So I have been given free reign to gut a Blues Jr and do whatever I want with it

[mort]

So if I use a Fender reverb circuit like from a deluxe reverb, can it go between the the treble pot and the PI? Just stick it in there with a 10pF cap and 3.3M resistor like on the Fender?

[Leo_Gnardo]

So if I use a Fender reverb circuit like from a deluxe reverb, can it go between the the treble pot and the PI? Just stick it in there with a 10pF cap and 3.3M resistor like on the Fender?

[ATTACH=CONFIG]42918[/ATTACH]

Referring to your schemo in #60, if you do it like that you won't have the reverb volume "follow" that of the volume control. Have another look at the Fender schemo, the reverb drive comes after the volume control. Some early Gibson amps didn't do like that, their reverb isn't proportional to the volume control setting, you have to adjust volume & reverb independently. OTOH you can leave the volume off, have reverb only and sound like you're at the other end of the tunnel, if that's your thing.

[mort]

So how about inserting the reverb between the volume control and PI?

[Leo_Gnardo]

So how about inserting the reverb between the volume control and PI?

That's more like it. Again look at what Fender did, you're going to have to add a couple of tubes, and doubtless the noise floor's going to increase in your amp as you will have to follow the lossy mixer with a gain stage. You might be able to squeak away with a single tube, say 12DW7 or ECC832 (one triode like a 12AX7, the other like a 12AU7) at the cost of having less drive signal to the reverb tank, sending a signal right off the volume control wiper, thru a small cap to the current amp triode (AU side), and from the same wiper to your 3M3 mix resistor. It's possible to vary the mixer resistors to balance dry vs reverb too. Gonna take some experimentation if you don't follow the Fender formula.

[mort]

My thought was to add a whole 12at7 to drive the reverb tank and use the other half of the 12ax7 cathode follower as the reverb recovery. That sums to EF86>1/2 12ax7>12at7>1/2 12ax7>12ax7 PI(or 12at7)>EL84 x 2

Sound about right?

Or am I still going to need that triode following the mixer?

[Leo_Gnardo]

My thought was to add a whole 12at7 to drive the reverb tank and use the other half of the 12ax7 cathode follower as the reverb recovery. That sums to EF86>1/2 12ax7>12at7>1/2 12ax7>12ax7 PI(or 12at7)>EL84 x 2

Sound about right?

That may well work. Again without that extra gain stage following volume control, you may not be "spanking the tank" the way Fender does. Maybe just as well, it usually sounds too much for me besides how many Fender players say "I can only turn the reverb control up to 2, 2 and a half, then it's too much." Just about all of 'em. One other thing's a little bit of a worry: crosstalk between the triodes in your 12AX7 might result in some squeeky feedback. If you encounter that, you might need to reduce hi frequency bandwidth with a disc cap or two in parallel with its plate resistors.

[mort]

Ok so here's the complete circuit rough draft. See any glaring problems?

[mort]

Made some changes. Added the gain stage just before the PI and dropped a triode out of the reverb driver.

Thoughts?

[g1]

If you have to use a single triode to drive the tank, then it would be a good idea to at least use a 12DW7 (like Leo spoke of in post #64) for better drive.

[mort]

I just might do that

[mort]

Just to update this thread, the amp got built using the last drawing and has been in the backline at a local blues club getting hammered on several times a week for about the past 8 months and hasn't had any issues. The reverb did end up being kinda weak but gets reverby enough for most players. I think I'm gonna ask the owner if I can pick it up and mod the reverb for more output and start making that amp as an "original" design.

[dchang0]

Thanks for the update.
BTW, I see from the last schematic you posted that you chose 500pF for the coupling cap into the reverb circuit.
I tried various values in that one spot and got a pretty big difference in tone, with some values considerably meatier.
You might try testing various values before messing with the rest of the reverb circuit.

[mort]

Well here’s an interesting problem...

I went to pick up the amp last night after the jam and the sound guy tells me that when you turn up the reverb it feeds back like hell. I get it home, dime the reverb and give it some volume and it plays fine.

The tank is bolted to the bottom of the cab and not in a bag. Seems the obvious next step is to put it in a bag. Is there anything circuit-wise to consider or is it probably as simple as giving it some vibration isolation?

[Leo_Gnardo]

The tank is bolted to the bottom of the cab and not in a bag. Seems the obvious next step is to put it in a bag. Is there anything circuit-wise to consider or is it probably as simple as giving it some vibration isolation?

Yes, a "tank sock" will help, but just a little. It also helps to cover the open side of the tank's metal box with a layer or 2 of corrugated cardboard you can razor out of a castoff shipping box. The rubber grommets and rivets supplied as tank mounts do nothing at this point - remove and discard them. Oh wait, keep the grommets if they're made of whitish translucent silastic, they may come in handy for some other thing later (says the guy who never throws anything away.) In extreme cases of tank howl, I've mummified the steel box with a layer of wide weatherstrip foam. Home Despot carries a weatherstrip that's intended for sealing the gap between pickup trucks and camper tops. It's about 1 1/4 inches wide & 1/8 inch thick, long enough to take care of several reverb tanks and/or other projects where it comes in handy. And under $10, what a deal. Cover as much of the metal reverb box as you can, also the cardboard cover, stick the whole shebang into a tank sock, then put a screw thru that assembly at each end. Now you've done as much as you can to mechanically decouple the reverb from the speaker cab.

Also, there's no harm in reducing the maximum reverb signal by padding* it before it hits the reverb volume. I know, most folks are used to dialing the reverb up to 3 or 4, and there's no point in going further because that puts you at the far end of the tunnel, why go there. If you pad the reverb so that 10 is just a tad more than any sane person could possibly need, there's only one thing that can go wrong with that: complaints from the folks who are used to dialing it up to 3 and assume there's something wrong if you have to go beyond that.

*Reduce the max signal available at the reverb pot by wiring a resistor in series with its signal terminal.

[mort]

Yes, a "tank sock" will help, but just a little. It also helps to cover the open side of the tank's metal box with a layer or 2 of corrugated cardboard you can razor out of a castoff shipping box. The rubber grommets and rivets supplied as tank mounts do nothing at this point - remove and discard them. Oh wait, keep the grommets if they're made of whitish translucent silastic, they may come in handy for some other thing later (says the guy who never throws anything away.) In extreme cases of tank howl, I've mummified the steel box with a layer of wide weatherstrip foam. Home Despot carries a weatherstrip that's intended for sealing the gap between pickup trucks and camper tops. It's about 1 1/4 inches wide & 1/8 inch thick, long enough to take care of several reverb tanks and/or other projects where it comes in handy. And under $10, what a deal. Cover as much of the metal reverb box as you can, also the cardboard cover, stick the whole shebang into a tank sock, then put a screw thru that assembly at each end. Now you've done as much as you can to mechanically decouple the reverb from the speaker cab.

Also, there's no harm in reducing the maximum reverb signal by padding* it before it hits the reverb volume. I know, most folks are used to dialing the reverb up to 3 or 4, and there's no point in going further because that puts you at the far end of the tunnel, why go there. If you pad the reverb so that 10 is just a tad more than any sane person could possibly need, there's only one thing that can go wrong with that: complaints from the folks who are used to dialing it up to 3 and assume there's something wrong if you have to go beyond that.

*Reduce the max signal available at the reverb pot by wiring a resistor in series with its signal terminal.

It does have a nice thick piece of corrugated cardboard between the tank and cab floor. And as for the reverb level, the effect at 10 sounds about like the average reverb level at about 3. Was actually considering increasing the reverb level(scroll up in the thread) for those that like it at 4 or 5 so now I have two objectives that are competing with each other lol.

What I may do is decouple as much as possible and then see if I can get the sound guy to meet me up there a little early before a jam with everything set up on stage and see if that helps the feedback situation. If so, then from there I can tweak to get a little more reverb effect and then meet with him a second time before a jam to test again. They mic the amps so the mic may have something to do with it... in fact I need to ask him if the feedback is in the amp or in the PA.