Kegger KT88 UL PSE?

whoaru99

Epic Member
After a fair amount of deliberation I think I'm going to forego further work on the Hammond H1C 6V6 PPP monos I've been tinkering with. Instead, convert them to KT88 UL PSE monos based on Kegger schematics/circuits.

Basically, all the main bits of the H1Cs except the oil capacitor and OPT will be repurposed. The OPTs I will use are the Edcor 25W CSXE 2.5K:8. Have a pair of those on the shelf.

Values and circuit look like I've got them compiled into a schematic correctly?

The voltages are what's predicted by PSUD for 75mA on (each) KT88 and 8mA on the driver. But, I think it will need higher value for cathode resistors at this voltage, yes?

KT88-PSE-UL.png
 
Yah I would go like say 560ohm on the KT88 cathode, and see how all the voltages shake out from there.
(my guesstimate if your supply is correct, you'll be 80ma per output including the screens, at 33w of diss)

That'll put yu at roughly 45v on the cathode, remember you'll drop 8-10v through the output tranny as well.
(and with single ended by nature of design, you'll be in class A all the way)

R17 I'd make that (2) 75k 2w in parallel there as well. An also drop R3 to 150K, make that 2w as well here.
 
Was also wondering about the 1.2k on the screens. Am used to seeing them lower value. Why 1.2k in this case?

Not like it really changes the cost by any material amount but would 1k be OK too, just to keep the p/n list a bit shorter?
 
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From the UL tap? I've been wondering the same thing. I didn't use it. Should I? What would it change, if anything?
 
It'll be easier I think to explain it with a drawing and a bit of simple math. :)

Basically you really do not want the screen with an higher voltage than the plate is,
But when you run UL output tranny that is what you get because of where the tap is.

Looking at the drawing, you see the plate gets the whole winding an the whole resistance.
The UL tap gets roughly 3rd of the winding and a 3rd of the resistance so less voltage drop.

(drawing attached)
So now we put our resistor in series with the screen, (an the screen drawing say 3.5ma)

With that 1.2k resistor in it, and 3.5ma from the screen you'll drop just an hair over 4 volts.

With the typical say 120ohm resistor an 3.5ma from the screen you'll drop hair over .4 volts.

So with our plate at 417v,
The 1.2k resistor gets us just under the 417v at the screen.
The 120ohm resistor gets us just under 421v at the screen. (leaving screen higher than the plate)

An yes this is something I tend to do more verse what many others do, they just leave screen high.
(just another tweak I run on SEUL designs, been many itterations, refinements/tests along the way)

And yes I'm estimating, with along the lines of what I typically see. (but you get the picture and the idea behind it there)
About an 8v drop from B+ to plate across the output tranny, will vary with different outputs, resistance an current drawn.
.
KT88-SE_UL_.GIF
 
Well that totally makes sense. Thanks. Guess I'll have to give it a try one of these days.
 
In my case I'm figuring about 440V a-k.

I was thinking about plotting a load line but a number of examples seem to indicate 6k is about right for 1 tube @ 450 a-k and Vg2=Va.

So, I thought 1/2 that or a bit less (my 2.5k) is pretty close for PSE at a bit less a-k voltage.
 
I'd love to see your progress, and completed project. I'm not quite ready yet, got a few projects to tweak, another Fisher to rebuild, and a vacation all in the way. But I've got another project idea, based similarly to yours. Like to build a KT88 PSE, possibly PPP, dedicated solely to bass. For all intents and purposes, a subwoofer amp I suppose. Meanwhile, enjoy!
 
I'd love to see your progress, and completed project. I'm not quite ready yet, got a few projects to tweak, another Fisher to rebuild, and a vacation all in the way. But I've got another project idea, based similarly to yours. Like to build a KT88 PSE, possibly PPP, dedicated solely to bass. For all intents and purposes, a subwoofer amp I suppose. Meanwhile, enjoy!

It's not going to be anything pretty out of the hole. Gonna try to make it work first in existing chassis before I go full bore on complete new layout.
 
That's what I thought I saw too, but it appears to be not too uncommon operating point for one tube at 5k or two at 2.5k?

I just wrote it off as something I must be misssing. Awaiting further comment from the collective expertise...
 
Lowering the voltage should be fairly easy in this one by using a smaller first cap. 3uf models what is listed, dropping to 2 or 2.5 reduces the voltage quite a bit as it moves closer to choke input PS.
 
With the voltages an supply you have posted, if it shakes out as you have predicted your right there..
(remember transformer loss an screen current as well, it all looks quite good to me here as it is now)
 
I need to get some parts ordered... :whip:

McShane seems to opine reasonably well about EH KT88s. Good enough for some "starter" tubes?

Also, looking intently at DC link film caps for the PS. They're basically for PC board mounting but I don't plan to etch a board. You guys use any of these? How did you mount them, a hunk of perf board or something on some standoffs?
 
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Yeah, good enough to get the project going and of decent function quality to use, but not necessarily $$$ TOTL in case something goes wrong. :)
 
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