JTM50 Blocking Distortion?

SDM · Post by **SDM** » Tue Jul 14, 2015 7:08 am

Littlewyan wrote:.... I did check the pi input and speaker output with a scope before the rebuild and they are out of phase...

They shouldn't be.

Littlewyan · Post by **Littlewyan** » Thu Jul 16, 2015 4:58 am

I removed V1 and hooked my signal generator up to the mixer resistors, same issue. Tried replacing all of the capacitors in the tonestack and PI (except for the output valve coupling caps) and still had the same issue.

The signals should be out of phase, if they were in phase you'd have positive feedback.

As all of the wiring, voltages, resistances and capacitors are correct my last idea is I have a dodgy valve socket. I do recall I would sometimes smell a very very slight hint of burning in the amp but not so bad that it alarmed me. I can't see any arcing on the valve sockets nor can I hear any popping or crackling through the speaker but they are the only parts I haven't changed. Except for resistors and pots but they all measure up fine. As the issue occurs with V1, power valves removed and the bias circuit removed it has to be something between the mixer resistors and PI.

I did find a small mark on the V3 socket between pins 6 and 7, but its very small and doesn't go all the way from one pin to another. Not sure what to make of it.

SDM · Post by **SDM** » Thu Jul 16, 2015 10:48 pm

Littlewyan wrote: The signals should be out of phase, if they were in phase you'd have positive feedback.

Not with a long tail pair PI.

A simple way to look at things is to just think of the PI as amplifying the signal difference between the grids. So, if signals are in phase (and ignoring a lot here) you've got one larger signal, one smaller. They are in phase, so have a fixed difference between them (again ignoring clipping and a lot of other things here), swing in a nice unison. The PI amplifies this difference, gain is reduced as the smaller signal is subtracted from the larger. Closed loop gain is set and negative feedback is at work.

If these signals are out of phase, the two signals are seesawing back and forth from large and opposite peak differences to 0 crossing points. The PI is trying to amplify this ever changing difference, but that difference is all over the place now and is even stronger than the original signal at the opposite peaks. This equals oscillation (audible or inaudible) and instability and you have positive feedback at work.

I'm tired, not a great explanation I know. If nothing else, just try the very quick and easy test of lifting an end of the NFB resistor as Neil previously suggested. See what happens.

Littlewyan · Post by **Littlewyan** » Fri Jul 17, 2015 1:28 am

Lifting the NFB increases the gain so its fine. If it was wrong the amp would howl like hell given the large amount of feedback. In fact positive feedback in these amps sometimes shuts them down!

Littlewyan · Post by **Littlewyan** » Sat Jul 18, 2015 11:57 am

Update

Today I disconnected the bright channel volume from the mixer resistors and connected it directly to the PI input cap. I also disconnected the tonestack. Anyway I can still recreate the issue. However it only occurs with the bright channel volume on 1. It doesn't occur with the normal channel at all unless I put a bright cap on it, so the PI only goes wrong when hit with high frequencies and the lower frequencies probably just stabilize it.

With the circuit wired up correctly I have driven the mixer resistors with an oscillator and still got the same issue so I'm saying its the PI. However all of the capacitors in the PI are ok and the resistors all measure up ok and the issue occurs without the power valves and with the grid leaks grounded and not connected to the bias supply AND with the nfb disconnected. So I'm saying valve socket.

Post by **VelvetGeorge** » Sat Jul 18, 2015 4:09 pm

Try adding filtering to the PI node. And try grounding the NF at the PI.

I hate to say it, but I suspect the OT. Not sure if you have eliminated that so far in troubleshooting.

George

Littlewyan · Post by **Littlewyan** » Sat Jul 18, 2015 8:05 pm

Can't be the OT as the issue still occurs if I remove the power valves and if I disconnect the NFB. The PI has 32uf filtering, tried replacing it but same issue occurs.

Littlewyan · Post by **Littlewyan** » Thu Aug 13, 2015 9:44 am

Just so you guys know, the amp is fine. Turns out after a few other members of a facebook group tested their JTM45s that its normal for the PI to react that way to a lot of high end. So simple solution, don't turn down the mids and turn up the treble

.

neikeel · Post by **neikeel** » Thu Aug 13, 2015 12:26 pm

Littlewyan wrote:Just so you guys know, the amp is fine. Turns out after a few other members of a facebook group tested their JTM45s that its normal for the PI to react that way to a lot of high end. So simple solution, don't turn down the mids and turn up the treble .

Littlewyan · Post by **Littlewyan** » Thu Aug 13, 2015 1:30 pm

Yeah obviously most people wouldn't play with those settings but what threw me is the issue was occurring all the time before I rebuilt the amp and when testing it I found the issue occurred with those settings so assumed it wasn't fixed. Ah well, live and learn!

I may convert the amp to 68 lead spec as its a bit too tame for me. Doesn't seem to have much grit. Might play around with the valves first though and might just try changing the NFB resistor to 47K. Its currently 27K on the 8Ohm tap.

neikeel · Post by **neikeel** » Fri Aug 14, 2015 9:09 am

Littlewyan wrote:Yeah obviously most people wouldn't play with those settings but what threw me is the issue was occurring all the time before I rebuilt the amp and when testing it I found the issue occurred with those settings so assumed it wasn't fixed. Ah well, live and learn!

I may convert the amp to 68 lead spec as its a bit too tame for me. Doesn't seem to have much grit. Might play around with the valves first though and might just try changing the NFB resistor to 47K. Its currently 27K on the 8Ohm tap.

Turn the mids right up to 10, the Treble to 8, Presence to 8, Bass 2, Channel 1 volume to 8, unused channel 2 on 4.

Plug in a good Les Paul and play

Littlewyan · Post by **Littlewyan** » Sat Aug 15, 2015 4:14 am

Yeah I've tried settings similar to those. The settings I settled on in the end were (channels jumpered) High Treble Vol 10, Norm Vol 4, Bass 0, Middle 10, Treble 8 and Presence 10. It was still a bit too smooth though. I'm going to up the feedback resistor to 47K to make the low end a bit more raw.

I've watched a lot of Johan Segeborn's videos and the 67 Marshalls sounded great but I suspected a lot of the resistors have drifted upwards, particularly the feedback resistor. Maybe Roe will comment, I THINK they are his amps. But those amps definitely have more grit.

Doug H · Post by **Doug H** » Sun Oct 04, 2015 8:39 pm

Old thread, probably sorted this out already, but looking at some of the old pics I bet if you strategically shortened the wires to your PT grids some of your problems would go away.

neikeel · Post by **neikeel** » Mon Oct 05, 2015 3:35 am

Doug H wrote:Old thread, probably sorted this out already, but looking at some of the old pics I bet if you strategically shortened the wires to your PT grids some of your problems would go away.

One of the first things suggested

Metropoulos Forum

JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?

Re: JTM50 Blocking Distortion?