Valve Jr. and Bitmo Mods

[henkejs]

After lurking here for several months, I finally got up the nerve to void my warranty and do some mods on my Valve Jr. head (v3). Since installing the Bitmo Trio and B-Verb kits, I'm really liking the new tones of this amp. The only problem is there's a fair amount of hum that wasn't there before. It's not really that bad, but I'd like to reduce it if I can.

 

Even with no input, there's some hum with the volume and reverb pots turned all the way down. It gets worse as you turn the volume up and worse still as you turn the reverb up. I was careful to locate all the components as instructed and keep the wires as short as possible, so I'm wondering what else I should look at.

 

One thing I did notice is that the wire connecting the reverb pot and the reverb block runs right past the big capacitors on the circuit board (that was the shortest route). I wonder if it would be better to use a longer (and shielded?) wire and route it away from the caps. I've also read here about problems with pots not making good contact with the chassis. Is that likely to be an issue here?

 

I'd appreciate any tips for trouble-shooting this problem. Thanks!

[m-theory]

It's not uncommon for there to be noise in reverb circuits. I suspect that you should be able to improve this by using shielded cable to and from the reverb circuit. Remember to ground only one end of each cable, and to use the same ground as the amp is using, or you'll likely introduce a ground loop hum.

 

Beyond that, wire dressing can be problematic as well. You can identify and resolve that by carefully probing and moving wires around with a chopstick, while the amp is on. If you hear changes in the noise with a given wire, try moving it to the quietest position possible.

 

Without having seen the schematic for the bitmo reverb circuit, I'd have to guess that the reason you're hearing the noise with the knob turned all the way down as well as up is because signal is always passing through the circuit, whether or not the reverb knob is turned up. You could eliminate that by installing a switch to completely bypass the reverb circuit when off, though the noise would still be there when you had the reverb circuit switched "on."

[henkejs]

Thanks, m-theory, I'll try your suggestions. I'm good at soldering and following instructions, but not very knowledgeable about the underlying theory. Could you be more specific about where to ground the shielded cable? According to the reverb kit instructions, one pin of the reverb block and one lug of the reverb control pot are both grounded at jumper JP2 on the main board. Would it make sense to ground the shield for the cable connected to the reverb control pot to the already-grounded control pot lug?

[m-theory]

Would it make sense to ground the shield for the cable connected to the reverb control pot to the already-grounded control pot lug?

That'll probably work.

 

So, with the wire that's going INTO the reverb circuit and the wire that's coming OUT of the reverb circuit and back to the amp circuit, use some shielded cable...George L works good, because it's very thin and easy to work with for this type of thing...

 

Only ground the shielded ends that are near that pot that you want to tie down to. The "hot" leads of those cables are replacing your current, single conductor wires, so just connect those where the others are connected now. Make sure you use tape or heatshrink to completely insulate the end of shielding that's not going to be connected.

[henkejs]

Thanks for the clarification. I'm hoping to get to this tomorrow -- will post back on the results.

[henkejs]

Just wanted to report on the the fix discussed here. I replaced the wires to and from the reverb unit with shielded cable and rerouted the new ones away from the capacitors. It made a tremendous difference! There's very little hum now unless you turn the reverb way up. Even then, it's not noticeable at all while you're playing.

 

Big thanks for the advice, m-theory. Now I can go fire up this little amp and really enjoy the new sounds.

[m-theory]

Glad to be of help. Rock on, dood.