which reverb?!

[makepeace]

Hi guys,

So next project on the list is to build a stand-alone reverb unit a la the famous Fender, but I'd like to try a different circuit with a lower parts count and overall lower price.

What I have at my disposal is the power amp (which includes reverb) of a hammond L-100 organ and various other spare components such as caps and tubes etc.

So, I have a tank equivalent to the accutronics 4FB1B1C (so its 1475ohms into 2250ohms), a 14H reverb inductor (or so I have gathered from the schem), lots of caps, big and small, 1 x 12BH7 and a whack of 12AU7's and 12AX7's.

Does anybody have any idea's as to what they would build with such a collection of parts? I'd like to end up with a really versatile verb that goes from subtle to very over the top, with as much control over the tone as possible. Think Dick Dale and his Del-tones as a reference for over the top

Would it be a good idea to take the L-100's current circuit and adapt it a little? Ie blend pot to mix dry and wet, and a couple of controls for what ever is controllable?

Remember the intention is to have a stand-alone unit that will act as an "effect pedal" between the guitar and the amp.

Here's what the L-100's circuit looks like:



Thanks!

[Chuck H]

Well, since you have a power amp to go along with the reverb circuit already in the same chassis, you could build a reverb amp and stuff it in a cab with it's own speaker. This works great for a couple of reasons. One is that you can place it anywhere in the room for ambient effects. Another is that you can still crank the host amps up for distortion without garbling the reverb since the reverb drive can be taken from a host amps output. You can add reverb to ANY amp whether it's cranked or not.

[makepeace]

Thanks for the reply Chuck!

You raise a very valid point, and I will have to give that some thought..

My original intention was to use the rest of the power amp to build an AC15 (maybe even a 30, apparently the PT's used in these organs were massively overspec'd) sans reverb and trem. I already have one ECC83 out of two for the top boost channel and PI, a pair of EL84's and the EZ81 recto. Just need another ECC83, an EF86, a choke and a nice OT (maybe even another pair of EL84's if I decide to be going AC30 :O). I am also planning to build a stand-alone trem/phaser out of the L100 vibrato/chorus/phaser-machine, which I also have, and a preamp out of the percussion amp.

Any ideas on some other verb circuits that use a 12BH and AX7? I do actually have a couple of spare 6V6's, so I could probably get a fender replica going if i can find an OT. I'm not sure of the specs in the fender, but do you think the OT from the L100's power stage would work?

But yeah, I would otherwise prefer to stay away from the fender and build something "new", so to speak..

Thanks!

[Chuck H]

Here's what I built for a friend/custormer (not the sort of custom build I would do for any customer). It's really the bomb. But does need more parts to build. Parts that might otherwise be used for an AC15 build.

[makepeace]

Thanks for the schem! I think I'll definitely try that at some stage even if I don't decide to do it for this build.. looks super interesting!

Anyone got any other ideas?

Cheers

[Chuck H]

Thanks. It filches power from the host amp output to drive the pan. And it's adjustable for amps from five to a hundred watts. Just imagine a cranked up 5e3 with clean reverb. You just can't do it with the stand alone types because the reverb gets distorted by the amp. At about fifteen watts it makes so much reverb you can go beyond BF wet mix with a fifty watt host amp.

[makepeace]

Ok, for the moment I've decided to stick with the L100's reverb circuit (in general) seeing as its already on the board and all I would have to do is strip the excess power amp parts, sort out the B+, and put in a mixing circuit and i/o jacks. Also might make the first 12AX7 stage switchable as think there might be too much overall gain otherwise, which would tend to make things inconsistent.

Here's what its looking like so far:



First and foremost, please let me know if there is anything fundamental missing that might result in death, or worse: something that doesn't work

And now.. How can I start fiddling with the gain stages and things to change things like dwell, intensity, depth, etc? Anything you can with a spring-tank setup, I'd like to have a go at.

Thanks

[makepeace]

Ok, so I think I've worked out the "dwell" story. Correct me if I'm wrong: a pot is placed between the input and the drive stage in order to vary the power transferred to the drive stage.

Is that the right idea? So in this circuit the pot (a normal volume control) is placed between the 12AX7 triode and the 12BH7 drive stage?

[makepeace]

hi guys,

I've added some controls to the schematic namely dwell, tone and mix.



The dwell control was copied largely from the fender verb (1M pot instead of 250k)

The tone stack is courtesy of Mark Hammer: AMZ Stupidly Wonderful Tone Control 2

The mix was of my own device, as was the pot instead of the "gain resistor"

Criticism appreciated, and any other ideas for other controls, and other ways of implementing the ones I've got down there.. What about varying the cathode resistors of the stages to reduce/increase gain?

Cheers

[makepeace]

Anyone?

I've put the circuit together (much like in the last schematic except I've left out the dwell and tone for now, but the mix pot is in there as per schem above. For some reason things aren't working, but haven't had a proper chance at debugging yet. The sound that comes out is like a weak gated drive/distortion, no reverb what-so-ever.. Last I checked my unloaded HV's were 315V on plates of the 12AX's and 335V on the 12BH7.

Cheers

[Enzo]

You have the dry signal path and the reverb signal path both through the unit. DO BOTH have the same bad sound? Or just the reverb?

Isolate the problem. Look up "Signal Tracer" and make one up and use it to listen to the sound at each point in the circuit.

Disconnect the reverb pan, plug some music into the unit input jack. Now there SHOULD be nice signal coming out the SEND jack. COnnect it to some other amp for a listen. And connecting the units output jack to some other amp, input a test signal to the RETURN jack - doesn;t have to be a strong signal - and se if that sounds OK coming through that half of the unit.

Whenever I hear someone has built something not working, my first reaction is look for missing resistors or caps. Especially a missing grid ground return resistor.

[makepeace]

Thanks for the reply Enzo. You're right, I should definitely invest some time in building a signal tracer.

Alright, I discovered the problem and managed to get it to work! As usual, it would have to be a silly mistake: I forgot the connection between the plates of the 12BH7 and the following circuitry (choke, cap etc)

So I have super lush reverb and tons of it. Its got bit too much low end, but I hope that gets improved when I put in the tone control at the end. Any other ideas on how to get rid of some bass?

The mixer is not working how I thought it would.. When I turn it all the way to dry, (ie 0ohms between the "dry" part of the circuit - coming from the resistor network ending with R319 just before the reverb drive stage - and the output), there is still a lot of reverb in the signal going to the amp (way more than a subtle amount). Does this mean that I need to make my mix pot larger? At the moment I have a 500k there. I put a 470k resistor between the wet lug of the pot and the wet circuit, but thats not helped things greatly.. I've also tried moving the source of the dry signal to before the 12AX7 stage (ie at the input), which didn't help much either.

Also, I've managed to get all of the HV's within 10V to those in the schematic, which is great, so everything is biased how Hammond would have liked.

Cheers!

[Chuck H]

This should solve the adjustment and heavy bass problems:

[makepeace]

Awesome thanks Chuck. Will give it a try. regards

[Chuck H]

This should allow you to adjust for full wet or full dry. Adjust the resistor marked R308 up or down to achieve unity gain in the full dry setting. The resistor marked 318 is a very large 4.7M. I don't even know why it's there, but, since it is you can make some much smaller value (like 220k and down) if you find you have too much reverb overall.