Help me specify a JTM45/100 clone

[BlueX]

I would like some help to specify a JTM45/100 clone, “Hendrix” circuit. Been searching Internet, and I see some of the frequent usernames here coming up, but some info is no longer available and there seems to be changes in availability for e.g. trafos from different suppliers. Therefore, I’m looking for some up-dated recommendations.

The plan is to build an amp where the relevant parts of the tone circuit are as close as possible to the original, to get close to that sound. I’ve no intention of building a faithful replica. Instead, I will follow modern electrical safety standards, and want good reliability. I can pay extra for components that have the right influence on the sound, but do not need vintage or NOS.

So, would be grateful if you can point me to useful schematics, and layout, as well as components you think are worth looking into.

I’ve built some pedal kits and a 5E3 kit, and that all worked out fine. After that I built an 18W amp using schematics and layout, and sourced the parts myself. Also for this build I plan to source parts myself.

 

[fitz]

British Style JTM45/100 "Experienced" - Ceriatone

Are you experienced?

ceriatone.com

 

[neikeel]

https://www.marshallforum.com/threads/my-45-100-jh-clone.101156/
Plenty there to be getting on with.
Use dual pole mains switch (live neutral) fuse both limbs of HT, IEC socket, Larry grounding.
That should do it for your specs.

 

[BlueX]

https://www.marshallforum.com/threads/my-45-100-jh-clone.101156/
Plenty there to be getting on with.
Use dual pole mains switch (live neutral) fuse both limbs of HT, IEC socket, Larry grounding.
That should do it for your specs.

Thanks! Don't think this thread showed up during my searches.

British Style JTM45/100 "Experienced" - Ceriatone

Are you experienced?

ceriatone.com

Have some reading now for the dark evenings ahead.

 

[playloud]

For the OT, I highly recommend Soursound. Noticeable improvement on the Marstran/Heyboer I had in mine previously (plus it hasn't shorted internally!)

Here's a direct link to the schematic above: https://www.marstran.com/HendrixAmpMod.gif

And pics of the Hendrix amp for reference: http://www.amparchives.com/folder/458/

 

[BlueX]

For the OT, I highly recommend Soursound. Noticeable improvement on the Marstran/Heyboer I had in mine previously (plus it hasn't shorted internally!)

Here's a direct link to the schematic above: https://www.marstran.com/HendrixAmpMod.gif

And pics of the Hendrix amp for reference: http://www.amparchives.com/folder/458/

Thanks!

Choice of trafos might not be the easiest decision. However, the only Marshall type trafo I can find on Soursound website is a 50W OT. When did you get yours, and do you have any model number?

 

[playloud]

Thanks!

Choice of trafos might not be the easiest decision. However, the only Marshall type trafo I can find on Soursound website is a 50W OT. When did you get yours, and do you have any model number?

It's not on the website I believe, but contact Bryan Sours and he should be able to make one for you. Model code is 1202-84 (like the original Drake). Tell him I sent you if you like

 

[BlueX]

A couple of questions on the Marstran schematics above:
- What do the labels for the OT secondaries mean (8 - 16 - 100V)? I see this also on the Dual OT 45/100 that @playloud posted yesterday (see below).
- For V4 and V7 there is a 1,5k resistor connected between p5 (I think) and a 220k resistor (and a 0,1uF cap) each. However, for V5 and V6 p5 is connected directly to these 220k/0,1uF. Shouldn't all KT66 be treated equally? (the Ceriatone layout, above, has a 1,5k resistor each to p5 on all four V4, V5, V6, and V7)
- There is a 0,05uF cap between recitfier diods and ground (snubber cap?). On the Ceriatone layout this is rated 1000V. Is that a relevant rating, or is e.g. 630V sufficient?
- Type of diods? 1N4007? (both rectifier x6 and bias x1)

@neikeel : I cannot see the snubber (?) cap at the rectifier on your build. Is it omitted or placed elsewhere?

Dual OT 45/100 + pinstripe cabs on Reverb

A little idiosyncratic, but definitely a good-looking stack! https://reverb.com/item/73544949-marshall-plexi-jtm-45-100-pinstripe-full-stack-circa-1965-66 Haven't seen a tremolo model with the dual OTs before. Wish there were more pics. I assume William from Houston isn't that William from...

www.marshallforum.com

 

[playloud]

A couple of questions on the Marstran schematics above:
- What do the labels for the OT secondaries mean (8 - 16 - 100V)? I see this also on the Dual OT 45/100 that @playloud posted yesterday (see below).

The original 1202-84 does indeed have those secondaries: no 4 ohm tap, but instead a 100V tap for PA line speakers (which is now a useless anachronism). The Drake 784-103, used in JTM45s, also had these secondaries. [Edited in light of neikeel's correction below.]

Most modern repros of the 1202-84 (Marstran, Merren, TAD) replaced this 100V tap with a more useful 4 ohm tap but - as Bryan Sours noticed when tearing down my Marstran - the 100V tap is still there, just buried internally. (In fact, it is an important element of the winding geometry). Bryan explained that adding the 4 ohm tap actually increases the arc potential (more wire in the same space, basically), so he suggested reverting to the original taps (and wire colors) on mine. You can make the case either way, but I never use the 4 ohm tap, so I went with the more period-correct option.

- For V4 and V7 there is a 1,5k resistor connected between p5 (I think) and a 220k resistor (and a 0,1uF cap) each. However, for V5 and V6 p5 is connected directly to these 220k/0,1uF. Shouldn't all KT66 be treated equally? (the Ceriatone layout, above, has a 1,5k resistor each to p5 on all four V4, V5, V6, and V7)

This is a classic Marshall 100W move: grid resistors for the outer-most power tubes only. My '69 plexi is the same. Putting them on V5 and V6 makes a lot of sense, if you're willing to sacrifice historical accuracy (fine balance!)

- There is a 0,05uF cap between recitfier diods and ground (snubber cap?). On the Ceriatone layout this is rated 1000V. Is that a relevant rating, or is e.g. 630V sufficient?

630V is sufficient - at least that's what been in mine for a while now and hasn't caused any issues. The snubber is optional though.

- Type of diods? 1N4007? (both rectifier x6 and bias x1)

I think Neil used (technically superior) UF5408 diodes in his, and a NOS "top hat" diode for the rectifier. I used a modern top hat diode from Valvestorm in mine - it looks the part (apart from being oriented in reverse), but won't make a perceivable difference when it comes to sound/performance. Valvestorm also supply 1N5408 diodes, which have higher current handling (3A)/are physically bigger than 1N4007s and what I used.

 

[BlueX]

Thanks @playloud , very good information as always!

The original 1202-84 does indeed have those secondaries: no 4 ohm tap, but instead a 100V tap for PA line speakers (which is now a useless anachronism). There was a 50W Drake OT (784-74) that also had these secondaries and was used in the first Drake-equipped JTM45s.

Most modern repros of the 1202-84 (Marstran, Merren, TAD) replaced this 100V tap with a more useful 4 ohm tap but - as Bryan Sours noticed when tearing down my Marstran - the 100V tap is still there, just buried internally. (In fact, it is an important element of the winding geometry). Bryan explained that adding the 4 ohm tap actually increases the arc potential (more wire in the same space, basically), so he suggested reverting to the original taps (and wire colors) on mine. You can make the case either way, but I never use the 4 ohm tap, so I went with the more period-correct option.

I need to look into this once I get response from trafo makers. So far I've only found JTM45/100 trafos at TAD, and the Ceriatone kit of course. Tempted to go vintage style.

 

[neikeel]

Just because it is in print above but no, the -74 was 4,8,16. The -103 was 8,16 and 100v
I used a snubber that went pop and never got round to replacing as it did not misbehave without. If you do I’d use 1000v. Remember ac and D.C. ratings of same component differ!
The 1k5 are just to stop oscillation. Not an issue with a 45/100 unlike split cathode Superleads.

 

[playloud]

Just because it is in print above but no, the -74 was 4,8,16. The -103 was 8,16 and 100v

Correct of course! I'll edit the above

 

[playloud]

Thanks @playloud , very good information as always!


I need to look into this once I get response from trafo makers. So far I've only found JTM45/100 trafos at TAD, and the Ceriatone kit of course. Tempted to go vintage style.

P.S. If you want to go the TAD route for the OT, I believe TAD themselves are out of stock, but Banzai (Germany) and Tone Factory (NL) have them - at least last I checked.

For the PT, I believe only Marstran (and Ceriatone?) do the 1204-43 currently. Merren used to.

 

[BlueX]

Just because it is in print above but no, the -74 was 4,8,16. The -103 was 8,16 and 100v
I used a snubber that went pop and never got round to replacing as it did not misbehave without. If you do I’d use 1000v. Remember ac and D.C. ratings of same component differ!
The 1k5 are just to stop oscillation. Not an issue with a 45/100 unlike split cathode Superleads.

Thanks, appreciated!

I'm gathering info before ordering parts. When looking at all pictures of old Marshalls it's tempting to make the build also visually correct, but I think that's a long stretch when starting new in this area.

 

[BlueX]

P.S. If you want to go the TAD route for the OT, I believe TAD themselves are out of stock, but Banzai (Germany) and Tone Factory (NL) have them - at least last I checked.

For the PT, I believe only Marstran (and Ceriatone?) do the 1204-43 currently. Merren used to.

Banzai and TF new to me. I will add to my "resources" list. Thanks!

 

[Marcomel79]

Look forward to your build!

 

[neikeel]

Might be a race with me if you don’t crack on as Autumn is coming and so is the Marstran 1202-84 and choke I ordered in Feb. for another JH clone.

 

[BlueX]

Might be a race with me if you don’t crack on as Autumn is coming and so is the Marstran 1202-84 and choke I ordered in Feb. for another JH clone.

I might already have lost. So far, no one has responded to my requests for trafos.

 

[_Steve]

Are Heyboer not taking direct orders anymore?

 

[BlueX]

Some more technical questions related to the Marstran schematics, if someone can help me.

Power rating for resistors.
- The four 470R to p4 (grid) on V5-7 are rated 5W on @neikeel 's photos (if I'm not mistaken). The corresponding 1k on Ceriatone layout also have 5W rating.
- There are three 10k (or 8k2) between the choke and the anods on V3, V2, and V1. These are rated 2W on Ceriatone schematics.
- Are 470R/5W and 10k/2W (or 8k2/2W) correct rating for the above?
- Any other resistors that need special rating, or is it OK with 0,5W CC or 1W CF/MF?

Recitifier snubber cap
- Is it OK to use two polyester 0,1uF/600V in series, to get 0,05uF/1200V? Marstran schematics only say 0,05uF, and Ceriatone layout 0,047uF/1000V.
- Difficult to find suitable through-hole caps