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Old 8th September 2006, 08:37 AM   #1
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Join Date: Mar 2005
Location: Bergamo
Default KT88 drivers

Hi all,

I'm building a SE KT88 amplifier.
At the moment I'm building the PSU, a 300+300 mosfet regulated HT PS.
The amplifier will be a parafeed, with a KT88 on top as a CCS (driven by a SS CCS).
THIS is the amplifier output stage; I have already tried the CCS with the KT88 pentode (on B+ 300V) and it seems to work fine.
The B+, as said, will be 600V.
Now, while preparring the PSU, I have to decide on the driver.
I have some 12AX7, ECL84, and can easily trade them with some 6SN7 or 6SL7.
I have also to decide the driver topology: SE, SE + CCS on top (the same of the output stage), SRPP, mu-follower?

My preferred solution is to use the ECL84 (or EL84) pentode stage, strapped as triode, in a SE configuration, maybe loaded with a CCS (a EL84 pentode driven by a SS CCS).

What do you think? which driver would you use for such a amplifier?

In Nomine Libertatis Vincula Edificamus,
In Nomine Veritatis Mendacia Efferimus.
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Old 8th September 2006, 07:20 PM   #2
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Join Date: Aug 2002
Location: Eire
Cant advise on what would be best, but loading the driver with a CCS will maximise your voltage gain and linearity. The LED biased two transistor CCS which SY developed works a treat.

Where did you get the design for your KT88 CCS, and how does it perform. I am currently converting my RH807 parafeed amp from CCS loaded to use chokes loaded - just need the chokes now !

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Old 11th September 2006, 03:32 AM   #3
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Join Date: Oct 2004
Location: Adelaide South Oz
As you seem to be happy with hybrid Solid State plus Tube circuits and designing around MOSFETS then I recommend that you look at the PowerDrive circuits on Tubelab's website.


The PowerDrive is a CCS loaded triode stage followed by a MOSFET Source follower to drive the output tube grid.
Its a nice fit with your existing circuits and is certainly high end in terms of performance.
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Old 11th September 2006, 07:27 AM   #4
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Join Date: Mar 2005
Location: Bergamo
The idea to drive (bias) a pentode with a solid state CCS comes from Kuei Yang Wang, we had a long thread about CCS here.
The schematic, instead, is mine.
I'm trying now a different version, with the use of a OP amp, in manner to have a feedback and improve the CCS dynamic and impedance.

I know the smart PowerDrive idea, George (TubeLab) made a wonderful job on it.

I already tried a home made dual PowerDrive (to balance the bias of two paralleded KT88) in a SE amp prototype, and it worked not too bad (it was a new design, and it needed lot of work to be fully functional); due to lack of time I never finished it.
Moreover, it was too complex for me.
This is the circuit.

I would like to keep the "sand" to a minimum: I mean, use the SS stuffs only where they are really necessary.
This means, in my current vision, using SS circuit only for a CCS, and try to use tube for everything else (also for the driver).
The CCS would be used only to improve the linearity, as you said, instead to use a resistor.

My idea is to go with the ECL84 pentode section, CCS loaded.
The triode section of the same tube will be used, instead, to create an active low-pass filter in manner to drive an external subwoofer amplifier.

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In Nomine Veritatis Mendacia Efferimus.
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