KT88 and Hammond Transformer Schematics

[jbldriven]

Is there anyone out there that has built 60 watt per channel KT88 amps using Hammond output transformers 1650n or 1650p. I am looking for proven circuitry and also need information on the line input transform in regaurds to voltage, do i use 400-0-400 and at what milliamp. If there is nothing existing available, is there any one who is willing to produce a design. I want to use hammond input and output with JJ kt88. I will be building 4 stereo units. I hope there is someone who can help. I sent here from the lansing forum, the moderator said this is the place where tubes experts reside. Don't make me go to china or japan

[Hojvaelde]

Hello

I are nearly finish building a Dynaco Mark III pp amplifier.

I an using a 1650N trafo as OPT.

My B+ is close to 500 Volts.

The schematic you can get at www.triodeel.com/schindex.htm

Regards

Benny

[aletheian]

Well, I'll let the experts do the talking, but i have built a couple ultralinear KT88 amps with the 1650n... have on right in front of me actually. Somewhere around 450v on the plates idling around 55mA or so per tube I think puts out about 60 watts. Ned has some classic design posted over at triode electronics: http://www.triodeel.com/schindex.htm That might be a good place to start...depends on what you are driving with though.

[aletheian]

Originally posted by Hojvaelde
Hello

I are nearly finish building a Dynaco Mark III pp amplifier.

I an using a 1650N trafo as OPT.

My B+ is close to 500 Volts.

The schematic you can get at www.triodeel.com/schindex.htm

Regards

Benny

There you go... doesn't get much more tried and true then a Dynaco...

[Sherman]

The B+ required and the current required depend on the design. A Citation II runs an idle current of about 100mA per tube, other designs I've seen go as low as 60mA per tube.

Voltages on KT88 designs are often between 400 and 500.

A 350-0-350 power trafo should give a B+ of 400 to 450 volts with tube rectification, higher with solid state. I'm using 350VCT trafos with my SE KT88 amps with a 5U4G tube rectifier and have a B+ of about 425 volts.

[jbldriven]

I have looked at the dynaco III amp circuits. I am trying to find a proven schematic with Hammond power tranformer complete with model number. I want to use the 1650p as the output. I look on hammond charts and see 350-0-350 or 400-0-400 volt transformers but i am not sure what amperages. What is the crossreference number for the dynaco III amp. I don't know enough to design engineer the circuits. Is there someone i can contact to have the circuits designed. I can't belive that there aren't circuits availble from hammond or someone who has built amps with these products. Where have all these transformer been used in the past?

[Sherman]

Originally posted by jbldriven
... I look on hammond charts and see 350-0-350 or 400-0-400 volt transformers but i am not sure what amperages. What is the crossreference number for the dynaco III amp...

Check out Angela Instruments (www.angela.com). Go to the 'Transformers' page for the specs on the Hammond iron, including current capability.

Triode Electronics has some Dynaco replacment transformers that are drop-in replacments size wise.

[jbldriven]

I have looked at the angle site and hammond. If the kt88 tube runs at 55 to 100 ma is the transform output based on 4 kt88 time the ma per tube eg 4 x 65ma=250 ma total. Therefore there is no 350-0-350 or 400-0-400 transform the will produce that current or is the ma 65 per push pull pair. Please be patient i have lot to learn.

[gingertube]

I've got a very nice prototype monoblock running using 4 off KT88 in Parallel Push Pull Ultralinear into Hammond 1650T trannies to give 135W.

Frequency response measured at 50W output was 65kHz at the high end and it was flat down to 22Hz at the low end but with some weird distortion setting in at around 18Hz due, I believe, to transformer core saturation. This would seem to stack up pretty well with hammonds claim of full power (120W) bandwidth of 30Hz to 30kHz

If interested then Go here:
http://www.evatco.com.au/project.htm

The 3rd project example down has a picture of the very agricultural prototype, the links for the schematic for the amplifier and the power supply and a link for the designers (me) comments. I did this amp around 2 years ago and rebuilding it to a decent chassis and doing the second channel is sort of gradually moving up the priority list. I also have PAT4006 Toroidal Output Trannies and tried them in place of the Hammond 1650Ts. The hughly higher primary inductance reduced lowe frequency 2nd harmonic distortion dramatically and the nominally 2100:5 impedance ratio compared to the Hammond 1900:4 required a bit more global feedback. Frankly I liked "Classic Tube Amp" sound that the Hammond imparted better than the pristine results from the Plitrons. My hip pocket liked the Hammonds a whole lot better too.

You will see that I used Cursio Audio Engineering (CAE) PCB1A boards for the front end. These boards were intended for upgrades of Dynaco MK3 Amps. The boards have a 6DJ8 (ECC88) cascode diffamp, with a current source tail and a high voltage regulator, These days you could also use 6N1P or probably even better, 6H30 Russian tubes (check pinout is suitable). I just added cathode followers to the output to drive 2 pairs of output tubes. For driving a single pair of output tubes the cathode followers would not be required and you could use the PCB1A boards as the whole front end.
P.S. I seem to recall that I had a problem with 6N1P not giving me quite enough gain.

Cheers,
Ian

[Sherman]

Originally posted by jbldriven
...the transform output based on 4 kt88 time the ma per tube eg 4 x 65ma=250 ma total. Therefore there is no 350-0-350 or 400-0-400 transform the will produce that current or is the ma 65 per push pull pair. Please be patient i have lot to learn.

No the 65mA is per tube so your calculations are correct. However you must have missed something when checking the specs of the trafos at Angela.

Go back to Angela and take a look at the 278CX. 400-0-400 at 465mA. That should do it.