally Posted by rayma View Post
The two transformer secondaries must NOT be connected together, because two of the supplies
are stacked in series. Both C1 and C2 will likely need to be several times larger for a 125W amplifier.

It may be better to use three identical circuit blocks side-by-side for this layout,
leaving some extra room for the Zeners in the screen supply block.

The two series supply blocks should be adjacent, with the grounded series supply in the center,
and with the screen supply on its other side, nearer the output common.
Whether or not the secondaries should be in series was not clear. It's an easy change to make.

I know that the caps may need to be much larger than the original 47uf in the schematic. That's why the footprint is big enough to accommodate 470uf caps.

I'm unsure if i can put the screen supply at the end of the board in a clean way. Putting it in the middle was relatively easy.

Quote:
Originally Posted by trobbins View Post
Why only one screen supply? Are you aiming for individual monoblock type channel separation, or happy to have some interaction?

Have you done a PSUD2 simulation of say the max continuous loading on the basic rectifier/filter to determine the ripple level, and then tried to settle on a fixed output regulated voltage that is suitably lower than the min raw B+ (due to ripple) and including mains voltage tolerance?

Have you then worked out the max power dissipation in your regulator FET when the raw B+ is at max due to mains tolerance, and the loading is max?
I'm attempting to go off of what people have done on big builds of the "big red board", but i want to use pete's universal driver boards.

The advice i got was to stick with the <400v supply for the driver board, and have a higher voltage supply for the output tubes.

I plan on two of these boards, in a semi-monoblock configuration. The channels will share power transformer iron but nothing else.

I have not worked out the max power dissipation in the regulator FET. Allegedly at least a dozen 125w versions of the big red board have been built, and this fet seemed to do the job.
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Old 21st May 2016, 11:23 PM   #6
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Click the image to open in full size.
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Old 21st May 2016, 11:24 PM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by ericj View Post
I know that the caps may need to be much larger than the original 47uf in the schematic.
That's why the footprint is big enough to accommodate 470uf caps. I have not worked out
the max power dissipation in the regulator FET. Allegedly at least a dozen 125w versions
of the big red board have been built, and this fet seemed to do the job.
A 470uF in place of the 47uF should be fine, but verify this. The FET's temperature depends
on the heatsink, current, and voltage drop, and should be designed to be within the margins.
The heat sink used is not likely to be very low in thermal resistance, though.

Last edited by rayma; 21st May 2016 at 11:27 PM.
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Old 21st May 2016, 11:34 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by ericj View Post
Click the image to open in full size.
I would make the pads for the large electrolytic capacitors larger in diameter,
more like the size of the pads for the heat sink mounting pins.
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Old 21st May 2016, 11:35 PM   #9
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Quote:
Originally Posted by rayma View Post
A 470uF in place of the 47uF should be fine, but verify this. The FET's temperature depends
on the heatsink, current, and voltage drop, and should be designed to be within the margins.
The heat sink used is not likely to be very low in thermal resistance, though.

So far, this is the heatsink I'm looking at:

530002B02500G - Board level heatsinks

Also keep in mind that there is one of these boards per channel.
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Old 21st May 2016, 11:40 PM   #10
ericj is offline ericj  United States
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Click the image to open in full size.

R10's previous alignment was a holdover from the first layout i posted . .
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