Epiphone Pacemaker 50T PT short

[maxkracht]

No idea where to find the best option, that 200v torrid is probably the closest electrically. They might have a 50va one that would work as well.

I think your ideal, is something like:

6.3v @ 2-3 A
180-0-180 or 350Vct @100-200mA (can probably go somewhat lower current without problems)

Your power tubes don't want to go over 250v, but you can probably exceed that a little bit without worrying too much.
Unless I'm missing something, the hammond line is mostly 550Vct
Some lower voltage but high current stuff that probably wouldn't fit, some lower voltage but no center tap that would require SS rectifier or a hybrid rectifier.
Do you have the transformer bolt spacing and hole dimensions?

[angelodp]

Hi Max, mount holes are 2"x 2.5". Chassis cut out is 2"x1.75"

Sorry if this is basic, can you explain the difference between 180-0-180 and a 350Vct. Is the 180-0-180 also a CT?

A

Then there is this?


https://edcorusa.com/products/xpwr013-5 ... 6-3v-4a-ct

[sluckey]

can you explain the difference between 180-0-180 and a 350Vct. Is the 180-0-180 also a CT?

It's simply expressing a center tapped winding in two different ways. 180-0-180 is the same as 360Vct and 350Vct is the same as 175-0-175.

[maxkracht]

Whoops, I meant to say 180-0-180 or *360Vct, can also have two separate 180v windings and connect them in series to make your own center tap, like that torrid transformer. Different manufacturers write it in different ways, and I get confused easily...

With a full wave SS rectifier, to find your estimated DC voltage, you take the 180 x 1.41 = 254v but with your tube rectifier you take off a bit. Steve said it is around 1.3 with that rectifier tube. I don't actually know how to find that number, but 180 x 1.3=234

For that Edcor it would be 255 x 1.3 = 332

You drop some voltage through the output transformer and the cathode resistor, so B+ can be a bit over 250v and keep the power tubes within their maximum plate to cathode voltage, and you can probably exceed that maximum without killing the tubes, especially if you have big enough screen resistors and don't bias it too hot. Probably still want to knock off a bit from the Edcor, but close enough. I'm not great at math/theory so maybe someone more informed will correct me, but you can ballpark most of this stuff.

[angelodp]

Thanks for clarification.

A

[maxkracht]

I got curious about the math. I think the 1.414 number I had in my head was for an "ideal" circuit without any losses, not a real world thing. Steves 1.3 figure is of course correct.

More accurate numbers for other rectifiers:

Diode (full wave): 1.37, GZ34: 1.36, EZ81: 1.30, 5U4B: 1.28, 5Y3: 1.25

https://thesubjectmatter.com/calcptcurrent.html

[maxkracht]

If you are going edcor, these are closer electrically. I didn't check physical dimensions.

https://edcorusa.com/products/xpwr068-4 ... -6-3v-2-5a
https://edcorusa.com/products/xpwr186-3 ... v-4a-5v-2a

[sluckey]

More accurate numbers for other rectifiers:

Diode (full wave): 1.37, GZ34: 1.36, EZ81: 1.30, 5U4B: 1.28, 5Y3: 1.25

Those numbers show the relationship between different rectifier types operating at only ONE load current. But if you change the load current (such as using a more powerful amp, those exact numbers no longer apply. That's because the voltage drop across a rectifier tube is not a fixed value. It depends on load current. Light load gives less voltage drop, resulting in higher B+, and heavy load gives more voltage drop, resulting in lower B+.

So, to say that a 5Y3 will always give a B+ that's equal to 1.25 times the secondary voltage is just false. The B+ value is dependent upon the load current ***AND*** the current capacity (internal resistance) of the PT. Change the load current and/or the PT and that 1.25 number becomes meaningless.

[maxkracht]

Meaningless, or just not accurate enough? I did not intend to imply any of these are exact, or static, figures but seemed good enough to find an appropriate transformer. That calculator gives rough estimates, but it seems like a handy tool if you understand the limitations.

[sluckey]

The only thing that is meaningful about those numbers is their relationship to each other. IOW, you have an amp that uses a GZ34 but the B+ is higher than needed. So replace the GZ34 with a 5Y3 and the B+ will be less.

For example, I have an amp with a 330-0-330 HT winding. To say a 5Y3 will give you a B+ of 330 x 1.25 = 412.5VDC is wrong and therefore MEANINGLESS because the B+ will be greatly influenced by the PT internal resistance and the load.

I actually have a perfect example of this. I have a fixed bias Deluxe Reverb clone and a cat biased 5E3 Deluxe clone. Both use the same PT and the same 5Y3 rectifier. The Deluxe Reverb has a 428V B+. Not too far from your calculated 412.5V. However, the 5E3 has a 345V B+. That's a long way from your calculated 412.5V! The reason is the load. The Deluxe Reverb is biased at 24mA per tube while the 5E3 is biased at 38mA per tube, much heavier load, so the B+ is considerably less due to the voltage drop across the 5Y3 and the PT internal resistance.

Here are the schematics of my example amps...

http://sluckeyamps.com/tdr/tdr.pdf
http://sluckeyamps.com/5e3/5e3.pdf

I've seen those numbers thrown around all over the net. And people who don't know any better tend to take them as gospel. Then they wonder things like "Why is my B+ too high?" I suppose some well meaning person actually took all those rectifier tubes and plugged them into his amp and came up with those numbers. Great. The numbers work well for his particular amp. But those numbers don't work in any other amp.

And that's why I say they are meaningless.

[angelodp]

At Edcor a standup, XPWR105 - 360V@250mA CT & 6.3V@3A. might just be the ticket. I would take Steve's advice and put the PT in the cab and run wires up to the chassis in a protective sleeve. It's about $125 shipped. Just about to pull the trigger.

Thanks A

[maxkracht]

I've seen those numbers thrown around all over the net. And people who don't know any better tend to take them as gospel. Then they wonder things like "Why is my B+ too high?" I suppose some well meaning person actually took all those rectifier tubes and plugged them into his amp and came up with those numbers. Great. The numbers work well for his particular amp. But those numbers don't work in any other amp.

I'm not religious, just assumed those were some kind of average value that could go 10-15% in either direction. I should probably be more suspicious when there is a number and I don't know where it came from. Need to spend more time reading my big red tube book instead of stuff on the internet... I am usually trying to figure out how to make what I have available work instead of shopping for something obscure and specific, so I don't think about this often.

In this situation, is it better to buy the transformer with a closer voltage but higher current rating, or slightly higher voltage with a current closer to what the amp is expected to need? Any useful rules of thumb?

[sluckey]

It's best to choose a PT with an appropriate voltage ***AND*** current rating for the load. Then cross your fingers. It's not often easy and not often kind.

[angelodp]

Given that what do you make of my selection at Edcor? It's between the unit that has two 6.3v windings and the unit with a single 6.3v and very close on the Secondary Voltage.

[maxkracht]

Maybe email edcor with your variables? If it's made to order anyway, perhaps they can make adjustments or at least steer you in the right direction.