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Old 23rd August 2004, 10:32 PM   #11
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I just now scouted around Lynn Olson's website and thought I would post a link to his design philosophy. Some nice tips for designing generally.
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Old 24th August 2004, 06:22 AM   #12
garbage is offline garbage  Singapore
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Quote:
Originally posted by SY
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.
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Old 24th August 2004, 02:33 PM   #13
jogas is offline jogas  Italy
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HI Frank,

thanks for the new schematic you posted.
I have a couple of questions about it, (due to my ignorance )..
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
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Old 24th August 2004, 02:39 PM   #14
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Hi,

Quote:
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.

Quote:
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,
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Frank
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Old 24th August 2004, 03:04 PM   #15
jogas is offline jogas  Italy
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When I will start to built it I think I use a very normal unregulated PS with just a delay on the 300b plates to allow the heater to be ready.
To avoid hum, I also think I'd better to use some DC regulators +5V and + 6.3

Ciao
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Old 24th August 2004, 03:06 PM   #16
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Quote:
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.

Quote:
200V should be O.K.
Looks like 100v from the diagram. While the tubes are getting warm the negative bias supply will protect them.
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Old 24th August 2004, 03:18 PM   #17
jogas is offline jogas  Italy
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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.
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Old 24th August 2004, 04:12 PM   #18
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Quote:
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.
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Old 24th August 2004, 04:37 PM   #19
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NE-2h, 105-125V, 1.9mA.

Hmm, i'll be damned if i can see what kind of difference will a diode voltage drop make to the firing voltage.
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Old 24th August 2004, 05:17 PM   #20
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Default Re: 300B pp schematic found, what about it?

Konnichiwa,

Quote:
Originally posted by jogas
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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