JTM45 with switchable Bassman mod?

[VintageCharlie]

Hi folks,

for some time i have been gathering the parts for an authentic JTM45 clone, but i wanted it to have a twist. I'm also interested in the tones of a JTM50 and JMP50. Hence i got a power transformer that can be switched for correct voltages of both models. Another idea would be to use an OT that has the 6k/8k AND 3k4 primaries (like the mercury magnetics Radiospares Deluxe OT, or maybe a custom one from Heyboer). This would allow me for authentic JTM45 tones with an easy way to get very close approximation of a JTM50/JMP50 (with a tube / ss rectifier switch). This also got me thinking that it might be interesting to experiment with a tweed bassman 5f6-a type thing in there too. The PT would fork out the necessary 700v, the OT would have the correct 6k primary for the 6L6. From what i see, there are very small differences to the Bassman and these few components could be made switcheable too (push/pull pots).

So what are the actual differences between JTM45 and the Bassman?

What i see it's the following:

- 6L6 (6k primary) vs KT66 (6k/8k primary)
- no 500pF mixer cap on the Bassmann
- 4k7 vs 8k2 2w power drop resistor
- 12AY7 vs 12AX7 in V1

Anything else apart from the speaker configuration and the OT secondary (2 ohm vs 4/8/16)?

I've read that the bassman supposedly has less negative feedback, but i see it too has a 27k NFB resistor?

Are the aformentioned differences the main cause for the cleaner, more organic, less middy and more bassy tone of the bassman, which gives it it's great clean tone, that has a nice contrast to the also great cleans of an JTM45? Is this really all there is to it to get the legendary Marshall sound from a tweed - one extra capacitor and one resistor change and a different v1 preamp tube?

I read this great article by a knowledgeable amp guy: http://www.stevesamps.co.uk/wp/?page_id=289

According to the articel, if i got it right, that's pretty much it.

Thanks guys and regards,

VintageCharlie

[statorvane]

Well, the power supply filtering is stiffer in the JMT45 vs the Bassman. Marshall continued to increase the power supply filtering in the JMT50 and JMP50 amplifiers.

[ToneMerc]

The negative feedback is different; the Tweed Bassman was connected to the 2 ohm tap verses the 16 ohm tap of the JTM45. The output transformers are vastly different 4K/2 ohm verses the RS 6.6-8-9K/ 4,8,16 or the Drake with the buried 100v tap.


TM

[VintageCharlie]

Thanks, forgot about that. Right - so that is something like this?:

Preamp is 8uF - 10uF for the Bassman and early JTM45 alike, right?

Other filtering sections:

JTM45:
PI / Screens / Mains
16uF 32uF 32uF

Bassman:
20uF 20uF 40uF

Keeping in mind the tendency of electrolytics measuring quite a bit higher than the nominal spec, it looks like PI and Mains in a Marshall would be quite close to the nominal specs of the Bassman schematic.
The screens seem to be the position that deviates the most and that might be worth to do as a switchable option between 32uF and 16uF (close to the nominal 20uF).

Am i halfway on the right track with this.?

As for NFB, i read now that due to the OT ratio differences between the amps, even though the JTM45 has the same NFB resistor value, it has in fact about 3x more negative feedback. Is this correct and if so, what value should be the NFB resistor in a JTM45 with a Radiospares or Drake style OT in order to get the equivalent amount of NFB that a Bassman has with it's 2ohm output OT?

I think NFB would be easily made adjustable by a trimpot instead of a fixed value resistor, correct?

So far, to my uneducated eyes, it looks like it would be possible to make such a switchable "bassman" mode without any too intrusive switching systems and stuff. Look like it would be possible to do it as in a rather simple fashion.

- 6L6 (6k primary) vs KT66 (6k/8k primary) - supposedly the KT66 is a british 6L6 variant, so it might not be much of imporance to change the power tubes at all - same primary impedance too, etc.?
- no 500pF mixer cap on the Bassmann - switch for taking the 500pF cap out of the circuit?
- 4k7 vs 8k2 2w power drop resistor - switch for another 8k2 resistor in parallel that results in 4k1?
- 3x lower NFB in the Bassman vs JTM45 - adjustable trim pot for NFB resistor?
- Screens filtering 20uF in Bassman vs 32uF in JTM45 - a dual 16uF cap and switch to go between 16uF and 32uF?
- 12AY7 vs 12AX7 in V1 - important change from what i gather.

Apart from the tube swaps necessary to go from one to the other model, would this be all that should be done in order to get an authentic bassman clone as possible, circuitwise? Only exception, of course, would be the OT in this case.

[VintageCharlie]

right, thanks for the correction - i mixed up the bassman OT primary with another tweed OT model.

So the big issue would be the 4k2 primary in Bassman vs the 6k6 or 8k primary in a JTM45.

I don't know of any multi primary tap OT that would have 4k and 6k. Closest that i know of is this: http://www.gabkits.com/transformers/mar ... -tap-p-456

Radiospares clone with 3K4, 6k6, 8k. Would be ideal to have the 4k tap in there somewhere. Might be worth a try to get a quote from Heyboer for a custom made OT.

Apart from that i suppose the Fender OT was built with different materials/techniques so it would not be a 1:1 bassman sound, but the closest possible, wiothout going with a bassman OT clone.

Other than the OT, is there something still missing from jtm45 vs bassman equation after all the aforementioned mods would have been done?

[statorvane]

This is a lot of mods. I would think building a Bassman with the stiffer Marshall filtering would be a start. Pop in a 12AX7 into V1 when you want the Marshall sound and a 12AY7 when you want a Fender sound. The V1 tube makes a big difference in the tone. Add a switch for the NFB resistor or make it variable with a pot.

[VintageCharlie]

I found out from Heyboer, that they would be able to make a Radiospares style OT with 3 primaries, which ever you specify for 110$. An OT with 4k, 6k6 and 8k primaries would be a heck of good base for a lot of experimentation with various circuits and mods - it would be able to take practically every tube in a push pull configuration - EL34, 6L6, KT66, 6V6, EL84, etc. And it seems to me like it might be worth playing with a JTM45 that can approximate a JTM50/JMP50 and a Bassman.
But, yes, you're right, that's quite a few switches and mods to get there... On the other hand, most of them should be doable with simple push pull pots instead of the usual pots - it's just adding a capresistor and removing a cap/resistor. Even the layout would not have to be changed for that - the extra parts can be populated onto the switches.
I would leave out the switchable filter mod idea, as that is probably not the most important tonal change?

I do prefer the drive and the semi clean of a JTM45 (judging from sound clips i've heard over the years), but the Bassman has a fantastic clean that i like more than any other later Fender model. I wouldn't want to build a whole new amp for that and will try to incorporate this idea into my JTM45. If it doesn't work as expected and the most magic lies within the OT, i will just revert it to a standard JTM45.

Thanks for all your advice and input! I will report back here if and when i manage to pull this off.

[statorvane]

but the Bassman has a fantastic clean that i like more than any other later Fender model.

I absolutely agree - no reservations here!.

FWIW, I added a couple of switches to my JMP50 clone to switch between split and shared cathode on V1 (P50 - T45) and a switch for low/high negative feedback. The power supply rectifier, filtering, and bright/norm channel mixing is pure JMP50 though.