Beano Geno
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- Mar 12, 2007
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VERY cool...thanks for sharing this! :salude
I was given an original RS Deluxe OT and built a JTM 45 around it, but wanted to have it 8/15 ohm switchable - it can be done! Looks like they've got it right in this one.
The RS OT is not, unlike more modern transformers, tapped for 8/15 or equipped with separate windings. What it has is three separate secondaries - one at 6 ohm, two at 4 ohm, brought out to 3 pairs of solder posts - visible in the top left corner of the photo showing the board. They've strapped it as per normal 15 (actually 14) ohm wiring, and presented it on the black and blue wires, but also brought out one extra point - the red wire.
The switch is a DPDT with both halves strapped together - so an SPST (common trick with slide switches as they can be unreliable) and gives you either the 6 ohm and both 4 ohm secondaries in series = 14 ohm, or the two 4 ohm windings in series for 8 ohm.
A second blue wire from the switch takes the wiper off to the output jack. The purple wire is the feedback tap to the Presence pot which is from the 14 ohm combined output.
My busy-day-at-work addled brain is telling me that possibly the phase will be reversed 8 ohm relative to 14 ohm. I used a DPDT switch to achieve the same result but preserve the absolute phase. Not entirely sure it matters!
So Marshall got it right - you've got an RS Deluxe equipped JTM 45 with 8/14 ohm switchable output!
If your cabs are 30 ohm I suspect (and the photo doesn't quite show it) the output jacks on the amp are wired in parallel so the two cabs together gives you 15 Ohms. Why the original owner wanted an 8 ohm switchable option is an interesting question.
Hope the above helps.
That is extremely helpful for me as well, thank you!
So Marshall got it right - you've got an RS Deluxe equipped JTM 45 with 8/14 ohm switchable output!
... I was at home in London last week with Joe B (PBUH) ...
You're welcome. I'm sitting here drawing coils and switches. This arrangement will flip the absolute phase when the impedance is changed but does preserve the feedback from the full 14 ohm setting. Might try it on mine as it's currently in bits here.
Shakti, there's a parallel thread running here.
http://www.marshallforum.com/index.php?threads/65-jtm45-matching-set.91144/
We're getting to the point where it's above my pay-grade. I'm anything but an amp tech, but I do have a spare Deluxe OT and will test resistance per the suggestion on the other thread.
WTF – windings, lugs, red wire, taps, series, parallel, data sheets, loads, primary, DPDT
Personally I just plug them in turn them on and play through them.
For goodness sake man, how does it sound, that’s what we want to know.