Adding Reverb to a 6G3 Brown Deluxe

[Fletcher Munson]

I couldn't find any previous discussions on this topic, so my apologies if I'm going over old ground.

I'd like to add the typical Fender blackface reverb circuit to a 6G3 Deluxe, and I'm not sure if I can simply insert it into the circuit or if I need to add another gain stage.

The blackface reverb amps typically have two gain stages with a tone stack in between before the signal is split into wet and dry paths. They are then joined together at the input of a third gain stage before going on to the phase inverter. I'm thinking of building a 6G3 with a single channel, then splitting the signal right after the tone stack, and bringing it back together at the second stage (using the usual 3.3M/10pf arrangement). Can this work?

[uneumann]

It may work, but not without a side effect of losing gain in the main dry signal path due to the 10M resistor.
There is also a weirdness in that the PI is fed from what looks like a 15K load resistor (split load with the 100k resistor).
Normally, you'd want to drive the PI from the plate side of the 100k.
You might also try doing this with the 10M inserted before the PI, but then you may need more gain in the pan recovery side.

IMO - when inserting reverb into a existing amp, it best to preserve the gain structure of the dry path.
That means doing something more like the 6G15 where the dry path gain is preserved and the pan (wet) signal is added on top of that.
Of course, that takes a bit more circuitry, but the result is better (IMO) since the amp behaves the same as before the reverb mod.

[nickb]

If you compare with the Deluxe Reverb you are missing an entire gain stage so the drive to the tank will be weak and than must mean the signal to noise ratio will be worse so I suspect it won't work very well.

If you remove the 220k after the reverb level that will give you back some of the gain. Also, if you feed the PI from the anode rather than the tap that should get you back to about where you were. The 0.02uF is superfluous.

[Chuck H]

uneumann has it right, that there would be a considerable gain loss simply inserting the classic circuit and that would change the tone. There are still a couple of ways you could do it though. That amp is capable of some preamp clipping now and that would be lost by just inserting a reverb as shown. Gain could be recovered by changing the split plate load feeding the PI tube, but the overall stage gain characteristics of the amp would change (for better or worse).

Another thing you could do would be to include a "mixing stage". Which is what most amps with reverb do. Since you're already sacrificing an input channel for the reverb recovery stage you can still do this adding only one tube. A 12dw7 (which has one triode like an au7 and the other like an ax7). If you use the au7 triode as the tank driver and the ax7 side as recovery you then have your amps 1/2 ax7 triode to use for the mix stage. I've built reverbs with the 12dw7 in this way and it works just fine.

Here's a thread where I helped someone add reverb to a 5g9. The full 'both triodes of a 12at7' was used in this one. But the same circuit could be built with just the low mu half of a 12dw7 driving the tank. And bada bing. You've only added one tube and it won't change the gain structure of the amp as it is now. I used a "virtual earth" mixer for this one and it's a good choice for you since it's almost invisible in the signal chain and it's low-ish impedance output would somewhat mimic the split plate load feeding the PI in the stock amp circuit.

https://music-electronics-forum.com/...-tweaks/52113-

[Chuck H]

If you wanted to try it your way, here is your schematic modified to work with what you propose. The overall amp gain will be about the same and the asymmetry of the clipping will be about the same (though opposite in phase). The preamp will not contribute clipping feeding the PI but the PI and power tubes would be the overdriven stages. I think the amp would still sound very similar regardless, except with reverb. It was necessary to alter some values to adjust for gain and wave form symmetry. How you derive voltage for the reverb circuit and how it is grounded will be important. Anyhoo... This is something you could try.

[Fletcher Munson]

Thanks Chuck. That's what I was looking for, but you've gone above and beyond. I really appreciate it.

In the mean time, while you were posting this I was working on the idea of incorporating reverb in the usual blackface manner. Here's a draft of that approach. We split the signal after the second gain stage, then join the wet and dry signals together at a third (mixing) stage. This should preserve the sound and feel of the 6G3 preamp and provide enough signal to drive the reverb. How does this look?

This design has one leftover triode (9 in total). So I could put the second ("Bright") channel back in, or add the leftover triode to the tremolo circuit to create a two stage tremolo similar to the 5G9 Tremolux.

This design should have a little more gain than a stock 6G3. I wonder if it would be useful to add a master volume, either before or after the phase inverter.

[Enzo]

Or just build a Deluxe Reverb amp and ditch the normal channel.

[Chuck H]

Well... I did some CAD stuff and what I saw was that there are problems with trying to preserve the preamp/power amp interaction of the original amp tone when adding a mixer stage. The reason is that the preamp clipping is quite asymmetrical and the response of the PI is tuned to match in the original design. Adding a mix stage inverts the signal feeding the PI and that makes the clipping look pretty horrible.

So... What's happening is that the original amp was designed from the preamp through the PI to provide an acceptably graceful clipping affect. Yes, these amps were not intended to clip when they were designed back in the day, but the designers of that time knew that moving gracefully into clipping was important for sound reinforcement amps. The mix stage inverting the preamp signal fouls this effort in the original design for the worse.

The only way around this would be to add yet another stage to reinvert the signal. Which seems ridiculous to me. Not to mention that the typical gain stage used as a mix stage adds considerable noise and distortions (not regular clipping, but other distortions that are not always desirable). So adding yet another stage for phase inversion is just a noisy, convoluted mess that probably wouldn't be very satisfying.

I hope I explained this well enough.?.

I would try the modified schematic I posted first and see if it doesn't offer up the goods. I think it will. And is the least intrusive way to the goal.

[J M Fahey]

If you want to "add" a reverb just ditch the old preamp and build a Blackface Reverb channel instead. Period.
Hey!!!! Thatīs what LEO did!!!

Or add the Reverb in the mix or as a standalone unit before it.

Or contact my friend Ronaldo Brito "Alien" in Brazil for one of his excellent Reverb pedals

https://www.youtube.com/watch?v=_H1-fvt7j88

[Fletcher Munson]

Hey Chuck, I can't thank you enough for your efforts here. My concern from the beginning was that inserting reverb into this circuit would have unintended / undesirable consequences. Your ability to model the circuit and make useful predictions is a huge time and headache saver. Again, thank you.

I understand - and largely agree with - those who say to just mod a Deluxe Reverb or "use a pedal." But I was asked by a friend if I could do this for him and that's why I've been exploring the idea. The heart wants what the heart wants.