Kay Model 777 Reverberation

[dbishopbliss]

I got this cool little reverb off of CL today. I plugged it in and it sound REALLY good. Not quite like a Fender Reverb. More plate-like than the drippy surf spring reverb sound but I dig it.


I'm wondering if it is safe to use...

• I'm thinking that I should probably add a 3 prong power cord but maybe that's not necessary.
• There doesn't seem to be any sort of fuse... maybe that's bad.
• The power is turned on using a switched pot. Anything to be concerned about?

There doesn't seem to be any information about these boxes our there. The circuit is different from the Model 710 that in the schematics library. What are your thoughts?


Thanks!

[dbishopbliss]

One other thing... if you plug the cable from the footswitch into you amp (yes, you read that correctly) it sounds like an overdrive/fuzz pedal. It's a cool effect but I'm not sure why it does that. Any thoughts?

[Williamblake]

Thanks for sharing. Would you mind measuring the tanks resistances? What voltage is the power supply?

[AlNewman]

Could be safe to use, but it wouldn't damage it to install a grounded cable and fuse.  May create a ground loop though, hard to say.

One other thing... if you plug the cable from the footswitch into you amp (yes, you read that correctly) it sounds like an overdrive/fuzz pedal. It's a cool effect but I'm not sure why it does that. Any thoughts?

Don't plug the amplifier into the pedal plug.   

[dogburn]

Since it's solid state, you are not going to be dealing with high voltages internally, so maybe you don't have to worry about the cord and a fuse. Measuring the secondary voltage on the power transformer would give you an idea what you are dealing with.

It does look intriguing and would make a fun project. The schematic gives most of the needed details, except for the transformer voltage and the reverb input/output impedances. Could you make a sound clip so we could hear what it's like?

[Lectroid]

Nice. It’s kinda like Fender’s reverb if you imagine triodes in place of the transistors.  And squint. 

Thoughts:

What they called “Depth” looks more like Fender’s Dwell control?  There isn't an Intensity knob like Fenders, off the recovery side.  How would you add Intensity to this circuit?

The tank output goes straight into the mixer TR2, so no dedicated reverb recovery stage at all?

The common Reverb foot switch works by tapping the reverb signal straight to ground, to turn the reverb OFF.  Pretty accepted way to do it.

But not as a built-in fuzz.   I'm with AlNewman on this--I wouldn't do this to my amp. The reverb signal on the foot switch jack has already been amplified by the reverb circuit.  If you can hear the added distortion so clearly, you are likely pushing a significantly hotter signal into the amp than what it's designed to deal with.

Good luck with the project. Let us hear some sounds.  And can we get the B+ "reservoir" voltage, and the resistances of the input and output coils on the tank?  Some of us have no life and we're bored.         

Thanks

[dbishopbliss]

Thanks for sharing. Would you mind measuring the tanks resistances? What voltage is the power supply?

When I measure from the green wire on one side of the tank to the green wire on the other side I get about 300R (298R).


The secondary of the transformer measures 16.3V.


I will work on some recordings later on.

[dogburn]

Thanks for the data! For measuring the pan, could you measure the DC resistance between the green and black wires on one end, and then between the green and black ones on the other? That would allow us to get a good guess on both input and output impedance.

I am really tempted to try to build a version of this someday - I've got a SurfyBear Metal, which is great, but it would be nice to have the option of another type of spring reverb.

[dbishopbliss]

Thanks for the data! For measuring the pan, could you measure the DC resistance between the green and black wires on one end, and then between the green and black ones on the other? That would allow us to get a good guess on both input and output impedance.

I wasn't aware that the resistance would change with the power on. I guess that is because those are tiny inductors not resistors:


Rounding up, 160R on one end and 300R on the other.

[dogburn]

Thanks - that's really helpful. Looks like an FB reverb pan (like 8FB3C1B) would be compatible with that circuit for someone wanting to try to build one.

[dbishopbliss]

Thanks - that's really helpful. Looks like an FB reverb pan (like 8FB3C1B) would be compatible with that circuit for someone wanting to try to build one.

What is an FB reverb pan?
Also, how did you determine that it would be compatible?

[tubeswell]

Thanks - that's really helpful. Looks like an FB reverb pan (like 8FB3C1B) would be compatible with that circuit for someone wanting to try to build one.

What is an FB reverb pan?

[dbishopbliss]

I made a video with some sound clips for people to check out.

[tubeswell]

Those are all PNP transistors, so when you try to fix anything (including the FWB rectifier diodes and the filter caps etc), remember its positive ground.

[dogburn]

The "FB" refers to the Accutronics codes for the input and output impedances of their reverb pans, which you can see in the chart Tubeswell posted. You can figure out what the closest match is via measuring the DC resistance on both ends of the pan and cross-reference on the chart. The standard pans used in classic Fender tube amps are the AB type, typically something like 4AB3C1B (some of the other numbers can vary depending on decay length and mounting position).

And thanks for the video - it's definitely different than the classic drippy Fender 6G15. More restrained and maybe a bit darker.

[dogburn]

While poking around on the forums, I came across this thread, which is worth reading through:
https://el34world.com/Forum/index.php?topic=29352.0

It's about an earlier Kay Reverberation unit (710), but has some similarities with your 777, and the guy did end up finding an FB reverb pan worked well.

[dbishopbliss]

I saw that. I don’t think there is anything wrong with this unit. I was just wondering if it needed any updates. If I recall Kay made a few widowmaker amps but it seems like this one is fine.

[Williamblake]

Would somebody in the know mind commenting how the circuit works, please? As far as i can guess left TR1  2N2613 is an input buffer, R6 is the mixing resistor, right TR2 2N408 amplifies the signal going to the reverb tank and left TR2 is an output buffer driving the tank. TR1 right amplifies the mixed signal driving the units output. So connecting your amp to the footswitch jack lets the signal driving the reverb tank drive your guitar amp. Is this really a distorted signal or is the distortion happening in your amplifier input stage due to very high signal level i wonder. Also you should have trails (reverb output while turned off with the foot switch) albeit the reverb tank does not seem to have that much decay. Begging to stand corrected.

[dogburn]

I'd also like to know more about how this circuit is designed. Also, in tracing the signal path, it looks like C2 might be drawn backwards - should it be the other way around?

[SEL49]

The attached pic should explain the circuit. I relabeled the transistors to make more sense. I believe all the electrolytic coupling caps are drawn correctly.

[dogburn]

The attached pic should explain the circuit. I relabeled the transistors to make more sense. I believe all the electrolytic coupling caps are drawn correctly.

Thanks for that - definitely helps with understanding the circuit. Would there be any way to tweak it to make the reverb stronger? It seems quite mild in the sound clips, so an increase in depth/intensity might be useful - though it doesn't have to be Fender 6G15 levels.

[dogburn]

I'm guessing changing the value of R7 would influence the amount of reverb, but I could be way off.

[SEL49]

Increase R6 and/or decrease R7 should give more reverb. But don't expect too much from this unit. It's probably already all it can be. You gotta remember that Kay was a very low end product.