Another JTM45/100 is born

[shakti]

I thought I'd put this up in the 45/100 section as well as the Marshall section.

Finally, it's come together. My dream stack since....forever.... This is a DIY JTM45/100 straight up clone with block end chassis, proper panels and plexi-style knobs, NOS mustards, RS silver micas, Piher resistors. Marstran PT and OT, CM 5H choke, RIFA electrolytics, GEC KT66s and two sweet Telefunkens and a Mullard in the preamp. Head cab by Jeff Swanson. The cabs are from a Marshall "Hendrix" stack, but with EC Collins pinstripe and MG repro logos. The slant cab currently has a quad of '70 G12M 55Hz, and the bottom currently has a quad of the stock Celestion G12C speakers.

I could go on and on about how the amp sounds. In short, it's absolutely the best amp I've ever played through, period. I have built a JTM45/100 previously using new production components, and it sounded fantastic as well, but this new one absolutely has the edge. I think a lot of the difference is the mustards, RS micas and not least the RIFA electrolytics, which I absolutely love. It sounds 100%, spot on, no ifs-ands-or-buts, to the Fresh Cream Spoonful tone...smoldering, thick, yet cutting, sparkling, clear, powerful sustain!

There are many people I should thank for parts and tips...don't know where to start, but you know who you are! I dedicate the amp to George and Metroamp though, for getting me started with this seriously addictive hobby. Cheers!

[myfoot]

Just a beautiful build Thor! You should be proud.

[Traviswalk]

That is awesome looking!! I'm starting my 45/100 build as we speak!!

[Wayne]

Haven't been here for a while.

That is just amazing stuff. Congratulations - you should be proud !!

Wayne.

[vanhalen5150]

That is really nice.

[shakti]

I'm having a hum problem with this amp. I get a very slight hum with the volume above 8, particularly on the high treble channel. It seems to be amplified with increased bass, and ever so slightly also by increased mids. The normal channel seems less susceptible. The amp has been built with Larry style grounding, just as my previous 45/100, which was dead quiet. I've tried reflowing the ground connections with no change. I've swapped preamp tubes with no change. Removing V1 makes the hum go away. Moving the grid wires around does not change anything.

I'm curious if transformer hum can actually be picked up by the circuit and be amplified? It sounds very similar to the actual transformer hum, so I'm not 100% convinced it's a ground hum. I had the PT oriented the other way in my previous build, could that be a potential source?

[shakti]

I'v been chasing this hum like mad, but no solution yet.

- tubes have been checked, and are not the reason
- I redid all the ground connections on V1 and V2
- I checked the volume pots, tried a couple others. If anything, it may have improved slightly when I swapped in another volume pot and tightened the nuts, but the difference is very small (if not imagined).

The amp is completely silent when switched on but with all controls at 0. There's only a very, very slight buzz which I attribute to not having a shield in the cab. With the tone controls at 0, the hum is not evident at all until above 8 on the high treble channel, and then it is only faintly noticeable. It's even less noticeable on the normal channel, again only above 8. The only other control which affects it is the bass control, which seems to amplify it slightly. With the bass above 3, the hum is gradually noticeable above 6 on the volume. It's never terribly loud, but it's there, and I don't get it with my Black Flag JTM100, nor on my previous JTM45/100.

As mentioned, pulling V1 makes it go away completely.

The hum sounds very similar to the transformer hum which you can hear with the amp on standby (which goes away when flipping the standby switch).

I really don't know where to look next... I should check the ground connection for the mains filter and PT center tap, but other than that I really don't know.

The only differences between this and my previous 45/100 was that I used PEC pots in that one, and Alphas in this one. I also had the PT oriented the other way on my previous amp. Could either of these be the cause? Could this be caused by transformer coupling? And could turning the PT 180 degrees somehow improve this? I don't want to start hacking away blindly...

I'm really open to all suggestions. The amp sounds terrific, and the hum is not very intrusive, it just bugs me because my previous amp didn't have this.

[JimiJames]

I've swapped preamp tubes with no change.

...tubes have been checked, and are not the reason

How about on the power tubes ?

....hum sounds very similar to the transformer hum

Same here but.....

I had something similar.... My tone was there but, the loud hum too.
I checked all the power tubes on the Hickok and all tested good with no shorts...
All tubes gave me a good bias reading ...
The one I had in V4 was too good !? It tested with the needle almost pegged off "good"
The other 3 tested just over good.
I replaced this hot tube and good bye hum, but I still don't know the "why" part.
Obviously it's not that simple in your case. I also use Larry's style grounding and I'm dead quiet like you.

[shakti]

Interesting...however these were the same tubes I used in my previous 45/100. I could try to swap them around though.

[shakti]

This is really frustrating...I haven't had time to check the power tubes, but while searching the forum for threads with possible fixes, I came up with this thread:

http://forum.metroamp.com/viewtopic.php?f=4&t=25258" onclick="window.open(this.href);return false;

Started by none other than yours truly, when I built my previous 45/100. It's like deja-vu all over again once more...
However this time I'm certain the B+ rail is correct, and it has been built with Larry grounding from the start.

[shakti]

Argh, this one is driving me nuts!

I've verified that it's almost certainly not a ground hum. I induced a ground loop by clipping together the PT CT and heater center tap (which is already grounded at grounding point #1, along with inputs and V1 cathode). The resulting ground loop noise is different from the hum I am getting. I've also checked all ground connections, and tried to physically move the grounding points, but no difference.

This led me to believe it might be a lead dress issue. I noticed I had the presence wire and bias tap wire running close together. Moving them physically apart got rid of the slight static buzz I got at idle, so now the amp is dead quiet at idle. However, I am still getting the very low-pitched hum with high treble volume above 7-8 and bass turned up. It is barely there at all on the normal channel. I've tried moving around other wires (tone pots, tube grids, output grids, choke wires), but this doesn't seem to affect it at all.

Is it time to start looking at the components on V1? Could a leaking coupling cap on V1 high treble channel cause this? Or how about the mixer bypass cap?
Or is my theory about transformer induced hum at all possible? The hum sounds so similar to the standby hum, but then it's somewhat strange that it only happens on the high treble channel (or at least much more noticeably).

[shakti]

Swapped channel 1 V1 plate resistor and coupling cap. No change.

The grid wire (from the 68k resistors) ran directly underneath the junction of the two 100k plate resistors, so I re-routed that so it ran in a loop around it, far away from any HT line. Also no change.

What's left to check now?

[shakti]

BTW, sometimes (not always) I'm also getting a loud pop when switching on and off standby. This didn't happen with my previous amp. Same type of switches.

[neikeel]

It may be a bit silly but I twist my ac wires to the lamp and route them along the floor of the chassis too. I also notice that on the 66/67 amps the PT primaries and switch wires are usually bundled and run wide around the outside of the chassis/PT, although the 50watters and 45s have them twisted and run up the inner aspect of the PT.

Without hearing the noise I think that some hum on this design of amp is not unusual when you have PT and OT so close together in that orientation. Why is the bright channel more affected? Why would the frequencies that are selceted and boosted by that channel present themselves through the speakers

[shakti]

Those are my same thoughts - since only the high treble channel is affected, and the noise goes away with V1 removed, it has to be something in or around V1. I've tried the plate resistor and coupling cap. Need to check input grid resistors and mixer resistors as well. I did drop in a plate resistor which turned out to be very noisy, and that resulted in a much more pronounced, but similar hum. I think it was the resistor, but it may have been a funky solder joint as I only threw it in hastily for a quick comparison. However, I've checked and re-soldered, so it should be good now. I could also try to lift the entire board and check the solderings on the underside as a last resort.

As mentioned, my previous amp had the PT mounted 180 degrees around from this, and I had the primaries all wired along the outer edge. But why would any noise from that area only affect the high treble channel?? I have also tried the amp with the indicator light removed, and there's no difference. Lifting the PT primary wires and moving them does not affect it either, but I can't move them very far.