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Re: JTM 45/100 - Build story
Posted: Mon Nov 19, 2012 4:35 pm
by VintageCharlie
Adrien, any new findings or other progress with your jtm45/100? Looking forward to some more clips and hope that it is setting in well.
Re: JTM 45/100 - Build story
Posted: Mon Nov 19, 2012 5:02 pm
by Xplorer
Hi Karlis, i've been quite busy and i'm away from home for work ( by the sea in britain, not bad !!!!! )
But i planned to build a second preamp, to compare it.
Before that i shouldput the stock values back, and do a probe test clip of it. Then, insert just the 33 uf back.
these mods brought some saturation, yes, but with stock values it shouldn't be dead clean like that. Putting back the 33uf will help me to see if the 16uf was a faulty one for example.
Before getting these mods back if needed, i want to make sure that it's balanced correctly with the stock values, hunting the faulty components or whatever if it is the case.
I've build the Xplorer fuzz ^^ , beautiful, i was a bit busy at it.
Later, i'll do real clips in a better environement.and i'll work on some bog clips
Re: JTM 45/100 - Build story
Posted: Fri Nov 23, 2012 8:50 am
by axeman
neikeel wrote:So you dropped in the higher reading V1 plate resistors - good, next put one of these in the place of the 8k2:
[url]ttp://
www.ebay.co.uk/itm/15K-CERAMIC-RESISTOR ... 563ba92b0a[/url]
You may have seen some original amps with them.
To lower the screens voltage that involves replacing the 470R (or 1K) resistors on the sockets with 5k6 or similar 5w RS resistors.
Having said that I do suspect you have a dodgy component somewhere. Did you measure the pot values before installation?
Playing and listening to mine I get the same break up at about 6 on the dial as you are getting at 10. Have you tried channel 2?
Just to get some gain at lower values have you used a bright cap - mine runs 100pF but my other 45/100 (with EL34s and -119OT) has a 5000pF as stock and it is not at all bright, it just acts as a bypass cap to bring in the gain lower in the volume pot register.
Also the Parkhead mod (470k resistor on vol1 pot between the lugs where a bright cap would be) adds thickness and tone at lower volumes if it feels a bit thin and add a 100pF cap for a bit of sparkle.
Can you use both? what would happen?
Re: JTM 45/100 - Build story
Posted: Fri Nov 23, 2012 9:14 am
by Xplorer
Both what ?
Re: JTM 45/100 - Build story
Posted: Fri Nov 23, 2012 10:35 am
by axeman
neikeel can I use the resistor and a capacitor at the same time for the parkhead mod?
Re: JTM 45/100 - Build story
Posted: Fri Nov 23, 2012 10:47 am
by neikeel
axeman wrote:neikeel can I use the resistor and a capacitor at the same time for the parkhead mod?
Yes both - gives sparkle and thickness
Re: JTM 45/100 - Build story
Posted: Sat Dec 01, 2012 10:00 am
by Xplorer
News ! so i've build a second JTM preamp, here it is.
the 27k CC read about 29k if i remember, and the CC 820 ohms measured higher, like 850 ohms or more.
i think that the 47pf is ok. the 560pf measured closer to 540 - 550 pf.
i'll reform this RS 32uf cap, it measured 32uf.
i used some sprague atom electrolytic caps for the rest, and i'll perhaps swap them for some others if i change my mind but for now they measure very close in tolerance.
Re: JTM 45/100 - Build story
Posted: Sat Dec 01, 2012 10:33 am
by julkke
Nice!
Re: JTM 45/100 - Build story
Posted: Sat Dec 01, 2012 4:50 pm
by JimiJames
M A N
That is James Bond All-the-way !
Re: JTM 45/100 - Build story
Posted: Sat Dec 01, 2012 5:15 pm
by Xplorer
Thanks guys. But this one isn't as nice as the previous one.
i've cut the paxolin like a pig this time, and it's a tad shorter on one side.
some RS turrets are nos but good, some are nos but not so shiny, and more difficult to solder.
+ the sprague atoms aren't good for the look, although they're great i think, for the essential. + things here and there ...
But good enough. got to implement it now.
Re: JTM 45/100 - Build story
Posted: Mon Dec 03, 2012 5:49 pm
by young flower
News ! so i've build a second JTM preamp, here it is.
the 27k CC read about 29k if i remember, and the CC 820 ohms measured higher, like 850 ohms or more.
i think that the 47pf is ok. the 560pf measured closer to 540 - 550 pf.
i'll reform this RS 32uf cap, it measured 32uf.
i used some sprague atom electrolytic caps for the rest, and i'll perhaps swap them for some others if i change my mind but for now they measure very close in tolerance.
The turrets used on the boards under this post - does anybody know where I can get these or similar ones in Europe? I´m starting a 100W build soon.
Already found some, like these:
http://www.tube-town.net/ttstore/produc ... -Pack.html" onclick="window.open(this.href);return false;
But I like the ones in the pictures a lot more. I think Brian H. from kurtsequipment is using the same kind for his reproduction boards.
Thanks
YF
Re: JTM 45/100 - Build story
Posted: Mon Dec 03, 2012 8:22 pm
by Xplorer
These, you'll have hard time finding them .. they're original old RS turrets. But the harwins turrets are easy to find, same model, different alloy. harwin H2073Z01
okay, some more liquid coming out of the 0,056uf 1000v red cap ( perhaps because of some unfortunate rare times i turned the amp on for a few seconds, before switching it off, when i realised a cab wasn't plugged on it. Once, it also had the aligator clips connected between V1 pin 1 and the ground
for a few seconds ) . that's what i was talking about in some older posts.
the amp still worked as usual after that.
removing the preamp board to check everything and instal the new preamp board, to compare it.
Re: JTM 45/100 - Build story
Posted: Mon Dec 03, 2012 9:29 pm
by BoogieEngineer
Nice. Hope it will work out for you this time.
Re: JTM 45/100 - Build story
Posted: Mon Dec 03, 2012 11:42 pm
by VintageCharlie
Good luck and i too hope that this time it works as it should or at least brings you closer to the solution.
Re: JTM 45/100 - Build story
Posted: Tue Dec 04, 2012 1:37 am
by emmjaydubya
Your persistence and patience is inspiring, I've been rooting for you on this amp the whole time you've been having problems, I hope it ends up sounding just like you want. It's a beauty!