Transformer Issue

[basschops1528]

Hey Guys,

I bought a power transformer years ago for the Hoffman 100W Plexi build and I believe the schematic that came with it might be wrong. I don't remember where I bought it from (along with an OT and Choke) but the schematics say Mojo Musical Supply so probably from Mojotone.
Attached is a picture of the transformer with ID and the schematic that came with it.

I just went on Mojotone.com and found the same PT (now 8 or so years later), and the schematic is totally different.
For comparison the following link can be used to avoid conflict with pictures below: http://www.mojotone.com/amp-parts/transformers-british-power/British-Style-Plexi-JCM800-100-Watt-Power-Transformer#.VUwPEJN0mv8 and http://www.mojotone.com/transformers/BRITISH-Plexi-800-100W-PT.pdf

Any ideas why the voltages on the HT secondaries are so different? If the original schematic does accurately represent my transformer will it put out too much for the Hoffman 100W Plexi PS?

Also on a side note, what's up with the green and white wires on the secondaries? (marked on the current schematic from the link as 103V).
Strangely these wires are on the primary side of my PT though I think I moved them myself after putting the cover back on.

Thanks for the help guys

[EL34]

Please stop reporting your post
I am receiving emails that you are reporting this post

You are posting this is schematics
please post in the board that I move this post to

[sluckey]

I bought a power transformer years ago for the Hoffman 100W Plexi build and I believe the schematic that came with it might be wrong.

Maybe. Use your voltmeter to verify.

Any ideas why the voltages on the HT secondaries are so different? If the original schematic does accurately represent my transformer will it put out too much for the Hoffman 100W Plexi PS?

There were different 100W Marshall amps. Some used a bridge rectifier that requires the lower voltage secondary. Some used a conventional rectifier that requires a higher voltage (approx. twice the voltage that would be needed by the bridge rectifier). Again, use your voltmeter to verify what the voltage really is.

Also on a side note, what's up with the green and white wires on the secondaries?

That winding is used for the bias supply. Look at a schematic.

[basschops1528]

Thanks!

[basschops1528]

Sluckey,

Once I measure the PT windings (not installed, just powered) and if they read 340-0-340 for the HT taps (blue-yellow-gray), will the transformer be usable for the schematic above, i.e. be compatible with the post bridge/B+, etc. setup? The B+ should be 450V from what I've heard (EL34s).

[sluckey]

Once I measure the PT windings (not installed, just powered) and if they read 340-0-340 for the HT taps (blue-yellow-gray), will the transformer be usable for the schematic above, i.e. be compatible with the post bridge/B+, etc. setup?

No.

[basschops1528]

Well that's unfortunate. I'm guessing I would need a 100W PT that puts out about half that voltage. Would this transformer work? http://www.mojotone.com/transformers/BRITISH-Plexi-800-100W-PT.pdf

[sluckey]

First, determine what the actual voltage is. But even if it is 340-0-340, it's still usable, just not with that schematic. Look at the JTM 100 series (1959 Super Lead).

[sluckey]

Would this transformer work? http://www.mojotone.com/transformers/BRITISH-Plexi-800-100W-PT.pdf

yes

[basschops1528]

Sluckey,

I like your straightforward approach haha. Thanks a bunch.

What would the reason be for my current transformer not to work?

[sluckey]

Sluckey,

I like your straightforward approach haha. Thanks a bunch.

What would the reason be for my current transformer not to work?

Take a couple minutes to actually measure the voltage. Then I can give you a more precise answer.

[basschops1528]

Sluckey, took the measurements today and I think I have good results. Turns out the company that shipped my PT sent the wrong schematic.  Below are the posted results. What do you think, is it gonna work? BTW I talked to a guy at Mojotone and he said the B+ caps are a voltage doubler, is that true? And what ratings would you suggest for voltage on the bridge.

[sluckey]

That wasn't so hard was it?    That PT is perfect to use with a bridge rectifier. I would use a 1000V/3A bridge or use four individual 1N5408 diodes to roll my own bridge (just because I have plenty).

BTW I talked to a guy at Mojotone and he said the B+ caps are a voltage doubler, is that true?

No. But the fact that the first two filter stages are actually made using two caps connected in series so the voltage rating for the filter is double that of a single cap. IOW, using two caps rated for 350V will give a voltage rating of 700V for that filter stage. This is common practice in many high voltage amps. Maybe Mojo man was talking about that?

[DummyLoad]

steve, we know with a conventional FW and CT grounded, 180-0-180 rms winding has a DC out ~245V. with a FWB and CT to 1/2 cap stack it's DC output is ~500V. so it's a FWB doubler, no?


--pete 

[sluckey]

You can think of the bridge as a doubler since it will produce twice as much dc as a conventional FW rectifier. Maybe that's what Mojo man was talking about? But it's not the caps that doubles the voltage and that's the question I was replying to.

[basschops1528]

Thanks guys, I appreciate the help a lot. Will be building this thing soon!

Btw, if I wanted to do that inrush limiter resistor (soft start) like I mentioned on the other thread, would I use two across the standby sw, or just one?