New Build - PAB-Jazz/Rock Problem and OD hum

[dancemyth]

Finished building my first Dumble.

It sounds amazing. Every time I pick up my guitar, the next thing I know, 2 hours have gone by. But I have two issues:

I have a very weird interaction between the Rock/Jazz switch and the PAB switch. Set on Jazz, the PAB has a big effect. However, switched to Rock, the amp is Much louder and the PAB has (almost) no effect. Never having played through a Dumble, I don't have a great sense of how these are supposed to interact, but clearly this is not it. Would seem to be a wiring mistake, but I have gone over the connections dozens (literally) of times, checking against both the layout and the schematic. I can post pictures, but as you know, it's very crowded around the relay and switches. I suppose I'm hoping someone has had this exact problem, and can point me in a direction.

Now to the second problem. The amp is dead quiet on the clean channel, even with the volume cranked. Just a bit of hiss through the FET circuit, but I suppose that is to be expected. However, the OD channel has a loud hum, even at low volumes (otherwise, it sounds gorgeous). I think my lead dress is pretty good. I've chopsticked everything. Reflowed my V2 pin connections. Checked my grounds. Swapped tubes. I know that I'm really fishing on this one, because there are probably a hundred things I could have done wrong to get a hum. The only other time I had this problem on an amp build, I found a bad ground connection somewhere.

Maybe you can help me from having to take it to an amp guy. I just read the amazing post about Larry Carlton borrowing one of you guys' amp, so I'm kind of embarrassed to post my silly little problems here. (In the future I will just create one post about a new build, so I don't clog up the joint with lots of threads). Thanks. - Dan

Disclosure: This is a Ceriatone kit. Yes, I know. Sorry. But I truly appreciate the help.

[pompeiisneaks]

I'd try checking the OD shielded wires to make sure the shielding is making good ground. Those are pretty important for removing stray noise getting into the signal path. OD tends to be a bit noisier. Mine's quiet a lower OD levels but the noise floor comes up definitely as I push it to higher gain territory.

As for the issue with PAB and the switches, not sure. It may be the way you've got it setup the PAB, as it lifts out the tone stack, lifts the frequency range you're using with the jazz setting a lot more than with the rock setting, they are two specifically different frequency responses, I.e. the tone stack 'impact' that you're using is minimal with the Rock setting but much more noticed on the frequencies around the Jazz setting.

I'm sure some of the guys that have built more than 2 dumbles like myself could offer a lot more help.

~Phil

[erwin_ve]

A few good detailed pics are helpful, otherwise it's shooting in the dark.

[xtian]

As said, if you lift the tone stack (meaning, high or infinite resistance to ground) PAB (which is itself a tone stack lift) will have no effect.

Post your schematic, and I'm sure we can suggest which components to check.

[dancemyth]

Took a long time to get back to this project, but I thought I owed a closing note to those who helped:

- I never did figure out the the deal with the PAB/Rock-Jazz interaction. It seems that if the Rock/Jazz is in the Rock position, PAB has very little effect (maybe because, as someone pointed out, both are lifting all/most of the tonestack). Also, the Treble control has essentially zero effect. Maybe this is just life with a Rock/Jazz switch. (I have yet to figure out how this tonestack works.) Anyway, it doesn't really matter - it sounds great in the Rock position, and I can get the right amount of treble with my guitar plus various bright switches (I also built a Dumbleator, so I have way too many bright switches)

- The hum problem took a little bit - OK, a lot - of debugging. I had put in a switch between V1 and V2 to allow me to turn off the local negative feedback on V1B. It turns out that this switch was the source of the hum. Maybe because it was too close to the V2A grid? Anyway, removing the switch and hard-wiring the feedback loop nice and tight to V2B solved the problem.

So, in the end, the amp sounds great. Really great. Just need to finish building the cabinet.....

Thanks for the help, guys!
Dan

[pompeiisneaks]

Great news.

If you do want to restore that switch, it's often super important to use shielded wire. Those wires early in the amps stages tend to be very prone to picking up hum from other areas. Shielding the wires means they pick up way less interference.

~Phil