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Old 10th November 2009, 02:32 PM   #11
Brit01 is offline Brit01  United Kingdom
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This concept of using a toroidal really interests me as OPTS are not available where I live.

Can you give me some more details of the specs of the toroids I need in general?
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Old 10th November 2009, 04:27 PM   #12
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Originally Posted by ystkkmkr View Post


Very cool. Thanks for the link. I don't for a second expect a 6SL7 running @1mA to be able to drive a 300B but this is a separate issue.
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Old 10th November 2009, 04:45 PM   #13
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Why do people insist on putting fruity things in the plate circuit, where they do absolutely nothing? The only thing supplying it with a CCS will do is make it fart.

Besides, move some things around (which according to Kirchoff, is allowable, despite what some more imaginative posters might believe), and you get PP again. If you must have class A guaranteed, you can put a CCS in the cathode circuit, where it will have mu times more effect than in the plate circuit. (Not that that's a good idea anyway, it still makes it fart more when driven into clipping. At least it farts due to cathode drive instead of running out of supply voltage.)

Oh, and what's the diode for? Looks like an excellent source of nasty harsh silicon clipping. Not that it will do much as a triode's grid is a better diode than a 1N4007 anyway. And the voltage divider, you don't need that for a cathodyne, just bias the first stage correctly in the first place.

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Old 10th November 2009, 05:24 PM   #14
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Default Toroids as OTs

Quote:
Originally Posted by Brit01 View Post
This concept of using a toroidal really interests me as OPTS are not available where I live. Can you give me some more details of the specs of the toroids I need in general?
Here is a recent thread - I started ...

Using Toroidal PTs as tube OTs - methods and sources
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Old 10th November 2009, 07:06 PM   #15
Fenris is offline Fenris  United States
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Originally Posted by Sch3mat1c View Post

Oh, and what's the diode for? Looks like an excellent source of nasty harsh silicon clipping. Not that it will do much as a triode's grid is a better diode than a 1N4007 anyway. And the voltage divider, you don't need that for a cathodyne, just bias the first stage correctly in the first place.

Tim
The diode protects the tubes at turn-on, when the cathodes are cold and not conducting. Just a bit of cheap insurance. It stops being seen by the circuit once the tubes start conducting. The voltage divider is there because it sets the DC operating point of the phase splitter while keeping the anode and cathode resistors the same, allowing for the most linear and symetrical operation of the phase splitter.
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Old 10th November 2009, 08:09 PM   #16
Fenris is offline Fenris  United States
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Originally Posted by Iain McNeill View Post
I'm a sand-guy so forgive me if I'm missing something here. If the primary windings were ground referenced rather than cathode referenced wouldn't this be equivalent to a PP output, assuming zero PSU impedance?
Standard PP works with the transformer in series with the tubes. In this instance the transformer is in paralell with the tubes. The disadvantage is a lower voltage swing and the need for a CCS in the plate (which is more difficult than in the cathode because of the higher voltages). But the output current doesn't flow through the cathode resistor. If the transformer were ground connected (and hence connected together), both the signal and output currents would flow through the cathode resistors, but this way they stay separate. A minor philosophical difference.
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Old 11th November 2009, 12:42 AM   #17
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Quote:
Originally Posted by Fenris View Post
The diode protects the tubes at turn-on, when the cathodes are cold and not conducting.
Protected from what?

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It stops being seen by the circuit once the tubes start conducting.
Ah, but not completely. Reverse biased diodes are crappy capacitors. Probably not noticable in the audio range, but it'll do strange things in the cutoff region (changing the slew rate with respect to loading).

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The voltage divider is there because it sets the DC operating point of the phase splitter while keeping the anode and cathode resistors the same, allowing for the most linear and symetrical operation of the phase splitter.
Ah, but the phase splitter itself is always biased at 1/4 V+ anyway, it has to be. If anything the excuse would be on the preamp stage, but it's actually more linear at lower voltages (further from cutoff). You just have to be sure to use a loadline that drops nice and low. Pentodes are good for this (also giving excellent gain, needed if you're applying GNFB), but regular triodes will also do well, as long as the plate resistor is big.

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Old 11th November 2009, 03:50 AM   #18
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Fenris - thankyou, I was missing something!
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Old 11th November 2009, 06:22 AM   #19
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Fenris,
Nothing strange with your circuit. It´s been done before. This is an example of a preamp:

Figure 18

Check it and see that the primary windings shouldn´t be returned to ground, instead to each other to get a better PP function. Minor adjustment!

Maybe this could interest you too:

http://www.nutshellhifi.com/library/Tube_Fest_Talk.html

Last edited by revintage; 11th November 2009 at 06:32 AM.
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Old 11th November 2009, 03:01 PM   #20
Brit01 is offline Brit01  United Kingdom
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These guys here in Uruguay want 70 USD for a 220/12V toroidal. You've got to be kidding.

I'll get a custom built traditional OPT for that price.
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