AB763 grounding scheme

[corkhead]

Hi all,
I will soon be launching into a Mojotone DR kit, and looking for a definitive guide in improving the stock grounding scheme. OR, talk me out of it, and tell me why the original is just fine!
I have read a LOT of posts about brass plates, copper buss bars etc, but the is no step by step or well documented path. If anyone can weigh in, I would be grateful. Thanks

[Roe]

the problem is not just bad grounding but that most of the preamp shares one filter cap, while the reverb shares a cap with the screens

[martin manning]

Here is is how I modified the AB763 grounding for a Deluxe Reverb: https://ampgarage.com/forum/viewtopic.p ... 77#p428877 No brass plate, no ground bus.

[ViperDoc]

the problem is not just bad grounding but that most of the preamp shares one filter cap, while the reverb shares a cap with the screens

I'm working up a Super Reverb AB763. I see the reverb driver is powered by the screen cap as you mentioned. If you were to run a "second B node" just for the reverb driver, would you need a 22uF cap, or could you lower it to 10 uF, say? I plan to run individual 22 uF for each preamp tube, and I'm going to incorporate Sluckey's tremonator.

[Roe]

the problem is not just bad grounding but that most of the preamp shares one filter cap, while the reverb shares a cap with the screens

I'm working up a Super Reverb AB763. I see the reverb driver is powered by the screen cap as you mentioned. If you were to run a "second B node" just for the reverb driver, would you need a 22uF cap, or could you lower it to 10 uF, say? I plan to run individual 22 uF for each preamp tube, and I'm going to incorporate Sluckey's tremonator.

it depends. If you use 1k between the nodes, I'd try at 33-47uf.

[corkhead]

Here is is how I modified the AB763 grounding for a Deluxe Reverb: https://ampgarage.com/forum/viewtopic.p ... 77#p428877 No brass plate, no ground bus.

I have pored over this thread and the accompanying picture for quite some time, and I am still unclear. Which is why I asked if there is a step by step guide or a definitive well documented path.

[pdf64]

The notes on the photo may initially seem a bit cryptic, but if read alongside the DR AB763 schematic, may eventually make sense.
https://el34world.com/charts/Schematics ... _ab763.pdf
eg BC- refers to the negative terminals of the B and C node caps of the HT supply, as per the labels on the schematic.
https://ampgarage.com/forum/download/fi ... &mode=view
In the light of that, is there anything you’re still stuck on?

[martin manning]

I've added a marked-up layout to the first post in the DR Replica thread.

[corkhead]

I've added a marked-up layout to the first post in the DR Replica thread.

Thank you very much! I will pore over it and hopefully my aged and rusty neurons will wake up, and it all makes sense.

Did you notice and improvement over the original AB763 wired as per the schematic?

I can't help but think replacing the input signal wires from the jacks to the pre-amp tubes with shielded wire grounded at only one end will further lower the noise floor at idle.

[martin manning]

Did you notice and improvement over the original AB763 wired as per the schematic?

If I had built an identical amp with the original grounding I could answer ;^) I can say that the amp as built with the new scheme is very quiet.

I can't help but think replacing the input signal wires from the jacks to the pre-amp tubes with shielded wire grounded at only one end will further lower the noise floor at idle.

Wouldn't hurt, may or may not make a noticeable improvement. To do the inputs right would require moving the grid resistors to the sockets, and then I'd probably want to ditch the low input. In the end I left it original, no problems.

[Roe]

you need a filter cap and ground for each of the 6 preamp tubes (pluss screens and mains)

[corkhead]

you need a filter cap and ground for each of the 6 preamp tubes (pluss screens and mains)

Why? Surely the filtered B+ (point D in links to schematic posted above) voltage has already benefitted from a choke and 2 dropping resistors prior to it. The ripple if any would be tiny?

[martin manning]

It's not just the ripple voltage; decoupling of the stages is also important. The last filter is powering no less than six triodes, where two or at most three is usually considered the best practice. It's not quite as bad as it seems, though. Typically only one channel is in use at a time, so the worst case is it supplies four active triodes, and the phase relationship of the reverb recover stage to the others is all over the map. I find there are no issues with the one I built, and in fact the original circuit design and layout, random grounding and all, has been working pretty well for almost 60 years :^)

[corkhead]

It's not just the ripple voltage; decoupling of the stages is also important. The last filter is powering no less than six triodes, where two or at most three is usually considered the best practice. It's not quite as bad as it seems, though. Typically only one channel is in use at a time, so the worst case is it supplies four active triodes, and the phase relationship of the reverb recover stage to the others is all over the map. I find there are no issues with the one I built, and in fact the original circuit design and layout, random grounding and all, has been working pretty well for almost 60 years :^)

Thanks Martin. My understanding is from the way the choke, 2 resistors and 3 capacitors are configured they form 3 consecutive low pass filters (B, C and D), successively dropping the voltage and further smoothing out AC ripple. Are you suggesting adding (say, 2) additional 16uF 450V electrolytics at the common high voltage junctions of the 100k screen resistors of the 7025 preamp stages, further adding to the effective capacitance to ground at node D?
If so, how would that be different from changing the electrolytic at node D to say 50uF?
And how can we decouple DC? Maybe I am thinking too much, or (more likely) don't understand the subtleties of AC to DC filter stages and the effects of loading them.
If you can point me to an online resource that can shed light on this mystery I would be most appreciative!
I am just looking to build a nice and quiet clone, using best practice from the current era, whilst staying true to the high tide mark of sonic design that this amp represents (IMHO).
Thanks in advance...

[Roe]

you need a filter cap and ground for each of the 6 preamp tubes (pluss screens and mains)

Why? Surely the filtered B+ (point D in links to schematic posted above) voltage has already benefitted from a choke and 2 dropping resistors prior to it. The ripple if any would be tiny?

its bad practice to let many preamp tubes share a filter cap and to have a messy ground path (and to power the reverb from the screens). Rebuild and decrease noise/hum. it works