JTM45 Impedance

[Littlewyan]

I may have a transformer for a JTM45 soon with an impedance of 6.6K. Now I did a bit of googling and it seems most people prefer 8K impedance in the JTM45 with 5881s and KT66s. What is the difference tonally?

[Roe]

6k6 gives more power, punch and bandwidth

[Littlewyan]

Thanks.

It seems most people prefer 8K.

[wyatt]

We have two different "eras" here.

The earlier offset and split-front JTM45's ('62 thru ~'64) used the Radio Spares output transformers. These were typically wired for 6.6K or even 4K (some combos) impedance. These amps have more output and more clean headroom thanks to the better match of the primary impedance (as much output and clean headroom as a 5F6a Bassman); and people find the "hi-fi" Radio Spares OT offers more clarity and sonic complexity.

The later JTM45's, particularity the '65 and '66 models, used a Drake OT and were wired for 8K. This is a bit of a mismatch, and results in lower out (30-35 watts) and therefore earlier overdrive and break up.

What's best? That's a matter of taste.

Ultimately...electronically...it's as simple as Roe states, more power, output and punch from 6.6K on the plates with KT66's, since the optimal match for output in a Class AB, push-pull is (IIRC) 4k per the tube data sheet.

[Littlewyan]

Thanks that really clears things up. Been doing a lot of googling but could never find a definitive answer as to when 6.6K was used.

[rp]

Thanks that really clears things up. Been doing a lot of googling but could never find a definitive answer as to when 6.6K was used.

+1 excellent.

[Roe]

the RS OT had taps for 6k6, 8k and 9k. Marshall are generally believed to have used 6k6 and 8k

[wyatt]

the RS OT had taps for 6k6, 8k and 9k. Marshall are generally believed to have used 6k6 and 8k

I've also read is that Marshall would wire up for 8K but wire but halve the secondary wiring up the combos to get a 4K primary.

But it's really hard to tell the speculation from actual first-hand knowledge on this stuff, and it's not like I get a chance to see too many (any?) early '60's combos. It's not something I explore either, I prefer the Drake 8K.

[Reeltarded]

The higher impedance sounds better. Has character. Feels like a Marshall.

[ToneMerc]

the RS OT had taps for 6k6, 8k and 9k. Marshall are generally believed to have used 6k6 and 8k

The stock 65' with the RS I worked on the tag was wired for 8K. I have even read that a few were wired 9K by mistake. I prefer the 8K myself.

TM

[Littlewyan]

Ok well basically I've already got the transformer but its in a TW Express Amp. I don't think its very well suited to that amp at all so may replace it with the correct Pacific TW Express OT and then build a JTM45 with the OT I take out. Don't really have a choice with regards to impedance as its hard wired to 6.6K.

You know I wonder if Angus Young's JTM45 uses 6.6K as its very different sounding to other JTM45s that I've heard. Very clear and punchy.

[wyatt]

You know I wonder if Angus Young's JTM45 uses 6.6K as its very different sounding to other JTM45s that I've heard. Very clear and punchy.

Which tracks?

The AC/DC-JTM45 connection is always over-blown.

[Littlewyan]

On the Black Ice Album. Then again thinking about it that sounds like he has a big bright cap and has the volume down low. There has always been question marks when it comes to what amps ACDC use. In interviews they said they never used Master Volume amps, however in the early days thats what they had on stage!

[johnnyreece]

Hmm...I always thought Angus used a Super Lead Plexi, and that Malcolm (from time to time) would use the 45...

[wyatt]

On the Black Ice Album. Then again thinking about it that sounds like he has a big bright cap and has the volume down low. There has always been question marks when it comes to what amps ACDC use. In interviews they said they never used Master Volume amps, however in the early days thats what they had on stage!

People always chase studio tones like they were recorded straight. They aren't. There is an great deal of time spent on choosing mic type and placement...the effect of which can be night and day. There is specialty recording equipment (most famously for AC/DC...the Diversity System). Often tape speed manipulation is used to thicken up tone without loosing clarity. But the biggest effect is that each track is heavily EQ'd to remove large chucks of the frequency to make each instrument sit in the band mix.

With Angus, the latter means removing all the low-end and lower-mids to keep him distinguished from Malcolm. If you were to isolate some of the most famous lead guitar work in the history of rock, you would release how thin and anemic the tone is after the engineer/producer get done with it. That's because it's only one part of a whole. Malcolm is there to thicken up the guitar tones.