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jogas 21st August 2004 10:25 PM

300B pp schematic found, what about it?
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I found on the net the attached schematic for a 300B P-P using a step-up transformer to drive a 6SL7.
What do you think about it?
Thanks for your opinion.

Sch3mat1c 22nd August 2004 12:21 AM

Will work fine.

analog_sa 22nd August 2004 07:54 AM

An interesting and unusual approach. Notwithstanding my doubts that a 6SL7 can drive a 300B, taking the phasesplitter to the input has certain advantages. Otoh, if this transformer is indeed a step-up it will impose increased drive demands on the preamp. And you need to correct for any imbalance of the driver stage.

jogas 22nd August 2004 10:22 AM


you are right, the step up transformer (1:4) has an input impedance of 600 Ohms, this will require a preamp with muscles to drive it.
I never saw 6SL7 and I have no idea about their capabilities to drive a 300B... I only have seen 6SN7 used for that.
Moreover, the VCC is 450V and the current is 60mA per 300B, (27W).
Is this biasing correct?
Aren't we close to the limits of a "normal" 300B?

analog_sa 22nd August 2004 11:55 AM

The 6Sl7 together with the step-up show an attempt to keep voltage gain high for just two stages. I still think that using a tube with 44k Rp to drive a 300B (especially in fixed bias) borders on the pathetic but some people seem to enjoy the anaemic sound this produces.
450v is a bit high but the 27W dissipation is certainly conservative.

SY 22nd August 2004 12:09 PM

If you're trying to run the outputs in AB2 or A2 (i.e., having the 300s draw grid current on peaks), the 6SL7 will not do the job. Nor will a 6SN7. You'll need something like a high-current cathode follower or a high voltage diver and step-down transformer. If you are trying to run in normal class A or AB1, the driver tube needs only to drive the Millered input capacitance. That's not too difficult.

I think that setting up the input for a 600 ohm input Z is unreasonable. If you use something like a Jensen J11P1 with an RC network as an input transformer/phase splitter, you can take that burden off the poor abused preamp and run a more normal input Z, though with no step-up gain. I've been using that transformer as an input in a little triode amp (15 watts) and been very happy with the performance. With a 6SL7, you should be able to realize enough gain to drive the outputs even with a 1:1 input tranny.

jogas 23rd August 2004 08:28 AM

... reading some specs of the 6SL7 I understood it is possible to squeeze up to 45 V rms. Is that right?
Is that enough to drive a 300B in class A?
What is the "right" voltage swing needed by this tube?
thanks for your patience to answer all my questions..

SY 23rd August 2004 10:54 AM

I haven't breadboarded or simmed your particular circuit yet, but in my amp's driver stage, there is no problem swinging over 120V peak (240 peak-to-peak) with a 6SL7. You want your driver to be capable of swinging somewhat more than the voltage required to take the output stage grid to 0V- in this case, if your driver can swing more than 45V peak linearly, it will work.

fdegrove 23rd August 2004 10:16 PM



What do you think about it?
While I like the inherent symmetry of the design and naturally having galvanic isolation at the input is a bonus, the choice of the 6SL7 as an anode follower to drive the PP 300B is rather unfortunate.

300Bs, like most other biggish DHTs like to be drive from a low impedance source to come alive. It really makes the difference between a rather dull, anemic presentation of the music and a live and vivid one.

Here's a schematic that should do the trick nicely and as it happens it can be driven from a balanced source as well:

300B PP

The circuit can easily be adapted to take cheaper_yet less powerful_valves such as the much more economic 2A3s if reducing the TCO is a deciding factor.


serengetiplains 23rd August 2004 10:23 PM

300B PP
You might also consider one of Lynn Olson's designs. He has spent several years working over and fine tuning 300B push-pull designs using interstage transformer coupling. Schematics and circuit explanations can be found on his website. I think he has begun to post in DIY Audio so probably could be reached here with questions about his designs.

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