KT88 Push Pull Idea... Questions - diyAudio
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Old 12th March 2012, 11:49 PM   #1
mltube is offline mltube  United States
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Default KT88 Push Pull Idea... Questions

Hello all, I have recently picked up a couple Hammond tone cab amps (parts)
And would like to know if anyone had some advice here.
The PS powered (6)6BQ5's, (1)12BH7, (2) 12ax7's, (2)6au6's, and one 12au7.
I am not sure as to the model of the tone cab.
The PS units are quite beefy and uses (2)5U4's for rectification.
Also I have all the matching OPT's two of them are really large, but they were from the Bass section... 2-6BQ5's each they are 8k p to p, but I thought for their size, I might try with kt88's triode strapped. I do not require huge power output, but always wanted to try this.
I am not sure if the power supply units powered anything else besides the tubes I have mentioned, so I am guessing 300-350ma on the high voltage leg, and 5v 6amp on the rectifiers, and 7-8amps at 6.3v since the tube draw is 6.66 amp.
Here are some photos and if you would, please let me know what you think.
I just do not know if the Large OPT's will handle the current of the KT88's, but I am willing to try .
The price was right so if something burns up in the experiment it will not be a big loss, but if it proves worthy, it may be sweet. I did not have the big money for fancy KT88's, so for the experiment I bought a matched quad of Sovtek's... Just under 100 usd.

Do you think that it may be a better choice to make 2 mono blocks, since I have to identical power transformer's? It will be a bit of over-kill, but will have more than enough juice to fly. Also, I have six smoothing chokes Identical to what is in the PS unit, More overkill, but will be hum free... Another tube considering for this is the 6dz7, as I have been sitting on a dozen for a few years, and they sound good. Would only need one power unit for that build tho. And the OPT should be close enough since they were for 6bq5's.
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Last edited by mltube; 13th March 2012 at 12:01 AM.
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Old 13th March 2012, 12:12 AM   #2
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Check out this link, lots of relevant info....

loadmatch3-pp-triodes
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Old 13th March 2012, 12:23 AM   #3
mltube is offline mltube  United States
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Thank you boywonder, that is good info... I am going to think about this for a while before commencing. I am mostly concerned if the opt can handle the current. (no spec sheet to work with) Most of the hammond opts that I have used in the past hve proven to be somwhat overbuilt, so I think that it will be ok, but I do not want to ruin them on the first attempt.
Thank You for the information.
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Old 13th March 2012, 06:22 AM   #4
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It seems like a workable plan to me. The PSU trans will probably cope okay.
The OPT's might be a bit on the 'small' size,but I'd give them a shot anyways.
If you cook them you can order some Edcor/Hammond transformers for not-too-much,or salvage some from somewhere,etc.
I'll SWAG they'd probably be okay at up to 40ma or so? (ymmv,etc.)

Any idea what the B+/HV secondary voltage is? (I assume 325-ish V,or so?)
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Old 13th March 2012, 02:09 PM   #5
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Quote:
Originally Posted by DigitalJunkie View Post
I'll SWAG they'd probably be okay at up to 40ma or so? (ymmv,etc.)

Any idea what the B+/HV secondary voltage is? (I assume 325-ish V,or so?)
I hope the iron is capable of more than that...

at 325V/40ma trioded KT88's won't even be breaking a sweat.....13W total with about 9W class A power.

Sticking a few numbers into Broskies PP calculator software shows that 475V at 60ma into 8K load yields about 30W with the first 20W being class A. 400V at 60ma yields about 20W all class A.

I can stick other voltages and idle currents into the software if you are interested.
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