JMP50 vs. 5F6 vs. JTM45

[wtk0315]

Well I'm starting research on my next build, but I'm still undecided. I remember playing on an actual Marshall 1987x reissue and I loved it when I was 14-15, but I'll have to listen again to see if I still want it.

So starting questions, is the JTM45 wired to run EL34s or KT66s, or can it do both?

And I've heard mentioned a switch to go from 5f6 to JTM45 sounds, how much extra cicuitry does that entail?

The reason I put all three is that Boothill offers a really affordable bassman chassis and parts kit. The JMP50 is not as popular, so far I've found one on triode and weber. Mojotone only has the JTM45.

Are the three circuits close enough that I could start with the boothill, and go either direction with the build? I know on the JMP50 it's SS rec, and I might have to order extra parts if Dave doesn't stock them.

Anyway, this is down the road, I'm just getting my thoughts out there.

 

[robrob]

From my website's 5F6A page:

Differences between the 5F6A Bassman and Marshall JTM45

The JTM45, Marshall's first amplifier, was an almost exact copy of the 5F6A.

The JTM45 was a head unit where the 5F6A came only as a 4x10" combo.

The JTM45 uses a higher gain 12AX7 in V1 where the 5F6A uses a 12AY7.

The JTM45 has 2.8 times more negative feedback because the feedback source is from the 16 ohm speaker tap where the 5F6A uses a 2 ohm tap.

The extra feedback changes the amp's playing dynamics by tightening the line between clean and overdrive. This makes it easier to use pick attack to induce overdrive. The extra feedback also makes the overdrive tone cleaner & tighter and is the main reason many rockers prefer the JTM45 over the 5F6A. This is why I installed the 3-way 5F6A/5E3/JTM45 Negative Feedback Switch on my 5F6A. The feedback level, tubes and speakers cause most of the tonal differences between the 5F6A and JTM45.

The JTM45 uses a 6.6k output transformer with 4, 8 and 16 ohm taps where the 5F6A uses a 4k with just a 2 ohm speaker tap.

The JTM45 uses a beefier power transformer so there's a little less voltage sag.

The JTM45 bias circuit is adjustable and supports the use of 5881, 6L6, EL34 and KT66 power tubes where the 5F6A was designed for the 5881. Early JTM45's came with 5881 tubes but later units came with KT66's and later still EL34's. [EL34 tubes require a socket jumper from pins 1 to 8 to tie the suppressor grid to the cathode. The power transformer must put out enough 6.3v current to handle the 3.2 amps required for a pair of EL34 or KT88 tubes.]

The JTM45's voltage dropping resistor after the choke is 10k where the 5F6A uses a 4.7k. This would lower the JTM45's voltages to the phase inverter and preamp slightly.

The JTM45 head was usually paired with Marshall speaker cabs which define the "British" tone.

Installing a 12AX7 in V1, adding a feedback switch for the JTM45 negative feedback level and using a Marshall style closed back cab will get you a very JTM45 tone out of a 5F6A Bassman. If your Bassman has adjustable bias you can even install a pair of KT66's or EL34's.

 

[wtk0315]

Thanks for the quick reply. I think I'll build it how you're describing for added versatility and nix the JMP50 idea.

 

[Wally]

One consideration for me would be 'what am I looking for' sonically. IF you are looking for AC/DC Angus Young type of tones.....and the tones of the majority of rock guitarists who used Marshall's, then imho the Md. 1987 with a closed back 4 x 12 and SS rectification is the way to go.
IF you are looking for the Clapton 'BluesBreaker' thing...then the tube rectified JTM 45 is the way to go....with an open back 2 x 12 combo cab??? IF you are looking for a tweed bassman, build that 5F6A.

 

[wtk0315]

I'll have to listen to demos. Like I said, out of the three I've played on the 1987x reissue and the tweed bassman reissue. I've never tried the JTM45, but it seems to be a popular build.

TBH though, I'm not all that picky on tone. I have a 5e3 circuit powered by 6L6GCs. I'd like to try EL34s and a full tone stack which is why the JTM45 appeals to me, as you can set it up for EL34s and also switch it back to a bassman set up. But I don't have a particular tone I'm after, I just want a head to be able to mix and match cabs down the road

 

[Wally]

Get robrob to work out a scheme whereby you can switch from tube to SS rectification and also a switch to lower the voltage the appropriate amount....and you need another switch to adjust the NFB loop's effect. Have two input stages --switchable of course....one for the 12AY7 and one for the 12AX7. Then, you have all 3 amps in one.

In any configuration, though, that basic circuit---5F6A and therefor the Marshalls patterned after the Bassman--- is worlds away from the 5E3...no matter what tubes are in that '5E3'.
As for EL-34's...the difference between them and the 5881/KT66 tubes is less of a difference than one might think when used in the same circuit and biased similarly.

 

[mcabe]

Get robrob to work out a scheme whereby you can switch from tube to SS rectification and also a switch to lower the voltage the appropriate amount....and you need another switch to adjust the NFB loop's effect.

Tubedepot has a kit they call the JTM45+. It has 3-way switchable negative feedback, and both solid state and tube rectification. You can find the schematic, layout, and build instructions over there. Something like this will probably be my next build.

 

[Wally]

mcabe, you missed the voltage issue that robrob mentions and the input stage tube. so, that Tubedepot thing is a JTM 45 with a switch to go to the MD. 1987. You can't get to the 5F6A without addressing that voltage situation and the input stage gain issue...nit-picking...but why not.

 

[wtk0315]

Now there's an idea. I wasn't sure if you could cram the SS rec and tube rec parts in the same chassis.

 

[wtk0315]

I'll probably still go with JTM45 OT though for the flexibility

 

[mcabe]

That would be a lot of switches! Not recommending the tubedepot kit per se, but it has a lot of the required options and could be adapted further. You could modify another kit or source the parts yourself. Freely available schematics and build instructions are nice.

It's really only one spst switch and a tube swap away from a bassman. Although, I guess the purpose is to make everything switchable, so tube swapping is out....

 

[robrob]

Here's a very simple rectifier switching scheme:

If you have rectifier socket backup diodes already installed like me you don't even need the two diodes in the layout, just run two wires from pins 4 and 6 to the switch.

 

[robrob]

Here's my layout of the mod:

A solid state rectifier would only raise the voltage about 10 volts compared to a GZ34 tube but you could use a rectifier tube with more voltage drop to emphasize the difference between tube and SS rectification.

 

[muchxs]

Thanks for the quick reply. I think I'll build it how you're describing for added versatility and nix the JMP50 idea.

One idea you might want to swipe from the JMP is split cathodes on V1. Otherwise you're stuck with the big fat shared cathode cap.

I fabricate my own custom circuit boards with a couple extra tie points. The 5F6A bias supply is a massive waste of space. Tightening up the layout makes room for the filter caps.

Tubedepot has a kit they call the JTM45+. It has 3-way switchable negative feedback, and both solid state and tube rectification. You can find the schematic, layout, and build instructions over there. Something like this will probably be my next build.

Love the build video! It's 15 minutes long. It plods along in minutiae up to minute 13.5 at which point the populated circuit board miraculously appears. It's all wired by minute 14.

I wish I could build 'em that fast...

Another funny tech joke... he's talkin' about low noise while poking towards the master volume that's where one of the input jacks would be.

 

[mcabe]

Another funny tech joke... he's talkin' about low noise while poking towards the master volume that's where one of the input jacks would be.

Haha. Never noticed the placement of the master volume before. Good eye!
But seriously, as a beginner I have found the videos at tubedepot to be really helpful. They have ones on soldering, making turret boards, building guitar cable, and more.

 

[wtk0315]

So both classictone transformers are spec ed with 6.3v 5a. But the bassman is about $8 more. And if I order from Dave I'm assuming the hole will be for a bassman size transformer, so the price difference would probably worth not hacking the chassis. Also classictone offers a 2,4,8 ohm OT.

What sonic difference will the choke values make? 4y for bassman and 20y for jtm45

 

[wtk0315]

Muchxs, what are the sonic benefits of splitting the cathode of v1?

 

[Wally]

Muchxs, what are the sonic benefits of splitting the cathode of v1?

I ain't much...but.....Splitting those cathodes allows one to voice those two triodes differently. That big cap...250-330mfd....passes more low end. The .68mfd that Marshall went to on one cathode yields less low end. So, you have two very different channels. Hendrix daisy chained the two channels in order to add 'meat' from that bassier channel to the brighter channel.

 

[wtk0315]

That's right up my alley then

 

[robrob]

Here's my 5F6A "Lead" channel mod. It uses the cathode resistor, bypass cap, and first coupling cap values from the 1987 Marshall to create a tighter, more modern clean and overdrive tone for one of the Bassman's channels.