JCM800 2204 50watt PT ?

[NickC]

Greetings,

I may have made a mistake in selecting a power transformer for a JCM800 2204 50 watt build.

I purchased the 100 watt version because it could handle more current for both B+ and heaters. I'd rather it be overbuilt and run cooler, if possible.

The JCM800 50 watt and 100 watt circuits use different rectifiers. The 100 watt PT also has a Bias secondary winding (not on the 50 watt PT). The 100 water uses a full-wave bridge, and the PT CT is taped off and not used (since the FWB is grounded).

Does a power transformer care what type of rectifier circuit it is attached to?

I attached specs for both the Magnetic Components 50 and 100 watt power transformers, and the 50 watt version schematic.

Is is okay to use the 100 watt PT for the 50 watt circuit if I tape off the Bias secondary wires, and keep all other 50 watt circuit components intact (relative to B+ and Bias supply)? I'd ground the PT CT as per 50 watt schematic (not full-wave bridge rectifier).

Thanks very much for any advice/guidance. Much appreciated!

[sabredude]

I've wondered this as well. My conclusion was that you should use the 100W PT for the 100W amp and 50 for 50. THis seems obvious but here's what I looked at:
Two different rectifier models will yield two different B+ and the secondary on the two transformers are significantly different unless this is a mistake by the docs.
I know that you didn't mention the OT's but if you did the same thing for the OT and bought a 100W OT, then just build the 100W amp. Won't make much of a difference in cost now.
If I remember, the 50 w uses a full wave while the 100W uses a full wave bridge rectifier which will have different voltage outcomes. I'd love to see some formulas and a better explanation. I've seen the Hammond guide that shows the output of the different rectifiers but in practical measurement, my voltages never seem to match what their math states they should.
I'm also unclear on trasnformers as I see them say 270v CT which I would assume is the equivalent to 135-0-135 but not sure.
I also noticed that between the two transformers, one has twice the voltage but less than half the current than the other. THis would also seem to indicate a different winding as opposed to just higher current handling.

Not sure that helped, perhaps when somebody smarter answers this, we will both learn!

[NickC]

I've wondered this as well. My conclusion was that you should use the 100W PT for the 100W amp and 50 for 50. THis seems obvious but here's what I looked at:
Two different rectifier models will yield two different B+ and the secondary on the two transformers are significantly different unless this is a mistake by the docs.
I know that you didn't mention the OT's but if you did the same thing for the OT and bought a 100W OT, then just build the 100W amp. Won't make much of a difference in cost now.
If I remember, the 50 w uses a full wave while the 100W uses a full wave bridge rectifier which will have different voltage outcomes. I'd love to see some formulas and a better explanation. I've seen the Hammond guide that shows the output of the different rectifiers but in practical measurement, my voltages never seem to match what their math states they should.
I'm also unclear on trasnformers as I see them say 270v CT which I would assume is the equivalent to 135-0-135 but not sure.
I also noticed that between the two transformers, one has twice the voltage but less than half the current than the other. THis would also seem to indicate a different winding as opposed to just higher current handling.

Not sure that helped, perhaps when somebody smarter answers this, we will both learn!

I interpreted (perhaps incorrectly) the two PT diagrams as:

345- 0 -345 for the 50 watt version (@150 mA)

and

350- 0 -350 for the 100 watt (@ 290 mA)

I based that assumption upon the voltage of the secondary B+ windings on the two schematics. Maybe a bad assumption on my part.


And yes, I hope we both learn something from this.

[martin manning]

The 100W PT (18024) is 350VAC CT, or 175-0-175. It is meant to be used with a FWB rectifier, so you would get roughly 350*1.414 = 495V. The CT is used to keep the voltages of the stacked reservoir caps equal (see schematic on the spec sheet).

The 50W PT (18023) is 690VAC CT, or 345-0-345. It is meant to be used with a FW rectifier, so you would get roughly 354*1.414 = 488V.

Using a 100W PT in a 50W amp means all of your voltages will be higher than if you used the correct PT. Higher filament voltage will shorten tube life.

[NickC]

The 100W PT (18024) is 350VAC CT, or 175-0-175. It is meant to be used with a FWB rectifier, so you would get roughly 350*1.414 = 495V. The CT is used to keep the voltages of the stacked reservoir caps equal (see schematic on the spec sheet).

The 50W PT (18023) is 690VAC CT, or 345-0-345. It is meant to be used with a FW rectifier, so you would get roughly 354*1.414 = 488V.

Using a 100W PT in a 50W amp means all of your voltages will be higher than if you used the correct PT. Higher filament voltage will shorten tube life.

Thank you Martin. Lesson learned. I'll save the 100W PT for another build and use the appropriate 50W transformer. Your help is much appreciated!