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|11th January 2012, 12:04 AM||#1|
Join Date: Mar 2004
Location: Albury NSW Australia
Does anyone know of Thomas Martens?
Hey. I stumbled across a post he had on audio asylum and wanted to contact him but no reply. I was hoping someone may know of him. I have copied what he has written and i have found that, for my ears, he is right as I have not been happy with my kt88 se amp at all and tried a squillion different driver/input tube combos
Here is what he wrote:
"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
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: You can see pics of my own
KT88 monoblocks (sorry site needs updating) on my blog:
No schematics yet in digital format, but get mail to you if interested.
Sincere best luck.
I was hoping to fill in the gaps between what he has written and a schematic he may have. Given he doesnt come up anymore on audio asylum I hope he is ok and I thought maybe might know of him or the amp he built.
"Better to say nothing and keep them guessing than to speak and remove all doubt."
Last edited by duderduderini; 11th January 2012 at 12:14 AM.
|11th January 2012, 04:15 AM||#2|
Join Date: Sep 2004
He has his own site
'Symmetrical equations are good in their place, but 'vector' is a useless survival, or offshoot from quaternions, and has never been of the slightest use to any creature.' Lord Kelvin
|11th January 2012, 06:52 AM||#3|
Join Date: Feb 2002
No, that's my site. My last name is Mayer, not Mertens
I don't know this guy, so cannot help
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