JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

[mikepukmel]

Hi All,

Id like to wire up a Mercury Radiospares OT to a JTM45 circuit
for 4 ohms. The stock JTM45 schematic has the negative feedback wire running to the 16 ohm tap, on the standard output transformer. Since this OT can't be wired for multiple impedances at the same time, what would happen if I ran the negative feedback to the OT wired for 4 ohms? Is there a fixup circuit I could use?

Thanks
MP

[frus]

The voltage on 4 ohm tap will be half the amplitude of 16 ohm tap (because the impedance is proportional to voltage squared)
So you want to decrease your feedback resistor (27k) to half the value. Try 15k or 12k standard value resistor

[g1]

Which OT in particular? It doesn't have a 16 ohm tap, or the 16 ohm tap will be unused?
If it's there but unused, you still wire up the NFB to it.

[the fatch]

The voltage on 4 ohm tap will be half the amplitude of 16 ohm tap (because the impedance is proportional to voltage squared)
So you want to decrease your feedback resistor (27k) to half the value. Try 15k or 12k standard value resistor

In general, moving from one tap to a tap with half the impedance, divide the feedback resistor value by the square root of two. This backs up frus, moving two taps lower means dividing by twice the square root of two - two.

[Chuck H]

Apparently the OT suffers the same limitation as the older off the shelf Hammond 16xx series in that it's prohibitively difficult to switch secondary impedance. It's probable that Mike can't derive a 4 ohm and a 16 ohm tap from the secondary. In which case he should follow frus's good advice in post #2

[Chuck H]

Here you go:

[Malcolm Irving]

Everything was nice and simple in the old days!

[Helmholtz]

Everything was nice and simple in the old days!

Well, original Radiospares OTs of early JTM45s had exactly this kind of secondary terminals.

[Leo_Gnardo]

Mike P, have you already bought the Mercury "RadioSpares Clone"?

[g1]

I guess if you want to clone something, you can't neglect the flaws.
This reminds me of when the MusicMan amps came out and didn't include the captive nuts in the chassis that newer Fender's had evolved to. Getting your fingers up in there is part of the blackface mojo I guess.

[Chuck H]

Getting your fingers up in there to get the nuts. I'm surprised Leo didn't jump on this opportunity. He's been enjoying some more "colorful" humor of late

[mikepukmel]

Sorry for the late response, thanks.

Leo, yeah I already bought the Mercury RS clone. There are quite a few bundles of wires coming out of that thing! I checked out some of the original JTM45 amps with the original RS and wow, looks like they hacked the holes in the chassis with a meat cleaver. I had read that they had some problems with arcing between the pins since they were all packed so tightly on that little board.

It has the wiring diagram Chuck posted above.
So, it can be wired for 4 ohms, I got the diagram on how to do that, but then its 4 ohms, and there are no other "taps".

Thanks frus, will stock up a few similar range values for alternate feedback resistors. (And I have an old clunker scope now, so maybe I can actually see the damage i inflict!)

[mikepukmel]

Mike P, have you already bought the Mercury "RadioSpares Clone"?

Yep! Already bolted in. Not sure if I did the "right thing" at this point, with all these wires. But it got a bunch of very good reviews. (Maybe it was the designers cousin).

[Leo_Gnardo]

it got a bunch of very good reviews. (Maybe it was the designers cousin).

Likely so. Wouldn't doubt Mercury has a claque audience right under their own roof to write "reviews."

[mikepukmel]

Likely so. Wouldn't doubt Mercury has a claque audience right under their own roof to write "reviews."

so, then, more wires coming out of a transformer isn't better?