More on Valve Junior hum

[ChewingAluminumFoil]

Mail ordered an Epiphone Valve Junior knowing full well I'd have to give it a bit of rework to deal with the well known hum issue. But a tube amp for $119 was just too tempting. Plus I guess I still like soldering.

 

Thanks to d gauss for all the tips elsewhere on this board. Here are my own observations. I'll probably do a web page with the details of my final mods shortly.

 

Main 120Hz buzz was solved by addition of 2 450v 100uf caps in parallel with the first 20uf cap in the existing filter chain. I tried putting caps downstream on the screen supply or the 3rd tap, but really hitting that first tap really was where I got the most.

 

This solved the constant 120Hz hum that was unaffected by the volume control.

 

Next I went to DC on the filaments. This is actually easier to do than you might think. I bought a 4A 400v bridge rectifier at Radio Shack and had some 1000uf low voltage caps around. Take the two filament supply lines and unhook them from the circuit board, connect 'em to the AC in on the bridge. Then take the + and - outputs of the bridge rect and connect those to the filament inputs on the circuit board. Finally solder a 1000uf cap across the +/- pins of the bridge rect (minding polarity of course), and poof, that's *most* of the hum.

 

Still some hum coming from the crazy grounding of the input stage, but it's livable. I did add a jumper from the 68k resistor that returns to the ground lug on the input jack and ran that over to the place on the circuit board that connects to the main star ground. I'm gonna order some insulated jacks from Digi-Key and that'll probably get me the rest of the way there as d gauss suggests.

 

Finally, tone-wise, there's a spot for a cap on the circuit ooard that wasn't populated. It's in parallel with the 1m resistor feeding the volume control. I put in a 500pf cap to add a bit of brightness and presence.

 

And I'm still fiddling with the input stage but I'm playing with different caps in place of the 1st 68k resistor try to take a little of the low end mud out. The interesting thing about this amp is the tube circuit is *so* simple, and it has zero high boost. Most guitar amps have high boost out the wazoo. I'm still playing looking for a good balance so that it won't go to mudville quite so quickly as you turn it up.

 

But fun little amp. Having a great time using it as a breadboard.

 

I'm going to order another and run them stereo. Should just about the right wattage to play with a drummer when run into some external 12" speakers.

[Ricochet]

The main hum on mine is 60 Hz. It comes from the heater traces on the PC board very closely paralleling the trace from the volume pot return to the V1b grid. Adding DC heater power as above would fix that. I'm leaving mine alone for now. I'm certainly not going to add a bright cap, as I think it's got a very bright tone and lots of presence from having no tone stack. I don't want it any brighter! I haven't heard any low end mud out of mine, either. At low or high volume. That's OK, folks are always going to have differing opinions about the sound of gear. Do what makes you happy. It's nice to have something nice and cheap that you don't mind tinkering with! If you substitute a cap for the 1st stage grid stopper resistor, though, you're likely to start picking up radio stations. That resistor works in series with the Miller capacitance of the triode to make a low pass filter that blocks out RF coming down the guitar cable. A cap of most any value will let the RF zip right through.