Quote Originally Posted by Ch0jin View Post
Not that I'm an expert, but I did quite a bit of reading on the topic while I built my JTM45, and I believe the difference between the original Bassman and the JTM45 are a little more substantial than that. IMHO, it breaks down like this.

The Bassman has a 12AY7 as V1 and the JTM has a 12AX7. That's a big difference of itself. I've just received a NOS 12AY7 so I can try it in my JTM style amp and see what happens. I would expect much less gain.

The two have completely different output transformers. The Fender with a single 2 Ohm tap, the JTM with multiple taps. They also have different primary impedance's.

I think the chokes were different values

I think both started with 5881 bottles, then the JTM pretty quickly went to KT66's

Then there's the speakers and the various open and closed back designs used.

So yeah, for sure the JTM was based on the 5F6-A Bassman, but I don't believe, based on what I've been reading, that they are as close as often mentioned on the net. Which is, of course, why they sound different. I read one post somewhere that said the difference in sound was down to the fact that one had American parts and one British. I laughed about that for a while.
Certainly a few more changes. The speakers did make a real difference. Same basic circuit, different results. A bit like a smart cover version that sounds original in itself.
In the early days Marshall (and by Marshall I mean Ken Bran) sourced parts from Henry's electronic shop in the Tottenham Court Road at the far end of the no 7 bus route from the shop in Hanwell. As Marshall grew the parts used became more consistent.