JMP and Power Transformer Question

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kfowler8
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JMP and Power Transformer Question

Post by kfowler8 » Tue Jul 02, 2019 12:50 pm

Hi guys,

I'm in the process of building a JMP/JCM 800 2204. I have an old 1968 Traynor YVM-1 PA head. This amp has 50 watts, 3 12AX7s, and 2 EL 34s. So pretty similat spec wise to my build. Other than some of the original mustard caps, I'm using the Hammond PT, OT, and Choke off the Traynor. All three are in good working order.

Some questions:

The PT has the following wires coming out of it:
- 2 Black (primary)
- 2 Red (secondary/High Voltage)
- 2 Green (filament)
- Green/Yellow (filament center tap)
- 2 Blue (bias supply)

For the two black primary wires, I assume it doesn't matter which I hook up to the fuse and which to the power switch? So basically there's no defined common or hot lead?

The red HV wires have no center tap. As I understand it since I'm running these to a solid state bridge rectifier, this is not needed, correct?

I won't need the 2 blue bias wires for this build. What's the best way to cap them off? Snip any exposed wire off and heat shrink the ends?

In the MetroAmp plans, there's an 8.2K 1 watt resistor in series with a 10K voltage dropping resistor going into the choke. What's the purpose of the 8.2K resistor? I don't see this on any of the stock JCM 800 2204 schematics.

Coming out of V1b pin 6 is a .022uf coupling cap that says you can use a .0022uf instead. As I understand it the lower .0022uf filters more the bass and allows the amps to focus on the mids and highs for a smoother distortion. Curious as to how much of a difference this makes?

Thanks so much for the help!

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Eric Klinger
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Re: JMP and Power Transformer Question

Post by Eric Klinger » Thu Aug 29, 2019 4:28 pm

Surprised no one answered, though it may be because you'd find all answers in the forum search.
For the two black primary wires, I assume it doesn't matter which I hook up to the fuse and which to the power switch? So basically there's no defined common or hot lead?
You are correct
The red HV wires have no center tap. As I understand it since I'm running these to a solid state bridge rectifier, this is not needed, correct?
Not exactly, what is the model of the PT? Hammond.... 290xx something or other? For a 50 watt, I believe it should be a 300-0-300 or even a 350-0-350... but those are with CT.

I won't need the 2 blue bias wires for this build. What's the best way to cap them off? Snip any exposed wire off and heat shrink the ends?
I would wrap them up, zip tied and heat shrink the ends, separately of course ;-)
In the MetroAmp plans, there's an 8.2K 1 watt resistor in series with a 10K voltage dropping resistor going into the choke. What's the purpose of the 8.2K resistor? I don't see this on any of the stock JCM 800 2204 schematics.
Hmmmm... In a 100 watt it is common to have 2x 10k resistors there to drop voltage going to the preamp. Schematics for 50 watts, that I have seen, do not include 2, but 1x 10k. Perhaps the specimen that George was recreating had those two values, 10 K and 8.2k? Together they simply would offer 18.2k resistance while their voltage handling would add up. I imagine, but do not quote me there, that if this was in a factory marshall, it could have been a random "closest to 10k on hand" thing... OR in this special case to get closer to a desired voltage drop.. ??
Coming out of V1b pin 6 is a .022uf coupling cap that says you can use a .0022uf instead. As I understand it the lower .0022uf filters more the bass and allows the amps to focus on the mids and highs for a smoother distortion. Curious as to how much of a difference this makes?
The difference would be a tighter overdrive. I prefer it - but this is a typical 1987, older spec. I thought you were building a 2204? Either way, it will tighten it up... great if you do a lot of crunchy palm mutes. but the .022 will lend a tad more beef if you do more chording.

Funny that the .0022 is an older spec... seems something more tuned for the 70's higher overdriven superleads.

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