JTM 45/100 - Build story

[Xplorer]

Thanks ^^

Well, i tried the amp this morning, with the different Ac voltage taps - 220 - 230 - 240 v ac ...
no much change, except for the voltage on the plates. ( so : all about the preamp board ? )
( by the way, the quad of kt66 doesn't seem to be very matched, since the mA seen accross the 1 ohm resistors varies a lot, sometimes close to 10 mA between the min and the max )

it doesn't have this bite needed, and i really have to hit the strings harder to get a little something.
at ten, if i play normaly, it still has a thin tone in the high frequencys, and it's like crystal, it's very clear. not ugly for sure but i'd expect more range with the amp : warmer, whooly, thick .... it doesn't !
i couldn't really do some jeff beck tones out of it, bity. at ten, it's surprising ...

Why the hell wouldn't this amp get there with stock specs ???

[Xplorer]

ok, as Neil advised, i used some CC 100k that measured 113k on the plated of V1. And i used a 10k CC 1w that measured 11,3k in place of the 8k2.
Well, this may bring a bit more break up but i'd like this to be earlier ...

You also advised me to reduce the voltage on the screens. How can i do that please ?

What else could i do ?

i'll post a list of voltages from my turrets, if it's ok to compare with your amp.

Thanks !

[Xplorer]

Here it is, all the voltages :

1 : -49
2 : 0
3 : 0
4 : 395
5 : 13,44
6 : -58,2
7 : -58,2
8 : 13,47
9 : 0,005
10 : 0,04
11 : 396
12 : 0,0008
13 : 0,003
14 : 0,0002
15 : 0,003
16 : 0,002
17 : 0,003
18 : 0,0002
19 : 0,0002
20 : 0,002
21 : 0,0002
22 : 0,0002
23 : 0,0003
24 : 1,57
25 : 227
26 : 237,7
27 : 1,27
28 : 193,6
29 : 192,5
30 : 340,2
31 : 340,2
32 : 28,5
33 : 45,2
34 : 30
35 : 267,5
36 : 245,2
37 :0,0015
38 : 496,8
39 : -59,7
40 : -73,45
41 : 499,5
42 : 132,8
43 : -59,7
44 : 394,5
45 : 43,7
46 : 0,0055
47 : 0,001
48 : 0,0004
49 : 339
50 : 0,0004

[Xplorer]

51 : 0,0003
52 : 0,0003
53 : 379,5 AC
54 : 379,5 AC
55 : 0,002
56 : 0,002
57 : 393,6
58 : 498,7
59 : 499,8
60 : 500
61 : 499,2
62 : 499,3
63 : 267
64 : 268
65 : 168,9
66 : 183
67 : 175,5
68 : 190
69 : 248
70 : 0,002
71 : 500,17

[Xplorer]

Ok, it has a bit more saturation but it sounds wierd to my ears ... and it doesn't feel right when playing. thin replace "bite", and the saturation looks like some kind of oscillation, modulation ...
this is how it sounds when played on a strat, at max volume. neck pickup, middle pickup ...
it doesn't even make me want to try to do a nice clip.

[Xplorer]

funny stuff but with a fuzz.
trying to play and play to burn the amp in.
so : allways on 10, plus fuzz close to ten.

[shakti]

That can't be right. Sounds like you have a flanger built in! Please try it with another preamp filter cap.

[Xplorer]

Yes but i actualy already did replace this preamp cap.
A flanger ... In which clip particularly ?

I should perhaps make sure that the jtm's power supply is completely right, tubes too, circuit too, and then build a second preamp board to compare. ..
And with the same components as karlis perhaps, as i have them.

If the problem is still there, then it might be some other stuff going wrong ..

[neikeel]

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.

[Xplorer]

Hi Neil !

- i bought this RS resistor : 15k 10watts ? ......... 10watts ?? wow !

- i don't have 5k6 in my hands, but i thought about using some nos RS 1k anyway, in place of the welwyns.

- yes, i'm using an Rs 560pf. i also have some Rs 5000 pf

- i checked the pots previously, it read ok on lugs 1 and 3 but as i used the solder gun for a long time on them ( when i had troubles in the beginning ) , could they be damaged because on the whiper ? i think it reads 1M on the lug 2 and 3 of the volume pot.

- it acts the same on the normal channel.

yes, i too think that i have a dodgy component somewhere, cause it should react just like your amp does !
But i'd really prefer to see it working with stock specs in the beginning, and then start to tweak it with what i've learned thanks to you and many of you guys, in this ( not ended ) build story.

i thought about making 100% sure that everything except the preamp board is right, and build another preamp board, as i planned to do anyway.
So that would mean making sure that :

- the pots are ok
- the power board is ok
- the tube sockets are ok ( although the voltages were close to correct ... )
- the wiring is ok ( i think i checked this already, and the amp couldn't work anyway if it wasn't wired correctly, right ?
- the tubes are ok
- replacing the welwyns with the RS 1k
- perhaps changing the 5k6 resistors : i have two, one between V4 and V5, another between V5 and V6.
- i'm using two 5k6 resistors, between V4 and V5 and between V5 and V6. i could change this probably ?
- place the new preamp board in place of the old one.

if somebody see something wrong on my layout .... cause i'm following it.
Any double advices on how to methodlicaly check each of these steps, to make 400 % sure that it's a good basis .... very much appreciated !

we could loose more and more time on trying to find what's wrong in this amp but it would be easier and faster to work on the amp from almost zero, with a good base i think, although it was very interesting to get some notions on how to debug an amp.
Here, all the best, neil, shakti, brian wallace, Roe, Karlis .... couldn't have the amp under their eyes, even if i tried to post clear pictures and tests. So i's not easy to debug it.

and building a preamp board is a pleasure anyway. there are not so many components in a jtm 45/100 too !
What do you think ?

[Xplorer]

oh yeah !! great clips !
i want this lol.

http://www.youtube.com/watch?v=OFlyLwbq ... sults_main" onclick="window.open(this.href);return false;

i didn't see it before. i know it's humbuckers, and i'm testing the 45/100 mainly with single coils on strat but , very nice.

[neikeel]

That is Ian (friend of mine) playing his SG through an EL34 equipped JTM45/100 with Drake 1202-119 and 1203-80PT, Drake -114 choke and dual rectifiers, sweet isn't it

He will be pleased that you like the clip!

[Xplorer]

Sure, it sounds just right !
Doesn't seem to be a 45/100 anymore ??? what's left of it ?
What do you think of my plan and its steps, to save my project ?

[neikeel]

Sure, it sounds just right !
Doesn't seem to be a 45/100 anymore ??? what's left of it ?
What do you think of my plan and its steps, to save my project ?

Use it play it!

As I said if you want you can send it to me and I will try to get it sorted - all you have to do it stump up postage to England (and back!).

[Xplorer]

Oh ! Sweet ! That's very kind of you , thanks ! i didn 't realise that you actualy proposed this before.
Yes, why not, i'll get some infos on the rates, and how i could pack it very well ( cause i don't have a box for this one, although i could use the SL's head box .. )

Anyway, before that, i'd like to try it with a new preamp board, stock, and check the rest of the amp properly, and find any hidden problem in : the pots, the tube sockets, the power caps board, the wiring ... scientificaly ! any advices to make sure that the tube sockets and power cap board are ok are welcome ! i'll also change the welwyns for the original RS 1k anyway.

i'll take pictures again , without the preamp board, and post them.
then, if it reacts the same way, it would mean that i missed the problem, and the preamp isn't involved.
after all, there are not so many components in such amp !

and if there's nothing i can do, i'd ship it to you to have a look, thanks !