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Grounding Basics

12/16/1997 11:22 AM
Don	Grounding Basics Hello all, Thanks for all the help with my previous questions. Well, I've completed my first homebrew PA to guitar amp conversion. Basically, I used the Bassman 5F6A preamp section and used the existing phase inverter (6SL7) and power section (four 6L6's, cathode biased drawing 45mA). It's working pretty well, but--surprise!--I have more hum than I'd like. Because I'm new to this, I decided to forget the DC filaments for now and concentrate on trying to get a good sounding, low-hum circuit working properly to learn the basics before diving into more complex designs. Anyway, a few questions about grounding: 1.) In the preamp section, I used the exisiting buss type ground, and this is terminated at the chassis at the same location as the ground on the power supply. Is this okay, or should the B- be connected to the chassis ground where the tranny and AC line is grounded? 2.) Because the input socket is located right next to the first stage, I just connected the 68K resistor from the socket to pin 2, the grid, of the first tube. I know this part should be shielded, so should I try to put all the parts on a remote board a la Fender and run shielded wires to the sensitive tube pins? 3.) The input socket is insulated from the chassis, so should I ground this to the preamp buss or run a wire directly to where the preamp buss attaches to the chassis? 4.) Even though the pots aren't insulated from the chassis, I ran a buss (copper wire) across the backs of all of them and then to chassis ground. Is this okay? Should this also terminate where all the other preamp grounds attach to the chassis? It seems I've got a ground problem that is causing the hum, and I've read everything I could find on grounding techniques, but it seems that I could use a good basic discussion of grounding because it seems to be a VERY important part of amp design. Thanks in advance for any help. Cheers, don daput@pe.net Idyllwild Brewing Company

12/16/1997 11:22 AM

Don

Grounding Basics

Hello all,

Thanks for all the help with my previous questions. Well, I've completed my first homebrew PA to guitar amp conversion. Basically, I used the Bassman 5F6A preamp section and used the existing phase inverter (6SL7) and power section (four 6L6's, cathode biased drawing 45mA). It's working pretty well, but--surprise!--I have more
hum than I'd like. Because I'm new to this, I decided to forget the DC filaments for now and concentrate on trying to get a good sounding, low-hum circuit working properly to learn the basics before diving into more complex designs.
Anyway, a few questions about grounding:

1.) In the preamp section, I used the exisiting buss type ground, and this is terminated at the chassis at the same location as the ground on the power supply. Is this okay, or should the B- be connected to the chassis ground where the tranny and AC line is grounded?

2.) Because the input socket is located right next to the first stage, I just connected the 68K resistor from the socket to pin 2, the grid, of the first tube. I know this part should be shielded, so should I try to put all the parts on a remote board a la Fender and run shielded wires to the sensitive tube pins?

3.) The input socket is insulated from the chassis, so should I ground this to the preamp buss or run a wire directly to where the preamp buss attaches to the chassis?

4.) Even though the pots aren't insulated from the chassis, I ran a buss (copper wire) across the backs of all of them and then to chassis ground. Is this okay? Should this also terminate where all the other preamp grounds attach to the chassis?

It seems I've got a ground problem that is causing the hum, and I've read everything I could find on grounding techniques, but it seems that I could use a good basic discussion of grounding because it seems to be a VERY important part of amp design.

Thanks in advance for any help.

Cheers,

don
daput@pe.net
Idyllwild Brewing Company

Replies:

ethan 2) It is good that you were able to... -- 12/19/1997 6:04 PM
me again I forgot, Since your chassis is gro... -- 12/19/1997 6:39 PM
rh if you've done your best at groundi... -- 12/20/1997 12:46 PM