Reverb brick install in fender circuit

[Apexelectric]

Working on a new project that needs reverb but I dont have any room for a tank, transformer and extra tubes to install a conventional reverb in the chassis. I figure I’d give the brick a try. I purchased a Spring reverb PCB from BYOC to keep the component install simple and compact and used their circuit design. I took the 6.3V filament supply and ran it trough a bridge rectifier and a 4700uf filter cap to get about 8.5V for the supply. Thought about a voltage doubler but the opamps should function ok at 8.5V. The reverb PCB has the rest of the power supply for the brick.

My big question is where and how to tap and insert it in the circuit for the best performance. Having a ton of reverb is not a necessity but getting the most out of the tone would be the priority.

The circuit I’m putting it in would be similar to a Brownface Fender with the harmonic vibrato except with a 6V6 output.

[sluckey]

I would put it just prior to the PI input coupling cap.

[Apexelectric]

So input and reinsert in the same area? Recommendations on a Reverb input coupling cap and resistor value between the two tap points?

[sluckey]

No need for a RC between the two tap points. But you need a cap (.047@630V) between the preamp and the input to your circuit. No need for a cap between your circuit output and the PI input cap.

[Apexelectric]

Let me clarify a little more. I was thinking about running the reverb parallel to the dry signal rather than in series so not like its intended design. Didn’t plan on having a switch like the pedal usually has. Wasn’t sure if we were on the same page.

Thanks for the help!

[sluckey]

Let me clarify a little more. I was thinking about running the reverb parallel to the dry signal rather than in series so not like its intended design. Didn’t plan on having a switch like the pedal usually has. Wasn’t sure if we were on the same page.

Thanks for the help!

Well, we ain't on the same page. My comments were based on the schematic you posted and the schematic of a 6G8A amp. Sorry.

[Apexelectric]

So do you think the typical Fender AB763 insertion method with a 3M resistor between the two points would apply to this application?

[2deaf]

The BYOC kit is a guitar level device, both input and output.  For an insertion in the amp signal chain you would have to reduce the amp signal for the input and then boost the signal that comes out of the output.  This is just an effects loop that is hard wired internally to only one effect with no jacks for external connections.

[2deaf]

So do you think the typical Fender AB763 insertion method with a 3M resistor between the two points would apply to this application?

It could be done.  Seriously attenuate the signal going to the input and hard wire it to R101/R1 while eliminating the switch.  Up R5 and R6 to 10K and up C3 to .022uF.  Change VR1 to 100K and use the wiper to connect to the AB763 circuit the same as the wiper on the reverb control in the AB763 does.  Connect a foot switch to the top of VR1 if so inclined.

Might have to do something about IC1b to keep it from going nuts.  Maybe remove R10 and replace R13 with a piece of wire.   

[Apexelectric]

Ive got no problem starting from scratch if it makes more sense. Just thought the pcb would be helpful but I guess I didn’t consider the signal level. I could possibly remove the first gain stage and it’s associated components but maybe I’ll start over.  It’s my first time using the brick so I was looking for a little guidance from someone who has used it in a build and had some input on what to avoid.

[tubenit]

I dont have any room for a tank, transformer and extra tubes to install a conventional reverb in the chassis

IF space is the concern, what about a Metro Zero Loss FX loop & then running a reverb and/or delay in the loop?

https://www.amazon.com/s?k=zero+loss+FX+loop&ref=nb_sb_noss

With respect, Tubenit

[Apexelectric]

Guy I’m building it for wants it to be onboard. But I will be putting an effects loop in it.

[TIMBO]

Hey mate, You could wire it in between the input jack and the grid resistor of the first triode, if you are going to use the pedal kit this will work.

[Apexelectric]

Guess I’ll try a few ways to implement it and see what works best. Thanks

[sluckey]

This circuit works in front of an amp and also in a FX loop.

[Tony Bones]

Executive summary: With an 18VDC supply and the right resistor choices, you can use the BYOC PCB anywhere in the signal chain where the signal is no more than 9V peak.

The brick itself will accept up to, but not more than 1.5Vpeak signal (3Vpp, 1Vrms.)

In the BYOC circuit, the signal is boosted by a factor of R4/R3 = 22K/10K = 2.2 before entering the brick  (IC2a). R14 and R15 provide the same amount of attenuation attenuation at the output. So maximum input signal to the stock circuit is 1.5/2.2 = 0.68V peak. You can increase that by using different values for R4/R3. But, be aware that the signal the opamps can handle is about 1/2 the supply voltage. With 9 volt supply, 4.5V peak. With a 12VDC supply, 6V peak. 18VDC, 9V peak. You can put the BYOC circuit anywhere in the signal chain you want, provided the signal doesn't clip either the brick or any opamps.

If you change R4 and R3 then you need to change R14 and R15 to match. If you need IC2a (R4, R3) to have a gain less then 0.5, then you can change R10-R13 to recover some gain in the output opamp (IC1b.)

[2deaf]

In the BYOC circuit, the signal is boosted by a factor of R4/R3 = 22K/10K = 2.2 before entering the brick  (IC2a). R14 and R15 provide the same amount of attenuation attenuation at the output.

R14 is 220 Ohms and R15 is 100,000 Ohms.

[Tony Bones]

R14 is 220 Ohms and R15 is 100,000 Ohms.

You're right. I saw what I wanted to see, not what was there.

IC1b can provide boost if the signal is attenuated before the brick.

[Apexelectric]

Thanks Tony, that’s the general idea I had in my head but much less specific. I figured there was a way to insert it in different areas, and in different ways, successfully. Appreciate the tech info.

See any issues with the power supply just being the bridge recto and a 4700uf filter cap fed from the 6.3V filament supply?

[sluckey]

You still need to understand that the reverb path and dry signal path are already in parallel on the pcb circuit. You don't need any 3.3M||10pF caps for attenuation and mixing. You don't need any additional tubes. Just break the signal path of a working amp and insert between input jack and grid of first preamp as Timbo suggested, or insert just prior to the PI input cap as I suggested (using coupling cap to the input of the pcb circuit. Either way, be prepared to do some fine tuning.

Is this all still an idea? Got anything on paper yet? Already built the brownface amp?

[Apexelectric]

All in the planning stages right now. Testing the reverb circuit first in another tube PA to get the effect dialed in. Trying not to blow up anything that I’m not willing to loose. Once I have the reverb worked out I can get to the design stage of the amp an planning on where it will fit in the chassis I have to work with. This is all going in an old Bell Sound Systems PA head that I’m converting. Going to gut it and leave just the transformers.

[2deaf]

If you change R4 and R3 then you need to change R14 and R15 to match. If you need IC2a (R4, R3) to have a gain less then 0.5, then you can change R10-R13 to recover some gain in the output opamp (IC1b.)

Changing R10-R13 changes the gain for the signal injected at the non-inverting input of the IC1b mixer.  As drawn in the BYOC schematic, the gain for the non-inverting input is 2 when the wiper of VR1 is grounded.  The signal is cut in half by R11-R12 and then doubled by IC1b, so there is unity signal going into IC1a and coming out of IC1b when the reverb control is turned clear down.  The gain for the non-inverting input jumps to 4.67 with VR1 grounded if you change R13 to 100K.

The gain for the non-inverting input of IC1b decreases as Vr1 is increased, but the reverb signal increases so that the overall combined signal is about the same throughout the range.
_____________

The input of the BRIC is rated at 1.5V_p, but I get some crackle above 0.4V_p.  The output is hard to quantify because it is all over the place and it has some resonant peaks. 

The input impedance is rated at 10K, but it tests out closer to 5K.  In any event, you better drive it with a low impedance source.
   

[Tony Bones]

See any issues with the power supply just being the bridge recto and a 4700uf filter cap fed from the 6.3V filament supply?

Even with Schottky diodes you're likely to get less than 9VDC. It might work OK if your signal levels are small.

[Tony Bones]

The input of the BRIC is rated at 1.5V_p, but I get some crackle above 0.4V_p.  The output is hard to quantify because it is all over the place and it has some resonant peaks.

That's interesting. I probably would have tried to use as much of the 1.5V as reasonable without clipping. This changes things. 

[Apexelectric]

Been having a rough time trying to get this to work in circuit. Effect is working but there is no volume. When the reverb circuit ground is isolated from the amp ground the volume comes back but the effect does not work.

I have it battery powered at the moment to eliminate the power supply as an issue I also have the input and output jumpered so there is no switch in the circuit.

It is inserted after the tremolo and before the PI as if it was in an effects loop.

Effect works fine as a stand alone pedal in the front end but that is not what I’m after. Any help would be greatly appreciated.

[Apexelectric]

Anyone willing to take this on?

[glass54]

Hi Apexelectric
Please be aware that if you're using the "standard" Heater wiring for a 6G8A, the bridge rectifier and 4700uF cap gives you approx +/-4Volts due to the centretapped heater supply. (I have used this successfully on many occasions  ).
Would you mind awfully posting the actual amplifier circuit as it would make analysis a little more rewarding.
Kind regards
Mirek

[Apexelectric]

I don't have a schematic drawn out, just the layout. Hope its enough to give you an idea.

As I mentioned, this is a solid state effect pedal, adapted to work in a tube amp as if it was run through a passive effects loop. I did wind up adding a .001 cap before the PI input not shown on the layout.

[glass54]

I see a few issues. (…and at the expense of some rantings  )
The rectified heater supply will only give you approx 7.7 V DC (floating) or +/-3.8V WRT Earth, assuming 0.6V drop per diode (Tony has already flagged this issue). I would suggest a doubler (minimum) and possibly a Tripler for the Positive supply. If you’re really clever you could use a DC-DC isolated converter  . Your power supply MUST have integrity in regards to proper earthing and sufficient voltage/filtering to run the Reverb module. I am assuming a 6G8A type circuit, as this is the best guide you have given us.
Ideally, as Sluckey and others have advised, the optimum insertion point is before the PI (reply #1, #3, #5 and 2deaf #7 and #9, etc) but we have a problem. Obviously the pre PI signal levels are excessive (up to a max of 5Vp-p) entering into the Reverb Module.
So now my difficulty with the Reverb module. It uses TL072 which are fine for line level signals when you want to save current consumption and driving peak indicator LEDs  . You can be quite successful running off a +12V supply modified correctly for 1/2V. Even more successful with Bipolar (say +/-12V) supply and this eradicates earthing issues with Reverb Module, but the module would have to be modified for a Bipolar supply.
This Reverb module uses IC1a as an input buffer, then followed by IC2a (gain as indicated by 2deaf) and processed by BTDR-3 stage which all add up to extra noise. You would need to attenuate the preamp input signal thus lowering your signal to noise ratio, then processing the signal prior to summing by IC1b. Tony has gone to considerable length to as to why you need a substantial power supply rail to maintain your dynamic range (signal from preamp to PI) and I am trying to encourage a good signal to noise ratio.
The TL07x family are not highly regarded for low noise applications and thus while the unit would work on the input to 1st tube, I would not use it for 2 reasons, the extra noise generated due to TL072 and most important you’re TUBE AMPLIFIER becomes a Hybrid!  The reason people use TL072’s is for low current consumption and portability generally at the expense of noise and when used with 9V supply or less at expense of dynamic range. You are not limited by low current so is this a good choice or do you modify the power supply capability on the Reverb unit?
You have already indicated issues with built in power supply, thus using battery power and from what you have described, there appears to be some problem with earthing and integration of the Reverb module into the chassis.
My final thoughts: To maintain the integrity of the Tube Amplifier concept, I would encourage you to put your energy into fitting a reliable effects loop. This would maximise your opportunity to do “pedal effects” without being stuck with a possible built-in “dog of a Reverb” and the signal path of the tube amplifier would have the utmost integrity. With all due respects, I’m with Sluckey and Tubenit on this one. And you are halfway there, ie output on the junction of 220k and 22k after the 0.022uF on the main board! Just trying to encourage you  .
You really need to “get it on paper”
(Buy the way, I have experience with a few Marshall DFx modules and the joys created  )

[pdf64]

I suggest not to use heater supplies for powering solid state circuits unless the heater winding has a CT that has a solid direct connection to 0V. Power tube shorts can cause balancing resistors to fuse and the whole heater circuit then gets pulled up towards the HT voltage; which obviously may damage the solid state circuit.