Converting JTM50 to JTM45

jasoncarmikle · Post by **jasoncarmikle** » Sun Jan 09, 2011 8:50 pm

How would I do this? Is it pretty simple? Where would I get the parts?

Thanks guys.

neikeel · Post by **neikeel** » Mon Jan 10, 2011 4:35 am

They are very similar

To give you specifics would need to know is it an original (in that case don't do it!) or a clone (whose clone?).

The preamp filtering on a 50 is a dual can with extra 10k dropper.
Obviously KT66s instead of EL34s.
PT is different and uses a 180k bias feed instead of 220k and the bias adjustment resistor may need changing to get the KT66s into the correct ballpark.

Some clones are different to others so a picture might be useful

jasoncarmikle · Post by **jasoncarmikle** » Mon Jan 10, 2011 9:11 am

I should have mentioned it was a Metro clone.

What do you mean the PT are different? Would I need to purchase a different one (and how much do those usually cost?)

Thanks.

neikeel · Post by **neikeel** » Mon Jan 10, 2011 10:49 am

A 50watt PT and OT will work (just they are different, OT different primary impedence (3.4k vs 6.6k and the 45 watt OT is a little saggier under heavy load).
A JTM50 is not a standard build - who made yours with what specs (ie trannies)?

jasoncarmikle · Post by **jasoncarmikle** » Mon Jan 10, 2011 11:10 am

http://img.photobucket.com/albums/v132/ ... nside3.jpg" onclick="window.open(this.href);return false;

There is a photo of the inside.

I'm not 100 percent sure I am going to convert it, I am in the midst of buying it right now....

Looks like a clean job though, might sound great as is.

neikeel · Post by **neikeel** » Mon Jan 10, 2011 12:37 pm

Yes, as I thought.

The preamp board is a hybrid:
-it is split V1 cathode like a post 68 JMP50 model 1987
-there is a 0.68uF bypass cap on V2a (gives mid boost) like a 68-70 JMP 50 model 1987
-bright cap on volume pot is a 100pF (same as JMP 50 model 1989)
-NFB resistor is 100k which is typical for a metal panel JMP50

Rest of it is stock for a JTM50, but need to see pic of rectifier and know what trannies were used.

Looks a nice clean build with good quality components, just a hybrid so you can feel free to tweak to your taste.

What sounds are you after and with what guitars?

jasoncarmikle · Post by **jasoncarmikle** » Mon Jan 10, 2011 1:48 pm

Some more pics of the inside and of the Mercury Magnetic Transformers.

http://img.photobucket.com/albums/v132/ ... nside2.jpg" onclick="window.open(this.href);return false;

http://img.photobucket.com/albums/v132/ ... ormer2.jpg" onclick="window.open(this.href);return false;

I'm just looking for a nice smooth, fat sound. Maybe Hendrix type clean stuff with a strat...and ACDC, Thin Lizzy type gain from a les paul.

neikeel · Post by **neikeel** » Mon Jan 10, 2011 2:10 pm

They are actually Magnetics Components from Triode Electronics (better classic vintage Marshall tone, IMO).
The OT is a 784-139 (JMP 50) type for EL34s and the PT is a type designed for JTM45 (will have rectifier tap and approx B+ of 440vdc).

Give the amp a trial with your set up and twiddle with the EQs. Whole range of tones can be extracted from one of those with a variety of mods

jasoncarmikle · Post by **jasoncarmikle** » Mon Jan 10, 2011 4:57 pm

Ok thanks.

If I were to change it back to JTM45 specs, what all would I need to replace?

neikeel · Post by **neikeel** » Mon Jan 10, 2011 5:22 pm

Strictly speaking?

The OT for a JM45 one and swap the EL34s for KT66s.
remove the split cathode on V1
remove the boost cap on V2a
replace the 470k mixer resistors with 270K
replace the NFB resistor with a 27k hooked up to 16ohm tap
replace the preamp dual axial with 16uF single axial, remove the 10k 2watt on the edge of the board and add a buswire link in its place.

None of it difficult, only expense is the OT and KT66s.

You should really give it a good try out before you buy/modify it, you might like it as is

6burgh · Post by **6burgh** » Mon Jan 10, 2011 5:41 pm

neikeel wrote:You should really give it a good try out before you buy/modify it, you might like it as is

+1, Neil's right. You can always roll the volume back on the guitar a little, or just lower the volume on the amp, or just remove the bright cap as an easy way to "clean up" (if that's what you're looking for) the tone without getting into lots of parts sourcing or soldering.

OnTheFritz · Post by **OnTheFritz** » Mon Jan 10, 2011 11:09 pm

Need to chime in, I built this amp! Did a double take with the board pic's and checked my archives.

I've got notes and have to check my old ebay listing to add anything, just saw this tonight.
My intent was to build a lead spec tube rectified 50 watter with a SS plug in option. I had done a straight up JTM-50 prior to this, and like others here who have done the same, it seemed to be lacking.

I used a mix of standard Sozo's, and Vintage Premiums. I love the .10 VP's in the phase inverter with the lead tone stack.
The MC PT put's out a healthy voltage, and it was about 460-465 with the tube rectifier. With the SS plug in rectifier it bumped up 20 volts. (re-bias)
This amp really screams, and sounded *brutally* fantastic when it left my hands. I hated to let it go! Great balance of punch, sag and gain.
As was mentioned, it would be no big deal to convert to 45 specs.

jasoncarmikle · Post by **jasoncarmikle** » Tue Jan 11, 2011 1:03 am

You put it on ebay? I was told this was made for the guy by an amp tech and he was the original owner.

Are you sure you built it?

OnTheFritz · Post by **OnTheFritz** » Tue Jan 11, 2011 1:52 am

jasoncarmikle wrote:You put it on ebay? I was told this was made for the guy by an amp tech and he was the original owner.

Are you sure you built it?

Absolutely 100% positive I built this. My photo's are identical.
Pisses me off if people try to misrepresent what they are selling.
Be right back..

jasoncarmikle · Post by **jasoncarmikle** » Tue Jan 11, 2011 2:04 am

The guy put this in his ad, saying the "builder" which I guess is you, said to him after building it.

"I employed a split cathode, the lead tone stack and mixer resistors, while retaining the .10 output coupling cap, which are Vintage Sozo's. They sound amazing in that position. I also used the Vintage Sozo's in most positions, with a couple standard Sozo's as well. AB carbon comps on the cathodes as well as the negative feedback resistor. (100k-8ohm tap) The PT delivers a tube rectified plate voltage of 465 vdc, and it really screams! The classic Marshall tones are all there, use the volume control on the guitar to clean things up, or let er' rip. I've installed the Lar/Mar PPIV Master Volume which works great. Really does give that cranked amp sound at reduced volumes. Of course it's at it's best when you get some air moving. Tube compliment is a pair of '59 GE 12AX7's from Tube Tramp in V1 and V2, and a Sino in V3. Output tubes are =C='s, and sound great. Tube rectifier is a Shuguang. The bias needs to be set accordingly. The SS rectifier bumps up the plate voltage to 480 vdc. Tightens things up quite a bit, the amp feels different, and absolutely roars. I like both, it's a mood thing! F&T filter caps, Magnetic Components transformers, and a Metro C1999 8H choke. Metro chassis and panels, and the head case is from Mojo via metro."

Sound about right?

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