ANOTHER Champ Vibro-Verb

[Spencer.sdk]

Hello all,

This is my first post, and I'd like to start by thanking everyone for the insane wealth of information on this forum. I've learned so much over the years-- just reading through threads has enabled me to answer all of my questions... so I've never really needed to post anything until now. I have many years of electronics experience; mostly restoring electronic test equipment and radios, and I've worked as a technician assembling electronics and boutique amplifiers... so I do know how to solder cleanly, and I have an understanding of the importance of lead dress; however, I have not yet attempted a scratch build. I'd also like to apologize for adding another "champ style with reverb" thread... but it really doesn't seem like there has been a conclusive build on this yet, and I'd like to narrow it down in one thread from start to finish!

I'm hoping to avoid rehashing any discussion on why on earth one would want to build a SE amp with tremolo and reverb (as there is in all the other threads)-- I just want to. Basically, I've narrowed down three designs, and I'm just hoping I can get some input on which seems to have the best chance at sounding half decent... and maybe any recommendations to improve the most viable design. The goal is to get reverb into a champ style amp, while keeping it as close to the tweed champ tone as possible. So, basically... I built off of a vibrochamp design, altered the power to supply to provide what I think will be slightly lower plate voltages, and added a "tweed" cap to the tone stack.

The three differences are just how the reverb is bypassed and mixed.

Version 1: uses a supro style reverb mix and an additional gain stage prior to the reverb driver. I'm not sure enough dry signal will make it through both the tone stack and reverb mixer. (Thinking about tapping the reverb prior to the volume and adding a dwell control... though I'll be space limited. I do like the possibility of 100% wet reverb, like a silvertone...just for fun).

Version 2: From research, this seems to be the design that might be most successful. Essentially, it's a SE Princeton reverb. I think this will significantly change the tone away from the tweed champ, and may provide too much overdrive... may need to reduce the input to the reverb mix; however, I feel like this has the best chance for success with minimal rework.

Version 3: Reverb bypasses the tone stack. Leaves an unused triode, though I'm wondering if the 12AT7 reverb driver could benefit from another gain stage prior.


I have the layout worked out for version 1-- it's possible to leave space to try all versions in the same layout. I'll share that after any discussion on the schematics. Also, this is my first go at using Jschem, or with drawing a complete schematic... so please be kind, and please both forgive and correct me on any convention errors!

Thanks for everything!
-Spencer

[Dave]

I'd use the vibrochamp scheme with Tweed Champ preamp and insert reverb between the preamp output and the phase inverter. Typical AB763 type tremolo scheme running through a 2.2m resistor. If you find that its too much reverb, or the tone is too affected by the reverb, you can decrease the 2.2m to whatever winds up sounding the best in terms of tone and reverb depth / balance.




Or… you could build it like a single ended Princeton Reverb but make the preamp look like a Tweed Champ. Either way of looking at it pretty much gives you the same thing.


Or… you could look at Tubenit's one tube reverb. You can find it all over the forum and in the library of information. Personally, I like to have big, thick, reverb on hand if I want it, and Tubenit's reverb isn't quite enough for me in that regard, but it seems to be for most people.

Dave

[Spencer.sdk]

I'd use the vibrochamp scheme with Tweed Champ preamp and insert reverb between the preamp output and the phase inverter. Typical AB763 type tremolo scheme running through a 2.2m resistor. If you find that its too much reverb, or the tone is too affected by the reverb, you can decrease the 2.2m to whatever winds up sounding the best in terms of tone and reverb depth / balance.



Or… you could build it like a single ended Princeton Reverb but make the preamp look like a Tweed Champ. Either way of looking at it pretty much gives you the same thing.
Dave

So, the version 2 schematic I posted above? AB763 uses a 3.3M reverb mix, and there is no PI in my build-- it's single ended-- that's kinda what makes this a challenge.

Thanks!

[Spencer.sdk]

And yes-- I'm also not a fan of the single tube reverb.

[Dave]

there is no PI-- it's single ended-- that's kinda what makes this a challenge.

Oh yeah, got a little ahead of myself there. I'll get back with you in a minute as soon as I finish punishing myself for that.


Dave

[Dave]

Yeah, so... I have never done it before, but if I were going to build it on a whim, I would use the Vibeochamp scheme and insert the reverb at the high side of the volume pot. This should give you the 100% wet you wanted.


Dave

[Spencer.sdk]

Yeah, so... I have never done it before, but if I were going to build it on a whim, I would use the Vibeochamp scheme and insert the reverb at the high side of the volume pot. This should give you the 100% wet you wanted.


Dave

So... you're recommending something like the version 3 schematic above then? 100% wet is not a necessity-- I'd prefer whatever gives this amp the best overall tone. But if it's this reverb mix that does it, the benefit is the possibility of 100% reverb signal when I feel like going into orbit. 

[Dave]

Well, that's probably fine too, but no that's not what I was saying.


If you hook up a standard Fender style reverb through a 3.3m resistor at the high side of the volume pot on a Tweed Champ circuit, you should get reverb that disregards the volume control. So, even if you turn the volume to zero on the amp, you should still have however much reverb you originally dialed in.


That could get irritating always having to adjust the reverb everytime you adjust the volume, but I was thinking about you mentioning that you liked the idea of 100% wet.


Dave

[Dave]

Actually if I were choosing from the 3 options you have schematics for, I would pick number 1. But, number one with the reverb tapped on the high side of the pot would give you 100% wet without the need for the mix control.


Dave

[Dave]

You know what, I think I got too wrapped up in what you were saying in your post and not what you posted in your schematics. I apologize. Too much coffee today.


Here's what I really think. Version 1 looks like the best version, to me, to get you where you say you want to be. Again, if you think there is too much tone sucking going on, you can always reduce the 3.3 / 2.2 (470k on your schematic) resistor and find an acceptable balance.


Dave

[PRR]

> avoid rehashing any discussion on why on earth one would want to build a SE amp with tremolo and reverb

It makes musical sense. It is awkward product engineering. A proper reverb (gain, driver, RT, recovery, mixer) is nearly a whole extra Champ inside the box. Will a small-amp customer pay a big-amp price?

I won't even mention that crappy reverb and trem pedals are now about good enough to use. Certainly better than 99% of your audience cares. (What audience?)

You are injecting LFO inside a NFB loop. NFB will work to stabilize gain, reduce trem action. Not-so-much with the small NFB of a AA-Champ power stage... but it's just wrong. Has this been demonstrated happily?

The stock AA-Champ has just-enough gain. Will the added loss of a reverb mixer make it hard-work to play?

Cathode injection is the last resort of a cornered designer. While we can still get photo-cells, why not use a roach?

The reverb path bypasses the main volume control. Seems to _me_ that every change of VOL will want a change of REV too. That's not how it is normally done. I see 100% Wet is a goal. Unless you have very strange arrangements, seems like a job for a Dry-Off switch.

[Spencer.sdk]

PRR--

I think maybe you only saw the version 3 schematic? It is the only one where the reverb bypasses the tone stack and subsequently, the volume control. You're correct-- it's not ideal, as any change in volume requires a change in the dwell control to maintain the same mix-- but it can actually be kinda fun to play around with. This is how the reverb works on a Sears Silvertone, albeit, the stock reverb is a joke on those... but I modded one with an AB763 reverb (as I'm using here) and it sounded great (and was fun to play around with the 100% wet reverb, though not particularly useful).

Version one and two of this build tap the reverb after the volume, and the difference is the reverb mix network and the application of additional gain after the tone stack. 100% wet reverb is NOT the goal, though I had also considered a dry off switch for fun. The versions are numbered in the order I -think- will work best, so the one you may be referring to is my last choice.

As for the LFO-- that's exactly where the signal is applied in the Vibrochamp. Maybe it's not ideal, but it sounds quite good on those. I prefer not to use an octocoupler here to keep the circuit simple, and I just like the way the vibrochamp sounds.

Anyway, I'd still prefer not to discuss why on earth anyone in their right mind would want to build such a ridiculous amp... it's not for sale, it's for me, and I just want to do it! Also, my audience is me... so... I care 

Dave--
Thanks, and no worries about the coffee! As for the 470k mix resistor; that is the value used on the supro SE reverb, which is tapped after the tone stack... so I decided to start there. However, there is more attenuation in the fender tone stack (though, the .022 "tweed/VC" bypass cap somewhat solves that problem... so I may decide to make that a permanent installation rather than have it be switched) so I'm worried that not enough dry signal will pass-- and reducing it too much will cause feedback. So we'll see.

[PRR]

> I'd still prefer not to discuss why

I am sorry to have offended you.

[Spencer.sdk]

PRR--

Not offended at all! I would also like to say, I appreciate your input-- I have learned a lot from reading many of your threads.

It's not that I don't want to explain my reasoning for wanting to build such an abomination; it's just that the other threads on the same subject have devolved into discussions on why it's worth doing it...as we're already doing about 10 comments in. I honestly can't really explain why I want to do it, I just do... and I'm probably not going to be easily talked out of it.

Anyway, I've done a lot of homework on this build already, including drawing three schematics and a layout... so I am aware that it's a ridiculous amount of hardware for a 3-5 watt SE amp.

SO, that being said... for what I want to do, I'm just hoping to get an opinion on which of the three designs might sound the most like a champ with reverb and tremolo, and what tweaks/redesign might help the better option get there. I also understand that it will be impossible for it to sound exactly like a champ with the reverb... I'm shooting for in the ballpark.

Anyway, thanks for the input and I'm not that easily offended. I was little offended by the audience comment though. 

just kidding