JTM45 Build

[Littlewyan]

The PI fizz cap is 47pF. I thought about the wires needing to be split up between the stages but the volume control wires and tone control wires are out of phase with each other so that shouldn't make much difference. IN THEORY. I've shortened them all anyway so the tone wires don't touch the volume wires anymore and I shortened the presence wire.

I do remember the wire going from the impedance selector to the NFB resistor being very long. It'll be a pain to shorten though so I've left that for the moment!

Just going to fit the amp into it's cab and then I'll test it again.

[rp]

Disconnect the NFB as a test. If that's it try a shielded run just on that. Time to google image old Marshalls and double check the layout. At least you know it can be done.

Exactly this was why I switched to ptp. Having built beautifully wired up amps and then getting oscillations and throwing my hands up. Granted I wasn't following proven layouts and there was no internet when I took that road.

BTW w/ ptp I don't need no stinkin' PI fizz caps

[Reeltarded]

Yeah yeah yeah disco the NFB and then fly it over the board if you have to just to get it away from errrrything.

I stick my resistor right on the pot and fly the wire to the selector.

[Littlewyan]

Ok haven't had much time to tinker as I spent ages drilling holes in the head cab to mount the chassis and I was sorting out the shield. Anyway thats all done!

So I tested the amp and still have the same issue after shortening the tone and volume wires. After playing with it a bit more I found that if I turn the Presence right down, the tone changes where I can hear the treble control actually working and the tone is clear. If I do it vice versa, turn the presence up but treble off, again tone is clear. Both up and it turns into a mess. I guess next thing to try is shortening the NFB wire thats underneath the board. Dam.

Edit: Just realised, the NFB wire goes across the power valves grid wires.....

[pdf64]

My experience is that these can be prone to oscillation and blocking distortion.
Have you fitted grid stoppers (eg 5k6) to the power tube control grids, at the socket terminals?
Consider reducing the power tube grid leaks down to 100k.
Suggest you twist any balanced signal wires, similar to heaters.
Shorter isn't necessarily better.
Where conductors in close proximity carry signal with gain and polarity flips, coupling between them can cause oscillation; best to keep such conductors well seperated.
Think of plate wires as transmitters, grid wires as receivers.

[Littlewyan]

Yeah grid stoppers are fitted. Ok I've lifted the board and now I have to make a choice. I can either try just shortening the NFB wire by 2" and leave it where it is or I can reroute it so it goes between the PI socket and power valve socket. In my 1987xl I believe it was routed with the secondary and primaries of the OT like I've done.

[Reeltarded]

YIKES!

We found the problem. If it touches the OT wires you lost the round. WAY far away.

I have a mutant super gain 4 holer open and upside down with the gains all dimed and the volume is at 5 right now and not one wibble. NFB is flying across the chassis in plane with the open side so it lays on the headbox when it's installed.

[Littlewyan]

But why should it affect the NFB if it touches the OT wires? As it comes off of the OT?

[Reeltarded]

The speaker taps come out and the pimary comes out but the NFB feed wire needs to be away from most other stuff.. from the pot to the resistor and then carefully routed sort of to the right and kind of short if it's in a stock configuration. This was the first build problem I ever experienced so I build everything with the resistor on the pot and the wire flying now.

[Littlewyan]

Well I just shortened it and it may have made a slight difference. I just turned off all of the lights, unplugged the guitar and turned every control onto 10 and couldn't see any change in the power valves. So its obviously not constant oscillation. I know routing the wire the way I have goes against what you say Reeltarded but many people have built these amps this way before :/

[Reeltarded]

Here is an example of what I don't do but it works. I wouldn't but it's the way they do it.

[Littlewyan]

O wow never seen that before! Ok there are a couple of tests I'm thinking of doing next.

1. Hook up a meter so I can watch the HT Voltage as this is happening to see if its going too low.

2. Do what you guys said and undo the NFB temporarily to see if the issue still occurs. I doubt it will as turning the presence down to 0 seems to make the problem go away.

[Littlewyan]

Definitely not a voltage issue. The lowest it went was 380V and that was on single note playing. When the issue occurred it only went down to 410V from 465V. Not sure if I'll bother disconnecting the NFB as like I said we know that turning the presence down to 0 solves the issue as well as turning the treble down to 0. I'm sure I remember reading that someone else had an issue where turning both the treble and presence up high caused an issue, but remember who or what came of it.

Edit: I should add that turning on the boost on my les paul makes the issue much worse and if I go without the boost and turn the volume down slightly it fixes the issue. As does turning the volume down on the amp slightly.

[Reeltarded]

The NFB circuit is subtractive to the overall sound. If you remove the NFB feed the amp will be about 20% louder and grits like an ac30, then you can see if the amp does it without..

I am thinking it doesn't but you never know until you do.

Hate to muss up your pretty amp.. just lift on end of the NFB resistor.

[Littlewyan]

Ah ha. It still does it without the NFB connected. Lifted the NFB resistor like you just said. Was quite disappointed with the gain, thought it was going to be a massive gain boost.

Don't worry its not messed up, I'm being extra careful!