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Aikido KT88 SET, etc.

Posted by Thomas Martens on January 26, 2007 at 09:08:03

In Reply to: Re: My Opinion on Aikido KT88 SET, etc. posted by JoshK on January 25, 2007 at 12:53:57:

Well, take a look at the grid resistor on the KT88 stage and the
cap feeding it: .22 into 100K. This results in a much higher freq
LF -3dB point than makes sense. Better a 220k (just fine for triode
conn kT88) fed by a .47 --much lower cutoff point. Also: look at
the 100uf cathode bypass on the 425ohm cathode Resistor. This
would better be a 470uf or even a 1000uf 100v can cap, which
would considerably extend the LF cutoff point here as well. Then
consider the UBT-2 OPT: a little light, with only a 5-lb weight.
This could tend to saturate at LF freqs, and does not extend as
low, IMHO, as it ideally should. Better would be the Hammond
1627SEA with about twice the iron weight, and MUCH better
power handling/LF response. Also: look at the puny 10uF filter cap
on the driver/voltage amp stages. I don't believe the rhetoric
about the PSRR of such a design at LF. I'd think you'd have some
considerable LF feedback from the output stage travelling back
into the lower stages through the Power supplu rail and mucking
up the transient response, etc. Also: what's up with running such
a lowish supply/plate voltage on the KT88, and then only
80mA of current? I'd think getting a full 500v supply voltage
and using a higher value bias resistor to allow 85-90mA to flow
at that level using the 2500 ohm primary for a very clean 8-9
watts would be more the ticket.Frankly, I seriously doubt the
design as drawn could produce a full clean 10 watts.

I could say more --but enough for now.

Anyhow, just my take on it.

-T.M.

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