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Old 15th June 2004, 11:37 AM   #1
jim is offline jim  Belgium
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Default Filament voltage of PP2A3

Hi,

I've a question concerning the filament-hookup of my (future) PP2A3 amp.
Are there any disadvantages if I wire it like in my picture.
It's easier to find a 6V transformer than a 2.5V one.

On the other hand, if I rewind an 120VA toriod with 4 separate 2.5V/2.5A windings, what thickness of wire should I use ?

Thanks,
Jim
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Old 15th June 2004, 12:58 PM   #2
benny is offline benny  Australia
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this idea almost would work.

obviously you'd know that ohms law dictates that the voltage is going to be split between your two series loads... as both loads identical, voltage will split evenly - so this is going to result it 3V per filament.

IF you want to do a seires hookup like this, then here's how to do it.

say your filamens draw 2A of current each, and they both need 2.5V... you have a suply of 6V, so split this will give 3V. Ideally you want 5V so when split this gives 2.5V... so, you want to drop the voltage by another 1V somehow, external to the filaments... so, use a series resistor. calculate value with ohm law...

EXAMPLE: you want a 1V drop, you have 2A current per filament, so total I draw is 4A.

R = V/I

R = 1/4

R = 0.25 ohms.

for power rating,

V(drop) = 1V, I = 4A

P = VI
P = 1 x 4 = 4W

hope this helps... if i'm wrong, someone correct me please.

cheers
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Old 15th June 2004, 12:58 PM   #3
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Your series filament setup will "work", if the 5 VAC supply floats. The problem is you can't do anything about hum balance. You will have to rely on cancellation in the O/P trafo.

How much stuff have you already purchased? I ask as 6B4Gs are equivalent to 2A3s except for the 6.3 VAC filament and Octal base. Between individual balance pots and O/P trafo cancellation, you should have quite acceptable residual hum levels using 6B4Gs. 4 separate 6.3 VAC windings should be an off the shelf proposition.
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Old 15th June 2004, 01:35 PM   #4
jim is offline jim  Belgium
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Hi,

Quote:
so this is going to result it 3V per filament
Yes, I know I have to drop another 1V.

I allready have 4 Sovtek 2A3 tubes so it's got to be 2.5V

Regards,
Jim
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Old 15th June 2004, 02:03 PM   #5
EC8010 is offline EC8010  United Kingdom
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If you put 2A3 filaments in series then one cathode will be 2.5V adrift from the other. As a minimum, you need a 2.5V 10A winding (assuming stereo chassis). Next best, 2.5V 10A with centre tap. Neither of these alternatives allow you to measure cathode current easily. Next best, 4 x 2.5V 2.5A; this will allow you to monitor cathode current easily by adding a 1R resistor between each winding and HT 0V. Best, 4 2.5V 2.5A centre tapped; this allows current monitoring and gives minimum hum. If it has a foil electrostatic screen between primary and secondaries, so much the better. Maplin (UK) do a 100VA transformer kit onto which you could wind these secondaries.
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