"Mullard 5-20 KT88 PP blocks!

[Eli Duttman]

TM,

If you use any sort of choke I/P filtration, true or pseudo, you will have 2 big inductors on the topside of the chassis. Mount the toroidal power trafo on the underside of the top plate, where it can't be seen. An "acorn" nut makes things attractive, "upstairs".

A very easy way to bring the B+ rail voltage down somewhat is employing a true choke I/P filter and FWCT rectification with a pair of damper diodes. ESRC is selling 19AU4s for $1 each. You'll have to buy 10 (big deal). Wire the heaters in series and energize them with an AnTek model AN-1236.

The 1st inductor in a choke I/P filter gets the snot badly beaten out. HEFT is the order of the day. The 21 lb. Hammond model 193Q should do. "Ordinary" stuff is fine for the 2nd inductor.

[Bandersnatch]

hey-Hey!!!,
On the choke input choke, there are vintage swinger chokes to be had in the usual markets, but they're a bit rare. I've designed a swinger with standard E-I lams that should work well in this service w/o the nasty saturation buzz. The gap is a wedge, longer at one end of the E than the other to give a slow and gradual saturation of the core as flux( both AC and C ) increases.
cheers,
Douglas

[TubeMack]

Originally Posted by Bandersnatch

hey-Hey!!!,
I'd suggest the Hammond 1650R 5k a-a output TX for starters. Very nice output, and in comparison to *MANY* custom and vintage units it came out ahead...let alone it isn't very expensive...

I'd also suggest dropping the power a few Watts to 45-50 and cut B+ to ~400V. It is much better to run the KT88 at 80-85 mA/tube without getting so close to maximum ratings the higher B+ would expose them to.
cheers,
Douglas

Well I didn't expect the Hammond recommendation! Hmmm. Excluding cost, would this Hammond be a better preforming choice than a standard Electraprint with the same specs? I won't be buying Super expensive outputs from Japan, but as these will be my main amps for the long haul, I'm fine with spending a little extra on such an important componet.

[Bandersnatch]

Originally Posted by TubeMack

Well I didn't expect the Hammond recommendation! Hmmm. Excluding cost, would this Hammond be a better preforming choice than a standard Electraprint with the same specs? I won't be buying Super expensive outputs from Japan, but as these will be my main amps for the long haul, I'm fine with spending a little extra on such an important componet.

yes.

That particular Hammond is *THAT* good. Totally put the brakes on building clones of the Peerless S-271-S I acquired and had torn down( shuddering while remembering the cost involved with that exercise ).
cheers,
Douglas

[Bandersnatch]

hey-Hey!!!,
I'd also suggest a different circuit...but check out Dave's post here:
Tube DIY Asylum - Hammond 1650R - Salectric - December 11, 2009 at 04:08:37

He's built an amp with them and could tell you how it worked for him. Straying from your current design is not what I am suggesting, at least not yet...
cheers,
Douglas

[Ultima Thule]

Originally Posted by TubeMack

Wow, I had all those same questions!

Please do join me in this build. It will make things easier I believe... I've spend a year researching, and believe this is the topology to go with for a "standard" KT88 PP amp. I also thought some good arguments were made for the driver tube choices. As for the schematic, i'm still unsure of the various stage voltages, (there was some debate over 520v), and if the CCS needs altering (cascode mosfet). I think all this was later changed by Zanfir, but the later schemetic is missing? If the 520v is needed for 60W output, i'd be inclined to stay with it.

Hey TubeMack,

ok I'm in too, let's build the nicest KT88 PP around!
Reread your first post and the goals for the project is quite similar for what I have lined out in my thoughts, even been thinking of mono-blocks here too!
I didn't aim for so high power with these tubes though it's probably not impossible but I would like to not run them so hot, I guess I would be happy if I get even lowish 30-40 Watts but maybe 50 is quite a sensible compromise and it's just 1 dB here or there and isn't really hearable.

Transformers,

agree we should go for something good but as you also said within reasonable costs as some are very expensive, but still the transformer is a very big part of the sound quality so should be high on priority as it's something one doesnt swap out after the amp is built.
I would prefer toroids through-out for several reasons because of the great bandwidth with toroids and that it's very easy to get (ironically though!) even with standard winding teqniques, the other reason is very low magnetic stray field meaning surrounding components will pick up less hum especially good thing in the power supply transformer but yet good magnetic connection between primary and secondary meaning less stray inductance especially important for the OPT. OTOH toroids doesn't have the "vintage sexiness" as ol' EI cores but square shaped cans can be found of course which is what I will go for if I use toroids.

Found one supplier of toroidal OPT's
Antek - Your reliable source of transformers, power supplies, and more.
eBay Store - Antek inc: Transformers, Enclosures, Power Supply
but they have very few models which is pity as they have a 70 Watt and 8 lb model which would have been on spot but it has a very low primary input impedance of around 2k and that's way too low I guess as I haven't seen anything near when it comes to KT88 PP amps, well I sent an e-mail inquiry about higher input impedance to them but just recently read from the very few post on DiyAudio regarding Antek that some have not had any good luck communicating with them. A Polish maker replied today they don't make output transformers, I truied to tell them I can give the winding specification but it seems the message didn't went through.
Hammond 1650R was mentioned, of some reason I got a feeling Hammond is not the pick from the very little I have read, like this thread: Japan vs US output tx read post #15. The good part is I found out I can get these locally to a reasonable price, but would still like to get more FB on Hammonds. Douglas link looks promissing so will have an eye how that thread develops.

Well, there's some food for thoughts for now, even if we wouldn't agree on all details I think we should still stick together and make it through, this is going to be a great thing and I think I won't build so many tube amps in my life(?) so this one is going to be something extra so I will for sure beside building an excellent amp also add som bling-bling as well to its aesthetical appeal. Oh, and don't worry about electronics/electricals and such things, I am a 'toob-noob' too but have some knowledge of electronics and I would be happy to give a helping hand as much as I can and I'm sure we will get lot more help from the forum members too.

EDIT: oh, and for the record if I didn't say it Option 1 looks like a great base to build on and I'm sure we can eventually add some further goodies and improvements!

Cheers Michael

[Ty_Bower]

Originally Posted by Bandersnatch

I'd suggest the Hammond 1650R 5k a-a output TX for starters. Very nice output, and in comparison to *MANY* custom and vintage units it came out ahead...

Any comments on the smaller Hammond 1650N? I've read good things about it, but never tried one myself.

[TubeMack]

Here are the related threads if you want to read up.

Help modifiying schematic

My new project, 2 x KT88 60W amp

Help with amp layout


After reading more, I would like to lower the B+ to the KT88's to 460V. Also, I think I'd rather stick with traditional vertical Trannies @ this time. As soon as at can get the Specs nailed, i'll purchase, or order custom from Electraprint.

As to the PS, I'd really love to keep it straight foward, and simple. I.E. diode bridge, single choke, caps, and resistors. As long as it performs well, i don't want to get too involved here. I'd therefore like to size the PS trans, just right to achieve this. Ultima Thule, How do you feel about 460V?

[Eli Duttman]

TM,

For a PSU with CLC filtration, you will not get a better value than the AnTek model AN-4T360. The rail will be in the 450 to 475 V. range, which is just fine for KT88s.

Be sure to use a CL150 inrush current limiter between the rectifier bridge and the 1st filter capacitor. 100% SS rectification allows the use of a large cap. in the 1st filter position. You can kill ripple freq. overtones such a large cap. generates ("hash") by inserting a LC section made of a high current RF choke and a 1000 pF. NPO ceramic or mica cap. between the large 1st filter cap. and the main filter choke. The RF component filter blackens the background nicely.

[TubeMack]

Eli, I like the sound of that last proposed PS. I'd like to move foward with that one. The rail voltage sounds good. The "Hash" filter sounds like a good option too!