JTM45 rectifier question

[SPeter]

You need to be careful with the grounding to minimize ghosting. Try the larry grounding mentioned on this board and detailed on the metro board. Also, you need good filter caps, esp for the mains and screens. try a 32uf that reads high for the mains. Most originals had 32uf on the mains, although I've seen pics of 40uf on a late 45. Screens could be either 32uf or 20uf. Finally, you need power tubes that are well balanced in terms of transconductance. And the a balanced PI matters if you crank the amp

I have a pair of 16/16 on the Screens (jumped so that's 32 right?) and a 32uf on the mains. These are the F/T brand valvestorm sells. Can you recommend a good reliable cap brand and source.
I read somewhere that with tube rectification 40uf should be the ceiling... Do you agree?

Edit: I used star grounding I think that is the same as 'Larry's", I will post pics of that too but I followed this (at least I think I did):

grounding scheme.jpg

[SPeter]

The added length of the PPIMV wiring can cause parasitic oscillations. Use of shielded cable or twisting the leads, is necessary. Post photo of your installation.

Pics of the Larmar install:

20190405-DSC03707-2.jpg

20190405-DSC03711-2.jpg

20190405-DSC03714-2.jpg

20190405-DSC03716-2.jpg

I uploaded them in large size over here, pretty big files you can zoom in any of the sections pretty good:
https://photos.app.goo.gl/MasK71jRMim4nbT69

[SPeter]

Photo of my grounding scheme. The pink mark shows that the ground of the fuse used to go to #5 last night I moved it to #4 to see if it made any difference. It did not:

grounding photo.JPG

[martin manning]

Here is my take on adding a PPIMV. I notice there are no "grid stopper" resistors on the power tube grids, which makes oscillations more likely. You could try putting 5k1 1/2 or 1W resistors on pin 5's (right on them, actually) and see if that helps.

[SPeter]

I just ordered 5w 1k resistors.

I have a 2w 1k flying resistor which I installed based on your recommendation. I noticed that it turned brown :O i removed it and the resistance still read 1k ohms. I put in a new one and played the amp for an hour. That one turned brown too... the 5w will solve that issue too hopefully.

Based on the sound clips do you think this is anoscillation issue?

I'm thinking to put everythin back to metro spec and see what happens.

[martin manning]

1k flying resistor was Roe's suggestion, and 2W rating was from the BYOC layout. 5k may stand up, but it seems like you are drawing a lot of screen current. That will happen if you are playing loud into high primary impedance. You can take that flying resistor out and replace it with a wire if you want to go back to the Metro screen supply set up.

The PPIMV could be causing parasitic oscillation, which can sound like what you are hearing, with the oscillation riding on the notes. Grid stoppers on the power tubes might cure that, or perhaps you can move the leads around. For example, see if lifting the orange and green twisted pair (grid wires) up and away from the output jack does anything.

Going all the way back to the original Metro layout might solve the problem, but I think the most likely culprit is the PPIMV installation.

F&T are very good caps, BTW.

Something else you could try, just to see what happens, is miss-matching the speaker to get a 4k primary load. That would require setting the selector to 16Ω and plugging into 8Ω. That should reduce the current load on the screens, and might clean up the sound.

[SPeter]

I moved the orange-green wire pair out of the way far away from the jack. No change.
The of things the next steps to try:

- replace the green orange wire pair to a shielded one
- remove the PPIMV completely
- add 1k 5W screen resistors
- restore Metro Layout
- dump the whole thing in the garbage can

Only the last step is set in stone

I was wondering if I could drop the B+ voltage closer to Metro spec would that help? If so how could I do that?

The amp is real quiet without signal everything dimed except the presence pot. I'm getting some chirping noise on when I dial up the presence pot all the way.

[Roe]

The added length of the PPIMV wiring can cause parasitic oscillations. Use of shielded cable or twisting the leads, is necessary. Post photo of your installation.

I'd start by moving the wires from the dual pot to the grids. keep them away from other wires and away from the speaker output

[martin manning]

I would also include on your list adding 5k1 grid stoppers on pin 5's.

Removing the PPIMV completely might eliminate the problem, but it has been used successfully in JTM45s. In general it looks like you have a lot of extra wire in there, which provides more opportunities for unwanted coupling and oscillations. Comparing the Metro photo of the finished chassis to yours makes that clear. A general clean-up wouldn't hurt.

Reducing the plate voltage is possible, usually best done using a big MOSFET. Search "MOSFET B+ Reducer." You'd need to find some place to put it and the heat it will generate. I'd go for the 1k screen resistors first.

[Roe]

I moved the orange-green wire pair out of the way far away from the jack. No change.

-keep the grid wires away from the output transformer primaries and secondaries. Shorten the wires and keep them fairly straight.

[SPeter]

PPIMV removed, Metro schemtics restored. I think there is a slight improvement but I'm not sure.

What should be the rating of the grid stoppers again 1K or 5.6K? The Ceriatone layout I found shows 5.6k. I have 1K 5W... so I hope it is that lol EDIT: I have 5.6k 2 watters too... those would be much easier to fit than these big ceramic block of 1K 5Watters

[martin manning]

The value for grid stoppers is not too critical, something around 5k is good. 1/2W is enough, but 1 or 2W will be easier to handle mechanically, if you can fit it in.

[SPeter]

They are in. 5.6K 2W pair. I can hear some improvement. The ghost notes are still there at full volume but much less pronounced.

I searched all over to find people with similar issues and I came across this:
http://www.marshallforum.com/threads/jt ... tes.18420/

To sum it up it looks like this is something I may have to learn to live with...

As the choke is part of the filtering. I wonder if a different choke would make a difference.
Mine is the 20Hy from classictone: 40-18059 (Marshall Style, 20Hy, 70mA DC, D.C.R. = 690 Ohms). This came in the set from Triode electronics

They offer an upgrade which is a 3H: 40-18058 (Marshall Style, 3 Hy, 250mA DC, D.C.R. = 112 Ohms)
Or the base model which is a 5H: 40-18032 (Marshall Style, 5 Hy, 120mA DC, D.C.R.= 115 Ohms)

Now I have to put the LARMAR back in as the family is complaining (the only way to test for this is full volume lol)

[martin manning]

Good progress, then. Do you have 1k 5W in for the screen resistors? From your post above, you have them on hand.

Have another go at the PPIMV as shown in my sketch, and tidy things up so they look more like the Metro photos.

Re the choke, the more Henrys, the more filtering you get. 20H is more than enough.

[SPeter]

Yes I do, should I try to replace the 470 ohmite ones to these 1k ceramic ones?