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Old 27th May 2005, 04:10 AM   #1
Raj1 is offline Raj1  United Kingdom
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Question using a plate choke?

Hi,

I have been thinking of replacing the bias resistor in this circuit with a high inductance choke (rather than a SS CCS). Can someone please tell me what sort of choke I need, and whether or not it needs to be custom wound, or if any of the shelf types are available like tango etc?


Thanks
Raja
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Old 27th May 2005, 04:47 AM   #2
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Um, chokes are designed (ideally) for zero DC resistance. It will short out your tubes, unless you provide additional grid bias.

I find it rather odd that a circuit would go to the trouble of using DHTs only to tie the channels together with one cathode resistor?? PP notwithstanding.

Tim
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Old 27th May 2005, 10:14 AM   #3
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Don't know about chokes, but...The 3A5 really should have battery powered filaments, espescially in this application. I've used these tubes and they sound great, but are very microphonic, and sensitive to filament induced noise. One D-size rechargeable battery per tube will last 10hours or so before recharging is needed. And these tubes can handle the voltage of fully charged batts.
I agree with Tim regarding the single bias resistor. What's up with that? Save 5cents? An upgrade to the given circuit would be one 300ohm bias resistor per channel, and separate batteries for the filaments.
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Old 27th May 2005, 03:45 PM   #4
Raj1 is offline Raj1  United Kingdom
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Hi,

Actually the designer of the stage has been asked the question of the shared resistor, basically 2 reasons,

1) the stage is actually intended for differntial operation

2) it actually sounds better with the shared resistor, I've tried both, the shared resistor sounds better to me..............

I've had no problem with hum at all, and agree that the tubes are senistive to the filament supply quality, maybe something I'll consider for the future. Though that does mean finding extra space for a charging circuit of some sort. The risk is a run down battery whilst the B+ is still applied......

so I can't use a choke in the tail?????
How could the circuit be adapted to do so???


Thanks
Raja
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Old 27th May 2005, 04:20 PM   #5
dhaen is offline dhaen  Europe
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Raja,

I'm confused... Where were you intending to use the choke, anode or cathode?
In either case, I cannot imagine why.
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Old 27th May 2005, 04:59 PM   #6
Raj1 is offline Raj1  United Kingdom
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Hi,

I was intending on using the choke to replace the cathode bias resistor. I was told that cathode bypass caps wouldn't work on a long tailed pair, but a cccs or high inductance choke would......

Thanks
Raja
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Old 27th May 2005, 06:03 PM   #7
Raj1 is offline Raj1  United Kingdom
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sorry, the whole reason I was investigating this , was to ask if it is possible to get rid of the degenrative feedback in the triode.......

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Raja
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Old 27th May 2005, 06:23 PM   #8
dhaen is offline dhaen  Europe
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Hi Raja,

I didn't mean to critisise I'm just curious.

If the cathodes of a long-tailed or push-pull pair share a cathode resistor, then degeneration is theoretically negated by cancellation. This is why you find some push-pull designs with no cathode bypass cap. However is practice, this cancellation is imperfect because the 2 sides can never be perfectly matched. It is though, pretty good in class A.

Depending on which camp you are in, you can believe that triodes are pentodes with degeneration. Although this models well, it's a concept I can't accept. In understanding something I start with the simple and work towards the complex, not the other way round.

I hope you can follow my reasoning.
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Old 2nd June 2005, 03:57 PM   #9
Raj1 is offline Raj1  United Kingdom
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HI,

Just a quick qusetion is it possible to use battery biasing in this circuit?

Thanks
Raja
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Old 2nd June 2005, 04:07 PM   #10
dhaen is offline dhaen  Europe
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Hi Raja,

The 'quick" answer is yes

The long answer will include all the possible methods of implementation....

Cheers,
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