JTM45 Build

[guitarmike2107]

Did your tech say he could not sort it or did he say it was ok as far as he was concerned?

[Littlewyan]

He said hes done all he can with it. He thinks the PI is supposed to behave that way. I've also posted this issue on Tube Guitar Amp facebook page and a guy on there is going to test his Marshall for me to see if it behaves the same way.

One thing I'm sure I recall is the PI only misbehaves when hit by a clipped signal. Sinewave is fine.

[guitarmike2107]

The JTM45 pre amp wont be distorting that much under normal guitar signal, if you stick your master volume before the PI you will see how little it distorts, so maybe trying to pushing a large square wave through is the actual issue?

I thought you said it sounded spitty with the guitar?

[Littlewyan]

Yes it did. I linked that with the PI swing as I could see the PI swing when the issue occurred. Granted it would happen very quickly but I could still see it happen. Hopefully someone can test their working JTM45 to see if that PI swing is normal. If it is I could just be looking in the wrong place.

Also don't forget I've got the 470K Mixer Resistors in my amp which can increase gain a tad. I think V2 starts to clip at 6/7 on the volume?

[pdf64]

Try measuring the voltage across each LTP 1M grid leak resistor, under static, 'large but ok sounding' signal, and 'problem' signal.
My guess is that the main input grid leak will move from 0V across its grid leak to a volt or two negative.
But I'm wondering if that may force a matching positive voltage across the other grid leak?

And I too was thinking that a temporary 'type 4' pre PI 1M master volume may help the investigation

[Littlewyan]

The LTP grid leak have a positive voltage across them at all times which is normal. They do change a bit when the issue occurs though.

[Littlewyan]

A few guys on the Tube Guitar Amp Builders FB group tested their JTM45s for me and confirmed this behavior is perfectly normal. With bass, middle on 0 and treble on 10 the amp will not behave very well. Well I'll be damned!

When I first built the amp it had this issue if the treble was above 0 even with the other controls up, when I rebuilt it it only happened with the bass, middle on 0 and treble on 10. Because I heard the issue with that configuration after the rebuild I just assumed it was the original issue. Whereas I probably actually had 2 separate issues. God dam it. Well thanks everyone for your help. Now its time to tweak to find what spec Marshall I like .

[guitarmike2107]

oh well, I was of the impression it just sounded bad, but actually its sounds normal, except for a setting that no one would ever use?

Does your express show the same thing then with the treble dimed and bass/mids turned down? I thought it didn't?

anyway glad you are able to move past it, think you need to build a JCM800 next..... or a Princeton reverb .. or a dumble clone.....

[Littlewyan]

The Express doesn't no but the Treble pot on that amp seems to control high frequencies and upper mid frequencies so its not quite the same.

Funny you should say that Michael, I like the idea of a JCM800 .

[pdf64]

From the symptoms, as an initial measure I suggest that the values of the caps at the LTP grids are reduced, eg by a divisor of ~4.

[joeboo88]

Ryan:
I hope when I reported back about setting my amp the way you wanted...my amp didnt sound great, but I couldnt get the same symptom you were having trouble with....But I believe that was before you rebuilt it again?

Anyways, i'm glad it is working....my amp seems to sound better and better now thats its broken in......I hope yours will too.
cheers for you and have an ale on us

[Reeltarded]

Every stage and the PI on any workable marshallish amplifier until there is no cosmic microwave background should have grid stops on the sockets. At least 33k input, 68k 2nd, and a 10k-whatever small you like on the PI sides.

Everyone should take a look at extreme control settings in the TSC.

You should edit the first post in this thread to link the 23rd page with a disclaimer about there not being a problem except user error, and a lack of accepting advice in a methodic manner.

I am fucking not kidding. We need to delete this before someone reads it.

[Littlewyan]

Miles, what the hell? Why the sledge hammer approach? I accepted your advice and added grid stoppers and they did nothing at all to help. What other advice did you give other than go on about grid stoppers that didn't help in this situation? If I didn't accept advice it was because I had a logical reason for it. Yes it was user error in the end but no one here seemed to know that those settings caused the amp to react that way. It was only when a couple of guys from a facebook group tested their JTM45s and confirmed it happens to them as well that I realised. Your response was completely unnecessary and is the kind of response that starts arguments.

Joe - Thanks mate, it was confusing as the issue changed slightly after the rebuild but got there in the end.

PDF64 - I'm going to experiment with smaller capacitors when I have a bit more spare time, at the moment I'm preparing for a gig on saturday and trying to get this amp fitted into it's cabinet so it can be a backup amp.

[ArrogantOwl]

Every stage and the PI on any workable marshallish amplifier until there is no cosmic microwave background should have grid stops on the sockets. At least 33k input, 68k 2nd, and a 10k-whatever small you like on the PI sides.

Everyone should take a look at extreme control settings in the TSC.

You should edit the first post in this thread to link the 23rd page with a disclaimer about there not being a problem except user error, and a lack of accepting advice in a methodic manner.

I am fucking not kidding. We need to delete this before someone reads it.

I just read this entire damn, thread, took me two hours, hoping to learn something. And this was the result. It doesn't help that I followed the same thread over on the aforementioned Tube Amp builder page on FB...

[JazzGuitarGimp]

Reel, did you get yourself another amp garage account?