JTM45 Build

[norburybrook]

Adding grid stoppers won't bugger up your lovely work and will be the first thing I'd do



As I said mine's as quiet as a mouse but ugly as sin inside



Marcus

[Littlewyan]

That DC was nothing, I still had it even with all of the caps lifted.

Tried replacing the PI 0.022uF Coupling cap but still same issue. George at Metroamp suggested un bundling the OT Primary and Secondary wires but I bundled these together on my Express build and it was ok?

[martin manning]

I still don't like the proximity of the power valve grid wires to the OT wires. I would twist the grid wires together and move them away from especially the OT secondary leads.

[Littlewyan]

They are twisted right up to the end of the board. Here is an updated photo to show you the latest wiring.

[martin manning]

I'd want the grid wires well away from the OT wires.

[rp]

Really doesn't look any different than a zillion other working Marshalls except around the output section and that's just a millimeter here or there. As much as you don't want to mess the nice lacing it's time to unbundle. But first definitely try what Martin said, maybe a few millimeters is all it takes, otherwise you could temporarily run those grid wires over the top, maybe w/ some dual shielded wire just to test that out. Why the floating swampers? Why not Black Face style off pins 5 & 1? Granted that would add a tiny bit or wire, but it also would force a reroute that might prove lucky.

[Littlewyan]

I found a problemo!

Got my brother to play guitar whilst I monitored and I found that the valve closest to the preamp is actually redplating a bit and I watched the bias voltage and whenever my brother hit a chord it very quickly went from -43V to -35V. Whereas the other valve is normal, bias voltage went from -43V to -50V and wasn't as erratic. So what does that tell us?

Also I tried moving the grid wires about whilst he played and they didn't make any difference. Even if I lifted them away from the secondaries.

[Reeltarded]

Interestong but not the HF osc you have. Swap tube positions and test it. The redplate follows the tube I am betting..

What is your bias and voltage?

[Littlewyan]

Tried different power valves the other day so I doubt its that. Just tried replacing the coupling cap for that valve but same problem. Just about to put the other coupling cap back then I'll hook my meter up and will see if the issue follows the valve just in case.

HT: 460V
Bias: 39mA last time I checked

Right just watched the bias voltage as I played and if I turn the guitar volume down the voltage goes only further negative as it should. Crank it and the voltage goes more positive at times and the valve begins to redplate. Will swap the valves in a sec, they're bit hot at the moment!

Edit: I can confirm that it does not follow the valve.

[Reeltarded]

With 34s? My bias on the bench right now is -39 on 388v..

Uhh

[Littlewyan]

70% dissipation? How did you bias them? I checked mine using the 1Ohm Bias Resistors and by measuring the voltage drop across the output transformer primaries. Also different valves can bias very differently. I stuck some GT EL34Ms in this amp and they were biased cold at 25mA each with -40V Bias Voltage.

One thing I tried was separating the secondaries and primaries as best I could and shortening the green grid wire, but this made no difference at all

Here is an updated photo.

[rp]

I stuck some GT EL34Ms in this amp

That explains your flying grid resistors. I'm rooting for you to find the problem, my 5G9 is going to wind up laid out like a JTM45 in a 12x8 chassis and I'll have pretty much the same wiring around the output stage. First amp w/ a board in 20 years, now you got me worried

[Littlewyan]

One thing I have noticed is my PI grids only measure at 14V whereas the Express voltage chart shows them to be 28V and Metroamp's 50W Marshall Voltage Chart shows them to be 36V. Are mine a bit low?

Also what could make the grid of my V4 EL34 go positive for a split second whilst playing? Its not a leaky capacitor as I tried subbing in another 0.1uF Cap. Unless that cap happened to be faulty as well but its unlikely. Plus the bias voltage is stable at idle. Someone on the Metroamp forum said its oscillation but I can't see how that would make the grid positive :S.

[Reeltarded]

If you don't stop the grids I am attacking Iraq.

Lift NFB. No redplate..?

This is a 20 cent fix we are spending days on. You have to rtust someone. I am done.

[lovetone]

I think you need to take a step back and start again....

I would go back to the scope and sig gen at 1Khz tone stack all controls on 5 disconnect the NFB.

Go through each stage checking for a clean wave form until you find where the fault starts.

If you think the fault is in the output stage disconnect the preamp and feed your signal straight in to the coupling cap of the PI use the level control on the sig gen. Bring the level up steady and look at the output stage waveform if its clean then you know the problem is up stream.

I use an 8 ohm 300 watt dummy load to keep the noise down, in the u.k you can use an old immersion heater element these are around 10 ohms.