65 Bandmaster Vibrato Amp - Need some experienced eyes

[AV]

Greetings all,
I am new to the forum and in the midst of my first tube amp project: rebuilding a blackface bandmaster my brother bought around 1970.  I have gotten my parts from Doug and am re-capping and replacing a few resistors, etc. I am fairly decent at this kind of thing, but far from an EE, so I do get confused by schematics sometimes. I am not exactly sure which model I have. The label is from Oct. '65, model AB763.  Im guessing there are variations to the AB763? This is two channel, one normal and one vibrato, but I have found many pictures and schematics for AB763 that are NOT like mine exactly (for example I have only 5 25uf/25v paper caps on the main board, not 7).

so, in the picture attached you are looking at the speaker jacks on the top of the image. Main spkr on the right, ext. spkr. on the left. Power cord would be in the upper right corner.
1. The EXT SPKR has the remains of a clipped black wire that someone removed. Im not sure where that should connect to.
2.You can also see a white wire that has been cut. Where did that come from?
3.You can see the pass thru grommet from the other side has 4 wires coming from the transformer that sits next to the filter caps: solid green and solid black, which go to the main speaker jack. The yellow, with green stripes, which goes to the ext. spkr jack, then continues on as yellow to the circuit board. And the white, which has been cut. If the nub on the ext.spkr. jack was white, this would be pretty obvious - someone snipped it to disable that jack - but the nub is black.  Were they connected? with a diode maybe?  I can not find a photo on the web with 4 wires coming from the pass thru, and the schematics I reviewed dont help me much.

what exactly IS the model and name of this unit? And what should I do with the unknown wires?  I can not wait to get this amp up and running - thanks for directing me to resources or sharing your experience in servicing the Bandmasters.
-Andy

[DummyLoad]

looks like the output transformer was likely replaced. can you confirm the output transformer date and manufacturer codes?

the hanging black wire would have been where the original OT common wire connected, the "hot" wire, if you will, looks to be connected properly. also, note that the tech decided that the replacement OT have more than one secondary speaker tap: note that he connected green tap to int. spk. jack and the green/yellow to ext. spk. jack. so the jacks are no longer common - so use one jack or the other and not both.
 
the yellow wire that connects to the ext. spk. jack and the green/yellow OT wire is the negative feedback loop to the 820ohm resistor on the ckt. board.

--pete

[sluckey]

Im guessing there are variations to the AB763? This is two channel, one normal and one vibrato, but I have found many pictures and schematics for AB763 that are NOT like mine exactly (for example I have only 5 25uf/25v paper caps on the main board, not 7).

AB763 is a whole family of similar amps. AB763 is not unique to the Bandmaster. The Bandmaster AB763 only has 5 25µF/25v caps. The reverb AB763 amps have 7 25µF/25v caps.

I agree with Pete. Looks like the OT has been replaced.

[Willabe]

Hi AV and welcome!

If you go down to the very bottom of this page you'll see our host Doug's schematic library link, just click on it and there's a ton of schematics.

In the mean time here's a link to AB763 Bandmaster, 2 pages, 1st electrical schematic, 2nd layout, just scroll down;

http://el34world.com/charts/Schematics/files/fender/Fender_bandmaster_ab763_schem.pdf

Highlighted layout drawing attached, 2 red circles are where the 2 grommet output transformer (OT) wire holes are in Fenders AB763 Bandmaster chassis with their original OT wire colors pointed to with blue arrows. Total of 5 'fly lead' wires, 3 for the primary, 2 for the secondary (input/output). The blue and brown are the start/finish of the primary wind (push/pull) for the power tubes plates (input to OT). The red is the OT primary center tap (CT where the B+dcv is applied to the OT) the black and black in the same grommet hole with the red/blue/brown wires are for/from the choke in the B+dcv power supply (PSU) that's between the B+dcv PSU from the power tube/OT CT primary feed and going to the power tube screen supply.

The purple arrow is the speaker jack link that DL pointed to, green arrow is negative feedback wire (-FB or more commonly, FB) from speaker jack going back to the eyelet board in this amp to be applied to it's phase inverter (PI) last 9 pin tube, right before the power tubes.                   
                               
                             
                                  Brad   

[DummyLoad]

trace the white wire back up through the chassis. take pics of where it terminates.


--pete

[AV]

Pete, Sluckey, Brad, et. al,
THANKS for the great information. I had seen the diagrams, but they bring up two more questions.

1. In the photo below note the blue & brown OT input wires coming from the tubes are switched. This appears factory and perhaps it is OK they are switched without consequence? Or are they causing the power tubes to feed the wrong (or at least unexpected ) channel? (I am planning to upgrade the resistors on the tubes.)

2. The EIA code (who knew there was such a thing) appears to indicate a quality maker, but a date of 1970??  That explains a few things, but not why the OT has 4 leads as opposed to the original, which it appears had only 2, correct? (black and green to the spkr jacks). So... what to make of the fact that 3 leads are now in use. Does the new OT have more "channels" ?  How does that work exactly?

3. Also, Pete said "note that the tech decided that the replacement OT have more than one secondary speaker tap: note that he connected green tap to int. spk. jack and the green/yellow to ext. spk. jack. so the jacks are no longer common - so use one jack or the other and not both."  [sorry, not seeing the quote function]
But I dont see how that EXT SPR jack will do anything as it only has 1 lead feeding it. Doesnt the hanging black wire need to be connected (perhaps over to the black on the main speaker jack?)

thanks again for taking the time to help me figure this out,
Andy

[Willabe]

1. In the photo below note the blue & brown OT input wires coming from the tubes are switched. This appears factory and perhaps it is OK they are switched without consequence? Or are they causing the power tubes to feed the wrong (or at least unexpected ) channel?

No not channels on the output, your amps output is mono even though you have 2 input channels.

The vast majority of guitar amps with 2 (or 4, 6 power tubes, 6L6's, 6V6's, EL34's....) are run in 'push pull' pairs. 1 tube (push) is for the top half of the signals sine wave and the other is for the bottom of the signals sine wave that are mixed together in the OT's secondary that feeds the speaker(s).

If the OT was changed then the blue/brown wires were not soldered up at Fender. They could be backwards, but most likely not, well if who ever put it in new what they were doing.

Tube power amps that use -FB (like yours does) if you get the OT primary wires backwards the -FB becomes +FB and the amp wont work/play/sound right. The easy way to check this out is to just temporarily disconnect the -FB wire. Guys like to leave these 2 wires a little long when the wire up an OT to make sure their correct then after they are sure they go back and trim them to the right length.

2. The EIA code (who knew there was such a thing) appears to indicate a quality maker, but a date of 1970??  That explains a few things, but not why the OT has 4 leads as opposed to the original, which it appears had only 2, correct? (black and green to the spkr jacks). So... what to make of the fact that 3 leads are now in use. Does the new OT have more "channels" ?  How does that work exactly?

3. Also, Pete said "note that the tech decided that the replacement OT have more than one secondary speaker tap: note that he connected green tap to int. spk. jack and the green/yellow to ext. spk. jack. so the jacks are no longer common - so use one jack or the other and not both."  [sorry, not seeing the quote function]
But I don't see how that EXT SPR jack will do anything as it only has 1 lead feeding it. Doesnt the hanging black wire need to be connected (perhaps over to the black on the main speaker jack?)

Pete already told you what the wires are, go back and reread reply#1

Your OT has a couple extra secondary load wires, might be wound with 4, 8 and 16ohm taps. Most Fender amps with a single speaker used an 8ohm speaker so they needed an OT with an 8ohm tap to match the speakers load. Most of their 2 speaker amps were set up at 4ohms, 2x8ohm speakers wired up in parallel = 4ohms. The OT's in those amps like a Twin Reverb, had OT's wound for a 4ohm secondary load tap.

Pete probably asked you to post 'output transformer date and manufacturer codes' so he could look it up and confirm what is is and what the taps are including their color coding.

The speaker jacks in your amp are switchcraft open frame that ground themselves to the chassis. So when you solder up the OT secondary wire to a speaker jack the OT is now grounded. (There's a little more to it as 1 jack has a switch built into it and the other doesn't.) When the OT was changed because it has 3 different speaker load taps they chose 2 of the 3 and separated the 2 speaker jacks (no longer 2 in parallel) and left 1 tap unused.

Here's the link to Doug's tube amp library of information, lots of info in there to sort out a lot of your questions;   

http://el34world.com/schematics.htm


                               Brad   

[Willabe]

note that he connected green tap to int. spk. jack and the green/yellow to ext. spk. jack. so the jacks are no longer common - so use one jack or the other and not both.

The speaker jacks are not 'common' (in parallel) to each other anymore as Pete said but the jack that the green wire goes to has a shorting switch that when nothing is plugged into it shorts the hot to ground. (See purple arrow in highlighted layout drawing for the wire that Fender had in place to make the 2 jacks in 'common'/parallel.)

So if you plug into the other jack the switch now shorts the OT signal to ground. This needs to be changed if you want to use the other jack that is for a different speaker load.

 

[Willabe]

trace the white wire back up through the chassis. take pics of where it terminates.

????????

In 1 of your pics it shows that your chassis was punched for 4 power tubes, Bandmasters had 2, could have been why they changed the OT?



                       Brad            

[Willabe]

Here's 2 drawings from Doug's store of OT's.


               Brad   

[sluckey]

In 1 of your pics it shows that your chassis was punched for 4 power tubes, Bandmasters had 2, could have been why they changed the OT?

That was pretty common. There may be some other chassis that came from the factory like that also.

[AV]

OK Brad, now I begin to see the light.  Pete's info was great, but without the background knowledge it didn't make sense to me. The jacks are self grounding, so now it does.

About the mismatched blue/brown push pull-- clearly IF the OT was replaced then those are not original solders (my mistake). The amp does function, as in it turns on and I get sound. So maybe that OT had non-spec leads? The colors don't match the schematic's specs. I'm not sure who may have worked on this in the decades past so it may have been my brother's friend for all I know (one of the soldiers looks pretty amateur). If the push pull is flipped are you saying I would not even get audio out? Or just muddy or out of phase audio? By removing the -FB lead these symptoms would go away, indicating the need to swap the blue/brown? (sorry if that is an obvious question). If I understand Pete correctly, defeating the -FB would be by removal of the yellow wire heading from my EXT Spkr jack to the circuit board.

As I consider the helpful schematics, it appears the non-standard (4 lead) OT is designed to supply power for various impedances, which if I infer correctly means I would use the Main Spkr jack for an 8 ohm load and plug into the ExtSpkr jack if I was driving a 4 ohm load.  That would be OK.  (Still need to build a cabinet and get speakers so that is the next area of research).

Finally, you are most observant. The chassis is punched for 4 large tubes, with 2 hole covered (pretty clearly at the time of construction). This is not common? If not I assume they just put some extra chassis onto the production line and covered the holes. I dont think it relates to the 4 lead OT, unless my reading of the date code is wrong, because the OT appears 5 years newer than the amp.

Again, many thanks. I will continue to check out the resources, but will likely be back with more burning questions on this smokin' hot amp!

-Andy

[DummyLoad]

Woodward - Schumacher was the winder for fender at the time. your OT is dated 1970 45week of production.

some research shows it's probably a replacement part sold by fender that covers several models. the p/n 017450

the taps are probably as follows:

white - 2ohm
green/yellow - 4ohm
green - 8ohm

probably means don't take in as factual. the only way to confirm is to measure OT ratios. to do that accurately you need a variac and decent DMM.

--pete

[DummyLoad]

trace the white wire back up through the chassis. take pics of where it terminates.

????????

In 1 of your pics it shows that your chassis was punched for 4 power tubes, Bandmasters had 2, could have been why they changed the OT?



                       Brad          

responding to the quote of ???????



see opening post question #2....


...where does my white wire go?    


--pete

[Willabe]

see opening post question #2....
...where does my white wire go?   

OK, now I get it.


                     Brad     

[eleventeen]

"About the mismatched blue/brown push pull-- clearly IF the OT was replaced then those are not original solders..."


You are waaaay overcomplicating things. Amp comes from factory with output transformer mfd by "X" and blue goes to one 6L6 tube and brown to the other 6L6, pin 3 (plates) in both cases. Fast forward some number of years, replace OT with an OT mfd by "Y".


The new OT will (should) have blue and brown wires intended to go to the two plates, as this is a standard color convention for ANY tube push-pull OT. However there is no absolute guarantee and there is no real way to predict which 6L6 gets the blue and which gets the brown. If the replacement was made using the exact model of the original, it would be different, you could wire up new blue to old blue and new brown to old brown. A bench tech should bolt in the new tranny and leaving the leads long and sloppy, solder it up one way or the other and fire up the amp. One way will produce bad juju---motorboating or weird noises....and the other way won't. It has to be made right, it cannot be predicted, and you just have to try it one way and flip the blue/brown wires around if you get bad sounds. It really isn't any more complicated than that...unless you wish to understand it at a deeper level, but even so, it isn't more complicated than that!

[AV]

Eleventeen,

THAT is what I wanted to hear, that a flipped push pull pair would be very obvious. I don't want to know any more! Seriously, just trying to understand how this works well enough to re-build it and enjoy it to its full potential. Im not going to re-wire the spkr jacks, and am not even sure I need to replace the tubes if it runs well. Have read up on the 'death cap' and Im leaving mine as a line filter as it will have a new grounded plug and I am comfortable with that, per several posts on this forum. I do need to read up on bias and figure out if that is something I have to do, but I am not looking for extra work. The 'new' (1970) OT seems to work fine and my only plan for that is to put some heat shrink on the clipped white wire, which has just been flopping around.

thanks a lot!
-Andy

[Willabe]

Im not going to re-wire the spkr jacks

note that he connected green tap to int. spk. jack and the green/yellow to ext. spk. jack. so the jacks are no longer common - so use one jack or the other and not both.

The speaker jacks are not 'common' (in parallel) to each other anymore as Pete said but the jack that the green wire goes to has a shorting switch that when nothing is plugged into it shorts the hot to ground. (See purple arrow in highlighted layout drawing for the wire that Fender had in place to make the 2 jacks in 'common'/parallel.)

So if you plug into the other jack the switch now shorts the OT signal to ground. This needs to be changed if you want to use the other jack that is for a different speaker load.

                                Brad   

[AV]

Ok, I missed that part as Pete had said: " so the jacks are no longer common - so use one jack or the other and not both. "  But you are saying I can't do that with the ExtSpkr jack, without rewiring, the ExtSpkr wont work.

The MainSpkr is wired for 8 ohms, and the Ext Spkr is supplied by the 4 ohm OT lead, so what do I need to change? Disable the grounding switch feature of the MainSpkr?  I know you pointed me to the labeled diagrams and I see what we're looking at, but at this point Im not sure if Im trying to put things back how they 'were' or set it up a new way. I likely only need one output, and that would be 8 ohm ideally, but it would be nice to be able to have that other jack for a 4 ohm load if the situation came up. I am very willing to work to figure out how to wire it, but won't be put off if anyone just wants to tell me what wire goes where for best results.  :)  Should I remove the yel/gr 4 ohm OT supply line and just re-wire the jacks as if I only had the black and green OT supply wires. What makes the most sense?

thks,
AV

[sluckey]

I would keep the option to use a 4Ω ext speaker (if in fact that is what it is). Here's what I'd do...

Look at the main speaker jack. The green wire connects to the hot terminal and the black wire connects to the ground terminal. There is a third terminal that is used as a switch. ***IF*** there is a jumper between the switch terminal and the ground terminal just snip it out. Now you can safely use either the main speaker jack or the ext. speaker jack, but not both.

I can't see the main jack clearly but I suspect whoever installed that multi-tap OT has already snipped that jumper.

[AV]

The reason I didnt see that 3rd terminal at first is because it has been folded over an soldered directly to the ground terminal. I will remove that solder bridge and hope the OT supply lines are 8 & 4 ohm (dont have the gear or expertise to test that).  If I do inadvertently run the wrong load or spkr config, would it damage the amp long term, or just result in reduced performance/volume?

[shooter]

I've asked that question - mismatched loads - and I believe it will have a life shorting effect if you run them wrong, at least in one direction, wants 8 you give it 4.  If you do a forum search on OT impedance there should be a good list of answers and probably more questions

[6G6]

Since the only difference between a Bandmaster chassis and a Dual Showman chassis is
the number of holes for the 6L6s, Fender sometimes just covered the extra holes.
If you ever need more power, change transformers, and add the other two 6L6s. 

[AV]

I would keep the option to use a 4Ω ext speaker (if in fact that is what it is). Here's what I'd do...

Look at the main speaker jack. The green wire connects to the hot terminal and the black wire connects to the ground terminal. There is a third terminal that is used as a switch. ***IF*** there is a jumper between the switch terminal and the ground terminal just snip it out. Now you can safely use either the main speaker jack or the ext. speaker jack, but not both.

I can't see the main jack clearly but I suspect whoever installed that multi-tap OT has already snipped that jumper.

So, I removed the jumper wire that was between the switch terminal and the ground terminal and the result was NOT GOOD. Sounded like a broken tone generator.  I put the jumper back and I get audio as expected, though only from the main spkr jack. The audio is not great, but I am pretty sure these tubes are not good either, at least the power tubes. They are very sensitive to vibration, look pretty burned inside, which may be normal? and are not a matched pair (one 6L6 GB and one ??.  Not much other unwanted noise, so Im thinking the caps replacement was a success.

I guess there was a reason whoever switched the OT also bridged the switch and ground terminals on the spkr jack in a very deliberate way (folding the leads together). Perhaps Im not understanding the true nature of the multi leads from that OT? The plan now is to reconnect the switch and ground and just use the main spkr jack since Im going to build a cabinet next and will just use an 8ohm configuration. Meanwhile Ill research the tubes and bias process. 
thanks,
Andy

[AV]

Greetings again - a few months back you folks shared some knowledge that helped me get started on my project. Just wanted to say thanks! now that it is finally done. The amp is working great, and sounds fantastic. I did replace the power tubes as they were mismatched, and the new components (caps as resistors) seem very solid. Then I had to build the speaker cab - I used 2x10 Eminence Legends and they sound better than I had hoped for. I wanted a smallish, nice looking cabinet I could leave out in the living room. Finally figured out my dovetail jig. With that done, it was clearly time for a new guitar, right? I wound up with a 2008 PRS Core Mira, which plays amazingly well. Needless to say I am very happy with the outcome. I know sometimes people give advice and you don't really know if it was ever taken or if it worked out, so again - thanks for your time an input!
Andy

[DummyLoad]

nice work and looks great. glad you got it sorted.


--pete