JTM 45/100 - Build story

[Xplorer]

ok, no problem. But i really don't see how to figure it out.
i'm not familiar with the good reflexes to have, to debug an amp.
the superlead went ok directly, following the manual
yes the resistor is a 470 ohms, i verified.
the mixer resistors are 270k.

i'll re do a voltage chart later.

[VintageCharlie]

I'm sure some of the more experienced guys here will take care of you and guide you through the trouble shooting process. What would be the first most logical steps to look into this issue guys? Keep us posted on the voltages!

[Xplorer]

At least, with a fuzz ( i just used the dunlop blue fuzz, stock ) it's heaven !!!!
it has a very different fuzz tone and dynamics compared to my SL. i don't regret to have build this amp, great choice;
if only mine also offered dreamy break ups and crunchs and saturations, more ranges ... with the strat straight in it.

[neikeel]

I'll try to listen to the clips to try and see what it is missing

[Xplorer]

Thanks man

[Xplorer]

Hey guys, i measured the voltages again, with all tubes loaded.
some preamp tube voltages seem much higher than the voltages of Dom, on his metro thread.

i wonder if there would be a bug somewhere, which wouldn't allow me to bias the amp warmer, stuck because of the mA seen accross the 1 ohm resistors. ?

Really i have no break up like the great clip of Neil, or some others , Karlis, etc.

[neikeel]

Just had a listen for the first time (working on call most of this weekend so could not open 'dangerous files' ).

Sounds quite weird! Really plinky and not right at all.

Yes voltages are high and -60 is too high in the bias circuit too.

What are your heaters at?

Have you tried plugging into both inputs high and low.

Also makes ne wonder whether you have mixed a pot up in the tone stack.

Do a quick test, pinch all the preamp valves from your Superlead and play?

I need to look carefully at where the voltages are adrift and why,it is as if something is not drawing enough current to pull them down and not giving you enough drive

Might be worth blowing a couple of kisses at SDM via a pm with a sound clip and link to your clearest pics - he is much sharper than me, I may take days to figure this one out!

[Xplorer]

Aaaah, yes you see, it doesn't sound right ^^ i knew it !

The heaters, when i measured the amp on standbye mode and only the preamp tubes loaded were at 3,3 VAC

yes, i played in both low and high channels.

i didn't try the 12ax7 new mullard from my superlead but i swapped all the preamp tubes for some others already ( other tubes )

Who is SDM please ?

what would be your reflexes, or method, to debug an amp like that ? so i could perhaps try and learn instead of anoying peoples lol.

[Xplorer]

Also makes ne wonder whether you have mixed a pot up in the tone stack.

What did you mean ? a wrong pot connection ?

[BoogieEngineer]

Also makes ne wonder whether you have mixed a pot up in the tone stack.

What did you mean ? a wrong pot connection ?

This was exactly a mistake that I made. The wire from preamp board to Presence pot was soldered to the Bass pot. It didn't change the sound too much but I had problem with strange noise coming from the sweep of PPIMV above 7 or something. Once I had corrected the connection, everything is fine.

[Xplorer]

Hope for a second, then i checked the amp and ... no , the purple wire from the preamp board is connected to the presence pot as it should like on the layout.

[BoogieEngineer]

Hope for a second, then i checked the amp and ... no , the purple wire from the preamp board is connected to the presence pot as it should like on the layout.

Well I mean you could make a similar mistake, maybe not that wire specifically

[Xplorer]

yes i know, thanks man.
i saw that you made your amp all by yourself and it works, i'm impressed !

[BoogieEngineer]

yes i know, thanks man.
i saw that you made your amp all by yourself and it works, i'm impressed !

Thanks, I got a lot of help to, and to be honest I am completely clueless as to how the whole thing works . I just followed the point to point diagrams. I suppose I was just a bit more lucky than you are now . I'm sure it will give you tears of joy once you've got it fixed.

[neikeel]

I have been through your voltage chart and it checks out pretty well vs mine (although mine has an early 530v PT) and the original chart that I compared it with (that was 560v B+).

Post some pics of your pre-amp tube wiring.

It sounds like it is missing a gain stage which is why my first reaction was to check and double check your tone stack V2a wiring.

I will see if I can see on your existing pics.

- check the heater on all pins (not a dry joint under on of the bridged red preamp tubes.

- is your V1 cap NOS or new? If NOS just lift one leg of it temporarily and retry.

- check you have the right pots wired up in the right order ie not slipped a 1 meg in the wrong place

- if still no better do this: open up your SL and have them side by side remove the grid wire from V2 on each and tape them up temporarily.
- take a wire and solder it across from the green wire that comes from your superlead 470k/470k/470pF and extend it to the correct lug on V2a.
- put both on a load with all tubes in with a temporatry earth between the two chassis as well
put speaker on the JTM45/100 and load box on the SL. Switch both on
- plug guitar into the SL input and see what you get out of the 45/100 speakers.

If you get full signal and fully responsive tonee controls etc then the problem is v1/v2A.

If it is no good do the reverse, feed the SL V2a from the 45/100 inputs into the SL after swapping load box.

This is to help when you have not got a 'scope and signal generator.

Post back when you have done these things (remember to bleed caps both times on both amps ).