Marshall JTM 45/100 reissue hum problem

[Randall]

Guys, I have a 40th anniversary JTM 45/100 Super 100 with a noise issue. Pro customer says it's unusable. It voltages run hot on these, so he uses a bucking transformer I built for him last year. Problem is this, with both volumes down, there is a noticeable low frequency hum and a higher buzz. Raising the normal channel increases the hum, and raising the bright channel raises the buzz. This is the case even with the first pre tube removed. I have checked all the grounds, and re-flowed all the turrets for good measure. It does it with either the outside or inside pair of KT66s removed, albeit at lesser volume. It also does it with all pre tubes removed, except the volumes and tone pots have no effect. I've tried jumpering in a fresh electrolytic at the PS filter caps to no avail. I request help on how to think about this one.

http://www.soultoneamps.com/images/JTM45100.jpg

http://forum.metropoulos.net/download/file.php?id=4349

[Jazz P Bass]

Have you looked for Vdc after any & all coupling caps?

Indicating a leaky cap.

I would also verify that the normally closed input jacks are closed.

[Randall]

All input jacks have been verified for proper grounds. What is the best advice to check for leaky coupling caps? Come on Enzo, chime in and kick my butt please!

[Enzo]

Have you looked for Vdc after any & all coupling caps?

Indicating a leaky cap.

Don't guess, find out where the hum comes from. Is it 60Hz or 120Hz? 120Hz is power supply ripple, 60Hz is most everything else, icluding bias ripple.

Try disconnecting the bucking transformer and running it straight, just for testing. If the voltages are a concern,. dial it down on a variac. This will tell us if the bucker is involved.

Scope the B+ and the bias supplies, is there any junk riding the voltage?

When you say pulled all the preamp tubes, did you include the phase inverter? (It is a small tube, but not part of the preamp. People tend to refer to all the small tubes as preamp tubes though) Does hum remain with phase inverter pulled? Verify you have good B+ on ALL FOUR power tube plates AND screens. If you have a bias probe, check each tube separately for current. Is any tube conducting zero, or is any way higher than the others?

[Jazz P Bass]

What is the best advice to check for leaky coupling caps?

If it is a stage coupling cap, you look on the non-plate side for unwanted Vdc.

[Randall]

With the PI tube in, preamp tubes out, I measure 120Hz with my Fluke at speaker. Not on bucking transformer. With PI out, lower level hum is still present, but can't measure frequency on my Fluke. Also, interestingly both volume controls have an effect, they introduce an odd distortion, maybe an inter-modulation type sound when turned up past 7 or so. This I don't understand.

The B+ and bias supply have a pretty 120Hz sine wave, agreed by my Fluke and scope. However, scope reads approx 45 V P-P, Fluke reads 15.9 VAC, the math isn't working for me here. Where am I erring? So it does look like PS needs caps I think. Should I do them all, or try to find the bad one, and leve the rest (assuming it is only one).

[Jazz P Bass]

Appears to me that you have some bad capacitors.

[Randall]

Bad caps it is. Three of the 8 ARS 33/500v filter caps measure bad. I'll replace them all. Might as well hit the bias caps while I'm at it. since 33uF/500v axial is not a popular value cap, I wonder if using 40uF/500v would matter?

[Leo_Gnardo]

Bad caps it is. Three of the 8 ARS 33/500v filter caps measure bad. I'll replace them all. Might as well hit the bias caps while I'm at it. since 33uF/500v axial is not a popular value cap, I wonder if using 40uF/500v would matter?

I'm sure you'll do fine with 40 or 47 uF, no worries. Less hum is always better by me.

Only the 'value weenies' would complain, you know "oooh it's different from the factory value, points off, hit you with a wet noodle!" Them... Plus original tolerances were often -20%, +80%. And I seriously doubt the factory measured the capacitance of every filter cap they installed. Got 40's? Go for it!

[Enzo]

45v p-p is 22.5v peak. RMS is 0.707 of peak. So 22.5 x 0.707 = 15.9. Looks like it works to me.

[Randall]

45v p-p is 22.5v peak. RMS is 0.707 of peak. So 22.5 x 0.707 = 15.9. Looks like it works to me.

This is a weak link in my understanding. The difference in p-p vs peak. Makes sense in that all my RMS calculations are coming up half. I'm missing something important here.

[Enzo]

Look at your scope. The trace rests in the center, that is zero. Your signal goes north to some peak and south to some peak. let us say it goes 5v positive, and then 5v negative. Those are each 5v peaks, and peak to peak is just what it sounds like: the distance from one peak to the other, in my example that is 10v p-p. From +5 to -5 is 10.

So if you have 10v p-p, that means either peak away from center zero would be 5v peak. And RMS is just 0.707 times whatever peak is by definition. Sine wave in the example.


That assumes a nice symmetrical waveform. I suppose it is possible some skewed signal is running through things, and the waveform goes from +8v only down to -2v. That would still be 10v p-p. But let's not go there.

[Randall]

I'm still missing why my measurements are coming up half. Big hole here.

[Enzo]

Your scope is centered I hope, so zero is the middle horizontal. You report 45v p-p, meaning on the scope there is 45v from the highest to the lowest part of the waveform. Assuming the waveform is more or less symmetrical, that means it extends up to a peak of 22.5v positive, and down to 22.5v negative. So peak voltage is 22.5 either way. The signal waveform never gets higher that 22.5v away from zero in either direction.

By definition, RMS is 0.707 times peak. SO if your peak voltage is 22.5, then your RMS would be 15.9v, which is what your meter is reading.

What do you expect the meter to read?



1 is peak
2 is peak to peak
3 is RMS

[Randall]

I was wrongly expecting my meter to read 0.707 of peak to peak, which explains why my readings were half of what I expected. Your explanation makes sense now. Thanks Enzo.