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Old 23rd February 2004, 07:06 PM   #1
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Default cathode bias in OTL??

Anyone experimented with cathode bias in a circlotron type OTL, but using a constant current sink in place of cathode resistor--or am I missing something or going senile!!

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Old 23rd February 2004, 08:31 PM   #2
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cathode bias in a high current OTLs is problematic as the cathode resistor will have too much voltage drop and bypass caps for high currents are not a good idea.

small cathode resistors (1-2R) can be used to introduce a small amount of degenerative feedback that will reduce current hogging at higher currents without reducing output power by much.

i don't think a CCS on each of the cathodes will work since in the output section you want current gain not voltage gain. in other words, you want the current to vary through the tubes, not remain constant.

every circlotron that I have seen uses fixed bias. however, because the bias requirements are the same for both banks of tubes (unlike the futterman), the biasing arrangement is usually very simple.
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Old 23rd February 2004, 11:15 PM   #3
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Hi,

Quote:
small cathode resistors (1-2R) can be used to introduce a small amount of degenerative feedback that will reduce current hogging at higher currents without reducing output power by much.
They can also double as current measuring TPs.

Quote:
banks of tubes (unlike the futterman), the biasing arrangement is usually very simple.
It's really just as simple, only it's referenced differently.

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Old 24th February 2004, 12:09 AM   #4
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One of the very few occasions where I don't agree with you Frank.

In a Futterman you need two bias voltages, usually a low negative for the upper bank and a higher negative voltage for the lower bank. So, must account for this in the bias PS.

Both banks of the circlotron use the same low negative bias voltage. They can both come from the same low voltage supply. It's not a whole lot simpler, but it is simpler.
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Old 24th February 2004, 12:48 AM   #5
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Hi,

Quote:
In a Futterman you need two bias voltages, usually a low negative for the upper bank and a higher negative voltage for the lower bank. So, must account for this in the bias PS.
O.K...Let's disagree on this one.

Surely I haven't seen all the variations of the Futterman designs but you CAN bias all those I've seen from a single negative supply just by referencing them either to the positive or negative part of the bipolar supply rails.

This doesn't change the voltage source at all but what you'd measure with respect to ground will make one think it involves different bias voltages, only it doesn't.


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Old 24th February 2004, 02:00 AM   #6
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I don't see it Frank, but you know French and I don't! Could you draw it for me?

Nonetheless, looking at all the activity on a vacuum tube forum in 2004, I might say that the more things change, the more they stay the same. . . . .
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Old 24th February 2004, 02:20 AM   #7
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Hi,

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I don't see it Frank, but you know French and I don't! Could you draw it for me?
As it's getting late...Maybe just an example will help already?

Follow the A and B connections and imagine what happens when they're connected.

I checked the Futterman 12B4A OTL and it use the same scheme, as does the Taki and some others I found (including my own, he, he)...

Any particular Futterman model you had in mind, BTW?

Here goes:

Cheers,
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Old 24th February 2004, 02:23 AM   #8
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OK. I see this. But this only works because the lower 6c33 is self biased with a 120R resistor and has a bypass cap (must be a big one).

Most of the Futtermans that I have seen are not self-biased, so the grid of the bottom tube must be held at some negative voltage below the negative rail. Thus, two negative supplies of different voltages, one for upper and one for lower.
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Old 24th February 2004, 02:32 AM   #9
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Hi,

Quote:
Most of the Futtermans that I have seen are not self-biased, so the grid of the bottom tube must be held at some negative voltage below the negative rail.
Yes...But, if you think about it everything you need is already there except for the bias voltage proper.
Which when we do the maths can be obtained by referencing it to the appropriate rail polarity.

This has the added advantage that both voltages will track eachother:

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Old 24th February 2004, 03:22 AM   #10
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Yes, this is what I would expect for a Futterman. There are two bias supplies, one is referenced to cathodes of the bottom tubes and the other floats on top of the output (to balance the drives).

But, in a circlotron, you only need one bias supply for both sets of tubes, since their cathodes have the same reference point. The bias supply is ground referenced and does not have to float on the output. You may choose to use two separate pots to control the voltages separately, but you still only need one supply.

Like I said, it's only a little simpler.
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