SET Asylum

Posted by Thom_M on May 13, 2008 at 22:05:09

In Reply to: RE: Thinking of converting PP KT88 monos to SET... posted by ABliss on May 10, 2008 at 18:14:35

ABliss,
I am sympathetic to your thoughts intentions. I just think there are
maybe better routes to go. I'm not "talking out of my ass" here, as I
spent two years breadboarding, measuring, and refining a KT88 SET
monoblock design. Several pics are on my tube audio page:

http://www.hotvalves.blogspot.com

I will suggest a few observations: By the time you get something working
half-way decent by reconfiging your push-pull amp, The chassis, KT88's
and just maybe the power transformer will probably be all that has not
been replaced. IMO, this is kind of an unnecessary sacrifice of a push-
pull amp. I'd suggest maybe spending $30-40 on a new Hammond steel
chassis box (NOT the aluminum ones, they are flimsy) and buy a good
step-drill bit to make clean large holes for your tube sockets,etc.
Go LARGER than what you think on the chassis size, as you will be very
suprised how quickly even a big chassis starts getting congested when
you start putting in the parts. I'd also buy a new Hammond power
transformer which gives you somewhat greater voltage and current
than what you think you need --you can always drop voltage down a bit,
but are screwed if you don't have enough to get the job done.

Anyhow, if you do decide to hack up your amp into a SET, a GOOD one,
then it will absolutely take some careful analysis, planning, and
calculations. You ABSOLUTELY will need new proper single-ended
Output Transformers. There is no way around this, as push-pull
transformers are made a totally different way, and will distort
unacceptably, particulary at low freqencies, due to saturation if
you try to run them single-ended. Also, the suggestion in this thread
that you simply disable one side of the push-pull circuit is a
VERY BAD IDEA, and you will very likely damage you P-P output
transformer --seriously.

FYI: KT88s when triode-connected and biased properly can sound very
good and give quite low measured distortion in SET mode. You have
choices as to how to run them. One way is to run about 150 mA on
each of them at about 250V plate-to-cathode voltage. This is the
low impedance OPT route. At the other end of the scale,
you could run 80mA per tube at 450V plate to cathode voltage as I
am doing. With the 80mA/ 450V route, you can use the excellent
Hammond 1627SEA OPT rated at 2500 Ohm primary, and simply feed
8 Ohm rated loudspeakers from the 4 ohm secondary tap on the
1627 for a proper match. This will give you extremely good,
and low distortion bass response even at full output (8W) down
to 15Hz, and also an upper -3dB rolloff point at 45Khz or so
if you build the rest of the circuit well. KT88 and Hammond 1627
OPT is a modest super-sleeper setup if done right. A good guideline
for SET OPT size is: 1.5 pounds MINIMUM of transformer weight per
watt of output power desired.

To sum it up: If your brother built a cool Push-Pull KT88 amp, I'd
simply and proudly keep it in the family and not butcher it
--seriously. Feel free to email me an image of the schematic of
your P-P amp, and I would tell you what I see/ what might be possible.
I would also be willing to offer suggestions/info for the building
of a whole new unit:

martens_thomas@yahoo.com

Cheers,

-T.M.

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