SSS #002 REDRAW DRAFT

[martin manning]

Here's what I found. There are a couple of updates to the baseline, and I corrected the revised step filter input and tweaked values to the low filter. Your rev 5 bias still doesn't look correct. That network is the same whether the bipolar or original driver supply is used, just different resistor values. The original didn't have grid stoppers on the driver triodes, and the amps that have been built seem to be fine, so I called those 220k's optional.

To be most useful, the schematic should be made to match Ryan's layout (GitHub), which I assume is near enough the same as Erwin's (?). The bipolar supply and revised step filters don't affect the layout, except for a new rectifier/bias supply board with the bipolar supply and the leads going to the driver tube.

[ViperDoc]

Here's what I found. There are a couple of updates to the baseline, and I corrected the revised step filter input and tweaked values to the low filter. Your rev 5 bias still doesn't look correct. That network is the same whether the bipolar or original driver supply is used, just different resistor values. The original didn't have grid stoppers on the driver triodes, and the amps that have been built seem to be fine, so I called those 220k's optional.

To be most useful, the schematic should be made to match Ryan's layout (GitHub), which I assume is near enough the same as Erwin's (?). The bipolar supply and revised step filters don't affect the layout, except for a new rectifier/bias supply board with the bipolar supply and the leads going to the driver tube.

Excellent. Thanks for your help. I'll have a V6 later today! I took the bipolar supply straight off the 2017 graphic posted somewhere here. Does that version pertain to a different amp?

EDIT: Where does the "V-" supply enter into the circuit? Is it not used in the edited bias?

[martin manning]

I took the bipolar supply straight off the 2017 graphic posted somewhere here. Does that version pertain to a different amp?

Different way to get to the same place, but the bias circuit shown here fails "cold" if the wiper loses contact.

EDIT: Where does the "V-" supply enter into the circuit? Is it not used in the edited bias?

Sorry, I covered it up. V- goes to the junction of the 100k cathode resistors and 75k. Same place you had it.

[ViperDoc]

THANKS. Is there no "V-BIAS" then, or does that inject at the junction of the 1Ms? You crossed out the "V-BIAS" on the V4 doc bias circuit. Thanks, Martin.

EDIT: Here's the bipolar Bias layout I originally used to draw it into the schematic.

https://ampgarage.com/forum/download/fi ... &mode=view

[ViperDoc]

V6 added to include Martin's corrections. Please review and let me know if you see anything. Much appreciated!

[martin manning]

Is there no "V-BIAS" then, or does that inject at the junction of the 1M's?

Yes that's the bias voltage for the driver stages, then the bias voltage for the power tubes appears at the driver cathodes.

[beasleybodyshop]

Bravo Viperdoc! I like how clean your schematic is. Kudos for annotating ground points as well. Too many schematics just assume.

[beasleybodyshop]

Is the thermistor as drawn going to mitigate inrush? Do you have a spec for this part?

[beasleybodyshop]

Also, if you can get preamp voltages and annotate that onto the schematic, that will be of big help. D style amps seem to be picky about where the preamp voltage at the plates is. JM2C. Looks great

[ViperDoc]

Is the thermistor as drawn going to mitigate inrush? Do you have a spec for this part?

Thanks, dude.

Yes, I usually add a thermistor on the 120AC hot after the switch because I generally delete standby switches on my amps. I usually use a CL80 (50W) or CL90 (100W), which I got on recommendation from pdf64. It helps prevent the current spike when lighting the amp up. As you know.

I hope to get the schematic into shape and then I will add voltages later. Great recommendation.

[ViperDoc]

Also, if you can get preamp voltages and annotate that onto the schematic, that will be of big help. D style amps seem to be picky about where the preamp voltage at the plates is. JM2C. Looks great

I won't be able to build and confirm voltages on this for a while. Can anyone offer target voltages for the SSS with Colgan bias, or will the standard targets still apply? I don't think I've seen a SSS voltage chart. Searching now....

[martin manning]

Can anyone offer target voltages for the SSS with Colgan bias, or will the standard targets still apply?

I can get voltages from a simulation, if that helps. There will be some variation depending on the bias supply and the tube choice for the reverb driver tube (12ATT7 or 5751). With the same assumptions, the sim voltages seem to match Ryan's layout.

BTW, I'm the originator of the revised adjustable bias, a suggestion to eliminate the need for a 2W pot which Ryan originally used: https://ampgarage.com/forum/viewtopic.p ... 50#p417450 A couple of posts down are the component value mods for the bipolar supply.

[ViperDoc]

I can get voltages from a simulation, if that helps. There will be some variation depending on the bias supply and the tube choice for the reverb driver tube (12ATT7 or 5751). With the same assumptions, the sim voltages seem to match Ryan's layout.

BTW, I'm the originator of the revised adjustable bias, a suggestion to eliminate the need for a 2W pot which Ryan originally used: https://ampgarage.com/forum/viewtopic.p ... 50#p417450 A couple of posts down are the component value mods for the bipolar supply.

Awesome, thanks Martin! If I recall, you and Ryan are on the same page. You both give each other credit for the upgraded bias circuit, so maybe I should call it the Colning Bias? Maryan Bias? Mangan? Manning/Colgan? Colgan/Manning? I will take directions.

[martin manning]

We can just say that the circuit has a pot added for bias voltage adjustment.

I'll gather up the voltages and post them, but we have a consistency issue to address. Ryan's layout has all of the small signal tubes oriented with the pin 1-9 gap in the same place, and for example the instrument inputs connect to the pin 2 grid, where your schematic shows the inputs connecting to the pin 7 grid. Schematics typically use "a" and "b" references, following a right to left (and typically signal chain) order, the same as the convention for tube numbers on the chassis. V1a is to the right of v1b, and V1 is to the right of V2, etc. See in the current Fender schematics for the AB763 amps, where V1a is the input triode, pins 123. I'm aware that tube data sheets typically refer to "section 1" as the 678 triode, but whatever, using "a" for the 123 and "b" for the 678 works fine. I suggest that you modify your schematic to reflect the layout, with the 123 triode as shown in the layout being "a."

[ViperDoc]

We can just say that the circuit has a pot added for bias voltage adjustment.

I'll gather up the voltages and post them, but we have a consistency issue to address. Ryan's layout has all of the small signal tubes oriented with the pin 1-9 gap in the same place, and for example the instrument inputs connect to the pin 2 grid, where your schematic shows the inputs connecting to the pin 7 grid. Schematics typically use "a" and "b" references, following a right to left (and typically signal chain) order, the same as the convention for tube numbers on the chassis. V1a is to the right of v1b, and V1 is to the right of V2, etc. See in the current Fender schematics for the AB763 amps, where V1a is the input triode, pins 123. I'm aware that tube data sheets typically refer to "section 1" as the 678 triode, but whatever, using "a" for the 123 and "b" for the 678 works fine. I suggest that you modify your schematic to reflect the layout, with the 123 triode as shown in the layout being "a."

AH, thanks for that info. Is the schematic's bias circuit correct?

I was under the impression that 678 WAS "a", as well as being normally the less noisier half of the tube, thus being preferred as the first to be used, no? The default setting inside KiCAD I believe is the way I have it set up, but I will double check that! Thanks as always, Martin.

EDIT: KiCAD does, in fact, list the 678 triode as the "A" section, and the 123 as the "B" section. I suppose in that case I'd have to swap the A/B designation in favor of the pins.