Having a hard time debugging this one...

[Musant]

Well this is interesting. Just stopped by the house and figured I would do the Enzo test. I had moved the amp to the other side of the shop to a staging area and just plugged it in where it sits and there was no hum (was probably 60Hz) but instead now I'm getting a washy sound that sounds similar to snow on and old TV set.

Try relocating your output transformer nearer to the output tubes and get it away from the preamp. Make sure that its laminations are oriented at 90 degrees to the those of the power transformer, and try swapping the blue and brown leads of the OT primary.

Best of luck,
Jim

[Justin Thomas]

My last build, I had an unbeatable hum I couldn't get rid of. Every time I tried the conventional "proper" way to fix it, I just couldn't kill it - it was the same or got worse. Eventually, in desperation, I don't remember how I arrived at this, but I isolated it to (I think) the PI, I took a two-foot piece of wire and used it to ground the stage. I found out that the longest, most wrongly-routed ground wire killed that hum 90%, apparently because the PT was sending out a wicked magnetic field, and I just had to cancel it with this long-ass piece of wire. So that wire layout and where I grounded it to completely defies grounding "wisdom," but it worked...

As it was a gutted amp and the tranny layout was already set, I couldn't rotate them... the hum is still somewhat there if you really crank it up, but a 50W EL34 stage drowns that out easily. No ill effect on guitar tone.

Justin

edit: IOW, sometimes some crazy-ass $#!+ just happens, and you gotta do some equally crazy-ass $#!+ to fix it...

Edit-edit: no scope...

[Enzo]

There are some Mesa amps with a small antenna wire sticking up from the circuit board to do just that.

[Musant]

My last build, I had an unbeatable hum I couldn't get rid of. Every time I tried the conventional "proper" way to fix it, I just couldn't kill it - it was the same or got worse. Eventually, in desperation, I don't remember how I arrived at this, but I isolated it to (I think) the PI, I took a two-foot piece of wire and used it to ground the stage. I found out that the longest, most wrongly-routed ground wire killed that hum 90%, apparently because the PT was sending out a wicked magnetic field, and I just had to cancel it with this long-ass piece of wire. So that wire layout and where I grounded it to completely defies grounding "wisdom," but it worked...

As it was a gutted amp and the tranny layout was already set, I couldn't rotate them... the hum is still somewhat there if you really crank it up, but a 50W EL34 stage drowns that out easily. No ill effect on guitar tone.

Justin

edit: IOW, sometimes some crazy-ass $#!+ just happens, and you gotta do some equally crazy-ass $#!+ to fix it...

Edit-edit: no scope...

It's possible to inject a hum signal at some point in the signal path such that it is approximately equal in amplitude and opposite in phase to the objectionable hum signal. But that solution is not exactly what I would call 'excellence in engineering'. ;-)

The bias network at the input to a split-load phase inverter can inject hum if the power supply is not adequately filtered.

And it's no secret that the power transformer can induce 60 Hz directly into the output transformer if care is not taken to avoid it.

I suspect in this case there may be a super audible oscillation masquerading as 'hum'. (470pf doesn't usually have much effect below middle C in tube circuits ;-))

Jim

[mort]

Hmmm.. just got back home (long day) and now the hum is back and the snow is gone. I thought it might have something to do with moving it to the other side of the shop and away from my laptop... but nope. I walked around the shop with it and the hum was consistent no matter where it was. I'm a little too tired to try anything more tonight but I'm thinking I'll replace the shielded cable just in case I accidentally sliced through the conductor jacket, thus allowing the shield a tiny access to the conductor. Will probably also try grounding the shields somewhere else.

Let's hope it doesn't come to moving the OT. I really don't feel like it's inherent to the layout because it was very plain that it started when that shield got grounded, and had no constant noise like that at all beforehand, only that it sounded bad when the guitar signal was applied.

I'm thrilled about the amp being mostly up an running though. This is one of the richest sounding circuits I've ever heard. It has a sturdy rubber type low end and a very glassy top. Scooped like a Fender but richer and thicker.

[nickb]

Hmmm.. just got back home (long day) and now the hum is back and the snow is gone. I thought it might have something to do with moving it to the other side of the shop and away from my laptop... but nope. I walked around the shop with it and the hum was consistent no matter where it was. I'm a little too tired to try anything more tonight but I'm thinking I'll replace the shielded cable just in case I accidentally sliced through the conductor jacket, thus allowing the shield a tiny access to the conductor. Will probably also try grounding the shields somewhere else.

Let's hope it doesn't come to moving the OT. I really don't feel like it's inherent to the layout because it was very plain that it started when that shield got grounded, and had no constant noise like that at all beforehand, only that it sounded bad when the guitar signal was applied.

I'm thrilled about the amp being mostly up an running though. This is one of the richest sounding circuits I've ever heard. It has a sturdy rubber type low end and a very glassy top. Scooped like a Fender but richer and thicker.

I'm not exactly in agreement with moving the OT unless it's picking up hum from the PT or its otherwise proven that there is a problem with the location. Right now it's away from the PT and on the other side of a screen. Your instability problems will likely arise from capacitive coupling. The OT lives on the other side of said screen which will prevent such coupling. Re-route the primary wires by drilling a new hole near the power tubes and run them on the other side of the chassis. Where they are in the piccie is just about the worst place you could choose.

[Enzo]

If you are worried about the OT orientation, pull the tubes and fire up the amp. If the PT and T are coupled, you will get hum in the speaker even without the tubes.

[mort]

Just following up with the resolution. Turns out the hum was just an insecure connection on the input grid tab. This amp is a beast. Mr Kelley sure knows how to design a good one, no doubt about that. I'll get some proper clips up soon. I've got 2 of my favorite local guitarists recording on 5 different amps tomorrow. Should be fun

[mort]

Here's a couple clips of teh amp with a different player on each clip.

The clips are informal and a little long but there is some good stuff in each.

The second is mainly noodling to see what the amp/guitar will do. He was checking out the switch positions on the Esquire...

https://drive.google.com/open?id=0B9...EQ1em9GbWM5VW8

https://drive.google.com/open?id=0B9...DkwekNyT0NMN3M

[mort]

Just got around to shipping this one off the other day. It was a casual build for a trade with a guitar building friend of mine. Here's the trade:








and as a bit of fun he added some actual vintage flare

[SimonAntony]

Just got around to shipping this one off the other day. It was a casual build for a trade with a guitar building friend of mine. Here's the trade:








and as a bit of fun he added some actual vintage flare

Hi Mort

Came across your post whilst researching the Jim Kelley amp clones, i'm planning on building one and was very interested in what you did here!

Did you by any chance create an updated circuit diagram at all? I'm trying to get a clean and updated copy as some of the ones i've found are not that clear - also any other spurious notes you may have made on the amp, pics are not showing anymore as the post is so old :-)

Cheers

Si