ODS Grounding Scheme

[benoit]

I've been searching, reading, and checking out photos of various dumbles and clones, trying to figure out the grounding scheme and I think I'm starting to figure it out. Tell me if this sounds about right:

One ground point right by the input that handles bus bar, input, and other preamp grounds.

One right next to bias pot for the bias pot and output tubes' cathodes.

One on or next to the right rear transformer bolt for the PT center taps, bias supply filter, and first HV filter ground.

One On/near the front right PT bolt for the relay supply ground and footswitch ground.

Anything wrong with this? Please take into account that at least as far as grounding scheme goes I'm trying to be as true to the originals as possible.

Also a quick question about the power resistor on the power supply board. Is that meant to be in place of the choke or what is it for?

[ayan]

I've been searching, reading, and checking out photos of various dumbles and clones, trying to figure out the grounding scheme and I think I'm starting to figure it out. Tell me if this sounds about right:

One ground point right by the input that handles bus bar, input, and other preamp grounds.

One right next to bias pot for the bias pot and output tubes' cathodes.

One on or next to the right rear transformer bolt for the PT center taps, bias supply filter, and first HV filter ground.

One On/near the front right PT bolt for the relay supply ground and footswitch ground.

Anything wrong with this? Please take into account that at least as far as grounding scheme goes I'm trying to be as true to the originals as possible.

Also a quick question about the power resistor on the power supply board. Is that meant to be in place of the choke or what is it for?

Sounds about right. The other ground is by the FX loop jacks, and that (in some amps) takes the ground for the PI tube. In other amps, it seems Dumble daisy-chained the grounds of V1-2-3.

The power resistor on the P.S. is NOT the one that replaces a choke. In amps without a choke, the cement block 25W resistor Dumble used is soldered to a terminal strip located between the pilot light and the presence control, flush to the front panel end of the P.S. board -- so, towards the front of the chassis.

Gil

[ic-racer]

I don't know if you have seen my rendition of the #124 schematic. Every grounding point and every visible wire on #124 is tediously documented. The numbers on the little diagram at the bottom match up with the numbers by the ground symbols on the schematic.

[benoit]

The power resistor on the P.S. is NOT the one that replaces a choke. In amps without a choke, the cement block 25W resistor Dumble used is soldered to a terminal strip located between the pilot light and the presence control, flush to the front panel end of the P.S. board -- so, towards the front of the chassis.

Gil

Interesting. But what IS that resistor? The only pic (a clone) where I can see somewhat clearly what it's doing is one where its connected to the screen resistors, but I've never seen a schematic I can find the resistor in. Still quite confused.

[benoit]

I don't know if you have seen my rendition of the #124 schematic. Every grounding point and every visible wire on #124 is tediously documented. The numbers on the little diagram at the bottom match up with the numbers by the ground symbols on the schematic.

Thanks, this schem is VERY informative. Most of my questions are cleared up. Still have a few though.

1. When you write "chassis" by the ground sign for a pot I assume you mean its simply to be installed so that it touches the chassis, no isolation washers. Is this correct?

2. I was confused about the grounds for the presence circuit and V2 cathodes. Are they meant to connect to the return jack?

[ic-racer]

1. When you write "chassis" by the ground sign for a pot I assume you mean its simply to be installed so that it touches the chassis, no isolation washers. Is this correct?

From what I can tell on the #124 pictures, yes, those jacks need to be grounded to the chassis for proper function. I think I can see evidence of the chassis being scrapped away around the jacks and I think those star washers on the back of the jacks help get a good ground in #124.

2. I was confused about the grounds for the presence circuit and V2 cathodes. Are they meant to connect to the return jack?

Mine is wired that way, but, after looking at the #124 pictures again, it is ambiguous as to wether V2 cathodes ground with V1 (at star ground #1) or with the PI (via the grounded send/return jacks). The circuit connection under the main board can't be seen.

I won't change the way my amp is built, but before I post my #124 schematic in the 'files' section I should change it to indicate that the exact grounding of V2 can't be determined by the pictures. Perhaps Gill can shed some insight here. He did indicate that in his BM build the V2 cathodes do NOT ground with the PI. https://tubeamparchive.com/viewtopic.ph ... 8&start=15