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    KT88 power and output transformers

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    CoffeeTones

    Posts : 2
    Join date : 2010-08-03

    KT88 power and output transformers

    Post by CoffeeTones on Tue Aug 03, 2010 12:17 pm

    Hi,is the A431 OT and P782 PT, from Mark 3 and Mark 2 amps needed to use the KT88 tubes and would tube swaps be possible with the B+ these would produce?

    Any other alternatives?

    Thanks

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88 power and output transformers

    Post by Bob Latino on Tue Aug 03, 2010 7:14 pm

    CoffeeTones wrote:Hi,is the A431 OT and P782 PT, from Mark 3 and Mark 2 amps needed to use the KT88 tubes and would tube swaps be possible with the B+ these would produce?

    Any other alternatives?

    Thanks

    Hi,

    What you need to use KT88 output tubes is a POWER transformer that is capable of supplying enough current to a KT88 without overheating. A Mark III's power transformer would have no trouble using KT88 output tubes. To run 6550's in a Mark II the only thing that Dynaco recommended (they don't mention KT88's but the same thing should apply) on page 12 of their Mark II manual was to raise the bias point to 1.68 VDC if you run 6550's. So - No problem IMHO in running KT88's in a Mark II.

    The problem arises when someone plugs KT88 (or 6550) tubes into an ST-70. This WILL WORK but the original PA-060 power transformer as found on the stock ST-70's will run very hot (at the very least) or cause possible damage to the original PA-060. Upgraded PA-060's as found on modern Dynaco kits from me or from Dynakitparts have a 5/8" higher stack lamination and use slightly heavier gauge wire. The moral of the story is if you want to run KT88's or 6550's in an ST-70 get an upgraded power transformer for your ST-70.

    Bob

    4MarkNY

    Posts : 18
    Join date : 2009-10-13

    KT88 power and output transformers

    Post by 4MarkNY on Mon Feb 13, 2012 6:28 pm

    The moral of the story is if you want to run KT88's or 6550's in an ST-70 get an upgraded power transformer for your ST-70.


    I'm not sure here Bob, if you mean that to accomplish this you need a newer model PA-060, or if you mean that you should use the p-782 power transformer.
    I'm finally around to rebuilding this second ST-70, after having the VTA board and components more than a year.I'm installing the brawnier filter cap too. I thought it would be nice to run 6550s in it, as I spare those for the Leslie tone cabinet....

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88 power and output transformers

    Post by Bob Latino on Mon Feb 13, 2012 6:51 pm


    Are you asking if the P782 POWER transformer from a Dynaco Mark III can be used on an ST-70 to run KT88 tubes ? Not really possible for 3 quick reasons I can think of.

    1. The P782 won't fit the ST-70 chassis mount holes.
    2. The P782 has one 6.3 volt AC line and is designed to create filament power for just TWO KT88/6550's. I don't know if it has enough reserve to run FOUR KT88's.
    3. The secondary windings on a P782 are about 410-0-410 VAC to create a B+ near 500 VDC on a Mark III. The ST-70 has secondary windings of 360-0-360 to create a B+ of maybe 440 VDC. If you use a P782 in an ST-70 the B+ will be too high.

    You can *try* running 6550 or KT88's in an ST-70 with an original PA-060 power transformer but I don't recommend doing this. If your original Dynaco PA-060 runs fairly warm running EL34's than it will run even warmer with 6550's. Any original PA-060 with a 1.5 inch stack lamination will be stressed above and beyond what it was designed to do.

    My recommendation is > If you intend to run KT88/6550 tubes in an ST-70, get an upgraded NEWER PA-060. All the newer PA-060's have a higher stack lamination (larger heat sink) and great inherent current delivery abilities (heavier duty windings).

    Bob

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: KT88 power and output transformers

    Post by burnedfingers on Mon Feb 13, 2012 10:57 pm

    Actually the 70 was designed to run 6CA7's/EL34's that draw 1.5A ea. The 6550's and KT88's draw 1.6A ea. Now, the amp was also designed to run a preamp that draws its power from the front octal sockets. So, when we figure the current draw that isn't being taken from the front end the amplifier is capable of running KT88's or 6550's. Mine is currently running KT88's and (4) 6Sl7's without a problem. The typical "the transformer gets hot" comment is total rubbish designed to sell the aftermarket transformer. I have run a number of modified 70's and NEVER lost a transformer. Sure the transformer runs warm and I have logged the transformer temperature over extended runs and never come up with a reading that would cause me to loose any sleep over.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: KT88 power and output transformers

    Post by burnedfingers on Mon Feb 13, 2012 11:03 pm


    Are you asking if the P782 POWER transformer from a Dynaco Mark III can be used on an ST-70 to run KT88 tubes ? Not really possible for 3 quick reasons I can think of.

    1. The P782 won't fit the ST-70 chassis mount holes.
    2. The P782 has one 6.3 volt AC line and is designed to create filament power for just TWO KT88/6550's. I don't know if it has enough reserve to run FOUR KT88's.
    3. The secondary windings on a P782 are about 410-0-410 VAC to create a B+ near 500 VDC on a Mark III. The ST-70 has secondary windings of 360-0-360 to create a B+ of maybe 440 VDC. If you use a P782 in an ST-70 the B+ will be too high.

    You can *try* running 6550 or KT88's in an ST-70 with an original PA-060 power transformer but I don't recommend doing this. If your original Dynaco PA-060 runs fairly warm running EL34's than it will run even warmer with 6550's. Any original PA-060 with a 1.5 inch stack lamination will be stressed above and beyond what it was designed to do.

    My recommendation is > If you intend to run KT88/6550 tubes in an ST-70, get an upgraded NEWER PA-060. All the newer PA-060's have a higher stack lamination (larger heat sink) and great inherent current delivery abilities (heavier duty windings).

    Well 1. is pretty easy to fix with a drill.
    2. is easy to figure out...load the filament line and plot the outcome.
    3. High B+ is not a problem...refigure the resistors to insure the proper plate voltages in the driver section.
    4. The dynaco was designed to run a preamp off the front octal sockets so running 6550's or KT88's can and does work.
    5. Extra money to burn? By all means purchase the big transformer.

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: KT88 power and output transformers

    Post by GP49 on Tue Feb 14, 2012 12:40 am

    From the number of burned up power transformers I encountered on Stereo 70s when I was in the repair business, I'd say the original PA-060 may have been INTENDED to run a full complement of tubes plus two PAM-1 preamps; but that the actual implementation of the transformer fell short. Transformer failures are NOT a figment of the imagination.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: KT88 power and output transformers

    Post by burnedfingers on Tue Feb 14, 2012 7:49 am

    As a repair tech myself that has thousands of hrs on the bench I can attest to a number of transformers spoiled because the owner tried to operate the amplifier with a fault. Usually a bad power supply cap. You unfortunately are no judge of how the amplifier was operated before it hit your bench and thus cannot come forth with an analysis based on something you simply do not know.

    I would be more than happy to supply real honest to goodness infrared temperature readings of the transformer under different operating conditions.

    The last eight or so 70's I have sold have been modified with an honest to goodness power supply. Power supply add on board, diode mods, different front end, individual tube bias adjustment, three wire power cord, and a single triode switch. They are all still running and the owners have no problem running 2)6sj7's and 2) 6sl7's some boards running 6SL7's and 6SN7's plus KT88's or 6550's.
    Now, instead of just pointing the finger I have actual experience running the different modifications. Yes, I had one large power transformer on an abortion of an amp I bought that used 4) 6dj8's in the driver board. I ran cool alright and for those faint at heart go for it.

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: KT88 power and output transformers

    Post by burnedfingers on Tue Feb 14, 2012 8:24 am

    After having posted this I remembered an email arguement that I got into with the designer of a board that was out a few years ago. He absolutely claimed that anyone purchasing his board needed to purchase a aftermarket transformer. I send him some information that I had gathered and he finally came back with an answer. He said he promoted the idea of purchasing an aftermarket transformer simply to keep anyone from trying to sue him if they experienced any transformer problems. His board ran 3) 6SN7's he is no longer in business but an amplifier that i built using his board is still operating with the original transformer.

    Westy56

    Posts : 58
    Join date : 2011-05-09
    Location : Albuquerque

    Re: KT88 power and output transformers

    Post by Westy56 on Tue Feb 14, 2012 12:12 pm

    burnedfingers wrote:As a repair tech myself.

    How close are you to Albuquerque? Smile

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