JTM45 Build

[Reeltarded]

Ok! This is good news because now you just have to push wire around until you find the culprit.

Been a while since I did a straight JTM.. let me go look back at your pics. Usually it's about signal wires near the treble cap or that wire from the treble pot vs something else.

hrrrmm..

[Reeltarded]

V1a plate so close to V2b grid. I pull the grids up slightly to get them out of plane with the plates and keep it all as eveny separated as possible.

But.. would you like to end this right now, today?

Put 33k grid stops on all three stage triodes. DONE.

It's a MUST on the input stages and a good idea on the second stage.

And your amp sounds fantastic now. Congrats!

[Littlewyan]

That wire is much shorter already, sorry should have uploaded a new pic. I have just shortened another anode wire though on V1A but I doubt that will fix it. I need to try different valves but its a bit late to be playing the amp now :/. I really don't want to resort to adding grid stoppers as there is no need for them in these amps. Not normally. Plus I'll just be covering up the issue rather than fixing it.

Edit: I should add that the cathode bypass cap I used for V1 is a NOS Phillips cap. Pretty sure its orientated the right way as the band around the cap generally means negative I believe? The other thing is it may not be properly formed?

[Reeltarded]

You think so, but that is the trouble.

You gotta put a stopper on the input stages. Got to do it. it will not even touch the response, but it will stop all the RF ringing in on your signal.

I have had to retro my vintage amps with small ones. The World is awash with RF.

[Littlewyan]

Yeah but I've got my Express here that has no grid stoppers at all and that amp runs like a dream.

Edit: It could just be blocking distortion in the output stage? Could I attach my meter to the bias circuit and see if the bias voltage changes? Surely that will tell me if the power valves are drawing grid current or not. Or even in the pre amp. I'll try different valves tomorrow. Its 11:50pm here and my parents are watching tv in the next room .

[Littlewyan]

Present. Here is a clip of it so you can hear for yourselves.

Tried a different cathode bypass cap on V1 this morning but no difference. Tried putting a jumper cable on the treble control connections in case its that but nah. Measured the bias voltage whilst playing and noticed it would go up from -40V to -47V as I was playing. Not sure if thats normal. Just about to try different power valves.

Edit: Different power valves made the issue worse. If we think about it, the issue occurs when turning the treble up, so its most likely either the PI or power valves that have the blocking distortion.

Edit2: Clipped in another PI Cap to replace the one I burnt but same issue. I could put a scope on the amp and push the front end with a big sine wave to make it distort, but would I be able to see blocking distortion on a scope?

[martin manning]

Turning the treble up lets more HF through... My guess is you have a parasitic oscillation happening. You might want to give Reeltard's suggestion of adding grid stoppers a try, if only as an experiment.

[norburybrook]

I have grid stoppers on mine.


Marcus

[Littlewyan]

Pants. Ok I'll give that a go later. I guess it would explain one thing, the amp cleans up with the guitar volume knob but maybe not as easy as I thought it would. I almost have to have the volume control set the same as I have it on my Express for cleans and this amp should clean up much easier than the Express. I could scope it later and see if there is any oscillation going on.

I suppose if I used shielded input wire I may not have had this issue.

Edit: I'm still a bit caught up on the bias voltage changing as I play the amp. Is this normal? As it goes from -40V to -47V when playing does this not indicate the power valves are drawing grid current?

[martin manning]

I'm still a bit caught up on the bias voltage changing as I play the amp. Is this normal? As it goes from -40V to -47V when playing does this not indicate the power valves are drawing grid current?

If this is happening when you are playing at moderate volume, it could be the result of a parasitic oscillation which is overdriving the grids.

I see you already have grid stoppers on the power valves, and the 68k's on the inputs are functioning as stoppers, if imperfect ones. I'm wondering about the routing of the power stage grid leads. They are running close to the OT secondary leads, and usually the resistors on the sockets are stood on end. You can also twist the leads together from the board to some point close to the sockets. Putting a scope on it it is a good idea.

[Littlewyan]

Just getting my scope set up ready to test. I don't think the issue is in the power stage as it still occurred when I disconnected the NFB. I reckon it could be the PI. Anyway I'll slam the front end with a sine wave to try and recreate the issue, if I can recreate the issue then I'll just keep moving up a stage to see where the blocking distortion is coming from.

[Littlewyan]

Ok I started off with controls all on 5 and volume on 10 and I got a square wave as expected. Then I turned the bass down, same wave, turned the middle down and got this......

[Littlewyan]

The scope bought it am afraid. I slipped when holding the probe and touched the screen grid connection which appears to have knackered the scope.

I'm really stumped at this point.

[Reeltarded]

ouch

Is that the PI? What says ya'll?

I can find trouble with ears and eyes but scope something and I'm out of good ideas.

[Littlewyan]

The scope output I uploaded here was off the speaker socket.

Edit: Ok if I turn the middle up to 10 I don't notice it as much :S. This is so weird.

Edit: Right lets follow the signal path here. With the bass and middle at 0 the middle control capacitor is being shunted to ground right next to the presence capacitor and the bias ground. Issue? I think its definitely something to do with the tone controls and perhaps grounding. Especially as it occurs when the tone controls are in a certain configuration.