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Herr Grau 22nd October 2009 05:06 PM

With a little help... KT88 PP UL Amp
Hi people,
it's been a while since I've built my last project and needed your help.
But at last I managed to build my Loesch-style Phono-pre.
Now it's time for a 'real' amp and after hours and hours of research I finaly setteled for this design:
Franz Wichlas KT88 UL PP Amp "Red Hot"
Schematic PSU

Firstoff: Does anyone see anything that should be done differently?

Then I got me some stupid questions (since propably noone will remember: I can build a circuit, but I merely understand the basics of electronics):
1. How much power do the unmarked resistors have to be able to tolerate? 0,5W? 1W?
2. I would like to use tubes for rectification (because of the 'inbuild' slow start and the so called "soft PSU" resulting from rec tubes) and build the amp as mono blocks, too. Other than the fact that I'll only need one 6,4V-lane per transformer: Can someone please re-write me the PSU-scheme?
(I just read at Emission Labs (here) that choke load should be preferred to capacitor load with rec tubes. If that's true, perhaps it's worth mentioning/implementing?)

I searched all over the net, but I couldnt find an explaination of how to calculate a filter circuit for a PSU. If you have something like that or could explain it, so that even I could understand, that would propably do it, too. But I guess, that'd be even more work... :headshot:

Thanks in advance guys,
regards from Germany,
Herr Grau

(P.S. whoever helps me earns free drinks whenever in my reach... :D )

SY 22nd October 2009 05:49 PM

Easy one first- for power supply filters, there is only one answer, PSUDII. It's a superb simulator. It can be found at the Duncan Amplification site.

As for that design... well... some people try to be too clever and sacrifice performance.

Herr Grau 22nd October 2009 06:02 PM

Which design would you recommend?
I'm searching for the definite amp for the next many years, so I'm more than willing to consider every option, of course searching for the best value. I just need a complete, working schematic which I can 'cook' the amp from.

If you don't think the design is 'the thing', than the PSU-work isn't necessary now.

I don't have too many wishes, I want a KT88 PP Amp with non-rare pre-tubes (personaly I do like the 6SN7, but thats not so important)(yeah, it shouldn't need an external pre-amp...) and tube rectification. It would be most terrific if the design would work with one of the standard supplied Sowter OPTs...

SY 22nd October 2009 06:19 PM

The 6SN7 is a fine tube for an input stage, as long as you run it the way it wants to be run for high swing. With this topology, the first stage has to swing the entire drive amount because the phase splitter is unity gain.

One thing to be aware of- if you have an ultralinear output stage with no feedback, the source impedance will be rather high. Some speakers are fine with that, some aren't.

I'm not terribly familiar with published diy designs, but I'd start by checking out Pete Millett's site and

Herr Grau 22nd October 2009 06:47 PM

I had checked the before mentioned sites, I checked too and googled quite a bit.

I found this one, but its a Williamson Circuit and I've read more than one post of you describing its immanent problems. So I threw that one out, too.

And of course I've stumbled across the Quad II which generaly was considered a very good amp. What about that one? How complicated would it be changing the circuit from KT66 and GZ32 to KT88, GZ34 and a senseful OPT? (schematic)

SY 22nd October 2009 06:50 PM

The Williamson is excellent as long as you change one of the RC time constants. There's a Dave Hafler article from the old Audio Engineering kicking around somewhere that has an explanation and a good circuit.

Herr Grau 22nd October 2009 07:42 PM

The only thing i could find is -> this
It doesnt say anything about RC time constants though, so I assume it's not the right thing. And even if it did, I'm very uncertain I could transfer it on the other circuit.

Anything to say about the Quad design?

Andrewbee 22nd October 2009 07:50 PM

For a KT88 there is also "Bill" from the Triode Dick's Page website.
look at the menu on the right for the TD's translation pages or something simailar for English. There is also a mono bloc version called (Trumpets blaring now) Mono Bill.

I have not heard it so cannot say what it sounds like but its 6SN7 input to ECC99 drivers :)

There are literally cazillions of circuits out there for KT88 PP (don't use Google Image search, you have been warned) but if you suffer from procrastination as I do, I would suggest only looking at perhaps half a dozen and then decide otherwise you will be forever comparing them against one another.

Good Luck

Gordy 22nd October 2009 07:55 PM


SY 22nd October 2009 08:11 PM

The article I had in mind appeared about a year later. I think it's in Audio Anthology- I'll check when I get home.

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