Bringing life back to a 1969 AB763 Twin Reverb Amp

[sssmoka]

Hello, all. I do hope this memo find everybody alive and well as the alternative seems rather void. I am in need of some advice...

I've been fortunate enough to have the opportunity to service an original '69 blackface Twin Reverb Amp that has been converted to a head at some point in time during its' life. I've been tasked with bringing her back to a functional state. The owner brought it in with symptoms of "nothing but noise and hum" when turned on. Owner also noted the amp had been stored outside in a shed for a number of years by a previous owner. By the grace of the tube amp gods, all it took was a new set of ten tubes to bring her back to life.

As can be seen in the photo below, the amplifier is missing Input Jack 1 of the Vibrato channel - which I found lying loosely inside the chassis with the threads warped. To boot, the jack I found wasn't the original - the substrate separating the connections is of a modern composition and is a lighter shade in color. The definitive tell-tale - the resistor on the jack isn't the original carbon comp - it's a carbon film (see photo).

Also, there's a gaping hole in the rear of the chassis where I believe the original owner may have possibly removed an AC receptacle - this picture was my reference; I just can't see an amp leaving the factory with said hole open and exposing deadly wiring - the AC lines from the power cord are right inside the hole.

As she currently is, my signal generator and oscilloscope show both channels of the circuit to be functional. She just needs that missing jack replaced in the Vibrato channel... I've got a few Switchcraft mono jacks with shunts in transit along with some 1M Ohm carbon comps.

My question: I've got her running a matched quartet of JJ Electronic 6L6 tubes - she had Sovtek 5881s in there; does this beast indeed require re-biasing, as I think she does? Also, any and all advice and help about restoration/maintenance is appreciated.









Thank you for your time.

- sssmoka

[Smitty]

This looks to be an AC568 circuit. It's got the funky output tube cathode network. I'm going to guess it's a 1969. Marking on the chassis is a hint. Here is a link to the schematic.

http://www.prowessamplifiers.com/schema ... _schem.pdf

I recommend replacing all of the mallory white plastic cathode bypass capacitors with good quality caps.

I also suggest restoring the accessory outlet you have mentioned. You can find one here.

https://www.ebay.com/itm/354970603025

You could also remove the polarity cap and install a three prong grounded power cord.

Check to ensure that the capacitors for the output tube cathode network are all non polar. Replacing with good quality makes a difference. The 100K vibrato cathode gets bypassed with a 5uF/25V not a 25uF/25V. See the AA769 schematic:

http://www.prowessamplifiers.com/schema ... matic.html

Yes. You will need to rebias that amp. It is the single 'wrongest' thing with the AC568. Very cold bias. May even be 25% of plate dissipation. I've seen them even colder. It's why folks don't like that circuit. Makes the amp sound sterile and lifeless. So easy to fix by changing just one resistor.

I like to leave the bias balance pot as is and increase the resistor on the diode board. Pull the 470R and tack in a 10K pot wired as a rheostat to dial in the bias (start at 10K and turn it down to increase idle current) remove the pot and measure it and then replace with a resistor. Remember to subtract the voltage drop across the cathode resistor (5-7 vots) from your plate voltage when calculating idle current.

I've found that JJ 6L6s bias well by removing the 470 ohm resistor on the diode board and replacing it with a value between 3K3 to 4K7 depending on how hot you want to run at idle. I like 70% of plate dissipation. See Randall's excellent note:

https://www.aikenamps.com/the-last-word-on-biasing

Power supply caps may need replacement if they are leaking. If not you can leave the dried out but not leaking power supply caps in to get that 'old amp' sound. To my ears this sounds like the amp is coughing when pushed hard. Some new good quality caps will wake it right up.

BTW, 68 and 69 Twins are my favorite. Of the dozen and a half Twins I own about half of them are from these two years. The first Twin I ever bought almost fifty years ago was a 1969 AC568 (almost identical to what you have on the bench) with EV SROs. Still have it.

If you are looking for some easy mods try removing the output tube cathode network for added bounce and change the negative feedback resistor from 820R to 1K5 to open things up.

Hope this helps.

[solderhead]

Did somebody say "SRO"?

1936a.JPG

I collect Twin Reverbs too, but at 18 you've got me beat! The SRO are by far my favorite gigging speaker in a Twin Reverb... I like them even more than my twins with the D120 and the EVM-12L. The nice thing about the SRO is that they begin to give you that glorious AlNiCo compression at useable volumes.

Regarding a schematic -- I can't read the values on that schematic on the prowess site. This one is a bit easier to read:

https://cdn.shopify.com/s/files/1/2325/ ... ematic.pdf

The best looking vintage Fender schematics by far are the ones that were re-drawn about 10 years ago by Bob Post. I tried googling up a link to find a source for the PDFs that I've got on disk, but I got nothing. I also tried uploading the complete Fender schematic PDF to this site but the board rejected it as too large. So here's a screen capture of the AC568 circuit from my PC display. click to enlarge. I captured it at 2500x1900 so it should be nice and clear:

twin-reverb-ac568-schematic.png

The hybrid cathode/fixed bias system used in the AC568 is definitely an oddball. IMO it's only saving grace is that it behaves rather nicely with poorly matched power tubes. If the OP's 568 were mine, then I'd revise the bias circuit by adding a proper bias offset pot instead of just using the bias balance pot.

Regarding that courtesy power outlet -- if you look closely at this pic of my favorite SF Twin, and you squint hard enough, you can see that in the mid-70s Fender provided grounded courtesy outlets to match the red-tipped grounded power cords. I'd definitely convert the amp to a grounded power cord and cover up that space with a grounded courtesy outlet like the one mentioned in the ebay link.

Regarding the lamp driver -- what's the point of worrying about the value of Ck? It's already subsonic with the value being 25.

[Smitty]

I agree with all of your comments. Thanks for the schematic!

SROs with original cones are something unto themselves. Original all-paper cones with fiberglass voice coil formers = yum. At 50 watts you've got to be careful, though. I provided the back line at a an outdoor blues festival and brought my first Twin with the original SROs. Little Milton dimed that amp and after twenty minutes it was the end of the road for one voice coil.

Fender changed to 5uF in that position some time in 68. It gets picked up in the AA1069 schematics. If all you have is 25uF use it! I think the most important thing is to nix those white Mallory capacitors. I like the Sprague 30D TE series for that vintage vibe. NOS parts test well and as new after decades. The acquisition of a DER EE DE-5000 LCR Meter (~$100) really opened my eyes to cathode bypass caps. Also helped me understand that the brown film/foil signal caps were fine and totally undeserving of all the negative hype.

I did an experiment with an AC568 Showman Reverb (yep no 'Dual' in the name yet but the 4 ohm output transformer) with blue molded caps compared to an AC568 Twin with brown Paktrons being the only difference. Manufacture dates were within a few months and the only change being the switch to brown caps. Lot's of measurement to ensure both tube compliments were as close as humanly possible. Used a Weber amp switcher to take the speakers out of the equation and used the SROs in the Twin. I didn't hear a difference. But changing out the white Mallorys? Wow! Instant tone upgrade.

I add a 10K CTS X201 trim pot to the diode board and keep the balance pot as is. I break the connection between the diode and the cap and insert the trim pot wired as a rheostat. Fits perfectly in those two eyelets. Best of both worlds.

[Smitty]

Just got this. Very clean and unmolested. Oxfords removed and replaced with what seems to be the most desirable 12L.

IMG_1466.jpeg

IMG_1467.jpeg

[solderhead]

I was lucky and found that Twin with the SROs just a few years ago in a little independent music store in podunk Illinois. It had a broken tremolo oscillator so I got it cheap, even with the SRO speakers! Somebody had serviced the trem circuit and had replaced the 3 ceramic oscillator caps with blue drops, but they didn't realize that they had overheated one of them, killing the circuit, and they never figured out what they had done. Even worse, nobody else was able to fix the amp, so it just sat there broken in the corner of a small town music store. I got the amp really cheap because more than one person couldn't fix it. Everyone wants to replace the opto bug or the driver tube, but nobody thinks about checking the oscillator caps with an LCR meter when everything else tests OK. I think that amp ended up being the best deal I've ever found on a used amp. I couldn't believe that the SROs had survived in a Twin all of these years.

Yes, an LCR meter is essential gear. It is a great device for breaking free of the internet lore about capacitors. I use a little B-K handheld unit (it was expensive) and I measure all caps before I consider replacing them. If they don't leak or have high ESR then I leave them be. Much to my surprise, that 70s twin in my photo still has it's original PSU caps in the doghouse because they all passed all of the cap tests with flying colors. Those old Mallory caps were great quality caps. 55 years later they still meet specs and the amp still sounds great. But as you said, all of the cathode bypass caps have to be replaced.

I agree about the tootsie roll caps -- they are fine caps that are every bit as good as the blue drops. They don't deserve the bad reputation that people give them. I think that people look down on them just because of their color.

Yes, I have to be careful when playing the SROs in an over-powered amp like the Twin. I know that if I abuse them, I'll blow them up and there's virtually no chance of getting an original recone kit. So I just crank the amp until the speakers are in the zone and I stop there. So far so good ...

[solderhead]

Wow -- that's one amp that's going to be both LOUD and HEAVY!

I'm nerding out on those speakers. Those are late production run, aftermarket retail, Series II speakers with the 200 W rating. You can tell they're late Series II because they have the flat black label instead of the metal medallion in the middle of the black magnet cap with the heatsink cooling fins on it. (IMO those are the most desirable.) OEM speakers didn't have the fancy label like that, they had bare metal backs with numbers stamped on them. Only the aftermarket retail speakers had the fancy labels. IIRC back in the late 70s/early 80s you could still get 12L with the black metal magnet caps on them, so if I had to guess, I'd say that somebody bought those speakers at retail in the 80s and put them in the amp. Have you looked at the date codes?

Personally I prefer the 200W Series II over the Series III because the HF response diminishes a little bit with the higher wattage voice coil. Same would be true for the new production Classic vs. Black Label. IMO the Classics are the ones you want.

I'd like to find a drip panel amp like that. This is a great time to be shopping for Twins -- nobody wants' high powered clean amps any more. I'm sure Leo never would have expected that the little Princeton Reverb student/practice amp would end up being more valuable in the aftermarket than the flagship Twin Reverb.

I recently picked up a Super Twin Reverb (6x6L6) with the OEM Pyle speakers for all of $350. Nobody wants an over-powered amp like that, but it *KILLS* with a Rhodes piano.

edit: here's the early version of the EVM-12L Series II with the heatsink and badge:

early-EVM-12L-series-ii.png

[Smitty]

No date codes but a serial number.

IMG_1468.jpeg

[BrownIsound]

Since were doing show and tell, here’s my ‘68…

IMG_1109.jpeg

IMG_1105.jpeg

These were factory recones from the early 90s (Mark IV industries) that sat unused in their shipping boxes until last year.

[BrownIsound]

But changing out the white Mallorys? Wow! Instant tone upgrade.

How would you describe the tone change?

I have a 1971 SuperSix reverb that is 99% original (other than the bias and power supply caps), and it is a great sounding amp, white Mallory’s and all.

[sssmoka]

This looks to be an AC568 circuit. It's got the funky output tube cathode network. I'm going to guess it's a 1969. Marking on the chassis is a hint. Here is a link to the schematic.

http://www.prowessamplifiers.com/schema ... _schem.pdf

I recommend replacing all of the mallory white plastic cathode bypass capacitors with good quality caps.

I also suggest restoring the accessory outlet you have mentioned. You can find one here.

https://www.ebay.com/itm/354970603025

You could also remove the polarity cap and install a three prong grounded power cord.

Check to ensure that the capacitors for the output tube cathode network are all non polar. Replacing with good quality makes a difference. The 100K vibrato cathode gets bypassed with a 5uF/25V not a 25uF/25V. See the AA769 schematic:

http://www.prowessamplifiers.com/schema ... matic.html

Yes. You will need to rebias that amp. It is the single 'wrongest' thing with the AC568. Very cold bias. May even be 25% of plate dissipation. I've seen them even colder. It's why folks don't like that circuit. Makes the amp sound sterile and lifeless. So easy to fix by changing just one resistor.

I like to leave the bias balance pot as is and increase the resistor on the diode board. Pull the 470R and tack in a 10K pot wired as a rheostat to dial in the bias (start at 10K and turn it down to increase idle current) remove the pot and measure it and then replace with a resistor. Remember to subtract the voltage drop across the cathode resistor (5-7 vots) from your plate voltage when calculating idle current.

I've found that JJ 6L6s bias well by removing the 470 ohm resistor on the diode board and replacing it with a value between 3K3 to 4K7 depending on how hot you want to run at idle. I like 70% of plate dissipation. See Randall's excellent note:

https://www.aikenamps.com/the-last-word-on-biasing

Power supply caps may need replacement if they are leaking. If not you can leave the dried out but not leaking power supply caps in to get that 'old amp' sound. To my ears this sounds like the amp is coughing when pushed hard. Some new good quality caps will wake it right up.

BTW, 68 and 69 Twins are my favorite. Of the dozen and a half Twins I own about half of them are from these two years. The first Twin I ever bought almost fifty years ago was a 1969 AC568 (almost identical to what you have on the bench) with EV SROs. Still have it.

If you are looking for some easy mods try removing the output tube cathode network for added bounce and change the negative feedback resistor from 820R to 1K5 to open things up.

Hope this helps.

I can't thank you enough for this plethora of information, Smitty! I owe you a multitude of gratitude, sir.

If I could pick your brain for a few - the LAST thing I want to do is screw up this vintage AC568. It's a functional piece of history that does not belong to me.

With that in mind...

1. replace the white Mallory cathode bypass caps - I can do that (the replacements are 25uf/25v sourced from Amplified Parts)
2. 5uf/50v Sprague cap for Vibrato cathode bypass is in transit
3. already placed the order for the accessory outlet using the provided link (many thanks)
4. new three prong grounded power cord installed - the polarity cap that's to be removed - she's on the diode/rectification board, yes?
5. ensure output tube cathode network caps are all NON-polar - roger that
6. I have the requisite value F&T caps to replace the AC filter caps - the ones in there seem to be working correctly thus far, as such, I haven't opened the doghouse - yet

Thank you for all of your advice and assistance, sir. You are vastly appreciated Smitty!

- sssmoka

[Roe]

the EVM 12l has lots of fat midrange punch that works particularly well with the fender mid scoop (around 450-500hz)

[Smitty]

With that in mind...

1. replace the white Mallory cathode bypass caps - I can do that (the replacements are 25uf/25v sourced from Amplified Parts)
I like the MOD caps from this page. Not sure about the no name caps.
2. 5uf/50v Sprague cap for Vibrato cathode bypass is in transit
3. already placed the order for the accessory outlet using the provided link (many thanks)
4. new three prong grounded power cord installed - the polarity cap that's to be removed - she's on the diode/rectification board, yes?
Black wire to the brass accessory outlet connection
White wire to the silver accessory outlet connection
Green to chassis ground
Remove the capacitor that is connected to the ground switch.
5. ensure output tube cathode network caps are all NON-polar - roger that
6. I have the requisite value F&T caps to replace the AC filter caps - the ones in there seem to be working correctly thus far, as such, I haven't opened the doghouse - yet.

Do you have a method for checking for leakage?

[solderhead]

Since were doing show and tell, here’s my ‘68…
IMG_1109.jpeg
IMG_1105.jpeg

These were factory recones from the early 90s (Mark IV industries) that sat unused in their shipping boxes until last year.

HEY! You gotta remove that JBL tag and replace it with an EV tag!

[solderhead]

But changing out the white Mallorys? Wow! Instant tone upgrade.

How would you describe the tone change?

I wouldn't say that TONE is the problem with the white Mallory cathode caps -- to me it's more about the fact that just about every white Mallory cathode cap that I've ever encountered has failed in some way by the age of 50. Even if they aren't leaky, their ESR is often high when they're decades old and it's best to replace them.