JTM45 Build

[Reeltarded]

Hey, I can hardly see right now.. you have it wired single node out front like the stock 45? how many nodes total?

Hey.. I may have an idea if you say single node and you have more than 4 filters or if you have a pair of filters tied together.

I may have asked this before but the whole thing has become deja vu all over and over again.

[Littlewyan]

V1 and V2 are seperated by a dropper resistor. Tried swapping the pre amp dual filter can for two seperate filter caps but same issue.

[Reeltarded]

Oh really? Hmmm.. cause.. I have something to show you if you will ignore the trem..

[Littlewyan]

If you mean V1 and V2 sharing a node yes JTM45s did that, I built a JTM50 (sort of). Late JTM50s had separate nodes for V1 and V2

[Reeltarded]

We found your problem. It's there in the string. Back up and redo that to 45 spec and watch it work.

Is it that it won't work? Nope. Everyone that does this creates their own style of error when spliting another node for V1.

Undo it and test.

[Littlewyan]

Why would this be a problem? I did it the same way it was done in later 50W Marshalls?

[Reeltarded]

I have no idea. There are 10 mistakes I have seen when people split off another node there.

It has to be something and as far as I can tell we tried everything else. This is the most common other thing.

If you had previous experience running this amp without a failure I would say idle it for 20 minutes and put your hand on the PT, but you don't have a baseline. I bet it's 20% hotter than it will be when the amp works.

Speculation. It's even odds here.

[Littlewyan]

What mistakes though? Its dropping the voltage as it should across the gain stages.

[guitarmike2107]

Have you tried bypassing V2 yet?

I agree that it sounds unlikely that decoupling V1 and V2 should cause this, but hey who knows and it is worth a try, it does look like you have decoupled V2 after V1 though. i.e.

V3- 10k - V1- 10k - V2, rather than the more traditional
V3- 10k - V2- 10k - V1

Would be easy to link out the 10k resistor to see, if it is wired stock we know it should work....

[Littlewyan]

V1 is decoupled after V2, I double checked. I haven't bypassed V2 yet, its on my list of things to do today.

Edit: I still haven't yet got round to this. I spoke to a local tech on Saturday who was very helpful and spoke with me on the phone for an hour. Awesome bloke. Told him the issue and he suggested double checking all of the components and wiring, if everything is in order then change all of the caps in the tonestack or any parts that may be suspect in V2. If there is still no change he said he will have a look for me.

He also suggested driving a signal into the tonestack but I don't have a signal generator powerful enough. He said he has one that can push out 25V! Anyway it is on my list of things to do. I was sorting out my Express today and my little JTM1.

[Littlewyan]

Put a jumper across the decoupling resistor but still same issue.

I also removed V1 and pushed a signal into the mixer resistors and had the same issue so that definitely rules out V1.

Just removed V2 and connected the oscillator to the tonestack and the PI anodes at idle are 10V apart. If I push a sine wave into it its fine, if I push a square wave into it they become 30V apart depending on where the tone controls are. Raising the treble control definitely made the anodes voltages move further apart but not by much. The signal was 24Vpp which may have been a bit weak for the job. I started with 1Khz and moved the frequency up slowly but the only thing that happened was both anode voltages started to decrease together equally.

There are few more checks I need to do. I need to go over all of the voltages in V2 to see if anything goes iffy when the issue starts. I also need to disconnect the PI again and run this oscillator through it as mine just wasn't powerful enough, but this one I borrowed from my Grandad should do the trick. We're getting close!

[guitarmike2107]

We're getting close!

I hope so! you have done well to stick with it, I would have pulled it apart and started from scratch by now

[Littlewyan]

Won't learn anything unless I find out what is causing it though.

Right I just pushed a signal into the input again and found that with a 250mV input signal the anodes stay 10V apart when the PI is behaving. If I hit the front end with a 1V signal they become 23V apart. also the input signal to the Tonestack is only 10Vpp and I was exceeding with the oscillator last night so I was driving the tonestack plenty hard enough.

So its either something extremely strange happening in the very simple V2 circuit, or its something in the tonestack throwing off something in V2. I know oscillation can occur in cathode followers in the MHz range so I wonder if my cathode follower is oscillating.

Anyway I've narrowed it down between the mixer resistors and the tonestack.

[Littlewyan]

Actually I got it wrong I wasn't driving the tonestack hard enough! I'm going to lift the PI Input Cap and drive the PI direct to see if it will misbehave.

It doesn't misbehave like it does when I push the signal through the input. The anodes start at 10V apart then then 100K side goes down to 190V and the 82K side goes up to 232V.

[Littlewyan]

I just replaced all of the capacitors in the tonestack and the phase inverter (except for the coupling capacitors to the power valves) and the issue still occurs. I converted the tonestack to the later 33k/500pF as I didn't have anymore 250pF Caps and the issue got worse. All of the resistors measure correctly so haven't replaced those (except tonestack of course).

After speaking to my Grandad all we can think of is the V2 valve socket is knacked but its a long shot. I can smell a very very slight burning sometimes in the amp, but its very slight. However I have no more ideas.