JTM50 Black Flag build, finally under way.

ivan H said:
I thoroughly recommend an amp build Robert, it is a very rewarding endeavour & having seen your guitar builds I'm sure your attention to detail will see you produce a decent build. Good questions too, & well answered Sysco. Yes, with a 50 Watter simple convection cooling is fine, no cooling fan needed. We see some extreme temperatures here (yesterday was 42*C/107.7*F & summer has just started). In these sort of temperatures I remove the rear panel from the headcab, just to allow better airflow. I've not encountered any heat problems.
Yes, OT phase reversal is "builder error". You see, most amp designs have a "negative feedback loop" around the power amp, where a portion of the output signal in injected back into the phase inverter. It is called "negative feedback" as it is 180 degrees out of phase with the signal at the input of the phase inverter. The presence & resonance controls are part of this negative feedback loop. If the OT primary leads are reversed, the feedback component being injected back into the phase inverter (or "driver tube" in a single ended design) will be positive (in phase with the signal on the PI input) instead of negative. This creates a "self feeding" instability where the amp will howl like a banshee. As Sysco said, pay attention to the OT primary colour coding on the instructional layout & it won't happen.
Look forward to hearing we've influenced another to start building. Cheers
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Thanks!!!!

i'm planning on a build after the holidays!!!!
 
Hello, that's really a nice build, congrats ! I ve built one myself and i see some difference concerning the filtering . Some seems to have the filter cap for the PI mounted on the wall with only one lug used and some have a 32+32 at this place with both lug used .If someone can tell me what is the original layout for this :) . Regards from Corsica France . Nicolas
 
nico56 said:
Hello, that's really a nice build, congrats ! I ve built one myself and i see some difference concerning the filtering . Some seems to have the filter cap for the PI mounted on the wall with only one lug used and some have a 32+32 at this place with both lug used .If someone can tell me what is the original layout for this :) . Regards from Corsica France . Nicolas
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Thanks for the compliment nico56.
The correct, or "original" positioning is for the main & screen grid dual filter capacitor to be mounted on the sidewall & the phase inverter's filter capacitor mounted between the (V6) GZ34 rectifier tube socket & (V5) power tube socket.
Due to my using a non standard grounding scheme (& the convenience of grounding the main/screen filters right at the capacitor clamp's mounting bolt), I have them them swapped around.
So "correct" is a dual cap can on the sidewall with both of it's capacitors used (main filter 32uf, screen grids 16uf). PI filter between rectifier & power tubes a single cap can 32uf.
An update on this build;
Some JTM50's used the 1202-118 power transformer, while others used the higher voltage 1202-133, which gave a tube rectified B+ of around 450V~470V.
I had used the 1202-118 as I had it sitting unused, however I now have a
1202-133 type that will soon be transplanted in there. Pics will follow when done. Cheers
 
Thanks a lot Ivan, that's really helpfull for me . I wonder how the higher voltage PT will change the tone of your amp . I know that's for the earlier JCM800 2204 the lower voltage PT made theme little less loud and bluesier some say . Great thread (y);)
 
ivan H said:
Thanks for the compliment nico56.
The correct, or "original" positioning is for the main & screen grid dual filter capacitor to be mounted on the sidewall & the phase inverter's filter capacitor mounted between the (V6) GZ34 rectifier tube socket & (V5) power tube socket.
Due to my using a non standard grounding scheme (& the convenience of grounding the main/screen filters right at the capacitor clamp's mounting bolt), I have them them swapped around.
So "correct" is a dual cap can on the sidewall with both of it's capacitors used (main filter 32uf, screen grids 16uf). PI filter between rectifier & power tubes a single cap can 32uf.
An update on this build;
Some JTM50's used the 1202-118 power transformer, while others used the higher voltage 1202-133, which gave a tube rectified B+ of around 450V~470V.
I had used the 1202-118 as I had it sitting unused, however I now have a
1202-133 type that will soon be transplanted in there. Pics will follow when done. Cheers
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If I keep reading this sorta stuff, it’s gonna make me want to build an amp, too.

I like these threads.
 
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smitty_p said:
If I keep reading this sorta stuff, it’s gonna make me want to build an amp, too.

I like these threads.
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I really encourage people to do so, it is extremely rewarding to play through an amp that you've build yourself & have it sound great & be reliable. If you can solder properly & follow something like this, you can do it.16267268948995989787738923844098.png
Of course, there are full, step by step instructionals available that can also be used. There are complete amp kits available from several vendors, amp designs ranging from 1 watt practice amps & single ended designs like the Fender Champ etc, ranging all the way through to designs like the SLO our buddy Sysco recently built & beyond.
Once you've built one amp, you will want to build more. Cheers
 
ivan H said:
Greenbacks. I've tried it with both the A & B cabs. I prefer this one with the A cab, though my '68 JMP50 clone (also built with NOS components, Iskras & mustards etc) I prefer with the B cab.
I'd like to try this with V30's though. I did try the JMP50 clone with V30's, but the JMP has stronger & more aggressive mids/upper mids than the JTM & I found it a bit much with the V30's.
Hopefully these pics are a little better SyscoView attachment 35203
View attachment 35204
Cheers
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Very clean build, excellent work !!
 
smitty_p said:
So, let me ask…are the prices on Cieratone the prices for a full amp or just the kit?
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I've not looked into Ceriatone products, but do know that both kits & completed amps can be purchased from them. Maybe someone else here may be more knowledgeable about the Ceriatone products. Cheers
 
smitty_p said:
So, let me ask…are the prices on Cieratone the prices for a full amp or just the kit?
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Ceriatone's website and ordering is a little bit confusing. Let's take their JTM50 offering: (Unassembled) Kit Package #2 supplies everything except for the head cabinet and tubes... $620 & 16kg of weight. Now open up Add On/Parts... Head cab is $175 & 21kg. Tubes are $100 & 3.5kg. Total now is $895 & 40.5kg of shipping weight. Ceriatone's shipping calculator shows UPS delivery from Malaysia to USA for 40.0kg is $276. At 50.0kg of shipping weight, then shipping jumps to $478. Head-scratcher, huh?
 
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here's some modifications to my JTM50 : New filtering scheme, more accurate with 32 uf main,16uf for the screen, 32 uf for the pi and 2x32 uf for the preamp .I ve also put some 0.1uf mustards and soon there will be some 0.022 uf too .I put a cv492 Tungsram on V1 and a sovtek 12ax7 lps on v3 .That amp rocks ! ;):dood:
 

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Cool. Be on the lookout for some RS (radio spares) silver mica's as well, & a 100pf Lemco ceramic disk for the volume control bright cap. If you are going with NOS coupling capacitors you may also want to look for some Piher or Iskra resistors (both were used, Piher in earlier JTM50'S, Iskra in later). IMHO, Pihers favorably colour the tone more than Iskra's, & are easier to find.
Question, have you tried both the 270k & 470k values for the channel mixer resistors?? 270k was the value used in earlier amps, 470k in later amps.
270k will be a tad cleaner sounding but will give more snap to the feel of the amp.
If you can find one, might I suggest a Mullard 161 series ECC83/12AX7 for the V1 position, or better yet, if you can find it, the same tube in a 7025. I was lucky enough to find a 161 series 7025 NOS/NIBIMG_20210724_063122.jpg
It now resides in the V1 position of my JTM50 build. The 161 series 12AX7's are less aggressive sounding than the 163's & really suit these amps. To me, Mullard tubes just make a Marshall type amp sound more Marshall. Cheers
 
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i'm not sure if i'm going to put nos parts everywhere, i just had the oportunity for the mustard so i pull the trigger :) (quite expensive by the way ..) .The tone is more to my liking so i think i'm on the right path .This amp is really appart to me on the Marshall history, sort of a transitional one between JTM45 and JMP, thanks to the EL34 you have the bark and grind but it's not as aggresive as later jmp, i like this .
I'm on the fine tuning process, i didn't try 270k mixers, but seems like 470 k are more suited for EL34 (that's what i read, i don't know for sure :unsure:) .I think next move is to try with a tube rectifier and why not swapping the mixers .If i have the oportunity to find iskra's or piher's i'll sure giv it a try :) .
For the tube yes, a nice Mullard in v1 would be great ! Where do you buy your NOS tube Yvan ?
 
ivan H said:
I really encourage people to do so, it is extremely rewarding to play through an amp that you've build yourself & have it sound great & be reliable.

Cheers
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I agree. When it is finally done and working correctly, it's awesome knowing that the sounds you are creating are truly yours. Especially, when it's reliable and you need to play as loud as your bass player who is using an Acoustic Black Widow plugged into an Acoustic 300/360 rig with 4 15" speakers.




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