JTM50 Blocking Distortion?

[Littlewyan]

I just finished building a JTM50 and everything checks out ok except I get some sort of blocking distortion which mainly shows when playing big open chords. When playing single notes or power chords or even the low E power chord you don't notice. Hit a big Open E and you get blatty distortion. When the amp does this I've got the controls set to Presence 10, Bass 0, Middle 5, Treble 10, Channel 1 volume 10 and Channel 2 volume 0. I can make the issue go away by turning the treble down strangely enough. All my leads are short and I can turn every control onto 10 and not get any oscillation without a guitar plugged in. So I'm at a loss really.

Any ideas?

[neikeel]

Pics

[Littlewyan]

You can't post pics on here anymore can you?

[Littlewyan]

Hopefully this will work

https://goo.gl/photos/e98dcUKt8BnUh5x56

I've got an audio clip of the issue as well but I'm not sure how I'll upload that!

The strange thing about the issue is it doesn't occur when playing chords on the Low E or when playing single notes. However I if whack an A chord or a big Open G or E it will happen. I think it also occurs when hitting high single notes hard and using a boost on the front end of the amp makes it worse and if I turn the Middle down to 0 it gets worse.

Voltages in the amp are fine, checked each HT node whilst playing and all sag slightly (except for mains which sags by about 100V). Tried replacing the 250uF Cathode Bypass Cap, the Treble Cap, the 0.1uF Cap in the PI that goes to the Presence control and even tried disconnecting the NFB but I still get the same issue. Tried different pre amp valves and power valves. I can turn every control up to 10 and all I get is hiss out of the speaker, no oscillation. Really am stumped here guys.

[neikeel]

Nice build!

Excellent clean work with good quality components.
You appear to have done all the things that I would suggest (treble cap, different valves etc).

Subtle point but the grid wire to V5 should be moved across away from the OT wires (with its resistor) into space and the loopy grid wire to V4 shortened.

Check the under board connections in the preamp - to check nothing loose (sometimes it is mechanical rather than electronic resonance!).

Make sure you have a good rectifier (some sag is normal) but a good NOS or nearly new Mullard is the definitive item.

[VelvetGeorge]

Unbundle the OT primary and secondary wires, these should always be bundled separately.

george

[Littlewyan]

Just responded to your email George!

As I mentioned in the email I found 3mV on the treble cap in the tonestack, .004V on the .022uF Cap that goes to the bottom of the treble pot and .004V on the PI coupling cap. When I turn the treble pot up to 10 I get 3mV on the PI Cap. Can't be the treble cap though as I changed this last night from a 250pF to a 270pF Cap?! I'll swap the .022uF Caps to see if that fixes it, if not I'll lift that Treble Cap off the turret in case something strange is going on underneath the board.

[Littlewyan]

The DC on the tonestack turned out to be nothing.

Would unbundling those wires really make much difference? As they'll still be next to each other and I bundled them together in an Express build that I did.

Here is an updated photo of the wiring.

https://goo.gl/photos/oeCDbRudTTEgWVnq8

[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 George with regards to the OT Primaries and Secondaries I have twisted the primaries together but left the secondaries untwisted on top. Did you think I had twisted the whole lot together?

[neikeel]

Red plating like that can be caused by parasitic oscillation due to cross talk between primary and secondary signals and from the grids of the output valves.

Suggest you , move the primaries away from the secondaries (Marshall seems to run them to the side under the bias caps during that period) and run the secondaries in a straight line from the OT hole. As before take off the last twist in the orange and green wires remove the loop in the green wire and move the orange wire away from the bundle too.

[Littlewyan]

O sorry I see, so you mean make the green wire a bit shorter so it looks like its going straight from the board.

At the moment my primaries come out of the hole and go to the right of the secondaries (if you sit in front of the amp facing the back). So do you mean swap them so the primaries go to the left of the secondaries? As in George's guide they go to the right of the secondaries.

[neikeel]

Here is my original:

[Littlewyan]

My power valve grids do run right next to where the secondaries come into the chassis. If I put them onto the other side of the board standoff then they'll be about half an inch away from them. My NFB wire by the way follows the secondaries until it gets to the top of the board, then it runs along the top to the NFB resistor. Is that ok?

[Littlewyan]

Separated the secondaries and primaries as best I could and shortened the green grid wire, no difference at all

https://goo.gl/photos/SquyKVoMKYW3bdag9

How could oscillation make the grid of a power valve go positive? I thought only leakage from a capacitor could do that?

[neikeel]

Sorry, but I really do not know what is going on from this distance.

It sounded very much like PO and yes cross talk across the grids can cause red plating.

All the things you did earlier sounded correct (the treble cap and its wiring would have been my guess until the EL34 red plating issue. I presume all of your output section components are in spec (I have to admit to putting a wrong value in the PI plate of a plexi not long ago (should have been 82k). Now I wear some +1 glasses for late night amp building (often only time I get free time!). I cannot see that sort of thing.
The fact that it is not a constant issue and only happens with certain control settings sounds like you have found a resonant frequency that is causing the issue, could be as simple as a dry joint, faulty internal component, poor ground on one tube socket, loads of irritating things to go through methodically one by one.

Please let us know how you have progressed!