2.5V Heater Tranformer-Best to Use One Per Channel? - diyAudio
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Old 31st December 2013, 06:52 AM   #1
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Default 2.5V Heater Tranformer-Best to Use One Per Channel?

Hello,

I just need to source one major component for my first tube amp project, a source for 2.5V for the 2A3 heaters. The spec calls for 2.5V @ 5A X2, which provides each of the four 2A3 power tubes with 2.5A each. I found a transformer rated at 40VA, which if I understand correctly should provide me with 16A of current at 2.5 volts. This is plenty for the heaters on all four tubes. Can I power them in this way, or is there a reason for keeping the channels isolated in this circuit? The transformer's original use is in some HVAC function. Is there any worry that it will work any better or worse than the offerings from Hammond or the like?

Thank you,

Rick
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Old 31st December 2013, 09:14 AM   #2
apnneto is online now apnneto  Brazil
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Take a look at 2A3 datasheet. Each tube is 2.5A. Four tubes will be 10A. 16A is more then you need. That's ok.
http://www.svetlana.com/pdf/sovtek/2a3-sovtek.pdf

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Old 31st December 2013, 02:27 PM   #3
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That depends on whether the output stage uses fixed bias (a negative voltage applied to the grid), or cathode bias (a resistor from filament, or center tap, or hum balance pot to ground).

If cathode bias , then you need separate windings for each pair of tubes.

Pete
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Old 31st December 2013, 03:52 PM   #4
lexx21 is offline lexx21  United States
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Edcor makes a power transformer that has two 2.5v taps. Take a look there.
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Old 31st December 2013, 05:18 PM   #5
kevinkr is offline kevinkr  United States
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Generally preferable in my experience with cathode bias to provide a separate center tapped winding for each tube. Each tube biases independently of all of the others resulting in relatively consistent plate current in all tubes.

You can get away with one transformer with fixed bias but lose the ability to measure bias current independently for each tube.

There is a significant advantage to DC heating particularly with a CCS, and this is to eliminate the intermodulation distortion from the filament supply current beating with the audio signal currents. This is quite visible using FFT analysis. (120Hz side bands either side of the fundamental tone)
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Old 31st December 2013, 05:58 PM   #6
seppoa is offline seppoa  Sweden
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Bias current can be measured independently for each tube, by connecting a mulitimeter set to current measuring, between the common HT connection on the OPT, and each anode. The resistance of the multimeter is much lower than that of the winding in the OPT, thus no error. A trick Ive learned from guitar amplifier constructor, technician and author Gerald Weber

Last edited by seppoa; 31st December 2013 at 06:01 PM.
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Old 1st January 2014, 04:20 PM   #7
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Thank you all for your suggestions. Some of them are well above me at this point in my learning curve, but I appreciate the effort none the less! It sounds like the safest thing to do is use two transformers, though I'll experiment with one initially and see how it goes. They were pretty cheap anyway...I hope I didn't misunderstand the item description.

Transformer Primary 120 208 240V Secondary 2 5 12 24 | eBay
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Old 1st January 2014, 04:23 PM   #8
MelB is offline MelB  Canada
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Quote:
Originally Posted by kevinkr View Post
Generally preferable in my experience with cathode bias to provide a separate center tapped winding for each tube. Each tube biases independently of all of the others resulting in relatively consistent plate current in all tubes.

You can get away with one transformer with fixed bias but lose the ability to measure bias current independently for each tube.

There is a significant advantage to DC heating particularly with a CCS, and this is to eliminate the intermodulation distortion from the filament supply current beating with the audio signal currents. This is quite visible using FFT analysis. (120Hz side bands either side of the fundamental tone)
You "can" measure the current to each tube at it's anode...carefully.
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Old 1st January 2014, 05:48 PM   #9
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The Hammond 302AX is specifically designed for 2A3/45 type amplifiers It has 2 2.5v windings with center taps. I've used this transformer and it works nicely.
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Old 1st January 2014, 06:53 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by MelB View Post
You "can" measure the current to each tube at it's anode...carefully.
Yes you can, but it requires taking the amplifier apart to do it or having pin jacks with high voltages present on the top plate. (I've done both - simpler and safer to do it in the cathode circuit if possible.) I like these things to be externally accessible so that I can change and bias new tubes without taking the bottom cover off. My amps run with plate voltages of 1kV..
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