Scope and Hum question

[zimm]

I converted my hot rod deville to a bluesmaster HRM, Its the best incarnation yet (started as Dlite/Hybrid A into nonhrm ODS-101 and has now morphed into a BM hrm). The only glaring issue I have at the moment is a low level hum (tolerable but annoying).

Hum level increases with increase in master volume, but not when switching between channels or boosts. It also seems to increase when I turn up the presence. If I pull V1 or V2 tubes alone its still there, pulling V3 alone eliminates it. Moving the lead dress around does not effect the hum level but I havent tried a DC heater supply to eliminate the heater signal.

I have acquired a scope but must admit Im not sure where to start and if this is the best way to troubleshoot the hum, or If its best to make up a grounding lead and ground out portions of the circuit. Can anyone point me in the right direction as to where I should be probing with the scope? Any other advice is much appreciated, thanks.

[BobW]

It could be a ground return issue around your MV circuit. A few pics, in particular showing the ground scheme would be helpful. Is the hum 60 or 120 Hz.?

[zimm]

Thanks. I will try to get some pics, I have been meaning to post some anyway. As far as the cycle Im not sure, thats why I asked the question about where I should be probing. I can get a 60 cycle trace if I probe the main but I can not pick up any AC traces when I probe randomly around the amp. 120 cycle would suggest some crossover from the bias supply? Ill also try to put together a lead with a cap and see if I can narrow it down by grounding out certain areas.

[zimm]

Probed through the amp with a cap to ground, hum was coming from the V3 grid. Followed the hum up to the pre amp jacks. If I unscrewed the jacks from the chassis the hum stopped, amp was silent. I put in some plastic spacers and ran a ground wire from the jacks to the ground buss. I dont exactly know what was going on, some weird ground loop, bad jack? Its quiet now.

['67_Plexi]

Probed through the amp with a cap to ground, hum was coming from the V3 grid. Followed the hum up to the pre amp jacks. If I unscrewed the jacks from the chassis the hum stopped, amp was silent. I put in some plastic spacers and ran a ground wire from the jacks to the ground buss. I dont exactly know what was going on, some weird ground loop, bad jack? Its quiet now.

Often, that can happen if your speaker jack ground and your O/T secondary ground are not connected together close to or on the speaker jack. Heavy current is then travelling through the chassis, the resistance of the ground paths cause a ground potential difference....blah de blah de blah.

Not saying that is it, just a common scenerio given the speaker jacks are close to preamp jacks, the ground potential could be elevated at that point. Actually grounding the preamp jacks at the same point as the speaker jacks is OK, providing the OT secondary connects directly to the speaker jack ground lug preventing the big A's from using the chassis as a conductor (even if the actual ground point isn't there). Make sense ?

Limiting any heavy current travelling distances through the chassis, be it speaker or filament current is always a wise idea for setting yourself on the way to hum and buzz free amp.

I'm jibbering now.....

[phsyconoodler]

'probed the amp with a cap to ground'?? Does that mean you isolated the hum by using a jumper consisting of a capacitor?

[heisthl]

The ground path of speaker jack to preamp out/poweramp in jacks is only an issue if you're grounding them (and the PI) together which can create an issue involving the relationship of the preamp signal(s) to the PI input. Dumble addressed this by grounding the PI at those preamp jacks on the builds that I've seen, as an alternate you can do it like you did - isolate the jacks and ground them at a point closer to the PI ground. These days I ground the OT, screens, first 2 filter nodes and speaker jacks to the chassis at points close to the output tubes and everything else to a point near the input jack (including the PI) and have not had to chase a ground problem since. (knocking on wood as I type)

[heisthl]

'probed the amp with a cap to ground'?? Does that mean you isolated the hum by using a jumper consisting of a capacitor?

If you use a big enough capacitor on a cliplead to ground it is a "safe" jumper to ground for any point on the signal path (.1 @ 600v works good). The only problem is it's not fool proof, many times it leads you on wild goose chases when you're hunting hum issues.

['67_Plexi]

The ground path of speaker jack to preamp out/poweramp in jacks is only an issue if you're grounding them (and the PI) together which can create an issue involving the relationship of the preamp signal(s) to the PI input. Dumble addressed this by grounding the PI at those preamp jacks on the builds that I've seen, as an alternate you can do it like you did - isolate the jacks and ground them at a point closer to the PI ground. These days I ground the OT, screens, first 2 filter nodes and speaker jacks to the chassis at points close to the output tubes and everything else to a point near the input jack (including the PI) and have not had to chase a ground problem since. (knocking on wood as I type)

The actual ground connection I make is near the PI, I actually ground the preamp caps at the same point. But because I have the OT secondary connected directly to the speaker jack, it doesn't use the chassis to conduct the speaker current, only the the very low feedback circuit current will flow down the ground wire that grounds near the PI. A practical demonstaration of Kirchoffs first and second laws as well as Ohms law.

[Normster]

If you use a big enough capacitor on a cliplead to ground it is a "safe" jumper to ground for any point on the signal path (.1 @ 600v works good). The only problem is it's not fool proof, many times it leads you on wild goose chases when you're hunting hum issues.

Been there, done that tracking down a reverb hum. Everything pointed to a bad bypass cap but the problem turned out to be insufficient grid load. Live and learn I guess.

[zimm]

Thanks for the help. Yes I meant I used a .1 uF 600V cap clipped to ground. It did seem to work for this particular instance but its good to know that its not fool proof.

My ground scheme is not standard, in fact most of the lead dress is not either, due to the tight space of the Deville chassis and the way I conserved the original face plate layout/ holes. My preamp in/out jack are on the front of the chassis and not on the rear as in many of the standard layouts here. My camera is on the fritz but Ill get some pics up so you can see the rats nest that is my lead dress. What you say about ground current makes sense.