Brownote Foxy 18

[amplifiednation]

Great job mike! That is quite the project. Blue Thunder lives!! Get some good pics out in natural light!!

[ampgeek]

That is some beautiful work there partner.
Well done!!
Dave O.

[Structo]

Here is Marshall version Moss put out there.
More gain.

Called Dirty 18

[neskor]

Sorry to revive an old thread, but I have one question.
On the layout grid leak resitors for EL84 output tubes are 1M, usually they are 220K or 470K.
Is this a correct value or error and what is the effect of 1M vs 220K/470K?

[martin manning]

1964 Mullard data sheet says 300k max. I would not go above 470k. Coupling cap and Rg RC filter will pass more bass with larger Rg, 10n and 470k would put -3 dB at 34 hZ, 270k would be 59 Hz. In the first layout with the MV, the pot resistances are in parallel with the 1M resistors on the board so it has at most 500k Rg.

[neskor]

Thank you Martin!
One more question regarding the Cut pot value.
Usually it is 250K with 0.0047uF, here it is 500K pot.
Why 500K value, is it because Rg is 500K (1M/1M master volume as you said)?
I don't have 500K SPST'. Can I use 250K SPST pot instead or better to stick with 500K (with separate switch)?

[martin manning]

This is the layout you are referring to?
https://ampgarage.com/forum/viewtopic.p ... 37#p196537

The schematic is as shown below (for one half only). No safety resistor is needed since if the wiper contact fails you still have the 1M to ground, and when the pot is full up you have 500k Rg, pretty much as if there is no MV in the circuit.

On the cut control, it goes across the outputs and allows the signals to cancel. It's separate from the MV, so You can experiment with the value of the cap and the pot. A 500k at max resistance would be getting closer to it being switched out.

[neskor]

This is the layout you are referring to?
https://ampgarage.com/forum/viewtopic.p ... 37#p196537

yes, that's the one

[dorrisant]

Does anyone have a schematic for that layout?

Or maybe just a detail drawing of the rotary switch... ?

Also, what is the value of the cabinet cap?

[sluckey]

I made a schematic for that layout several years ago when I was considering building that amp. I changed my mind and built my Dual Lite instead. Here's my schematic. It's available in Visio format also if needed.

[jbrrrrr]

Mike, do you have a bunch of 1k's on hand?

You can make a 5.1M pretty easy!

I know who I'm not asking for help when I get stuck

[Colossal]

I made a schematic for that layout several years ago when I was considering building that amp. I changed my mind and built my Dual Lite instead. Here's my schematic. It's available in Visio format also if needed.

The PI plate load resistors are shown as the usual 82k/100k, however, the 18watt and Vox PIs with the 47k tail resistor usually have 100k/100k. Do you think the choice of 82k/100k was intentional to get some mis-match in the output balance or a layout mistake on Brownnote's part?

[sluckey]

The 82K should be on top and the 100K should be on bottom. Was it intentional or a drawing error? I suspect we'll never know.

[Colossal]

The 82K should be on top and the 100K should be on bottom. Was it intentional or a drawing error? I suspect we'll never know.

Hi Steve,

I was referring to the part choices for the PI. With a Voxy or Marshall 18W PI, the large 47k tail resistor (56k in some early 18W amps) improves balance and the plate loads are typically 100k/100k. In the Plexi PI, with the 10k tail resistor, the 82k/100k pair are to (somewhat) restore the imbalance caused by the 10k tail.

I am wondering if the 82k/100k choice (or at least as drawn) was intentional on Moss's part to cause some imbalance in what is otherwise a Voxy PI and which should have 100k/100k resistors.

[sluckey]

Who knows what he was thinking. I just drew the schematic to match the layout and never thought too much about it since I never built it.

I also drew schematics for his Dirty 18 and Lite IIB. He used two 100K plate load resistors in both of those amps.