Mixing black and brown

[jordan86]

I know it’s a fashion faux pas to mix black and brown but curious how it might work out in a fender deluxe?

The brownface has always intrigued me. Thinking it’d be fun to split the V1 cathodes and convert the normal channel to blackface tone stack spec. 100k plate load. 1.5K cathode. Would need to find a place for an extra control (bass + treb now - maybe sacrifice an input) and would surely lose a lot of gain. Imagine it’d be Clean up to 9.

Would like to push the gain up a bit on the bright channel to compensate/contrast. Keep the 220k/2.7k scenario. Maybe adjusting the voltage divider off V2 plate would accomplish that? Plus would help out the struggling blackface normal channel.

Could still jumper them for more flavors and layers. Sort of like a proper two channel amp scenario (clean fender + brown/marshally). 

Thoughts?

[sluckey]

That's the same idea I had when building my Phoenix Project! I was originally gonna combine an AB763 preamp and 6G3 preamp for an All American Dual Lite. I changed my mind during the drawing stage and decided to do a Fender/Ampeg Dual Lite instead. Here's my original preamp schematic...

[jordan86]

Wow, Sluckey. I’d say great minds think alike, but wouldn’t dare disrespect you by dragging you down to my level 

So it looks like you scrapped the trem for independant second stages. I’d prob keep the trem. Any issues with shared second stage? I love your hot switch idea.

Last question, I assume cathode bias is not an option with the bias vary trem? That would be the ultimate to me but not a deal breaker.

[sluckey]

I had strict guidelines for my Phoenix. I had a 12.5" x 6.5" chassis with holes for 3 noval sockets, and 6 knobs on the front panel. Tremolo was never even a consideration.

Referring to my schematic, you could mix the two preamps by putting the 220K mixers between the volume control wipers and the grid of the second triode. This frees up a triode for your tremolo. That one triode tremolo works very well with cathode biased amps. I have two such amps...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf
     http://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf

Or, you could use my schematic as drawn and add another tube . This would allow you to use the 6G16 Vibroverb Tremolo circuit, which is an improvement over the one triode tremolo. This has more appeal to me.

[jordan86]

Thanks, Sluckey. Yeah I’m not advanced enough quite yet to source my own chassis and all that so I’d prob just pick up a 6g3 kit and have Doug do an eyelet board for me.

[acheld]

Of course you're advanced enough to do this!

This fella will fabricate a chassis inexpensively to your specs.   https://seasidechassisdesign.com

All you need is a drill, a step-drill bit, a de-burring tool and a hacksaw.

Admittedly, I use a router now to cut the PT hole, and a drill press makes the hole drilling more accurate, but you can get good results with very basic tools.

It takes some effort, but then you are not limited in your design.

[brewdude]

I’ve been under the impression that the LTP PI differences between the brown and black fenders was critical to there individual tones beyond the plate/cathode loads and tone stack differences.

[jordan86]

Great point. I’ll look into that. May even compare to jtm45 ltp scenario to land somewhere in the best of both.

[jordan86]

Had a fun quick little dive down the LTP trail. https://www.aikenamps.com/index.php/the-long-tail-pair  The 6G3 is not too far off from the AB763 or the Marshall LTP. A few minor value changes, which equate to a cooler bias - if I am reading right.

Got me curious about a presence control though. That might be nice seeing as it's a pretty "Marshally" Fender. I was thinking about just a NFB switch originally - on/off. Could anyone could tell me about possible issues with defeating NFB on an amp with a presence control? I did a simple switch like this on my Princeton AA1164. Put it in the second speaker jack hole to break NFB. What happens to the Presence control when you break that NFB connection?

[sluckey]

It no longer works.

[brewdude]

The AB763's use a 12AT7 in the PI, while the 6G3's have a 12AX7.  I am not experienced enough to know how much this matters with the differences in resistor values... that is, do the value changes simply compensate for the tube swap? 

[thetragichero]

The AB763's use a 12AT7 in the PI, while the 6G3's have a 12AX7.  I am not experienced enough to know how much this matters with the differences in resistor values... that is, do the value changes simply compensate for the tube swap?

i use this handy dandy calculator to suss out long tailed pair phase inverters: https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/calculator/

[PRR]

I thought the 12AT7 was called in when Fender had more than two fixed-bias power tubes?

[jordan86]

This is becoming a very helpful thread. Thanks for LTP calculator!!!

Is it correct to assume that you want both halves of the LTP to be exactly matched? Or is some mismatch desirable? Looks like 8k Rt produces the closest match with a 12AX7.

[sluckey]

I thought the 12AT7 was called in when Fender had more than two fixed-bias power tubes?

I think the 12AT7 showed up at the same time as black tolex.

[roseblood11]

Referring to my schematic, you could mix the two preamps by putting the 220K mixers between the volume control wipers and the grid of the second triode. This frees up a triode for your tremolo. That one triode tremolo works very well with cathode biased amps. I have two such amps...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf
     http://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf

Or, you could use my schematic as drawn and add another tube . This would allow you to use the 6G16 Vibroverb Tremolo circuit, which is an improvement over the one triode tremolo. This has more appeal to me.

Can I use the two triode tremolo from the 6g16 in a 6V6 amp like the 6g3 without any changes?

[sluckey]

Can I use the two triode tremolo from the 6g16 in a 6V6 amp like the 6g3 without any changes?

yes

[roseblood11]

@sluckey: The HOT switch in your schematic would cause a loud popping noise... Wouldn't it be better to add coupling caps and place the switch after them?
I don't really understand, why Fender used that voltage divider anyway. To save a few cents? Why didn't they take the signal from the anode and add a coupling cap and a voltage divider? Maybe even a pot? Would that affect the sound? Power supply noise should be reduced significantly.

[HotBluePlates]

I don't really understand, why Fender used that voltage divider anyway. To save a few cents? Why didn't they take the signal from the anode and add a coupling cap and a voltage divider? ... Power supply noise should be reduced significantly.

Power supply noise is largely insignificant once you're that far along, including in my actual 1962 Deluxe.  It's only a few millivolts at the filter cap.

Meanwhile, it's kinda elegant to assemble a voltage divider at a location & using parts that already needed to be present.  In the 1960s amps, there are resistors that mix Dry and Reverb in a manner akin to channel-mix resistors that are a "hidden voltage divider" to adjust the relative strength of each signal.  They're not really "hidden" but folks fail to notice their function.

[sluckey]

@sluckey: The HOT switch in your schematic would cause a loud popping noise...

Jordan wanted to push the gain up a bit. The switch was just to illustrate how he could easily do that. It was never meant to be an actual switch. Just an idea. That circuit was never built.

[roseblood11]

That's what I thought...
Do you think that it's a good idea to take the full signal at the plate, add a coupling cap and a 1M volume pot? And rename the original volume pot as "gain"... This would allow more control over the distortion, and it would be possible to match the volume with the AB763 channel.
I will build the amp, with both channels, 6g16 tremolo, PI from the 6g3, power amp with 1k5 and 470R resistors, and the channel link switch that you described in your AC15 pdf, because it allows to use both channels for different signals. Direct signal into 6g3 and delay/reverb into the ab763 might be interesting...

[sluckey]

I'd probably build it stock to evaluate. If necessary I'd experiment with your idea.

[dwinstonwood]

...take the full signal at the plate, add a coupling cap and a 1M volume pot?

roseblood11, I've tried three different combinations. No 220K voltage divider resistors, just the top resistor, and both resistors before the pot. It's easy enough to try them all to see what you like best.

[roseblood11]

I´m planning to use these transformers: 125P33A (=22749), 160mA@330V, 4A@6.3V, 3A@5V, because it fits into the chassis, and a 1760J, 4k prim., 4/8/16 Ohms sec., because I already have it.

I understand that I should use the 4 Ohms tap with an 8 Ohms speaker and 6V6`s.

But could I safely use 5881`s instead? That would add the option to use two 8 Ohm speakers in parallel at the 4 Ohms tap, and it would be an interesting option soundwise, to use the Tung-Sol 5881 reissues.

2*0.9A+4*0.3A = 3A heater current, or 3.6A, if I use a DC heater supply for both preamp tubes, so that should work?

Would this bias circuit work with 5881`s? https://tedweber.com/media/kits/lenora_schem.jpg

[sluckey]

I'm using a 1760J with 6L6s. Should also be a good match with 5881s.

Would this bias circuit work with 5881`s? https://tedweber.com/media/kits/lenora_schem.jpg

It will work. You may have to drop the value of R34 to get the negative bias voltage high enough to work with 5881s. If lucky you may have enough range on the bias pot to cover 6V6s and 5881s.

I prefer a two stage filter on my bias supply. Like this...

     http://sluckeyamps.com/phoenix/phoenix.pdf
 

[roseblood11]

That's the same idea I had when building my Phoenix Project! I was originally gonna combine an AB763 preamp and 6G3 preamp for an All American Dual Lite. I changed my mind during the drawing stage and decided to do a Fender/Ampeg Dual Lite instead. Here's my original preamp schematic...

I`m thinking about a simple way to add channel switching with just one relay, without loosing the option to use both inputs separately. What do you think about this:
- Inputs are linked via the switch.
- Relay shunts one or the other channel to ground. (Where? Left side of the mixing resistors? Or between first and second stage?)
- a switch at the back breaks that ground connection -> footswitch doesn`t work, but both channels can run in parallel.

[roseblood11]

What other amps are there that combine blackface and brownface channels? The only one that I found is the 3rd Power American Dream.
It would be interesting to see some schematics...

[roseblood11]

I will use separate inputs for both channels, and Sluckey`s switch to jumper both inputs to run the channels in parallel - see picture.

Is it possible to add a third position, to run both channels cascaded? 6G3 into AB763 should make a pretty good high gain sound.

The topology would be very similar to the Damocles by Chasingtone: https://chasingtone.com/damocles/

My amp will have a volume pot after the second triode stage of the 6g3 channel, and a LarMar PPIMV, so, there will be various options to control the gain structure...

[DummyLoad]

I thought the 12AT7 was called in when Fender had more than two fixed-bias power tubes?

I think the 12AT7 showed up at the same time as black tolex.

Seems like that is the case -  My 1967 Super Reverb uses a 12AT7 for the LTPI.


--Pete

[DummyLoad]

My take on the proposed. Uses Super Reverb transformers, except choke, I like a little stiffer regulation for the screens and preamps.


Respectfully,


--Pete

[roseblood11]

Interesting.
Did you already build this amp?

Why didn't you choose the LTP?

How does the 6g3 channel sound with the third, fully bypassed gain stage?
I wonder why you kept the 220k voltage dividers, as there is a volume pot anyway?
And did you really use that switch at the plate? Will cause a very loud crack, and it's not decoupled from dc... As you had to add an additional coupling cap anyway, a volume control after that cap (instead of the switch) wouldn't hurt... And it would give you full control over PI distortion.

It's interesting that you wired the mid pot in the AB763 channel as a divider, like Marshall does. I recently tested a 68 custom deluxe reverb and liked the voicing of both channels, so I'll make them switchable (bass pot with dpdt push/pull).

[DummyLoad]

Did you already build this amp?

No. If I were to do so, what I presented is where I'd start.

Why didn't you choose the LTP? 

More gain - LTPI has an Av of ~25 - gain stage + cathodyne is about ~45. 

I wonder why you kept the 220k voltage dividers, as there is a volume pot anyway?

Looking at the original 6G3 closely & envision the normal volume pot fully CCW - There's a 220K + 220K divider to ground at the grid summing amp - I replaced the LTPI with a gain stage (also summing amp) and cathodyne pair. You could just insert a MV between the summing amp and cathodyne, however, that adds another cap impacting NFB loop phase.

--Pete
 

[DummyLoad]

IF I were to bradboard the concept, I'd spin these up for comparison with my previous post.

The previous post has elements of the 5 series (tweed deluxe) 6 series and AA-AB series amps. 

--Pete

[roseblood11]

At what point would you shunt a channel to ground to silence it, in the dual master volume version?

I`ve got the problem that I only have one relay for a simple channel switching, but I also need separate input jacks and the options to use both channels separated or linked. So I think, that it would be the easiest solution to link them via toggle switch and then shunt one or the other chanel to ground with a footswitch. (If the footswitch isn`t connected, the path to ground would be disconnected, so that both channels are active...)

[DummyLoad]

Four plans I'd prototype on a breadboard, IF I were to build.

--Pete

[roseblood11]

Why would you keep the switch at the plate, if it`s followed by a volume pot anyway?

[DummyLoad]

Why would you keep the switch at the plate, if it`s followed by a volume pot anyway?

Why not? It's an experiment.




--Pete

[Sonny ReVerb]

This was my take on a Brownface/Blackface hybrid. The schematic is at the bottom of the first post.