Two brown tone-stacks on contemplated build

[luthierwnc]

Hi All,
I've been toying with making either a 6G3 or 6G11 for a winter build and wondered about combining them.  Attached is a cut-and-paste schematic that uses elements of both.  It could be either for 6V6s or 6L6's so I added an adjustable fixed-bias with a wide sweep on the pot.  The mix resistors going to the PI use a voltage divider in case one side is too much hotter than the other.
If anyone spots something or has better ideas, I hope you'll comment.  Thanks, as always, Skip

[tubenit]

I think your idea is just fine. 

Having said that, I have found over the course of doing a number of builds that I end up favoring one tone over another. An example for me would be a pentode/triode mod where I almost never used the triode mode and ended up removing the pentode/triode switch on several builds.

I'm thinking that you may end up preferring one tone stack so much more than the other that one of them isn't used much?
And that may not be the case?

I would be inclined to go with the 6G11 tone stack and use a midboost switch or a preamp boost  PAB like they do on some Dumble amps for that "2nd" tonal option.  I think that might somewhat approximate the 6G3 tone?  You could simply install one of those on a mini-toggle.

Then I'd use the extra tube for a one tube reverb.

In other words,  build the 6G11 with midboost or PAB and one tube reverb.

Just a thought to consider.  I think you should build whatever you want though.

with respect, Tubenit

[d95err]

There’s an error in the schematic. There is no output from the second channel; the lower part of the mix pot is connected to B+.

[ac427v]

Very interesting idea. I think the 6G3 channel has the potential to be much hotter than the other channel depending on tweaks to the split load of the second triode. That is a plus in my book if you set it up more like a Marshall 18 watt. You could use the voltage divider pot as a prePI master volume for the hot channel and separate mixer resisters to reduce the MV effect on the 6G11 clean channel.

[luthierwnc]

Thanks for catching that d95err.  I've made the correction.
I also wondered about putting a pot on the bias wobble and finally figured that when these were new, you went to the 5 and dime for a pair of blackplates and slapped them in.  Given the variations on tubes today, you are adjusting both the bias and the trem.  This may find a happy medium.  sh

[tubeswell]

Besides what d59err said about the output from the brown channel being drawn wrong, you might want to think about other types of channel switching, because grounding the grid of each input channel won't address noise from the grid of each tone stack recovery stage when the 'off channel' is supposedly 'off', and the 500k 'mix' control is somewhat redundant if the idea is to only switch either one channel or the other.

[sluckey]

Grounding the grid of the unused channel (via input jack) works very well for all those two channel blackface Fender amps.

The switch he's using is a 3 position switch that will allow selecting either channel or both channels. Works very well when wired correctly. His is not. Here's the correct wiring...

     http://sluckeyamps.com/supro/supro.pdf

[tubeswell]

Grounding the grid of the unused channel (via input jack) works very well for all those two channel blackface Fender amps.

Yes it does. But in conjunction with the mix control, its a bit redundant, or vice versa

[sluckey]

IMO the mix control is not very useful. I prefer two fixed resistors. The volume controls do a good job of blending the two channels together.

[luthierwnc]

Steve, should I get rid of the hi-lo inputs?  The switch is the same as yours but I put a break in the signal line to show it doesn't touch.  The mixer pot is a trimmer -- set and forget.  Could just as well be two fixed.  If so; I don't suppose I really need coupling caps before the PI.  Food for thought.
I don't know when I'll get to this but my subscription to the Emachine DXF conversion utility lapses in a week so I'm roughing chassis designs for the future.

Cheers, sh

[sluckey]

The Hi-Lo jacks are fine, but you are missing a ground to the switch. I would leave the coupling caps because they prevent the plate voltage on one preamp tube from mixing in with the plate voltage of the other preamp tube.

[luthierwnc]

sloppy copying.  I wrap the resistor around the jack the same way every time.  Thanks again, sh

[sluckey]

...I would leave the coupling caps because they prevent the plate voltage on one preamp tube from mixing in with the plate voltage of the other preamp tube.

[2deaf]

The 4R and 8R wires need to criss-cross on the OT like a 70W Super Reverb.

Eliminate the 100K to ground on the bias.  Change 12K to 22K.  Change 50K pot to 25K pot.

Do you have a source for that 350K pot with a 70% tap?

[sluckey]

Weber has that 350K with 70K tap pot.    https://tedweber.com/wp354at/

[tubeswell]

Taking the same number of components from your 1st schematic, here's another idea.

You have choice of single or parallel channel inputs, where the DPDT switch reverses the single channel choice, and a common anode mixer output where the 10kL mix control tweaks* the mix going to the PI by altering the gain boost between the channels - without the need to use mixing resistors to combine channel outputs. (I've included a cathode bypass on Ch A input to indicate that there could even be a potential gain difference between Ch A and Ch B, but YMMV)

* assuming you also have separate volume controls (with the tone stacks) in each channel


You could also plug an ABY pedal in front of the amp - into both input jacks (which overrides the single-parallel input jacks) - creating ability to easily foot-switch the channel choices to either or both, without the need for any relays. If you use an active ABY like Morley's one, https://www.morleyproducts.com/aby-mix-mixer-combiner/, you have additional pedal gain options


These are based on ideas are from Merlin Blencowe's (1st edition) designing tube pre-amps book

[luthierwnc]

Thanks gents.  I've got a lot to think about.  I'm also fairly sure I've got a 350k tapped pot in the bin.
This started as a stock 6G3 build and then I started messing with making one of the channels significantly different than the other.  Whatever happens, I want an uncompromised Brown Deluxe option.  That morphed into this.  I may just do the 6G3 but this is interesting in the pixel stage.  I could put a pair of relays to stomp a/b/y but I don't think this is that type of amp.
The project is down down on the list but I appreciate everyone chiming in and wish you all luck with your projects.  Cheers, Skip

[tdvt]

I am wondering if this was ever built?

I had a friend ask about a BrF/BlkF hybrid TS set-up & I had come up with a similar topography while thinking about it.

A search landed me here.