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spritzer
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updated power supply board with a pair of resistors for high voltage divider.

impedance of the divider needs to be 1M or more.

Awesome, thanks! It will be a tight squeeze in the case, but this might now be a drop-in replacement for my Exstata.

Just one last question...... what is the approximate voltage headroom required for proper regulation?

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Just one last question...... what is the approximate voltage headroom required for proper regulation?

AH yes, the age old question. About 12 volts. So an unreg of 412 volts.

So if you pick the transformer correctly you want about 440 volts with the load.

The 450v caps are rated 450v, 500v surge.

If your AC line is 10% high, then you are pushing it.

and if your AC line is 10% low, then you drop out of regulation.

Better is to go with the 500 volt caps which are 550 volt surge.

For a while the 500 volt caps were not available, but it seems people

are making them again.

Otherwise you have to go with the kgsshv supply which puts 2 x 450v caps in

series, for lots of headroom. This also increases the physical size a fair amount.

For the exstata you really want +/-300, so you set the zener strings for 300v

and then you have plenty of headroom with the 450v caps.

Edited by kevin gilmore
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updated power supply board with a pair of resistors for high voltage divider.

impedance of the divider needs to be 1M or more.

HE60 bias is 600V, so that should run fine on 580. Same as ESP950

If memory serves, the ESP950 sounded best at 580, at least on Marc's (now HPA's) hot rodded KGSS.

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Awesome, thanks! It will be a tight squeeze in the case, but this might now be a drop-in replacement for my Exstata.

Just one last question...... what is the approximate voltage headroom required for proper regulation?

Yeah, I love that casework, so why re-invent the wheel?

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AH yes, the age old question. About 12 volts......

For the exstata you really want +/-300, so you set the zener strings for 300v

Excellent. With the existing power supply and transformer the unreg DC is 335-340V for the 300V output, so again, a drop in replacement.

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And you have the nice benefit that the supply won't blow up if one of the amp boards goes south.

(or north if you live on the other side of the world)

And its also current limited, so if one of the amp boards decides to short, no flying parts.

I might be able to squeeze another .25 inch on the width if its that close.

Edited by kevin gilmore
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I might be able to squeeze another .25 inch on the width if its that close.

Heh, I've measured it up and would still be an inch over in either direction without major surgery, so don't make any modifications on my account!

No matter, my Exstata can keep being used as-is until it kerplodes. The money can go into some fancy casework for a 'Bate'.

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At the risk of making a fool of myself I'll play "Devil's Advocate" a bit:

1) The 6BL7 has a Miller Capacitance of around 100mmf. A triode-connected 7189A (and I assume a 6P14P-EV) has about 20mmf. Is this a concern using a 12AX7 with a 330K plate load as a driver and a grid resistor of 680K? Wouldn't something like a 5687 driving a 47K grid resistor be a better match?

2) Looks like the 6BL7 is operating at about [email protected]. Again comparing a 7189A , the 6BL7 plate curves look substantially less linear at this point.

3) As the output tubes of an Egmont, I had examples of 6BL7 that with the same cathode resistance, varied by as much as 50V between plates. Typical was about 20V. Section to section matching was quite poor among 20+ samples. Does this matter?

4) RV1-4 look like they will be exposed to full neg. rail voltage. The only pots I'm aware of that can withstand this sort of voltage level are the big 2.5W types.

Don't get me wrong, I like this circuit a lot. And I'm a big fan of the 6BL7, which I've used many times as a driver. I'm actually surprized that nobody makes a similar commercial amp. Tart it up a bit with some CCS plate loads and a fancy case and you've got a $3K. product.

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1) i can do a board for 7189a's. But its going to be bigger because its now 8 tubes instead of 6.

Also don't know what a 7189 is going to do at 800 volts. And its not a triode.

3) The bias pots take care of any differences. Self bias for these tubes rarely work, same

as 6ca7 which mikhail tried to use with self bias.

4) bias pots are between -430 and -400 volts.

Edited by kevin gilmore
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The pots won't be a problem at these voltages and with this setup (same as on the ESX) you can adjust each output to 0VDC.

These improvements can be added to the Egmont as well plus some feedback would make it quite good. Pretty much the improvements I did to my ECC83/6S4A version.

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