A #102 Attempt - First amp

[Richard1001]

I would like to help with your hum problem, but can't realy get a good view on the photos. So i just list the most probale causes i can think of.

- on the first triode: are the cathode resistor and grid resistor connected to the same grounding point? (Actual same point) Edit: or if you use diffrent wiring: the input jack ground the same point as the kathode resistor and fetboard ground?

- on one of the pictures i saw a board with what looks like a bridge rectifier and it seems to be connected to the heater supply. If so: how is the AC heater supply balanced?

- did you try shielding the top of the chassis? By this i mean putting a piece of aluminium on top of the chassis covering the input / fet / V1 section. (Like it would be shielded in the cabinet) This way you can see if the hum is a wiring problem inside the amp or hum induced from the outside.

- I think Martin already said it but it could be a good idea to disconnect the fetpreamp (including the relay or switch) from the input all together and first concentrate on getting rid of hum just on the amp itself. If pulling v1 eliminates the hum, the problem could be in the wiring of V1 (and the wiring of components connected to V1), the heater supply, or everything that comes before the V1 tube (fet board wiring, relay etc). If you disconnect the fetboard and relay and the hum is gone, you know you have to look at the fetboard and its switching.

[martin manning]

Good suggestions.

A couple more questions and ideas:

The hum is present with or without anything being plugged into the input jack, correct?
Is there any effect on the hum when switching the FET relay?
Is there any effect if you move the heater leads above the V1 socket?
What happens if you ground the grid of V1a or V1b? I would do this by jumping from the terminal strip lugs (where the 33k and 220k grid resistors are soldered) to the input jack ground. That should isolate the problem to V1a, V1b, or the input jack/FET/relay circuit.

[ijedouglas]

Did you get the hum resolved?

[Vertigo]

Thanks for asking but not yet. I've been frustrated by it and have put it out of mind for the past few days. I did cover the top with an aluminum plate, which made no change. Removing the input from the terminal strip also made no change. I think I already wrote that it is quiet with V1 out, so there aren't many places left to look. I just need to devote the time to it. Incidentally, I picked up a used Boss Waze Tube Amp Expander last week and the hum is much less apparent when listening through headphones than through a speaker.

And Martin, to answer your questions.
The hum is present with or without any in the input jack.
I actually haven't built the pedal yet, so I'm not switching relays, just the toggles at this point. Switching the FET boosts the volume of the hum but it's in proportion to the boost in volume that I dialed into the FET
There's no effect if I push around the heater leads on V1

I haven't tried grounding the grid of anything on V1. But that will be my next move before working my way through Richards suggestions.

Thank you all. I appreciate the help.

[Vertigo]

Good suggestions.

A couple more questions and ideas:

The hum is present with or without anything being plugged into the input jack, correct?
Is there any effect on the hum when switching the FET relay?
Is there any effect if you move the heater leads above the V1 socket?
What happens if you ground the grid of V1a or V1b? I would do this by jumping from the terminal strip lugs (where the 33k and 220k grid resistors are soldered) to the input jack ground. That should isolate the problem to V1a, V1b, or the input jack/FET/relay circuit.

I grounded the grids (at the terminal strip) to the jack ground and neither one made a difference in the hum.
After pushing the heater around a little harder, I can make it scratch and pop a little bit and the hum will slightly decrease in volume.

[martin manning]

Here's a pic of the V1 socket in my 124-ish amp. Note the grid components are well away from the heater wiring. It may be worth reworking yours a bit.

[ijedouglas]

Silly question...do your other amps plugged into the same socket have any hum? I had a bud who chased his tail for a week before we worked out that he had terrible power (plus a Tesla charger in close proximity) and all his amps hummed

[Vertigo]

Here's a pic of the V1 socket in my 124-ish amp. Note the grid components are well away from the heater wiring. It may be worth reworking yours a bit.

That did it! Thank you. I moved the 220K/500pF much farther away from the heaters and that did the trick.

While I was in there, I pulled the "ayan OD entrance mod" out as well and...I think I need to put that on a switch to A/B it to really decide if I like it there or not. With the treble reduction network removed from the FX loop, I'm not sure it's needed anymore.

Anyhow, HUGE thank you to everyone offering suggestions to troubleshoot the hum problem. I'm relieved to have that taken care of. Now I can move on to building the cab for it.

[Vertigo]

Silly question...do your other amps plugged into the same socket have any hum? I had a bud who chased his tail for a week before we worked out that he had terrible power (plus a Tesla charger in close proximity) and all his amps hummed

Not a silly question at all. That's one of the first things I tried to isolate because my Demeter is pretty noisy too and my wife keeps plugging her laptop charger into the same socket. The answer is that the two noisy amps are still noisy in other sockets. The 102 is fixed now, but it'll probably be a good while until I can sort out the Demeter.

[martin manning]

Great! Glad it's solved.

[ijedouglas]

Awesome!

[Vertigo]

we're in remote learning hell right now and I can't string together enough time during the day to even think about attempting to do my real job. Luckily, I can sneak into my shop for a few minutes at a time to cut plywood.

As I write this, I realize that I don't have nearly as many photos as I thought I did. I cut some rectangles, did a bit of layout for the dominos and just for fun, decided to inset the support blocks for the chassis. It seemed cleaner than using screws and gave me more opportunities for short distractions...then I realized that I didn't have a quick and easy way to route the groove. Rather than give in, I swapped the jaw location on my mill vise and just cut it on my milling machine. I'm not a big fan of wood dust in a metal shop, but the vacuum kept it pretty tidy.

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In between trying to teach long division and how to capture detail in writing short stories, I'd run out and chisel a corner. Simple yet gratifying but far from perfect.

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I don't know how everyone cuts the profile in the front edge of the top panel. If I were to do more of these, I'd make a template but it seems like overkill for a single. I layed it out with a plastic circle template, routed the bulk of the straight edge in the center, got the curves close and then finished them with a modelers rasp and sanding block before routing the roundovers.

Laying out the holes for the chassis after the fact might not have been the most efficient choice either but it worked out OK. The front holes are 3/16 but I drilled the rear ones 1/4" so the bolts can float a little bit to make up for the error in the hole locations.

I actually have a sapele board that I was planning to use to make the cabinet, but I'm not confident at all in my ability to cut dovetails. What usually happens when I make a temporary thing is that it becomes permanent. I happen to have a yard of Alcantara that I bought ages ago to recover the seat on my motorbike but James Renazco caught wind of it and made me a custom seat so I've never found an opportunity to use the fabric. It's a hideous blue and it might just be perfect for this if I can get it to mold to the profile of that top edge.

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Where is everyone getting the Vox grill cloth? MojoTone seems to be the only place that has it in stock, and only in brown.

[norburybrook]

I do it my hand/eye with my jigsaw then go over with the router. I also screw and glue and then fill with car body filler, no fancy dovetails/biscuit joints. It's going to be covered in tolex/suede so I don't think it maters what's underneath as long as it's strong.

Blue Suede is good..here's mine.

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M

[Vertigo]

That looks great, Marcus! It's quite similar in color to what I have. It looks good!

[dimitris]

You can find black vox grill cloth on ebay.