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Amp Of The Month (1): 1965 JTM45 Matched Set.

shakti

Member
Joined
Jan 17, 2007
Messages
494
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!
 

springhead

Member
Joined
Feb 12, 2016
Messages
262
That is extremely helpful for me as well, thank you!

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.
 

F-Hole

Well-known member
Joined
Sep 2, 2015
Messages
2,171
Springhead, I can't thank you enough for your thoughtful post. Sincere thanks......:salude

I'll try to take some more photos tomorrow and post them. I've been talking with tech friends about this one, and it has most of them thrown.

Great to have you here, please stick around.

So Marshall got it right - you've got an RS Deluxe equipped JTM 45 with 8/14 ohm switchable output!

It's just beginning to sink in, doubt there's another.
 
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springhead

Member
Joined
Feb 12, 2016
Messages
262
You're welcome, glad to have helped. And I'm not going anywhere - I want to see some more photo's!
 

Unbound Dot Neck

Well-known member
Joined
Jun 14, 2004
Messages
1,623
Head spinning , John and Springhead on the wiring ohm nuances.
:salude
I really don't know how this stuff works, but can appreciate those who do !

Love it,
:jim
 

goldtop0

Well-known member
Joined
Aug 19, 2003
Messages
8,931
Whoa hold the horses........... that's something else and with the unique 18 watt you recently got........:2cool

Thanks for doing amp of the month........this is real drool material:lol

John the grille cloth on the speakers, is that what's called vynair or is it a lighter pinstripe?
 

MM1959

Member
Joined
Sep 23, 2014
Messages
74
John, Congrats on a very exciting score! Fantastic story and pictures.

I have learned a lot from you over the years, so I know the “Amp Of The Month” will be another terrific education!


Major props to Rob.
:salude


 
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shakti

Member
Joined
Jan 17, 2007
Messages
494
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.

Just so I get this exactly right; the turrets on the OT secondaries are wired as they would be for 16 (14) ohm operation, with the black and blue wires being the "common" and 16 ohm wires respectively. Then there's an additional red wire which runs from the lower right hand turret (as viewed in the photo), which can be thought of as the 8 ohm wire/tap, is that correct? So theoretically you could use a standard impedance selector and use the black wire as the common, the blue wire as the 16 ohm tap and the red wire as the 8 ohm tap, right? Absolute phase reversal is a minor drawback, but not really critical unless you are using it simultaneously with another amp.
 

springhead

Member
Joined
Feb 12, 2016
Messages
262
So in 14ohm mode all three windings - 6,4,4 are in series. You can see the links on the right hand two pairs of lugs. The red wire is on the junction of the 6 ohm and one of the 4 ohm windings. So they are just switching between 4+4+6 or 4+4. The negative feedback remains on the top end of the 6 ohm winding, and always 'sees' the three windings in series.

On the RS data sheet in 8 ohm configuration they show the two 4 ohm windings in parallel for 2 ohms, then this is in series with the 6 ohm winding. So all three windings are still in play, compared to Marshall's method which is only loading the two 4's. I'm trying to decide if that's an issue. The windings would have to be rated to handle the increased power, compared to the data sheet method which spreads the load across all 3. Other than that the correct load will be refelected to the primary so all is well there. I guess as the original owner asked for an 8 ohm option it has been happily used in that mode in its long life time!

The alternative is to use a DPDT switch and replicate the two RS 8/15 ohm options, at the expense of the feedback also moving. On my build I made the feedback variable so can compensate.

I'll have a chat on another forum with some transformer brains.
 

springhead

Member
Joined
Feb 12, 2016
Messages
262
I've registered and replied over there, see what he says. The impedance ratio is indeed the square of the turns ratio, but I don't get why he's referring to them as 4 or 1 ohm windings. Maybe I'm being dim! As my amp is mostly in bits I might resort to stripping all the secondary wiring off and getting a sig-gen/scope on there. Got to get back to my day job first!
 

kerryboy

Well-known member
Joined
Jun 20, 2002
Messages
1,199
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.
 

ourmaninthenorth

Well-known member
Joined
Mar 28, 2009
Messages
7,119
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.

If you don't get the technicals sorted out with vintage gear before you turn it on, there will be sounds you don't want to hear...

Only flick that switch when you're sure.
 
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