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300B pp schematic found, what about it?

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Hello,

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.
 

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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.
 
Analog,

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?
 
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.
 
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.
 
... 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..
 
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.
 
diyAudio Senior Member
Joined 2002
Hi,

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.

Cheers,;)
 
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.
 
SY said:
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.

hi SY

care to reveal the operating points of your 6SL7?

i am going to build a drd 300b driven by 6SL7, but it seems that i'd have to run it at it's recommended operating point of 250V, -2V bias, 2.3mA and both halves in parallel in order to comfortably drive the 300b at an operating point of 300V, -61V, 60mA.
 
HI Frank,

thanks for the new schematic you posted.
I have a couple of questions about it, (due to my ignorance:D )..
On the bottom of the schematic there are two neon LP-1 & LP2.
I understood LP1 can be considered as a voltage reference... but at +200volts?
Moreover, what about LP2 with a diode in series that does not allow it to work?
Is that possible LP2 is just necessary at the beginning in the warm-up when the +400V is not yet available?

Thanks again
 
diyAudio Senior Member
Joined 2002
Hi,

I understood LP1 can be considered as a voltage reference... but at +200volts?

Yes...I have no idea where to buy those in Europe but they seem available in the U.S.
200V should be O.K.

Moreover, what about LP2 with a diode in series that does not allow it to work?

I haven't studied the diagram but it looks to me as if the diode is indeed there to protect the cathode at startup, it's completely harmless otherwise.;)

BTW, if you look at part 2 & 3 you'll notice the regulated PSs as well, it's not a must and the 6336 can be replaced with a 6C33-C if you opt to build it as shown.
All in all a quite impressive piece of work, I'd say.

Cheers, ;)
 
I haven't studied the diagram but it looks to me as if the diode is indeed there to protect the cathode at startup, it's completely harmless otherwise.

Harmless? Seems to me it will stop all current flowing through the driver tubes.

200V should be O.K.

Looks like 100v from the diagram. While the tubes are getting warm the negative bias supply will protect them.
 
Hello Analog,
I don want to act as a designer but, as I understood the 6SN7 cathode current is flowing thru the two R 23,2K and 22,1K at 6.6 mA.
On the other side the center of the 6SL7 cathodes is biased at 100K * 2 mA = 200 Volts, (correct?)
Moreover having 100K on the 6SL7 plate with 1mA each tube, we also have 100V on the 100K reisstors and the rest, around 110V on the two half of the 6SL7.
 
I don want to act as a designer

You seem to be absolutely right. I wrongly assumed that the neons are used as voltage regulators and this is clearly not the case. The bias voltage is applied before the tubes start conducting and the neons keep the negative voltage low to save the tubes. The diode is then properly biased and seems to delay the driver stage slightly.
 
Konnichiwa,

jogas said:
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?

The schematic was developed by an italian compnay to offer a simple upgrade for a very cramped chassis Cary Audio build PP 300B Amplifier, using the existing Valves. It will work okay(ish) but you can do much better. Expect reasonable average sonics from this, no revelations. BTW, IIRC the transformer used has no stepup but is a simple 1:1+1 Phasesplitter.

Sayonara
 
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