AB763 - Twin Reverb

[toddincharlotte]

Hey All.
I am new to the forum, I a player but noodle in guitar/amp/pedal design and building.

I recently bought a "Twin Reverb Head". At least that's how it was billed to me when I bought it. It housed in a Dual Showman Cab and I believe the trannies/Chassis are from the same.


I was wondering if someone could eyeball this gut shot and tell me if it's even close to an AB763 Twin. I can provide the cap values if necessary.

I wasn't sure if you guys get this a lot and if you frown on players posting gut shots asking these kind of questions then I'll buzz off

[Leo_Gnardo]

I recently bought a "Twin Reverb Head". At least that's how it was billed to me when I bought it. It housed in a Dual Showman Cab and I believe the trannies/Chassis are from the same.

I was wondering if someone could eyeball this gut shot and tell me if it's even close to an AB763 Twin. I can provide the cap values if necessary.

[ATTACH=CONFIG]41504[/ATTACH]

Welcome aboard. Twin Reverb and Dual Showman Reverb are the same amp, the difference is Twin comes in a 2x12 combo and Showman is a separate head, much less of a PIA to move. Note in your second photo, on the right side there's a small bypass cap that looks blown up, and in parallel a smoked resistor. Possibly this resulted from a reverb drive tube that was shorted. Looks like someone did a thorough replacement of caps, all those orange drops plus Sprague Atoms in the power supply. So far so good. What we don't see, was bias filter cap replaced? The tech was so thorough, it probably was. But if it wasn't, it's time to do that. Also, the shorting bars on the output jack and input jacks of vib channel appear to be sprung. Replace the jacks as I don't think bending the bars back into place is going to work. Bending is iffy, new Switchcraft jacks don't cost all that much and can work for decades. I mention Switchcraft because I'd recommend avoiding other brands.

From what I can see this looks like an AB763, good amp & well worth having & maintaining.

[Justin Thomas]

Check the layout diagram of an AB763 Twin Reverb against the BOARD in your amp. If the eyelets match up, any alterations should be easy to restore. Agree with Leo on checking the bias caps and using Switchcraft jacks.

Whether it's an AB763 or not, it's still worth having and maintaining. I actually PREFER the Silverface amps, the "mixed-bias" ones, too. Got a front shot of the chassis, and a back shot? What are the numbers stamped on the transformers? It can help narrow down the date...

Justin

[toddincharlotte]

Welcome aboard. Twin Reverb and Dual Showman Reverb are the same amp, the difference is Twin comes in a 2x12 combo and Showman is a separate head, much less of a PIA to move. Note in your second photo, on the right side there's a small bypass cap that looks blown up, and in parallel a smoked resistor. Possibly this resulted from a reverb drive tube that was shorted. Looks like someone did a thorough replacement of caps, all those orange drops plus Sprague Atoms in the power supply. So far so good. What we don't see, was bias filter cap replaced? The tech was so thorough, it probably was. But if it wasn't, it's time to do that. Also, the shorting bars on the output jack and input jacks of vib channel appear to be sprung. Replace the jacks as I don't think bending the bars back into place is going to work. Bending is iffy, new Switchcraft jacks don't cost all that much and can work for decades. I mention Switchcraft because I'd recommend avoiding other brands.

From what I can see this looks like an AB763, good amp & well worth having & maintaining.

Wow, thank you so much for the quick reply. So, you are spot on about the cap/resistor meltdown. I retubed the amp recently and some of the preamp tubes were used, sourced from eBay. I read the tube chart as 12AT7 in v3 and that's what I put in there but it went bright and i smelled the meltdown. I will get in there and replace those two components.

Also, interesting you picked out those bad jacks. I have noted the following on the amp.
1) I have to use the External Speaker tap for the main, the Speaker tap does not work.
2) Normal input on the vibe channel does not work, I have to use the #2 input
Do you think replacing both jacks would help the amp?

Also, I don't think the bright switches are working in either channel but they appear to be in the circuit.....are those something that go in the amps? I guess I put a voltmeter on either pole to test?

[toddincharlotte]

Check the layout diagram of an AB763 Twin Reverb against the BOARD in your amp. If the eyelets match up, any alterations should be easy to restore. Agree with Leo on checking the bias caps and using Switchcraft jacks.

Whether it's an AB763 or not, it's still worth having and maintaining. I actually PREFER the Silverface amps, the "mixed-bias" ones, too. Got a front shot of the chassis, and a back shot? What are the numbers stamped on the transformers? It can help narrow down the date...

Justin

Hey Man,

Thanks for the reply. I do have some chassis/tranny shots and will post them shortly....have to run some code before lunch.

[Leo_Gnardo]

1) I have to use the External Speaker tap for the main, the Speaker tap does not work.
2) Normal input on the vibe channel does not work, I have to use the #2 input
Do you think replacing both jacks would help the amp?

Yes deafinitely, replacing those jacks would likely resolve the problems. They take a beating in club & tour use, and maybe some wires and/or resistors were torn loose besides - you'll be restoring them when replacing the jacks.

Also, I don't think the bright switches are working in either channel but they appear to be in the circuit.....are those something that go in the amps? I guess I put a voltmeter on either pole to test?

Applying a voltmeter - the ohm meter function for this - is more technology than you really need for this. What I mean is, your ears will tell you all you need to know, and without fiddling with a meter & test leads. Note the bright switches function is only obvious at lower settings of the volume controls, they do little at higher settings and nothing if you're dialed up to ten. You can shoot some cleaner spray in those switches, but after 40+ years of corrosion why not put in fresh ones at the same time you refresh the jacks. Add 'em to your shopping list. FWIW I mostly buy from Antique (tubesandmore.com) but there are plenty other good suppliers. Mojo is sort of in your neighborhood for instance, if your charlotte is the Carolina one.

[toddincharlotte]

Yes deafinitely, replacing those jacks would likely resolve the problems. They take a beating in club & tour use, and maybe some wires and/or resistors were torn loose besides - you'll be restoring them when replacing the jacks.



Applying a voltmeter - the ohm meter function for this - is more technology than you really need for this. What I mean is, your ears will tell you all you need to know, and without fiddling with a meter & test leads. Note the bright switches function is only obvious at lower settings of the volume controls, they do little at higher settings and nothing if you're dialed up to ten. You can shoot some cleaner spray in those switches, but after 40+ years of corrosion why not put in fresh ones at the same time you refresh the jacks. Add 'em to your shopping list. FWIW I mostly buy from Antique (tubesandmore.com) but there are plenty other good suppliers. Mojo is sort of in your neighborhood for instance, if your charlotte is the Carolina one.

Did not know Mojotone was in near Charlotte!!! Eureka!

Thanks for the input, I will get to work on this over the holiday. I will replace the switches too as there is definitely no noticeable difference upon engaging the switch. I also play a AA864 Bassman and AB763 Bandmaster and almost always use the bright for humbuckers. The "difference" is not apparent on the amp.

Thanks for the input, holding out a hope for this guy now....

[Leo_Gnardo]

Did not know Mojotone was in near Charlotte!!!

Woops, not IN Charlotte. Mojo's in Burgaw, a ways to the east (210 miles give or take) but still close enough anything you order ought to arrive next day, 2 at most.

The bright switches should be as apparent as they are on your Bassman & Bandmaster. One minor possibility, the 120 pF bright cap may be damaged, but it's much more likely that corrosion on the slide switch contacts is preventing them from working.

[Justin Thomas]

Isn't Hoffman in NC? And Tube Depot in TN... Swart I think is in NC?

Justin

[toddincharlotte]

Isn't Hoffman in NC? And Tube Depot in TN... Swart I think is in NC?

Justin

Swart is in town. Not sure about the others, I will check it out. Definitely need to get something rolling as I'd like to get after this over thanksgiving break.

[Leo_Gnardo]

Isn't Hoffman in NC? And Tube Depot in TN... Swart I think is in NC?

Righto, Hoffman's about 75 miles east in Pisgah, another excellent outfit (and bless Doug Hoffman + contributors for having that extensive schematic library on the website.) Swart builds excellent amps, if he'll sell you a couple parts at a fair price so much the better, and it wouldn't hurt to say hello & try to make friends. It's hard to know what attitude you'll get from small scale builders, sometimes they're hard pressed to begrudgingly squeek out a spare minute, others can be very friendly because after all you are a potential buyer for their finished product.

[toddincharlotte]

Welcome aboard. Twin Reverb and Dual Showman Reverb are the same amp, the difference is Twin comes in a 2x12 combo and Showman is a separate head, much less of a PIA to move. Note in your second photo, on the right side there's a small bypass cap that looks blown up, and in parallel a smoked resistor. Possibly this resulted from a reverb drive tube that was shorted. Looks like someone did a thorough replacement of caps, all those orange drops plus Sprague Atoms in the power supply. So far so good. What we don't see, was bias filter cap replaced? The tech was so thorough, it probably was. But if it wasn't, it's time to do that. Also, the shorting bars on the output jack and input jacks of vib channel appear to be sprung. Replace the jacks as I don't think bending the bars back into place is going to work. Bending is iffy, new Switchcraft jacks don't cost all that much and can work for decades. I mention Switchcraft because I'd recommend avoiding other brands.

From what I can see this looks like an AB763, good amp & well worth having & maintaining.

He Leo,

I'm putting a cart together online and noticed the value of the smoked cap is 25uf 25v but the mojo layout calls for 25uf 50v. Is it OK to use the mojo part here?

Also, I can't make out the value on the resistor, I'm looking at two different layouts online, one indicates 2.2k [mojo] the other looks kind of like 8.6k [original fender layout].

[Jazz P Bass]

If it is the reverb drive socket (plates are in parallel & wired to the reverb transformer) then the cathode resistor is 2.2K.
The 25uf bypass capacitor can be a 50 volt value.

I would look at that socker real good seeing that a tube smoked in it.
Either the socket & the circuit has issues or you had a bad tube.
The 12AT7 does go in the reverb drive socket.

Aside: Layouts are great but you really should try and 'read' the schematic.

twin_reverb_ab763_schem.zip

[toddincharlotte]

If it is the reverb drive socket (plates are in parallel & wired to the reverb transformer) then the cathode resistor is 2.2K.
The 25uf bypass capacitor can be a 50 volt value.

I would look at that socker real good seeing that a tube smoked in it.
Either the socket & the circuit has issues or you had a bad tube.
The 12AT7 does go in the reverb drive socket.

Aside: Layouts are great but you really should try and 'read' the schematic.

[ATTACH]41513[/ATTACH]

Thanks for the input man. I'm leaning toward a bad tube because I think it worked fine before I retubed it the other day [i'm not a big reverb guy so i don't recall if i tested it when i got it, i thought i did]. I'll check through the circuit to be sure.

What's a good setting on my multimeter for testing these circuits? I think I used 2k or 20k for building pedals.

[Leo_Gnardo]

the value of the smoked cap is 25uf 25v but the mojo layout calls for 25uf 50v. Is it OK to use the mojo part here?

Also, I can't make out the value on the resistor, I'm looking at two different layouts online, one indicates 2.2k [mojo] the other looks kind of like 8.6k [original fender layout].

What Jazz P Bass said ^^^. DO inspect that V3 socket carefully, make sure there's no charred arc debris, miswiring, etc or you may smoke the replacement parts. 50V cap is perfectly good. Personally I like to leave out the bypass cap, then the reverb signal isn't smacking the tank quite as hard. I like to back off from the full surf reverb Fender designed in, but that's me. Easy enough to try it with or without & decide which you like better. I've seen values for the cathode resistor vary from 470 to 2K2. Higher value, lower current thru the drive tube, this may help it last longer. But 8.6K looks way out of line, it's not even a standard resistor value. Mind you it wouldn't stop the circuit from working but gotta think "where'd they get that?"

I use all the scales on an ohm meter when working on tube amps. Sometimes you have to measure 47 or 100 ohms, other times a meg ohm or more. Don't be lazy, turn the switch.