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SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream. |
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SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream. |
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In Reply to: Driver for SE KT88 operating at 450 vdc UL or triode posted by Neff on January 23, 2008 at 13:50:01:
A single section of 5687 is the best match I've found sonically by
far for a KT88 SET running high voltage. My KT88 SET reads 450V plate
to cathode, and draws 80 mA with an approx 550-Ohm (an adj. pot in line)
resistance stack for cathode bias. OPT is the Hammond 1627SE (2.5K)
fed from about 519V HT.
I tried MANY MANY different drivers, for example: single and parallel
6SN7 at many different operating points, 12B4A many configs, 6SL7 many
configs and others. The 6SN7 is a great tube for driving 2A3 but NOT what
you want for KT88. KT88 is most linear as SET when operated at 250V,
but then needs a whopping 160mA. No problem, even though
somewhat less linear with 450V plate-to-cathode, (do not run more than
85mA at this voltage, 80 is good) proper selection of a driver tube with
complementary harmonics will cancel much of the distortion and result in
greater linearity in actual operation. I use a 5687 (one section only)
operating at 195V plate to cathode and drawing 19mA. The 5687 is set up
with it's cathode elevated at +85V to allow direct coupling from a 6J5
(same as one section of a 6SN7) input stage. output of the 5687 is fed
through a .47uF coupling cap to the KT88's 220K (this is a good value,
do not go larger than 220K) grid resistor.
The above is a VERY synergistic topology which results in excellent
frequency response, transient, response, linearity and harmonic
balance.
Whatever you decide, I strongly suggest you feed the driver's plate
load resistor from a high voltage B+ (mine is at about +510 supplied
from main 519V B+ through a 20H (181 Ohm)Hammond choke and 47uF Solen
filter cap)to ensure a maximum low distortion voltage swing. You will
need to achieve at least a good clean 200V peak-to-peak output from
your driver to adequately drive the KT-88 (more is better). KT88 SET
can equall or rival best 300B circuits IF set up properly.
Settting up and chosing the rigth driver config is, IMO, the key to
making a SET work right. There is no one universal "best" driver
tube for all SETs. It all depends on the operating points used
on both driver and output tubes, and which type of tubes selected.
This is why it is so hard to say what are universally "good" tubes.
Feel free to send me e-mail: thomas_martens@ekit.com with any
questions, and also I will be happy to send my personal phone number
if you desire to discuss your project. You can see pics of my own
KT88 monoblocks (sorry site needs updating) on my blog:
http://www.hotvalves.blogspot.com
No schematics yet in digital format, but get mail to you if interested.
Sincere best luck.
-T.M.
P.S. I agree with all of Paul Joppa's stated perspectives
regarding this subject.
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Follow Ups
- Absolutely the best for KT88 SET ( I've tried many)... - Thomas Martens 11:31:14 01/26/08 (0)
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