• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

2.5V Heater Tranformer-Best to Use One Per Channel?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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
 
Administrator
Joined 2004
Paid Member
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)
 
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 I´ve learned from guitar amplifier constructor, technician and author Gerald Weber :)
 
Last edited:
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
 
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.
 
Administrator
Joined 2004
Paid Member
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..
 
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..

Agree 100%

Is this a push-pull or parallel design? I'm thinking if it is push pull A/C would be just fine on the cathode. The Hammond 302AX looks perfect.
 
The design I'm using does not need a transformer with a center tap, and...I was able to buy two of what I did off a vendor on E-Bay, 'Supco SXT101s' for $40.00, including shipping(!). I'll test them for their current capacity as soon as I get them. I've attached a schematic for the amp I'm putting together.
 

Attachments

  • 2A3 Amp.JPG
    2A3 Amp.JPG
    111.5 KB · Views: 71
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.