JTM45 Build

[Littlewyan]

Also I just found that with the 2Vpp signal into the mixer resistor and the Treble on 10 the signal going into the PI input is quite clean. However on the other side its distorted with the top and bottom chopped off but the flat part on the top and bottom is thickish and is moving.

[Littlewyan]

Don't know if this means anything but when I have the high treble volume on 10 and the treble on 10 but everything else 0 the PI anode side with the 82K resistor swings up to 300V whereas the other side stays around the 200V mark. Measured the bias voltage on the grids of the power valves and V4 starts to go positive as well. However if I bring the middle and bass up with the treble its fine :S.

Also just now the amp blew a fuse because of it, think it may have red plated and I just didn't see it in time.

Edit: V5 has died. Dammit.

Edit2: New valves in, same issue. Strange how V5 blew considering it wasn't the one red plating.

[martin manning]

What happens if the power valves are removed? Does it remain stable?

[guitarmike2107]

Good point Martin, I would ask the same thing if V1 is removed too.

Ryan, have you scoped the amp with nothing plugged in and everything cranked full?
Is the amp stable running wide open by itself?
Is it stable running wide open with the guitar plugged in and guitar turned down fully?

Did you try disconnect the power valve cathodes from the chassis and run a wire direct to the cap can? I recall you been concerned about the quality of that connection

I read through some of the older posts to see what I had missed, Last time I had an issue with an amp that was affected by the guitar pot position the problem was outside the amp.... Just a thought...

[guitarmike2107]

also what were your under board connections like? I normally don't solder to turrets on the underside of turrets as I struggle to get a good connection, if you look at the metro amp and originals they brought the wires through the board and wrap around the turrets, Miles posted a good picture earlier showing this, makes it much easier for you and others to spot errors.

[Littlewyan]

With power valves removed the PI voltages do exactly the same thing. 82K side swings positive really high and the 100k side swings negative and goes down to about 160V. It also only does this if I slam the front end. Unfortunately the oscillator I've got only puts out 2Vpp and nothing else, so its either slam the front end or under power the mixer resistor.

I did scope the amp with everything cranked but nothing plugged in and picked up nothing but noise (smaller than 10mV on the speaker output!).

The amp is stable with everything wide open guitar plugged in or not. I didn't disconnect the power cathodes I simply ran a jumper lead from them to the chassis. I checked all under the board connections with a meter and all came up ok.

So we can rule out the power section then I guess.

Edit: Just found that with the power valves out if I measure the voltage on the end of the grid leaks and turn the treble up the voltage goes a bit more positive by a few voltages and then back to where it was. Yet the actual bias voltage on the bias circuit stays put. Surely this is a leaky coupling cap? I already tried changing the one coming off of the 82K resistor but not the 100K resistor one. Would this cause the issue? Bit strange how both sides do the same thing though if only one coupling cap is faulty and the actual bias voltage doesn't move.

[martin manning]

Did you try a larger cap across the PI plates?

[Littlewyan]

I did try a 100pF Cap clipped onto the plates in addition to the current 47pF. Did you see the edit on my previous message?

[martin manning]

Yes, I'm thinking that effect is asymmetrically clipped signal coming through the coupling cap.

What happens if you put a small cap across the PI plate load on the side with the odd behavior?

[Littlewyan]

The one that swings positive does the same thing with 100pF across the 82K resistor.

Across the 100K resistor, same thing. Swings way negative.

[martin manning]

Can you get idle voltages around the PI pins, including the PI supply voltage? Measure grid voltages from each grid to the junction of the 1M's and 470R (i.e. across the 1M's).

What about the 0.1uF cap at the non-inverting grid? Any chance it is defective?

[Littlewyan]

From memory the idle voltages are roughly 200V each side and the grids are 28V. 336V rings a bell for the HT. Not sure about the cathode.

When you say non inverting side do you mean the 100K side or 82K side? As I already tried swapping the 82K side coupling cap. Could try again tho to be sure.

[Reeltarded]

oh no... is that the melty one, Ryan?

[martin manning]

I'm talking about the 0.1uF cap that goes from the 100k side's grid to the 27k FB resistor/10k tail resistor/presence pot junction.

[Reeltarded]

one of his .1u got a little iron rash.. wasn't sure which it was...