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SE 45 DIY Amp Question

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Doug Weiss

Member
Hello all, I have been gathering parts for a simple stereo SE45 amp per the JE Labs schematics shown. Was wondering if I use a PT with a CT 5 volt winding, could I use each side of the tap to power each of the 45 filaments? Will there be any unwanted interactions, since the 45s would share one leg of the filament supply. I could also run both off the same half of the 5v supply, as it is a 3A tap. I was planning to use a 6AX5 rectifier, and one of my PT options has plenty of amperage on the 6.3v tap to run the preamp tubes and the rectifier tube. I'll be running old Triad HSM-79 outputs, and the PT will require a little tweaking to get the voltage right, but that's easy enough. The goal is to minimize extra iron on the chassis where possible, and that's where I'd like to use that 5v CT option on the PT. Thanks!
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I don't use directly heated tubes, but here's my guess. I hope that other people with more experience can chime in.

I don't think it will work. The cathodes of each tube won't be isolated from each other. They'll have continuity as they will both be connected to the center tap of the 5V winding. The cathodes of directly heated tubes have signal on them, and that will cause crosstalk.

I also am not sure about having two hum pots connected to ground on the same heater winding. I'm not sure if that would cause problems, but I've never seen it done, and heard that you should really only have one reference to ground on heater windings.

Well, I've probably exposed my ignorance on the topic more than anything, but I hope it helps, even if it's not the answer you're hoping for.
 
I don't use them either but he is basically correct^^^
You want an independent filament transformer for each power tube.
 
Squidward said:
I don't use directly heated tubes, but here's my guess. I hope that other people with more experience can chime in.

I don't think it will work. The cathodes of each tube won't be isolated from each other. They'll have continuity as they will both be connected to the center tap of the 5V winding. The cathodes of directly heated tubes have signal on them, and that will cause crosstalk.

I also am not sure about having two hum pots connected to ground on the same heater winding. I'm not sure if that would cause problems, but I've never seen it done, and heard that you should really only have one reference to ground on heater windings.

Well, I've probably exposed my ignorance on the topic more than anything, but I hope it helps, even if it's not the answer you're hoping for.
Click to expand...

Appreciate the thoughts. It's really a question about the filament/cathode combo, as most push pull tube stereo amps share a common ground across all of the power tube cathodes, whether fixed or cathode biased.
 
trainbuftony said:
I don't use them either but he is basically correct^^^
You want an independent filament transformer for each power tube.
Click to expand...

Thanks for the input. If it weren't for the fact I've found two published schematics with a shared filament trafo, one from JE, the other from Glass Audio, I wouldn't have even considered it. Too bad I can't just dig into the PT and separate the CT into two leads. Well, I could, but that's a tedious chore. Just trying to keep the build simple.
 
Doug Weiss said:
Thanks for the input. If it weren't for the fact I've found two published schematics with a shared filament trafo, one from JE, the other from Glass Audio, I wouldn't have even considered it. Too bad I can't just dig into the PT and separate the CT into two leads. Well, I could, but that's a tedious chore. Just trying to keep the build simple.
Click to expand...
Shared transformer isn't the issue as much as shared winding, as I understand it. I have some 10V CT transformers but the 10V side is two separate 5V windings, in that case it would work, and perhaps that is what is used in the examples you found? With the two tubes essentially shorted to one another at the cathode it will probably "work" but with significantly less than desirable results.

Edit: In my case it was for 300Bs needing 5V, in your case it would be a 5V transformer with two separate 2.5V secondaries.
 
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I'd go do a google search and find a 120/5 transformer rated for the current of one 45 filament, buy 2 of em, and by the time they're at my door and the governor's fee is paid I'd expect to have $50 invested.
They'll be about the size of a golf ball and you'll need to figure out where to mount them.
 
Selmerdave said:
Shared transformer isn't the issue as much as shared winding, as I understand it. I have some 10V CT transformers but the 10V side is two separate 5V windings, in that case it would work, and perhaps that is what is used in the examples you found? With the two tubes essentially shorted to one another at the cathode it will probably "work" but with significantly less than desirable results.
Click to expand...

Thanks for the thoughts. The JE Labs and Eric Barbour versions both share the same transformer filament winding. In fact, both use a combined cathode bias resistor and bypass cap. Again, I'm just trying to simplify as well as work with what I have on hand, and saw the CT 5v winding as an option. But it may not work the same way unless they share the same pair of windings, i.e. leave one side of the 5v leg unconnected and use the CT and the other side to get 2.5v for both tubes.
 
trainbuftony said:
I'd go do a google search and find a 120/5 transformer rated for the current of one 45 filament, buy 2 of em, and by the time they're at my door and the governor's fee is paid I'd expect to have $50 invested.
They'll be about the size of a golf ball and you'll need to figure out where to mount them.
Click to expand...

Appreciate the thoughts. Unfortunately, the only 120/2.5v trannies I've found so far are larger than golf balls, and that's why I'm exploring all options before I start cutting holes in one of two chassis I have to work with.
 
I was looking for the JE Labs 45 schematic yesterday when this topic came up.

It said, and I quote, "The filament supply calls for a pair of 2.5V @ 2.5A transformers".

I understand your thoughts about how other amps (push pull) cathodes are tied together with no ill effects, I can even think of a few single ended ones as well. I guess you could try it, probably nothing would be harmed, but it would seem that best practice is each filament to have it's own discreet winding.
 
Selmerdave said:
Can you post an example? The schematic above is for one channel.
Click to expand...

Here are two examples. Note both use a shared cathode bias / bypass components. I just happened to like the SRPP version from the original post better, and it doesn't require the CT filament tap.
 

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Squidward said:
I was looking for the JE Labs 45 schematic yesterday when this topic came up.

It said, and I quote, "The filament supply calls for a pair of 2.5V @ 2.5A transformers".

I understand your thoughts about how other amps (push pull) cathodes are tied together with no ill effects, I can even think of a few single ended ones as well. I guess you could try it, probably nothing would be harmed, but it would seem that best practice is each filament to have it's own discreet winding.
Click to expand...

There are a couple of JE Labs versions. This one shares the filament trafo. No doubt the isolation is better with separates, but I could also do dual monos on the same chassis if it matters.
 

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Triad magnetics F1x transformer makes 3 amps at 2.5v
Costs $13
Mouser has 20+ in stock ready to ship.
The fact that it makes twice the required current means it will just run cooler, and you may need to shunt some extra power in operation.
Perhaps theres a better suited unit, but for well under $50 you should have what you need.
 
trainbuftony said:
Triad magnetics F1x transformer makes 3 amps at 2.5v
Costs $13
Mouser has 20+ in stock ready to ship.
The fact that it makes twice the required current means it will just run cooler, and you may need to shunt some extra power in operation.
Perhaps theres a better suited unit, but for well under $50 you should have what you need.
Click to expand...

You could also run a 2A3 with the 2.5VAC filament current. Same as the transformer I mentioned above.
 
trainbuftony said:
Triad magnetics F1x transformer makes 3 amps at 2.5v
Costs $13
Mouser has 20+ in stock ready to ship.
The fact that it makes twice the required current means it will just run cooler, and you may need to shunt some extra power in operation.
Perhaps theres a better suited unit, but for well under $50 you should have what you need.
Click to expand...

Thanks - I was just on Mouser's site, and this one didn't display. Sometimes their search is a bit wonky. Also looked for a 5v with dual secondary, and they displayed a Hammond option, but didn't see any Triads. Will poke around some more.
 
Squidward said:
You could also run a 2A3 with the 2.5VAC filament current. Same as the transformer I mentioned above.
Click to expand...

I originally wanted to make this a multi output tube amp, but the little OPTs I have max at 40 ma, and I don't want to take a chance with the 2A3 at 60 ma. I have some Hammond 127SE waiting for them.
 
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