problem with JTM45 clone

Repairs, trouble shooting, modding and so on.

problem with JTM45 clone

Postby flatart » Fri Oct 23, 2009 7:54 pm

I have a problem with a JTM45 clone I've just finished with some spare parts, never faced this problem before.
When I set presence control to 10 (max) I have a very bad sound with a very strange kind of sustain feedback.
In attachment I put a short mp3: in the first part you can hear the amp with presence control at 10(max). In second half you hear the amp with presence at 0 (min).
For the mp3 please go to this link: http://ampgarage.com/forum/viewtopic.php?t=9659

Has someone already faced such a problem?
Once I had a similar issue (but not the same) and I solved it putting grid stoppers on output tubes, but this amp has already grid stoppers so... HELP!
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Re: problem with JTM45 clone

Postby Brian Wallace » Fri Oct 23, 2009 8:02 pm

I think you need to be logged into that site to see your attachment. Can you host it somewhere and post the link here?
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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Re: problem with JTM45 clone

Postby adrian » Sun Oct 25, 2009 7:23 am

I have no access to your sound file either, but what you describe could be parasitic oscillation.
Do you have grid stopper resistors (typically 5.6k) at the power tube sockets?
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Re: problem with JTM45 clone

Postby flatart » Sun Oct 25, 2009 11:52 am

yes as I said I had grid stoppers. Now I removed them.
I've tried a lot things, some with sense and others (I admit) with no sense, but really this issue is driving me crazy:
- I changed presence cap
- I changed 0.1u PI caps
- removed feedback
- I removed 47pF cap between anodes of V3 (PI)
- Moreover I did some other tests on output tubes section: I removed 8K before screen resitors and so I burnt 470 ohm output screen resistor (only 2W). replaced with 5W
- I burnt my tester (left on output plate while playing)
......
Anyway I found a trick that solves the problem but to me it's unexplainable.
I connected one long white cable to pin 1 of V3 (phase inverter) and I put this cable near the input side of the chassis, but the other lead of the white cable is not connected to anything. Consider that if I move this white cable towards the power section the problem comes back.
Maybe this trick makes sense for someone of you.
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Re: problem with JTM45 clone

Postby Brian Wallace » Sun Oct 25, 2009 1:29 pm

The resistor feeding your 470 ohm 5W resistors should be a 1K, the 8K resistor is part of the power rail feeding the rest of the amp.

Have you tired twisting your wires from the PI to the output tube sockets?
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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Re: problem with JTM45 clone

Postby flatart » Sun Oct 25, 2009 6:51 pm

the resistor feeding 470ohm was 8K too (reccomended in yahoo group...) anyway now I removed it, as it was in original JTM and I got only a couple of 1K 5W going to the screens (I have no 470 5W...).
Sure, wires from PI to output tubes are twisted, they have been always twisted. I also tried another stupid thing: I swapped OT wires in the output tubes and obviously also wires coming from the PI inverter. I moved ground connections of 16+16 can near preamp chassis zone, I tried to move every wire inside the chassis but notjing changes: the only way to have the amp playing in a decent way is the trick I told you before: cable to pin 1 of V3 (phase inverter) and the other lead near the input side of the chassis, also after swapping wire from OT...
For today I have had enough of this fuc...ing amp and I dont want to burn it or destroy it with a hammer
goodnight everybody
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Re: problem with JTM45 clone

Postby David B » Sun Oct 25, 2009 8:38 pm

flatart wrote: the only way to have the amp playing in a decent way is the trick I told you before: cable to pin 1 of V3 (phase inverter) and the other lead near the input side of the chassis


Isn't that an old radio trick to suppress oscillations?
any pictures of the wiring?
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Re: problem with JTM45 clone

Postby flatart » Mon Oct 26, 2009 11:20 am

someone on Metro forum seems to have a similar problem and fixed it removing 1ohm bias resistor (!!!???) of output tubes.

http://forum.metroamp.com/viewtopic.php ... ng#p275076

It really makes no sense to me but I will try this too. After that I think I'll have to replace all signal wires with shielded cable.
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Re: problem with JTM45 clone

Postby David B » Mon Oct 26, 2009 12:48 pm

Oh yeah forgot about that, I take those things off
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Re: problem with JTM45 clone

Postby flatart » Mon Nov 02, 2009 7:02 am

after lot of tries and tests I eventually limited the issue to reasonable levels, even if the problem didn't disappear completely.
Here the main changes that helped me:
- shorten ground presence directly to pots ground wire
- shielded cable for some lead signal wires
- shorten wires that were too long
- moving wires to get a better layout
My target was to get a JTM45 clone that even with all the pots wound up at 10, it should sound good with no hiss.
I know that I will never use this amp completely wound up, but this is a way to measure the accuracy of circuit building and layout.

Now the question is: do you think that this is an achievable goal with this kind of amp?
I hope everyone of you can answer based on his own bulding experience

For example with my clone if volume is equal or lower than 6 I have no problems whatever tone and presence I set. If volume is between 6 and almost 9 I start to get some hiss and with presence over 7. If volume is at 10 (max) I can't get a good sound out of the amp, whatever setting I use.

here there 3 samples, I hope that you can download and hear them (problamatic link):
http://mio.discoremoto.alice.it/flatart
- test JTM45: volume1 at 6, volume2 at 3 (chained), bass 5, mid 10, treble 10, presence 5, just a strato plugged in, no effects, nothing, just playing with strato volume pot
- test JTM45-2: volume1 at 8, volume2 at 3 (chained), bass 5, mid 10, treble 10, presence 5, just a strato plugged in, no effects, nothing, just playing with strato volume pot
- test JTM45-problem: everything wound up at 10

thanks everybody for your preciuos help
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Re: problem with JTM45 clone

Postby neikeel » Mon Nov 02, 2009 12:55 pm

I do not think anyone would expect an accurate JTM45 clone to be able to be used with the presence and bass at 10 without making horrible noises.

The noises you describe would be typical of PO that your 5k6 swampers would help with and careful routing of the presence wire either side of the NFB resistor. Some valves ar emore prone to problems too (I had a pair of GEC KT66s that were fine in other applications but made this noise in my RI when some Harmas and Mullard El34s did not.

Lead dress is crucial, but you may also have been slightly unrealistic in your expectations for EQ settings :wink:
Neil
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Re: problem with JTM45 clone

Postby adrian » Thu Nov 05, 2009 10:18 pm

Some of the obvious mistakes are impossible to see without pictures :wink:
I've seen people going nuts by trying to troubleshoot a powder coated chassis :shock:
Sometimes it's something very simple like a missing ground at the speaker output... (just speculating...) :idea:
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