Why don't you ask Toroidy ?
KT88 schematics exist in hundreds, just take Dynaco MkIII as one. IT may not
be perfect with Toroidy transformer, that's why you should ask them.
Sorry we have not replied sooner to your cordial question. The suggestion to ask Toroidy is good; I would like to know their answer too.
But I respect your question and the answer is not very simple. It depends - on how much you know, how much experience you have with building high voltage amplifiers, and how much you wish to spend.
I assume you have some experience, and your are looking for a design that provides good “bang for the money”, as Toroidy transformers do.
My suggestion would be a classic Mullard design - a voltage amplifier (12ax7, 12at7, 6/12sl7, or 6/12sn7) with a 6/12sn7 LTP and fixed biased KT88s. Beyond that you could find a large number of designs with Solid State current sources, regulated screen designs etc., followed by transformer connected amplifier stages, but in my opinion you will be better off starting with a simpler design. Keep in mind that most modern music sources have higher output than in the classic tube period, so you don’t. eed as much gain and feedback as was common in the 50-60s. If you search on “kt88 mullard amplifier schematics” you will find many designs. One of my favorite “modern designs”, although I have not personally built it, is:
https://www.triodedick.com/Bill/schema_versterker.GIF
Good luck and please follow-up with you progress.
Yes, asking Toroidy for a proven schematic is a good thing to try.
Otherwise:
Take a good push pull amplifier that has lots of negative feedback, proper compensation network(s), etc., and works very well.
Now, replace the output transformers with one that has some different characteristics:
High frequency phase shift
High frequency rolloff
Primary inductance
Different core saturation at a given low frequency and amplitude.
In that case it may, or may not, work without some changes in the amplifier's
negative feedback circuitry, and/or its low frequency poles.
Be prepared to adjust as necessary.
Your mileage may vary (YMMV).
Otherwise:
Take a good push pull amplifier that has lots of negative feedback, proper compensation network(s), etc., and works very well.
Now, replace the output transformers with one that has some different characteristics:
High frequency phase shift
High frequency rolloff
Primary inductance
Different core saturation at a given low frequency and amplitude.
In that case it may, or may not, work without some changes in the amplifier's
negative feedback circuitry, and/or its low frequency poles.
Be prepared to adjust as necessary.
Your mileage may vary (YMMV).
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Why would that be weird? Toroidy make transformers, not tube amps.
There are so many tube amp designs out there, there is simply no way to tell you "build this!". It's like if you bought a box of nails and now ask around what kind of house to build with the nails.
Weird indeed. Toroidy also manufactures tube amplifiers. They are sold with the Fezz Audio brand name. The KT88 push-pull model is called Titania. The user manual is available on the fezz audio web site; the schematic diagram is not included, unfortunately. On the last page of the KT88 Titania user manual, the CE declaration of conformity is signed by the Toroidy Transformatory owner.
That link shows a nice simple circuit. Just curious about the large difference in plate resistors for the LTP ECC99. 30Kfor the upper, approx 75K for the lower.
The lower parallel pair gives a load of about 37.2k vs. 30k for the upper load resistors. (1/Req=1/R1 + 1/R2)
Without a negative supply in the LPT tail the “undriven” side has lower gain, so some designers increase that load resistor. I agree it seems a bit high, but could be based on testing/simulating the design, which I have not done. Triode Dick is a clever designer, and without further investigation I will accept those values.
Edit: Old Hector got it before I could post.
Just for another option, the EL34 variant of the Baby Huey was designed around the EL34 but has been built with KT66, 6V6, and KT88 tubes (all with the appropriate adjustments). A KT120 variant has also been built if I recall correctly. There was a Group Buy for PCBs made for ease of building.
A KT88 variant was built using Edcor output transformers with a primary impedance of 4.2 k Ohms as well as Hammond 1650N OPTs and those are 4.3 k Ohms. Your Toridy transformers are probably spec'd out around that mark probably.
EL34 Baby Huey Amplifier
The EL84 Baby Huey is a successful and well documented amplifier here on diyaudio.com (gingertube started it all off I think) and the EL34 Baby Huey is right up there following along with it. Many examples have been built, there is good support in the thread, and the many modifications have been documented. Wouldn't hurt to take a look.
A KT88 variant was built using Edcor output transformers with a primary impedance of 4.2 k Ohms as well as Hammond 1650N OPTs and those are 4.3 k Ohms. Your Toridy transformers are probably spec'd out around that mark probably.
EL34 Baby Huey Amplifier
The EL84 Baby Huey is a successful and well documented amplifier here on diyaudio.com (gingertube started it all off I think) and the EL34 Baby Huey is right up there following along with it. Many examples have been built, there is good support in the thread, and the many modifications have been documented. Wouldn't hurt to take a look.
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