Metro JTM45 ghosting revisited......

Flames1950 · Post by **Flames1950** » Mon Mar 13, 2006 9:08 pm

Since many of us have had ghosting with the stock filtering values on these, I thought I would share some recent experience for the better on the subject.......
On original JTM45's you may note that there is a 1K 2watt resistor in between the screen voltage tap and the 470K screen resistors. I asked George once why he did not include this resistor, and he said he didn't like the feel of the amp, he thought it stiffened it up too much. The Marshall RI also does not have this resistor........
For the hell of it, I gave it a try Saturday. The amp was a touch stiffer I suppose, but actually seemed a little truer to the classic Clapton "Beano" tones. But the real treat was the elimination of almost all my ghosting issues!! Virtually no ghosting to be found.........
It does require removing the yellow wire from the 32/32 filter can to the power tube sockets, and replacing it with a longer wire that goes to the 8.2K resistor on the board on the power tube socket side (remove the yellow wire from this point to the power tube sockets as well.) Place the resistor next to the 8.2K, with one lead soldered to the 8.2K's bare lead between the resistor body and the turret on the tube side, and the other lead soldered to pin 6 of the tube socket where the short wire from the board to the tube socket used to be...........I know, I shoulda taken pics, but that would have been too easy........
It also brings me back to the question of whether my JTM45/100 would benefit from the 1K screen resistors instead of the 470's.........

BashCoder · Post by **BashCoder** » Mon Mar 13, 2006 9:15 pm

Pics, man!!! Pics!!!

Flames1950 · Post by **Flames1950** » Mon Mar 13, 2006 9:17 pm

Damn, and I just put the back panel back on that amp this afternoon......I'll try to get some pics in the next couple of days, but you can see plenty of examples in JTM45's in Marstran's pictorial history too.

Flames1950 · Post by **Flames1950** » Mon Mar 13, 2006 9:18 pm

I should also note that even my neck pickup didn't elicite much ghosting at all....it was damn near unusable at full volume on the guitar before.

BashCoder · Post by **BashCoder** » Mon Mar 13, 2006 9:20 pm

Oh yeah, and for what it's worth I have 1K Welwyns for my screen grid resistors in my JTM45/100, and the amp is haunted with ghosts when I crank it. Will this mod of yours exorcise my amp?

Flames1950 · Post by **Flames1950** » Mon Mar 13, 2006 9:23 pm

I don't know what it will do for the 45/100. I just tried it on the JTM45 since it was true to the originals and I was bored.
Too bad on the 1K's on the 45/100 though, I thought that changing to those from the 470's I'm using might have a similar effect. I still love that amp too much, ghosting and all, to get too upset about it though.

BashCoder · Post by **BashCoder** » Mon Mar 13, 2006 9:23 pm

Yeah, neck PU has the worst ghosting for my single coils and that's the sound I use most. Heck, a lot of the time I really like it -- some serious fatness and harmonic complexity. But then I go up the neck and play lead lines and it's it's like I've got an octaver on, but not quite in tune.

Billy Batz · Post by **Billy Batz** » Mon Mar 13, 2006 9:35 pm

I dont know if I totaly believe the conventional wisdom anymore. I dont think upping the screens is the only thing. I have an order to do another early 67 for a friend and Im 'adding' a few 'extras' to his order so I can mess with all the filters in my 67. Ill try moving the screens to 50u which I doubt Ill leave it that way even if it works but from paralleling cans with the screens Im convinced it wont totally cure the problem. I also am upping V3 and the preamp a bit. Ill go every way I can to see what gets me the most with the least change in tone.

Flames1950 · Post by **Flames1950** » Mon Mar 13, 2006 11:52 pm

Here ya go Bash, I didn't wanna go to bed and get some sleep anyway.

Note that my yellow wire from the cap can slinks under the board now and over the edge so that the screens are supplied through the resistor off the board now.

It's not exactly like upping the screens filtering, Dan, it's limiting the current draw; now the filters won't have to work as hard to supply the current demands.....sonically I don't think it has as much a stiffening effect as upping the filtering. Again, it took a turn towards even more authentic "Beano" tones......but I've never seen this done on a 100-watt so I have no point of reference for what it would do to the tone in that application.

BashCoder · Post by **BashCoder** » Tue Mar 14, 2006 12:23 am

haha -- you da MAN!! Thanks, bro.

Post by **VelvetGeorge** » Tue Mar 14, 2006 10:50 am

Great info, Flames! Glad that works for you. I should try it again sometime too.
I swear I must have sharp fingers or something, because I can dial an amp in for me and then someone else plays it and it's pure mud. Crazy ghosting.
It's not that I'm a finess player, I can assure you of that!
I use .010's and 2mm picks too.

On ghosting: anything you do to the amp that will reduce the ability of the signal to modulate the power section will help. Raising the screens filtering is a great place to start. So is more regulation to the screens. A larger choke will help, somewhat indirectly.

I'm interested to see what happens with the 100 watts......

VG

BashCoder · Post by **BashCoder** » Tue Mar 14, 2006 11:15 am

Thanks George - I'll try this in a few days and see what it does to my 45/100. The circuits are similar enough that I might expect a similar result. So that's what does it, huh? The signal is modulating the power section -- makes sense. So then the signal rides on top of that power, which contains an image of the signal and both make it through.

By more regulation, do you mean add another diode bridge?

Post by **VelvetGeorge** » Tue Mar 14, 2006 11:23 am

Regulation by means of:

A. current limiting larger resistance in the B+ line.

B. higher filtering more current on tap to resist modulation.

george

Billy Batz · Post by **Billy Batz** » Tue Mar 14, 2006 1:12 pm

Traynor and HiWatt use that same solution. Apparently splitting the current limiting resistors to the screens isnt as efficient as using one large resistor. I think HiWatts actually split it so that theres a common 470 at 10W and 4 100 at 1W on each tube. Ill have to try that. Im just worried if thats OK to do on a 100W Marshall?

Flames1950 · Post by **Flames1950** » Tue Mar 14, 2006 4:37 pm

See, at most I could go to 1K's on the screens in the JTM45/100, because I've seen many pics of examples with 1K's (both old and new resistors) to make me think many did have this value instead if the 470's......but anything more would be changing the amp too much for my tastes.....I'm just wierd that way, and if it ghosts then goddamit it will ghost and everyone in the front row wil be too deaf to know any better!!!
I only did this to the JTM45 because that was the way most originals came, at least the KT66 ones....I don't see the resistor on EL34 amps and didn't look around at 6L6/5881 amps.