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PostPosted: Fri Mar 19, 2021 3:31 am 
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Hi all,

I put together a reverb for the OSD based on the Fender Twin Reverb circuit. It sits in the effects loop and sounds pretty darn good, although there are some tweaks I'll need to perform to make it more useable. I don't have a finished schematic/BoM/etc for it yet, but I'll put them together of there's interest.

It uses a 12AY7 with parallel triodes to drive a Mod 4AB3C1B 2-spring long decay tank. Recovery is done with one triode of a 5751 tube; the other triode boosts the mixed wet/dry signal to match the level of the input signal to the output signal. It has two controls: tone (which allows me to brighten the wet signal but has no effect on the dry signal) and a level control, which determines the amount of wet mixed into the dry. The driver transformer is a MOJO776.

It's powered from B2 (approximately 300V) and drops that voltage with RCRC of 820R / 10uF / 22k / 47uF to give approximately 250V at the plates of the tubes.

Here's a video clip of me testing it out: https://www.dropbox.com/s/bwhdwpd7vbadhwk/IMG_6808.MOV

And some photos. Please let me know if you'd like more details.

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PostPosted: Fri Mar 19, 2021 9:45 am 
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I definitely want to be in the loop on this one. A couple of questions. How do you switch reverb in and out? I’m assuming you use your effects loop out and in to feed the reverb unit?

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PostPosted: Fri Mar 19, 2021 11:07 am 
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coco wrote:
I definitely want to be in the loop on this one. A couple of questions. How do you switch reverb in and out? I’m assuming you use your effects loop out and in to feed the reverb unit?


This isn't going to be a solution for everyone, but I think it could be a solid base to build on.

There isn't any switching at present - it just connects to the effects loop send/return jacks and is always in circuit because I don't play without at least a smidge of reverb at all times. Note that there's no buffer or anything on the effects loop, it's just the standard unbuffered loop between v2 (or v3 if using OD) and the PI.

This causes some issues. For example, due to the effects return coming after the master volume control it's impossible to stop bumps of the tank making sounds out of the speaker, even when the volumes are turned all the way down. It turns out not to be a problem in practice, but it's something to be aware of. The standby switch deals with the issue perfectly.

There's also some unavoidable noise added to the amp due to the tank/springs acting as an antenna. I've been super careful in my layouts, cable routing, ground loops, and shielding, but still if you're in a noisy environment it's a little noticeable. Normally my amp is dead silent - the barest hiss, no hum. With the reverb in circuit there's a slight increase in background hiss and, in the wrong room, some slight 60 cycle hum. It's less than my 5e3 style amp, but noisier than the OSD without reverb.

Turning the wet mix all the way down gets rid of the noise completely (i.e. it's quiet when 100% dry).

Then there's the issue of compatibility. Because I'm using an unbuffered loop, my reverb circuit is working with huge signal levels when the amp is cranked. For 100mV pk-pk at the amp's input jack it's easy to get 60V pk-pk at the effects loop send, and because my circuit outputs the same level as its input, there's no way to daisy chain other effects without adding some complex buffering / signal attenuation / boost. That's not an issue for me because reverb is the only thing I'm putting in the loop, but it might be a problem for others. I'm sure a slight redesign would be easy and could accommodate something like the Trinity Dumbleator-style buffer.

The pot that I'm using for wet/dry mix (250k) is too large. All of the useable reverb (for me) falls within the first few degrees of turn of the knob. I plan to experiment with 25k/50k/100k to see what gives the best range of mix.

I'll post more photos soon.

Finally, today I'll be in jam practice for 3 hours or so and it'll be cranked most of the time. I'll let you know if it behaves or blows up!


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PostPosted: Fri Mar 19, 2021 11:38 am 
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All very relevant design factors.
Looking forward to your post-jam report.
I have thought of using the Fx loop board to drive a reverb. That would be useful.

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PostPosted: Fri Mar 19, 2021 11:43 am 
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Regarding switching, I'm thinking it would be easy to just put a switch in the "bypass" hole of the OSD's effects loop. Job done.

And regarding the level mix knob, instead of swapping it for a lower impedance one I think I'll run a shielded wire from the reverb circuit box to the OSD chassis and put a 250k pot (or whatever value sounds best) in parallel with the existing level/mix pot. I could mount the new pot in a hole next to the EXT jack on the OSD chassis; that'll be much more convenient for dialing in the amount of reverb I want. Reaching into the rear of the amp is a bit of a pain in the ass.


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PostPosted: Sat Mar 20, 2021 12:24 pm 
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coco wrote:
All very relevant design factors.
Looking forward to your post-jam report.


The jam was a great success! Noise wasn't an issue, nothing burned to the ground, and it sounded great. We're trying to organize another session today and I expect no problems with the OSD/verb :D

I can confirm that it's a pain in the butt to have the level control inside the back of the amp, so I'm definitely going to look at ways of bringing it into a more accessible location. I want to be able to adjust it easily depending on room size/boominess/etc. I'll post updates on that when I get a chance to do it.

coco wrote:
I have thought of using the Fx loop board to drive a reverb. That would be useful.


Yes! One of my first ideas was to use the Trinity FX loop board with something like a Crazy Tube Circuits White Whale trem/reverb (real springs in a large pedal format) but in the end I wanted to have a go at using real old school tube-driven reverb. For those less inclined to shoehorn an entire reverb tank/circuit into their amp, a buffer/pedal is a very reasonable option that will sound fantastic.


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PostPosted: Sun Mar 21, 2021 3:50 pm 
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Very nice.

My only suggestion/input would be you might want to find a more positive-locking solution for your HV and filament supplies for safety reasons. Maybe even running those through some flex conduit screwed-in at each end? But there are other more elegant solutions as well that can give you a nice twist-lock or screw-lock plug/jack.


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PostPosted: Sun Mar 21, 2021 6:32 pm 
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Maybe use a push pull pot and pull to bypass and turn to adjust send or return.Put it in the bypass hole in the Fx loop.

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PostPosted: Sun Mar 21, 2021 7:10 pm 
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dtp wrote:
Very nice.

My only suggestion/input would be you might want to find a more positive-locking solution for your HV and filament supplies for safety reasons. Maybe even running those through some flex conduit screwed-in at each end? But there are other more elegant solutions as well that can give you a nice twist-lock or screw-lock plug/jack.


Thanks! And yes indeed. I can get some good mil-spec Amphenol twist-lock connectors off eBay for not silly money, which is what I'll do once I'm sure it's a keeper.

coco wrote:
Maybe use a push pull pot and pull to bypass and turn to adjust send or return.Put it in the bypass hole in the Fx loop.


It turns out there's a spare core/pin on the footswitch cable/connectors, so I'm going to run the reverb attenuation out to the footswitch. I don't believe it'll be noisy given the circuit design. As for switching, I think a simple toggle at the FX loop bypass hole should do the trick nicely.


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PostPosted: Sun Mar 21, 2021 10:40 pm 
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Amphenol! I was racking my brain trying to remember that name when I typed my response! :lol: Sounds like you are already thinking in that direction.

I'd probably remove the standby switch and put the reverb level knob in its place.


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PostPosted: Mon Mar 22, 2021 12:59 pm 
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dtp wrote:
Amphenol! I was racking my brain trying to remember that name when I typed my response! :lol: Sounds like you are already thinking in that direction.

I'd probably remove the standby switch and put the reverb level knob in its place.


That's actually genius because the switch can be relocated to the rear panel next the fuse, which is already connected to the switch!


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PostPosted: Mon Mar 22, 2021 2:52 pm 
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bushwhacka wrote:
dtp wrote:
Amphenol! I was racking my brain trying to remember that name when I typed my response! :lol: Sounds like you are already thinking in that direction.

I'd probably remove the standby switch and put the reverb level knob in its place.


That's actually genius because the switch can be relocated to the rear panel next the fuse, which is already connected to the switch!


Or you could just bypass the standby... it's a relic from the old days of extremely high voltages and current-inrush from radio broadcasting equipment and was just blindly copied-over by Fender and other early amp makers because... well they just basically copied their circuits directly from the tube receiving manuals of the day without asking any questions as to what & why - or they just copied Fender's designs.

But yeah, if you want to, next to the fuse would be a good place for the standby switch.


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PostPosted: Mon Mar 22, 2021 9:19 pm 
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Or get a 3 position switch to replace the power switch. OFF-STANDBY-ON like we have in the Triwatt.
And on the standby switch, agree with dtp, BUT , only for tube rectified amps which the OSD is not. Bit of a shocker to hit hi voltage on the plates before its warmed up. It is always debatable.

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PostPosted: Fri Mar 26, 2021 12:54 pm 
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Yeah a three-way switch would be even better if you want to keep the standby.

The main problem with standby switches are when you use them as a "mute switch" for extended periods - like between sets and such. Just turn the amp off.


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PostPosted: Fri Mar 26, 2021 4:42 pm 
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dtp wrote:
The main problem with standby switches are when you use them as a "mute switch" for extended periods - like between sets and such. Just turn the amp off.


:thumbsup: Couldn't agree more! I treat them as a soft start. I had plans to build a simple slow start circuit to delay switching on the HV stuff until 30 seconds had passed on the heaters, but decided it's a bad idea to add complexity just to offset my own impatience and laziness.


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PostPosted: Wed Aug 25, 2021 1:47 pm 
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I've got an update on the tube-driven reverb!

No matter what I did I couldn't get the noise down to an acceptable level, so I ditched the tube drive. I know, I know, that sucks. What doesn't suck is that I bought a Crazy Tube Circuits White Whale tremolo/reverb pedal, which is a real spring reverb in a small box. The reverb tank is driven by op-amps instead of tubes and works beautifully, silently, and sounds great.

White Whale: https://crazytubecircuits.com/white-whale

Image

I'm building a front panel for the ODS that incorporates all the controls on the White Whale and I'll modify it to run pots/switches to the panel while I keep the whale hidden in the bottom of the amp. Another mod I'm going to do is to re-use the Accutronics 4AB3C1B long 3-spring tank (it's the same impedance specs as the whale's short tank) and put the whale/accutronics tanks on a switch so I can toggle between them for alternative sounds.

I'll post pics as I go, but for those wanting a good reverb/trem solution (as long as you're using the Trinity Dumbleator effects loop buffer) the White Whale is proving to be excellent.


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