Fender Twin Reverb (1972, CBS) - suggestions for “refurbishment”

[Silas]

Hi guys,

I’ve picked up a 1972 fender twin reverb, in good nick and entirely original (down to tubes even)

Powered up slowly on the variac and sounded rather nice in testing. I plan to replace all the electrolytics and the dropping resistors before using further.

Anyone have any suggestions of things to look over, be aware of?

I’ve ordered a replacement board for the filter caps, as it seems to have got damp and bowed so thought worth changing. Does anyone know if fender ever put anything exotic in their boards (or anywhere else…) to look out for….? Asbestos etc….?
My understanding is they were all vulcanised cotton - which is what I’m replacing it with.

Some pics attached.

Cheers

Silas

 

[Flatpicker]

Those filter caps are the main thing, so you are headed down the correct path. Also you my want to replace the electrolytic caps on the circuit board (they're the white ones). Only other thing I can think of is make sure it has a 3-prong AC power cord. For great info on these and other mods, checkout this website: https://robrobinette.com/Amp_Stuff.htm

 

[CJ]

the master volume with the pull boost thing, I would trash that as it sounds hiddy.

If the caps show no signs of leakage or if the plus side has no bubbles on the ends then I would leave them. But that's just me. Only seen a few of those caps go bad. And sometimes after I put in new filters the sound changes for the worst. If it sounds good now...

But if you play for money is plan to flip the amp, I would replace them.

I like the 70/350 caps, Fender upped the value over the years and that can change the sound.

The original fiver boards were wax dipped to protect the circuit and the board from moisture. Things get hot in there and when the amp goes from the stage to the 2 am parking kit, the temp change can cause moisture to develop.

I usually clear coat the boards.

here are some caps from a 75 Twin that just came in, the 86 uf was probably on the bottom leg,

 

[CJ]

and the 25/25 Mallory caps were tested. One had some leakage at 25 VDC but it went down to zero at 3 volts. I chopped it open and found it to be very dry. Word has it that the pre CBS Sprague 25/25 caps used before the white Mallory 25/25 caps were better and i agree with this. So I would change the 25/25 caps and the two 70/350 or whatever is in there as they do 90 percent of the work.

if it was for a customer, i would change all of them including the bias supply cap(s).

if you really want to do a good job, pull the brass plate under the pots and clean it, what a pain but you won't believe the corrosion that is under there due to dis-similar metals.

I would probably change the 4 pwr tube sockets also as there was a time period where all of those had weak tension and could be conductive as well as be more likely to arc from screen to grid (pin 4 to pin 5)
new screen resistors are a good idea regardless of if you change the sockets.


i would ditch the polarity cap also. (under the gnd sw)

it might be hard to get the bias right if the bias matching circuit is used (balances current but does not help to set idle ma) you can either keep the balance circuit and adjust the resistors to get the idle right or change to the pre CBS circuit and hope for balanced tubes.

some folks say that the pre CBS inverter circuit is better but i hear no difference. this involves the 47 K vs 82K and 100K plate resistors and a few other resistors.

 

[soulsonic]

I've worked on a handful of silverface Fenders over the years. Definitely replace the large brown filter caps no matter what - since you weren't greeted with a constant loud buzz, they aren't toast yet, but I guarantee they will be before too long. Not even worth considering keeping them.

The black fiberboards in old Fenders are the WORST. I just hate them. They always get warped and they suck up moisture badly, which can lead to annoying noises. I've even seen them get moldy. I recommend when you replace the board for the filter caps, DON'T get "period correct" fiberboard. Try to get stiff garolite board, or even better use modern FR4 material for the board. Hoffman Amps (el34world.com) sells very nice thick board material, but you could even use FR4 perfboard. When buying new electrolytic caps, don't waste money on the Sprague Atom caps that the old timers always seem to recommend. There's nothing special about them that would magically make the amp sound better (other than being new and fresh) and it isn't what Fender used anyway. F&T caps are good upmarket choice, but Illinois/CDE is good, and the generic house brand MOD caps sold by Amplified Parts/Antique Electronics Supply are just fine - I just recapped a '69 Super Bassman with the MOD caps and it sounds exactly as it did with the old caps before they went bad.

Other things to look at: The 100K 1W carbon comp plate load resistors can be a source of annoying noise. I think Fender made an improvement by bumping up to the 1W resistors over the 1/2W ones the older amps used, but carbon comps are still notorious for going noisy when used as plate resistors. Replace them with good quality 1/2W or 1W metal film. Dale RN60/RN65 are a good choice, as are the equivalent by PRP if you want to feel fancy. They don't NEED to be 1W resistors - in fact you can maybe get away with 1/4W, that's what Peavey always used. But 1W resistors will inherently have lower noise and they also have a higher voltage rating than the lower wattage types. I do not recommend metal oxide resistors for plate loads. I've used them before on a lark and it was one of the few times where I felt the resistor type was making it sound harsh for some reason. If you want to be thorough, you should consider replacing all the plate load resistors with metal film, but it's specifically the 100K ones on the first input stages that I think contribute the most noise.

Absolutely replace the 470 Ohm screen grid resistors that are strapped across the 6L6 tube sockets. The heat from the tubes cooks them over the years and it will be only a matter of time before one fails. And again, carbon comp is a poor choice here. Replace them with larger 5W wirewound resistors. I've used Mills when I want to be fancy, but the cheapy "sand block" types are just fine. Also on those sockets are the 1.5K grid stopper resistors; good idea to go ahead and replace those with RN60 metal film while you're at it, since they get cooked and drifted off spec as well.

The Bright switches tend to go bad. Start out by giving them a good cleaning at least. You can find new replacements, but watch out that some of the newer slide switches don't line up with the old screw holes for mounting them, despite them being otherwise the same size.

Judging from the photo, this amp has one of the "bias balancing" bias controls. You'll get a better range of adjustment by rewiring the circuit to be like the older blackface style bias control. You don't need to replace the pot to do this, just a rewiring of the circuit. I believe the above linked Rob Robinette site shows how this is done. This is another thing I did to the Super Bassman, and it works as it should.

Sometimes these old silver rectifier diodes go bad. This is just a few pennies worth of 1N4007s to replace. But since you've powered it up and it seemed fine, they're probably fine.

 

[CJ]

backup material from Gerald Weber>

 

[CJ]

took apart one of those old Mallory 20uf/500V caps, and an FT 22/450,

the Mallory has 3 layers of paper between the plates, everything looked ok, discolored part is from glue in the bottom of the can that secures the paper/foil assy. no goop ion the FT,

the plates of the FT are smaller due to advancements in the chemicals i suppose,

the gary foil on the Mallory was 4 mils and the lighter gray was 3 mil,

the FT gray plate was 2,5 mils and the brighter foil was 1 mil,

does the F$T have better ESR due to the shorter path that the electrons have to travel, due to the shorter foil and with the lead taps in the middle instead of the end? i don't know i never worked for Sprague or Cornel Dublier.

 

[CJ]

i have bought about 20 F & T caps and one went bad, opened up, but i still use them , the price is nice,

Sprague Atoms are a fortune but probably higher quality than F & T,

here is another option>

 

[soulsonic]

Speaking of cutting up electrolytics, I remember years back - probably around 2007 - someone, I think on the Hoffman Amps forum, cut open a current production Sprague Atom and found a tiny little thing inside this huge case; compared to an old one which was packed full. The proof was that the construction technique of the new ones was very different than how it had been and they were putting them inside oversized casings to retain the old appearance and "fit." I don't think this proved it was any worse or better, but it was clear they were not being made the same way they had been. My overall impression was that you were paying extra for a typical modern cap hiding inside a big case.

Browsing around on Mouser to see what the scene is like these days for axial high voltage from big suppliers, I spotted the Kemet PEG series rated at an astonishing 27500 hours at 105C... wow

 

[CJ]

The axial caps are way expensive and choices are way below radial caps.

You can stick a radial cap inside an old Mallory casing and run some nice cloth leads out, red and black. Then fill it up with the hot glue gun,

The Sprague axial 25/50 caps are great for the cathodes, about 3 bucks at Hoffnan.

 

[Silas]

Thanks guys, loads of really helpful info here!!

Definitely changing the electrolytics - they're bulging slightly in my filter section anyway.

Great to know re common resistor failures too, makes sense to do those whilst I'm at it.
Noise wise the amp is pretty quiet on the whole, so it bodes well there. I don't think it's been used a huge amount if I'm honest. At least it didn't explode during the initial slow power up, was half expecting some smoke.

Interesting about some of the blackface mods, I had been looking them over at Rob Robinnette's site - considering it for sure. Cheers for the Gerald Weber shots too, very simple & clearly illustrated.

Re capacitors, I bought a mixture of F&T and Sprague, price dependant. The 25uf on mouser were pretty cheap as you say. Can't remember if I over-ordered, perhaps one could be cut open to see if the outer shell is purely decoration...

Bright switches are currently okay, and most dials adjust cleanly, middle control in the vibrato section is a bit rough, but hoping I can bring it back to life.

Rather nicely there's no ground switch/death cap on this model, safer from the get go...

 

[oldsmobuick]

Clip out the death cap, add 3-prong power cable, replace doghouse caps (watch for the backwards one), delete master volume, check resistors around tubes, check tubes, replace speakers (they are terrible), and that’s it! Don’t do too much and keep the old parts for the weirdos who like that crap when you go to re-sell.

 

[soulsonic]

Then fill it up with the hot glue gun,

Unwise. Electrolytic caps need to be vented to release the gasses generated during use. That is why they have vent holes on their rubber end caps. Sealing them up provokes an early death.

 

[Rusan]

it might be hard to get the bias right if the bias matching circuit is used (balances current but does not help to set idle ma) you can either keep the balance circuit and adjust the resistors to get the idle right or change to the pre CBS circuit and hope for balanced tubes.

Silas, the first thing I looked for in the chassis photo was if they'd added the 1/4" line output between the vibrato pedal jack and speaker jacks yet in '72. I removed the line out jack in a friend's '77 Twin and installed a voltage pot, so it has fully adjustable bias now. Too bad yours doesn't have the line out, that is, if one wasn't planning on using it and could part with it.

Speaking of cutting up electrolytics, I remember years back - probably around 2007 - someone, I think on the Hoffman Amps forum, cut open a current production Sprague Atom and found a tiny little thing inside this huge case; compared to an old one which was packed full. The proof was that the construction technique of the new ones was very different than how it had been and they were putting them inside oversized casings to retain the old appearance and "fit." I don't think this proved it was any worse or better, but it was clear they were not being made the same way they had been. My overall impression was that you were paying extra for a typical modern cap hiding inside a big case.

I think it was around 2005 or 2006 when I ordered some new Atoms to recap an amp, and noticed they felt very lightweight. I compared them to some I'd had on hand for a few years, and there was definitely a weight difference. I held the new ones by their leads and thumped on them with my fingernail to see if I could hear/feel a difference, and they all rang with a pingy, boing resonance. The older ones didn't; they were much denser and deader. Sprague definitely changed their recipe at some point.

 

[Khron]

Sprague definitely changed their recipe at some point.

Why would they use (or waste) more material than they need to, to get the (same) job done?

 

[rock soderstrom]

Unwise. Electrolytic caps need to be vented to release the gasses generated during use. That is why they have vent holes on their rubber end caps. Sealing them up provokes an early death.

Really? I think an electrolytic capacitor emits almost no gas in normal operation, only in the event of a fault or polarity reversal does this occur to a greater extent . Modern electrolytic capacitors have predetermined breaking points and degassing valves to prevent an explosion. This is therefore a safety measure, which may be suppressed by potting it with hot glue etc...

 

[soulsonic]

Really? I think an electrolytic capacitor emits almost no gas in normal operation, only in the event of a fault or polarity reversal does this occur to a greater extent . Modern electrolytic capacitors have predetermined breaking points and degassing valves to prevent an explosion. This is therefore a safety measure, which may be suppressed by potting it with hot glue etc...
View attachment 142637

No, they do emit gas at all times in use. As current flows through the wet electrolyte solution, gas is generated. Closing the vent creates a fault that will make it fail much sooner than designed. I learned these things from a quality assurance engineer from the auto industry and it is backed up by study of manufacturer datasheets. Some manufacturers even move the vent to the top of the cap so that the base can be secured with glue without obstructing the vent.

 

[rock soderstrom]

No, they do emit gas at all times in use. As current flows through the wet electrolyte solution, gas is generated. Closing the vent creates a fault that will make it fail much sooner than designed. I learned these things from a quality assurance engineer from the auto industry and it is backed up by study of manufacturer datasheets. Some manufacturers even move the vent to the top of the cap so that the base can be secured with glue without obstructing the vent.

I am fully aware that electrolytic capacitors have minimal out-gassing in normal operation (I used the word "almost"!), because this leads to the drying out of the capacitors. However, these are very slow processes which, in my opinion, should lead to virtually no reduction in lifetime. Incidentally, outgassing takes place with or without a pressure relief valve. This is actually intended for the event of a fault and should not be closed. That would be my first reason for refraining from potting.

Your statement/experience makes me curious, do you have any further links or studies? I like to learn, without any irony.

 

[soulsonic]

This is a thread about refurbishing a Twin Reverb, and I do not wish to see it further devolve into a discussion about the reasons it is unwise to block a capacitor's vents.

 

[Quantum J Audio]

I had good results increasing the bias cap from 50v 50uf to 100uf 100v. I think the silverface also has a 10uf to ground on the bias circuit. If not present worth putting in, If I remember correctly I went to 25uf to ground, both these made noticeable improvements to hum.

I also elevated the filament heaters with noticeable improvements.