2A3 to 45 Cathode Wattage? - diyAudio
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Old 8th November 2011, 10:11 PM   #1
batsong is offline batsong  United States
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Default 2A3 to 45 Cathode Wattage?

Thinking about switching from 2A3 to 45. Have a few questions:

Would you simply add another cathode resistor in parallel (eg. another 750R)? Or replace the existing 750R with a single resistor (1K5)?

Is a 20W rating necessary for the cathode resistor?

A switch enabling one to change 2A3s to 45s 'on the fly'?

Any help will be appreciated. Thanks.
-Gabe
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Old 8th November 2011, 10:22 PM   #2
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Quote:
Originally Posted by batsong View Post

Would you simply add another cathode resistor in parallel (eg. another 750R)? Or replace the existing 750R with a single resistor (1K5)?
Assuming that you want less current, do you mean "in series"?.......paralleling a pair of 750R resistors gives you 375R...
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Old 8th November 2011, 10:42 PM   #3
batsong is offline batsong  United States
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Thanks. Meant series.
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Old 9th November 2011, 01:30 AM   #4
batsong is offline batsong  United States
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Sorry about that. I have been juggling PS capacitor values for this amp. Trying to decide whether to implement Ultrapath or W.E. and substituting quality film for electrolytics, while trying to save space and expense.
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Old 9th November 2011, 04:19 PM   #5
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Default I'm no expert but......

Since nobody has jumped in here........

Assuming SE operation and looking at Duncan Amps TDSL data, it appears that both tubes like about 250V on their anodes, and the 2A3 needs a 750R cathode R to keep the grid at -45V for 60ma idle. The 45 needs a 1500R cathode R to keep the grid at -50V for 36ma idle.

TDSL Tube data [45]

TDSL Tube data [2A3]

IIRC, the 2A3 is essentially a doubled up 45 so the above makes sense (to me anyway)

Using ohms law, for 2A3 P=(Isquared)(R) so .06 x .06 x 750=2.7W and for 45 P=2W. I would use at least a 5W R....

I suppose that you could have a switch to swap tubes, just be careful or you will fry the 45's on the 2A3 setting!

Also, according to the TDSL data, the heater current of the 2A3 is 2.5A and the 45 is 1.5A, so your filament voltage may be high when running the 45's.

Last edited by boywonder; 9th November 2011 at 04:21 PM.
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Old 9th November 2011, 07:24 PM   #6
batsong is offline batsong  United States
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Thank you for all of the work. Very informative.
Filament voltage measures 2.7V.
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