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papparazzi 2nd July 2006 08:43 AM

Using 6L6 for 300b driver

As a long time user of JeLabs type single 300b amp. I was wondering
how to use available kt66 for driver to 300b tube. Looking at schematics found on net ,Komuro San's circuit found on well known
site seems to be interesting

I have only two Hammond chokes 159p 10h/120 ma iwas wondering
could i use these? Available voltage is about 400-350 VDC so resistor loaded ,isuppose is no good??
Input tube could be 6sl7 type.

Thanks for your ideas!


Giaime 2nd July 2006 08:55 AM

I have a 404 error on your link :whazzat:

This is the correct link:

papparazzi 2nd July 2006 09:02 AM

Yes its correct link


rcavictim 2nd July 2006 10:24 AM


Those 159P chokes at just 10H will not have enough L. If you cannot source any other parts perhaps by partially disassembling these chokes enough to remove gap spacing material to close the gap you can get more L and still have enough current capability before saturation. I suspect you could at least double the inductance that way, perhaps slightly more. If you can treble it you're all set.

Try to identify the outside winding connection and use this towards the driver tube plate as (I always take special care to do this assuming) it has less shunt capacitance to the core than the inside of the winding.

papparazzi 2nd July 2006 06:12 PM


Those 159P chokes at just 10H will not have enough L.
Yes three times more inductance would be suitable,but what if you
get your voltage gain from first stage and use 6l6 type only as
current source, is H so important then?

Rcavictim : For me as a hobbyist only, your advise wasnt quite clear for me?


Miles Prower 2nd July 2006 07:26 PM

I wouldn't use any of those designs. I'd use something like a 6AU6 (or a 12AX7A or 6SL7 differential, depending on how much gain this would require) front end. since you keep hearing about how "colored" the 300B is, you're gonna need gNFB to fix that, and you need sufficient open loop gain.

Next, use a trioded 6BQ5 with a power MOSFET on top for a hybrid Kimmel DC coupled to the 300B. That would be able to slam enough current into the 300B input/Miller/stray capacitance at high frequencies, and push it into Class A2 for better transient behaviour.

Magnetics in the signal path means peculiar behaviours when applying the gNFB.

rcavictim 2nd July 2006 08:03 PM


Originally posted by papparazzi

Rcavictim : For me as a hobbyist only, your advise wasnt quite clear for me?


The Hammond (and all other common) chokes are wound placing a coil inside an E-I laminated steel core. There is a small thickness air gap beytween the E and the I made out of paper or some other non magnetic material. You will see it easily by examining the choke core at the top and bottom. It keeps the iron core from saturating but it also reduces the inductance. Inductance will rise as the gap is made smaller with maximum achievable L with the iron squeezed together tighly with clamps. Since you are going to put less than rated (120 mA) current through this thing if used as a plate choke (maybe 40 mA on the 6L6?), then you can close the air gap by at least half, probably more. Use thick paper from business cards or something sililar. This will make more inductance while setting saturation current lower.

Access to an L meter will be very helpful to do this and get both chokes the same.

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