JTM100 build taking off! IT LIVES!

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C J H
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Re: JTM100 build taking off! (.. and some sideprojects)

Post by C J H » Fri May 03, 2013 6:19 pm

Did the testing tonight.. After blowing 2 fuses just by plugging the AC cord in the jack I knew this was gonna be a though one.
That turned out to be two wrongly orientated diodes. I then had a hard time getting the tubes to bias in range and noticed HEAVY redplating. Had 150k (JTM/KT 66 value) instead of 27k in the biascircuit, felt really stupid when I stumbled on that one and might have toasted a quad of tubes. Fired her up with some other tubes and got bias in good range. Plugged in a gat and turned the volume up.. turned it up some more.. maxed it. All that came out was whispers, it was reaaaallly quiet. So yeah, I´m in bed now, was feeling too tired to keep working on an amp but does anyone have any pointers on where I should start looking tomorrow? I´ve figured i should go over the pots again and also double check that i haven´t put a 470k resistor between the 1meg ones.. but other than that?

I´m pretty bummed about the tubes, if they´re fried, but I might as well get used to it, this one has 532 VDC on the plates.

Jake
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C J H
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Re: JTM100 build taking off! NEED SOME HELP

Post by C J H » Sat May 04, 2013 12:51 am

Just measured the OT, primary to secondary (pin3 och v4-7) reads between 19.7 and 21.5 ohms.. all good on the OT i guess?


Voltages on V3 are off the charts now.. :palm:
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neikeel
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Re: JTM100 build taking off! NEED SOME HELP!

Post by neikeel » Sat May 04, 2013 10:01 am

If the V3 voltages are very high what are your V1 and V2 voltages? Are your wiring links to these valves ok (current draw in these usually drops the PI voltages).

Post a couple of pictures of your work so far.
Neil

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C J H
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Re: JTM100 build taking off! NEED SOME HELP!

Post by C J H » Sat May 04, 2013 12:32 pm

LATE NIGHT EDIT:
Looked through some of the pics I´ve taken with my phone during the build. I think I might have missed the jumpers connecting the positive terminals of the mains caps together.. they´re only jumpered closest to the switches, going to ground.
So effectively my mains are currently 23.5 uf, could this be it?



Thanks for chiming in. Yes, the voltages on V3 are high but V1 and V2 are good. So can´t take any pics or check for exact voltages but all measurements were good except for V3 and bias, the latter was sorted when i changed the dropping resistor.

I´ve gone over the wiring and checked all grounds (using a multimeter testing for continuity) .. no luck so far.

To describe the little sound i get further its REAAALLY quiet, very fizzy and fades in and out (kind of like a ghost note) and i only have sound if i put volumes arounf 8. More or less and its dead quiet.. I´m thinking a cap might be bad?

I will take some pics and go over wiring around V1-3 again tomorrow or next week.
Jake
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C J H
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Re: JTM100 build taking off! IT LIVES!

Post by C J H » Sun May 05, 2013 7:38 am

Alright all sorted. After looking it all over once again this morning with well rested eyes I quickly found the culprit. I had missed the 10k resistor following the V3 grids, what a dumbass..

With that fixed i rechecked all the voltages and adjusted bias accordingly, ended up with 540 VDC on the plates :whistle:
Plugged in and bright channel was still a bit funky, low output and no breakup, howling if I took the master above 25 %. Switched to normal channel and there it was, what a beast! Way more aggresive sounding than my old build (10xxx series) and a bit wet sounding, 3D if you will.. I recorded a quick clip when plugged straight into the normal I with my les paul, cab is my BW:d 1960 ax with reissue 25w. Don´t mind my sloppy playing, or the mic clipping (used my iphone). It gives a good feel for the sound of the amp though. It turned out to be a bad solder on the birght ch mixer resitor, easy fix! I was to occupied playing to take clips on bright channel now that its all runnning smooth.. oh well, maybe later.
CLIP:
http://speedy.sh/W4pu6/Memo.m4a" onclick="window.open(this.href);return false;

And some pics:
Image
Guts
Image
In the box
Image
On top of 2 vintage BW:s, back.
Image
On top of 2 vintage BW:s, front with strat.

Neikeel, big thanks for being around through pretty much the whole build, axeman for cheering me on the whole time and all you others who have written posts on the forum thats been of huge help.
Jake
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neikeel
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Re: JTM100 build taking off! IT LIVES!

Post by neikeel » Wed May 08, 2013 8:58 am

Excellent!

It does sound nice and crisp with a decent amount of grunt. I would expect it soften a little and develop a bit more of those complex harmonics when pushed later.

Were those clips normal channel?? Sounded more like bright channel on about 6 to me!
Neil

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Re: JTM100 build taking off! IT LIVES!

Post by Roe » Thu May 09, 2013 10:01 am

any redplating with 540v on the plates?
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
http://www.myspace.com/prostitutes" onclick="window.open(this.href);return false;
Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

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C J H
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Re: JTM100 build taking off! IT LIVES!

Post by C J H » Thu May 09, 2013 2:54 pm

Nope, tubes seem to be holding up so far..

I will get better clips up when I have some proper rec equipment around, the sound quality with the iphone doesn´t do any of the amps justice

Jake
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