JTM45 build. Authentic Tone!

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MHProd
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JTM45 build. Authentic Tone!

Post by MHProd »

DISCLAIMER: The way I hear is different from yours! The following is simply a compilation of suggestions based on my experience. Let’s remember that no matter how many details are shared, it will never account for the totality of unspecified factors (guitars, pickups, cabs, tubes, settings, speakers, etc…) around what constitutes “tone”. After a few years of building amps as a Sunday afternoon hobbyist, I really believe in taking the time to TRY things out for yourself when it comes to anything guitar related.

OK guys, here are the results of many, many hours of A/B’ing a JTM45 clone to 3 original specimens I am very lucky to own. For a few years now, I tried to recreate the tonal characteristics of the original amps into a clone and trust me, it was a rather frustrating experience. However, I am now confident the mods I made to the clone bring the modern reproduction of this amp into the same ballpark as the originals. I decided to call the mods “Ballbreaker specs”.

Background: Since 2008, I’ve been the owner of 3 beautiful original JTM45s. A ’64 coffin logo/split front, a ’65 block logo and a ’66 script logo. The 65 and 66 sound very much alike. The 64 has the RS Deluxe OT where the low medium and medium response is a bit more scooped than the 2 others which have OTs made by Drake. All 3 are set up the exact same way (component value, close inspection did not reveal any significant drift) and all have NOS glass. After talking to many different people and making a lot of research, I was able to confirm all 3 amps are genuine and original amps made by Marshall and none of them are lash ups from the British amp mafia (most coffin and block logos often seen for sale are fakes made from ‘donor’ amps. Be very careful should you consider buying one. The “Bluesbreaker” and 18 watt combos are even worse.)

Goal: I wanted to be able to recreate the tone and dynamics of the original amps AND making it sound a bit more precise in the low end. On my originals, I have no problems turning the bass on 4 or 5 and the chords ring out beautifully although not very precise in the low end (not talking about mud here). The type of glass you use in the amp is also a very big factor in the final result, more on this later.

Straight up clone results: A few years back, I decided to build a clone. Piher resistors and mustard caps everywhere. A friend gave me a Merren Audio PT he wasn’t using and I chose to buy a MM OT. The build went well, the amp worked just fine but it didn’t sound nowhere near like my originals (much stiffer sounding, way too bright). I started exploring forums and was recommended a Marstran OT made by Heyboer. I switched the OT and the amp sounded better (the clean in particular, overall tone was much sweeter) but still, nothing like my originals. As you pushed the amp past 6, it became very compressed and didn’t have anything to do with my original examples. It wasn’t sounding bad at all, fairly thick and juicy leads, but the tonal response and dynamics were way off. Finally, I got in touch with Merren and ordered his OT. That was a big step forward for tone.
Even though it was getting closer with Merren’s transformers and the original layout/component values, the amp still sounded very different from the originals : the bandwidth was much narrower, mids too present, low end was nowhere near as open and the top end wasn’t sounding sweet. Also, the amp sounded flatter (dynamic range), louder AND was overdriving quite a bit more compared to the real ones. Again, it sounded like a good amp but it was not behaving or responding like an original. At this point I had tried different type of glass, including NOS everywhere. The amp stayed on a shelf for a while, I would plug it in every so often, but I quickly went back to my originals when I was in the mood for these types of sounds. I went on Youtube and listened to many different clones from hobbyists to builders and I felt a bit relieved as I failed to hear any “clones” which successfully captured the sound of originals.

After reading posts on different forums, I decided that maybe I could change things in the amp’s layout to compensate for what I was hearing vs. the originals. I couldn’t tell you what was my logic behind trying different layouts, there was a bit of shooting in the dark for a while there, but eventually, the results became promising and I started playing the amp more and more, giving myself weeks of breaks to come back to it with fresh ears.

I performed the last changes a few months ago and I now feel comfortable with the results meeting my expectations of getting the amp to display most of the attributes of my original JTM45s. I believe the combination of the following changes take the amp much closer to a real specimen:

Pots: measure them! The original pots all measure 10% above their value. Most modern pots measure 10% shy of their value!
Merren Transformers
Change V1 820r/250uF to 1K2/1uF
Drop the mixer Rs from 270K to 220K and use a 560pF cap on the bright channel (100pF on Vol.1 pot)
Change the preamp filtering to 33uF
PI: Change the 10K/820R to 47K/1K2 (ala Vox) and lower the coupling caps to .068uf (I wouldn’t go lower). Also in the PI, change the 220Ks to 120Ks.
Add 1K 2W resistor before the screens and use 620R resistors on the tubes (instead of the usual 470R).
Up the usual screen grid 5,6K value to 12K
Drop NFB resistor from 27K to 18K on 16Ohms tap.
Mains filtering is 47uF, screens 33uF, PI 22uF
DO NOT use Silver Micas. Ceramic only. I’ve used NOS and measured them before soldering.
Last but not least, (assuming your OT has a primary Z of 8K) install a conjunctive filter of 10K(5W or more)/2200pf(1Kv or more) between the plates (pin 3 V5 > 10K > 2200pF > pin 3 V4).

Caps: all NOS mustards (I believe you could get away with Sozos and still get somewhat similar results). For the filtering, I used a mixture of FT and Rifa.
Resistors: I used NOS Piher and some AB Carbon Comp everywhere. I’m pretty sure any good quality modern carbon film will be just fine.

Make sure your PT is right! 150ma is correct (200ma won’t hurt but don’t get more than that!). Also, make sure it is capable of bringing the B+ around 445V on the plates, just like the originals. Anything less than 430V will affect the character of the amp and will make the low end muddy.

Glass: NOS is always best. EVERYWHERE! At least, in the preamp, V1 and V2. For the GZ34, I strongly suggest Sovtek. Avoid using the chinese 5AR4! It changes the amp’s response by quite a large margin. I also tried different tubes in the preamps (AT7, AY7, 5751…) to compensate for the dynamics and got mixed results. I went back to AX7s.

Original KT66s are amazing sounding tubes. In the original amps especially, there is a huge difference between NOS and reissue glass. In the build, the difference is still there but a lot less. What sounds very good to my ears is a pair of Chinese KT66 biased at 80%. I’ve played the amp like that for long periods of time and the tubes are holding up just find. I did try the Russian Genalex KT66s with different bias settings but was rather disappointed with the results. Also tried various NOS and new 6L6s but quickly went back to KT66s.

The amp is now very enjoyable to play. It is still a little different from an original but it is a lot closer for sure when it comes to authenticity of tone. The low end is not muddy and the bass pot is very useable (played with original greenbacks in a closed cab and silver alnicos in an open cab), the break up/overdrive characteristics are virtually identical to my originals and the top end is sweet. The mids are still a little more pronounced than my originals. The harmonic content also seems to be a bit richer in the originals.

Again, all of this is based on my experience playing original examples of the JTM45 vs. a regular clone. The goal is not finding out what is good sounding or bad sounding but authenticity in sound. As I mentioned earlier, a lot of clones end up sounding like very good amps. I just never really heard one that sounded close to an original. I guess this post is mostly intended for builders who are not satisfied with their clone.

I am currently in the process of re-building my ProTools set up and I will post A/B clips in a few weeks (might be months unfortunately as I am quite busy).



Last edited by MHProd on Fri Feb 10, 2017 5:40 am, edited 1 time in total.
Roe
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Re: JTM45 build. Authentic Tone!

Post by Roe »

that's a little strange. anyway, the pots read 30% high, not 10%
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dcribbs1412
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Re: JTM45 build. Authentic Tone!

Post by dcribbs1412 »

Great info MHProd
Congrats on some fine vintage amps and using them to create your clone
I'm in the middle of a jtm45 build
Interested in your clips when you get them done
I too am looking to get as close to what my idea is of the classic JTM45 tones.
Will be using your info in the near future
Thanks

D
MHProd
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Re: JTM45 build. Authentic Tone!

Post by MHProd »

Roe wrote:that's a little strange. anyway, the pots read 30% high, not 10%
In my originals, the pots were 10% to 15% overspecs. Not sure if you're refering to taper or specs..?

I mostly use alpha pots in my builds which are often underspecs IME.
MHProd
Posts: 167
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Re: JTM45 build. Authentic Tone!

Post by MHProd »

dcribbs1412 wrote:Great info MHProd
Congrats on some fine vintage amps and using them to create your clone
I'm in the middle of a jtm45 build
Interested in your clips when you get them done
I too am looking to get as close to what my idea is of the classic JTM45 tones.
Will be using your info in the near future
Thanks

D
I say, try the original layout first, then the mod. It's all about taking the time to try different things.
Roe
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Re: JTM45 build. Authentic Tone!

Post by Roe »

MHProd wrote:
Roe wrote:that's a little strange. anyway, the pots read 30% high, not 10%
In my originals, the pots were 10% to 15% overspecs. Not sure if you're refering to taper or specs..?

I mostly use alpha pots in my builds which are often underspecs IME.
On average, the pots on late 60s amps seem to read +30%. A 250k pot will read 325k. Its not about the taper
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Turbojunkie
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Re: JTM45 build. Authentic Tone!

Post by Turbojunkie »

Cool thread! Looking forward to sound clips...

Out of curiosity, are you using Merren's replica of the Drake 784-103 OT in your build?

Have you noticed any change in how your built amp sounds as it gets more hours on it, or any opinions on the concept of some components having a "break-in" period?

Thanks for posting!
MHProd
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Re: JTM45 build. Authentic Tone!

Post by MHProd »

Actually it's his RS ot but with 8k primary.

I can't really say about the break in time... The OT maybe but I built and rebuilt the amp many times and tried many different tubes. So... I really can't say.

Clips won't be coming anytime soon I'm afraid... No proper set up yet.
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Turbojunkie
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Re: JTM45 build. Authentic Tone!

Post by Turbojunkie »

Ah, you were lucky enough to get his RS...I sent an email last week inquiring about his RS & 784-103 OT's for JTM45...but he's no longer winding the RS versions "at this time"...didn't give a reason in his reply.

Once again, it was fun and informative to read how you approached the build in detail, and I appreciate you taking the time to post the process.

TurboJunkie
MHProd
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Re: JTM45 build. Authentic Tone!

Post by MHProd »

It was a long and at times frustrating process!! I'm sure his 103 would be perfect too.

I'd love it if others tried the mod and post their opinions.

I also built a 15w 2x6V6 version which sounds fantastic.
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Phil_S
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Re: JTM45 build. Authentic Tone!

Post by Phil_S »

I am surprised to hear what you have to say on the MM output transformer. At MM, they generally claim to have unwound an original (maybe more than one) to determine how to duplicate it. I wonder what they used for a model. In any case, I want to suggest it is worth a phone call to MM to see what they have to say about it. You never know where that might lead. OTOH, you can always sell it on eBait and get most of your money out of it.
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norburybrook
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Re: JTM45 build. Authentic Tone!

Post by norburybrook »

I built a JTM45 with Dagnall transformers, sozo caps, Chinese KT66's and it sounds very good, in fact a good pro guitarist friend said it was the best amp he'd ever played.

I've AB'd it against Marshalls best and it holds up very well indeed. The JTM 45 is a great amplifier, most people don't realise how good the clean sound is.

Unfortunately I have no idea what I did when making it to make it sound as good as it does :D !! that goes for most of my amps though!


MC
Roe
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Re: JTM45 build. Authentic Tone!

Post by Roe »

Phil_S wrote: Fri Feb 17, 2017 8:50 pm I am surprised to hear what you have to say on the MM output transformer. At MM, they generally claim to have unwound an original (maybe more than one) to determine how to duplicate it. I wonder what they used for a model. In any case, I want to suggest it is worth a phone call to MM to see what they have to say about it. You never know where that might lead. OTOH, you can always sell it on eBait and get most of your money out of it.
I've had a few 45s on my bench, including an original 66, RIs, metro and Ceriatones. The worst had a MM OT
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Re: JTM45 build. Authentic Tone!

Post by gsb »

Hi !

I build also a replica jtm45.My OT is Shinrock von German winder-Drake with 8k.What do you meen 1k for Screen -where you put resistors on -Pin ?
Are you used electrolyt cap on V1 or poly.? If i understand you used 100p bright cap an 560 mixer cap on bright channel and nothing on normal channel just like original schematic.
Thanks gsb
Roe
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Re: JTM45 build. Authentic Tone!

Post by Roe »

most original had a shared 1k flying resistor for the screens in addition to 470R individual screen resistors. The flying resistor was wired between the choke and the 470s
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