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Old 10th November 2008, 11:39 AM   #1
diyAudio Member
Join Date: Mar 2004
Location: Albury NSW Australia
Default input/driver stage for kt88/6550

Hey there
Well I just finished my pre amp running c3g's and 6sl7's for the phono stage and 6G6G in triode mode (srpp) for the line stage.
My next quest is to build a PSE for either 6550/kt88.
I have a stack of d3a's and 6g6gs.
I figure one d3a should have enough drive for the 6550 or do I need a second stage there.
I was hoping someone could offer an opinion re this rather than beam me a schematic
"Better to say nothing and keep them guessing than to speak and remove all doubt."
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Old 12th November 2008, 05:29 AM   #2
diyAudio Member
Join Date: Oct 2004
Location: Adelaide South Oz
G'day bloke,
No answers - that surprised me.
Not really my forte' but FWIW.

6550/KT88 in SE Operating Point for single tube

Va 250
Vg2 250
Vg1 -14.0
Ia 140mA
Ig2 12mA
Pout 12.5 W
Zout 1K5


Va 400
Vg2 225
Vg1 -16.5
Ia 87mA
Ig2 4mA
Pout 20 W
Zout 3K

For parallel SE (two tubes) divide Zout by 2

Worst case you ONLY need to swing 2 x 16.5 = 33V peak to peak.

So you want an input/driver gain of about 20'ish assuming you don't want to use global feedback.

I would simply use George's (Tubelab) Simple SE design. That is not strictly a single stage - its a single gain stage plus mosfet source follower. The D3a would be ideal for the gain stage.
Why? Because the max Rg1 (maximum grid1 to 0V resistance value) is quite low (100K I think - check the data sheet) and this presents a significant load on the driver particularly for parallel output tubes where you need to drive 2 of those Rg1 in parallel (50K). The MOSFET Source follower fixes this for you, it will laugh at a 50K load.

Hardest part might be finding good output trannies.


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Old 12th November 2008, 10:22 AM   #3
diyAudio Member
Join Date: Mar 2004
Location: Albury NSW Australia
Default I forgot to mention triode mode as well

Hey Ginger.
Well its heating up in Albury NSW so I bet Adelaide is way hot.
I was surprised as well that no one has answered.. Maybe they dont like me..
I have procured 2 hammond 1642 se output trannys with U/L tap.
I have heard an amp my mate made where he is driving a pair of genalex reissues with a 6n1p into an EL84 the the pair of Genalexes.. I reckon its overdriven as I head it on an NAD cd player and it distorted at all but the lowest volume..on my cd palyer.. a Marantz cd50 with variable output, it sounded quite nice as I could lower the output of the cd player.. he was running the aforementioned hammonds so I went with them. there Primary impedance is 1.5K
I am not a big fan of mosfets, I know I know I should be, but I have tried the ccs thing and just didnt like the sound... Sadly or otherwise I much prefer srpp configurations. I was thinking maybe 2xD3a's in srpp driving the kt88's or the d3a ( I also have e 180f's and E 280f's) into a 6g6g then the pair of kt88's
Maybe I need to do a test rig and see how they both perform.. personally I think a single stage will suffice, given that I have a preamp.
Good to hear from you and thanks for your help.
"Better to say nothing and keep them guessing than to speak and remove all doubt."
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Old 12th November 2008, 11:15 AM   #4
SY is offline SY  United States
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A single D3a, no SRPP, no cathode degeneration, CCS plate load has astonishingly low distortion at swings much greater than what you need. And a low output impedance, just to ice the cake.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 12th November 2008, 11:22 AM   #5
diyAudio Member
Join Date: Mar 2004
Location: Albury NSW Australia
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Hi Sy
I have some Ixys 10M45's which I will try.
will let the forum know how I went
Thanks for your help
"Better to say nothing and keep them guessing than to speak and remove all doubt."
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Old 12th October 2010, 03:21 PM   #6
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Join Date: Nov 2004
Location: Den Haag

I'm also looking into drivers for a SE 6550. D3a looks nice, but the grid voltage curves only go from -3 to 0 volts, and I'm afraid that this is not enough for the preamp I use (6SN7 in srpp). It has no problems driving my EL84 power amp without a driver. I don't want to have to turn the volume knob all the way down.
Also I would like to make the new power amp direct coupled..
I've been studying driver requirements for a while now, but it's still very hazy, so any help would be appreciated!
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