JTM45 output w/ KT66's -- please clarify how I should label the impedence selector

[mort]

It's the classic JTM45 circuit w/ a bias pot added and using KT66's and a Classictone 40-18039 OT

Here's the OT Marshall JTM 45 Output Transformer

It appears that this transformer is designed to be used with KT66's for 4/8/16 ohm switching, but on the schematic that I used CLICK it shows 8,16,100v

Two questions: what would the 100v ever even be used for? And am I looking at this correctly to say that the Classictone is simply wired differently and I should just mark the selector as shown in the Classictone info?

[wyatt]

Exactly as the Classictone specs show. 4-8-16 and don't use the 100v. The Marshall schematic does not apply, you are not using the same part.

The 100V is a throwback to an old way to run long speakers cables, but it requires a dedicated cabinet with another transformer at the receiving end. It's only included on these OT's because the originals had them. Some companies leave the 100V off, others "bury" it in the transformer.

[mort]

ok cool. I think I had seen on a drawing somewhere where the same OT was used for EL34's or for KT66's and it said the secondary would be 4/8/16 for the 34's and 8/16/100v for the 66's, and it just had me second guessing a little bit. Or maybe that info was erroneous anyway... can't seem to find it now.

Thanks for the clarification.

[wyatt]

ok cool. I think I had seen on a drawing somewhere where the same OT was used for EL34's or for KT66's and it said the secondary would be 4/8/16 for the 34's and 8/16/100v for the 66's, and it just had me second guessing a little bit. Or maybe that info was erroneous anyway... can't seem to find it now.

Thanks for the clarification.

IIRC, Marshall re-purposed a 50-watt OT for the JTM45 RI, so the rule of thumb was to off-set the impedance switch when using KT66's. That's not an issue here.

[Enzo]

Just ignore the 100v thing, and don't try to rationalize a use for it. 100v is one of the "constant voltage" speaker distribution standards. 70.7v is most common here. That system is used to power large numbers of speakers without having to make convoluted series parallel connections to create a certain impedance. CV outputs are used in "PA amplifiers", like Bogens and such.

[g1]

That '66 JTM45 schematic you linked is a bit irksome to me. It's a real Marshall re-issue schematic that has been doctored by Marstran (they inserted their name in it).
They have a custom transformer which includes the 100V tap (because they claim it affects the tone) but it's buried and they've also added a 4 ohm tap. (JTM45 Output Transformer)
So it would be wired exactly like the re-issue, with 4,8,& 16 ohm taps, but they alter the re-issue schematic to show it wired up with 8,16, and 100V taps ?