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

[heatwalk]

Back to the original topic - having serviced vintage fenders every couple weeks for our music store and having seen quite the spread of work done inside old amps, not once has the thought of replacing the entire board been necessary… what a hassle you would make for yourself. Yes they can warp and bend over the years but servicing the eyelets is much less invasive than a rebuild and has never not worked out except a single amp that had turned into a salt lick and was conducting on the rectifier board. Doing that extensive of a job is limited to people on forums with too much time on their hands and the payoff I cant imagine is ever worth the labor.

Once the caps are replaced turn it on and take a chopstick and tap around on the board and wiring and find microphonic joints. This era has stranded core so less of an issue compared to solid. Some joints will visibly have little holes forming. Its important to remove existing solder first instead of simply reflowing. I use the little red rubber solder sucker bulb as you can get a good amount removed and they are easy to unclog with an allen key or screwdriver vs the button style suction guys.

The more things you replace the less value it will hold when you sell it down the road, and the more rebuilding the higher chance you create new problems for yourself. If plate resistors are crackling replace them but otherwise I would leave them alone. You can always take the few minutes to replace them later if need be. The 470Rs on the power tube sockets do burn open somewhat often so if you are feeling inclined those can be worth replacing.

If you have a high wattage iron you can help with grounding by soldering the leads running from the cathode caps to the brass plate by scoring the chassis in the middle of the leads and soldering the wires there, but if your iron cant keep the heat it doesnt really work out.

 

[Alva Goldbook]

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

The board was never wet. Fender's fiber boards just bow over time. It's what they do. Leave the original board in.

You should replace ALL the electrolytics, particularly the filter caps, regardless of whether they have failed yet or not. There's no reason to leave them in, they've done their job, and the amp will sound better with new caps. This is a fact, anyone who says otherwise doesn't know what they're talking about.

The board will also have small electrolytics caps, 25uF 25-50v. These are mostly used as cathode bypass caps for the preamp tubes. They need to go. The coupling caps, most often the blue molded caps are usually still good and should stay.

You can replace the cathode bypass caps with new electrolytics, or you can look into MLCC's.

 

[CJ]

i can attest to the labor involved in swapping out a turret board having built a Vibrolux from the ground up from a Mojo board . I left some space inside the cap covers when i stuffed the radial caps in there, i used to build pipe bombs as a kid, fill it full of match heads and crimp the ends with a huge vice, put a ciggy fuse on there and run like hell, good Lord i am lucky to be alive after all the dumb crap i did, God looks after fools and drunks,

Fender used to dip the boards in wax, CBS quit doing that, which is why you see more warpage on the 70's stuff.

I have ruined many an amps sound by changing parts. Leaky coupling caps can work magic if they leak right,
If it sounds good, play it til it blows, because once you start changing parts, the magic will be gone forever.


There was an Epiphone chassis i gutted and built a EL34 circuit on, it did not have reverb so i cobbled in a way to add a pan, but it worked funny, when you turned the reverb up a bit, the gain went up too. It sounded like heaven on earth at the right settings, talk was getting around town. People were coming to my shows to witness the new kid on the block with the magic amp. Them i took it apart and tried something new without making a schematic. Curse myself everyday.

I have played about 100 Twin Reverbs, there was only one that was magical, it was the at the Git Center in San Jose, CA. Never heard anything like it since. Too old to play loud, use a Princeton Reverb with an Emmy Alesendro converted to Amperex EL84's and Fisher output xfmr. Right now i am installing a Fuzzrite Germanium circuit board into an Epiphone Les Paul set up for C# and a overwound P90, feeding it into a Creepy reverb stomp box and the sound is unreal. My job is my hobby, how lucky can one man be?

And dont forget to check our site for all the coolest equipment in town>

https://reverb.com/shop/capital-city-guitars

 

[CJ]

my favorite mods to a twin reverb are RCA black plate power tubes and a 12AY7/6072 for the preamp tubes,

sad to learn that Gerald Weber passed away,
here are some of his mods for a super reverb but most of them apply to a Twin except the rectifier tube>

 

[Silas]

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.



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.

Hmm yeah I was wondering where I could add something on the chassis for a negative feedback defeat circuit too... may have to think inventively for that.

Bias wise I may just add a bias trimmer (internally) additionally to the bias balance as suggested on Rob Robinnette's site.

 

[Silas]

my favorite mods to a twin reverb are RCA black plate power tubes and a 12AY7/6072 for the preamp tubes,

sad to learn that Gerald Weber passed away,
here are some of his mods for a super reverb but most of them apply to a Twin except the rectifier tube>

Great, thanks CJ, very interesting

 

[CJ]

here is an early 60's Sprague capacitor, 8 uf at 250, measures 11.5 uf and has absolutely no leakage from the git go.

there is no evidence of voltage breakdown,

the lead attachment points look perfect.

there is plenty of fluid left on the paper.

i am sticking the one i did not cut up back in the Standel Reverbalux that is on the bench.

do you always need to install new caps in everything? not always.

 

[heatwalk]

I am with you CJ that a lot of the old fender filter caps are surprisingly ok from a performance standpoint. I just get told to routinely replace them by store owner so we can sell as recapped but I can’t remember encountering a shorted filter cap in a vintage fender in the past few years. Ripple on the first stage sure so worth replacing if its humming away. Even with leaky crust on the ends of the vintage caps sometimes the amp will still be quiet.The failure rate of gray Illinois caps on PCB era is universally worse even though they are decades younger. The caps that I find the most issue with in the olds ones are the yellow astron caps used in vibrato circuits which have have grown tired over the years making for a sleepy vibrato. Trying to get a harmonic vibrato working on a brownface while keeping old caps in can prove quite challenging. Replacing the cathode caps on the verb and vibrato preamp usually improves performance with respect to those functions more than a full filter recap.

Anyone do much work on old Sunn amps? Those Mallory multicans are probably most notorious IMO. And the chassis of Sunns are always so rusty, its like every one was kept in a damp barn for a few decades and home to spiders and mice. Had to drill through one of the chassis bolts last year to get it out of the head sleeve.

 

[Alva Goldbook]

Old Sprague filter caps should go in the bin. There's no reason to keep them. Their performance even brand new is poo compared to modern made caps. Their age makes them even worse. Electrolytic caps are one of the few electrical components that have a stated life span. Even brand new their life span was maybe 500-1000 hours. Axials made today have a life span of 2-4 thousand hours. Radials have up to 20,000 hours. Every time I repair an amp and have to replace the filter caps I'm stunned by how much better the amp sounds with new filter caps. It sounds like it did when the amp was brand new. Old filter caps on the other hand sound brittle and lifeless. Electrolytic caps are a necessary evil. In an ideal world, they wouldn't be made. It's just difficult to get that much voltage and capacitance into a component without it being huge and expensive. Film caps do exist that replace them, but they're overly large. But their performance is far, far superior. I've been replacing electrolytics in cathode bypass caps with MLCC's and they sound astonishingly good.

 

[ccaudle]

I've been replacing electrolytics in cathode bypass caps with MLCC's

What dielectric? Do you parallel several, or can you get single MLCC in appropriate capacitance values?

 

[Rusan]

The failure rate of gray Illinois caps on PCB era is universally worse even though they are decades younger.

Back in the early 2000s when I worked on a lot of music gear, I encountered some Illinois electrolytics in 4 or 5 year old Fender and Peavey amps that had deteriorated to the point they were buzzing.

On the topic of high voltage axial electrolytics, I recently ran across the Vishay/BC 04x ASH Series caps. They have an advertised 5K to 15K hour lifespan rating at 85°C, and are priced similarly to the competition. I rarely work on tube gear these days, but they look promising. Has anyone used them?

 

[danfrank]

Hmm yeah I was wondering where I could add something on the chassis for a negative feedback defeat circuit too... may have to think inventively for that.

I typically rewire the ground switch in the back for stock/altered/no GNF. It’s a hole/switch thats already there and becomes obsolete at its old job with a 3 pronged power cord.

 

[Alva Goldbook]

What dielectric? Do you parallel several, or can you get single MLCC in appropriate capacitance values?

I've found values as high as 47uF, 25v. Plus, non polarized.

https://www.mouser.com/c/passive-co...sive-components/capacitors|~Voltage Rating DC

 

[Alva Goldbook]

Back in the early 2000s when I worked on a lot of music gear, I encountered some Illinois electrolytics in 4 or 5 year old Fender and Peavey amps that had deteriorated to the point they were buzzing.

On the topic of high voltage axial electrolytics, I recently ran across the Vishay/BC 04x ASH Series caps. They have an advertised 5K to 15K hour lifespan rating at 85°C, and are priced similarly to the competition. I rarely work on tube gear these days, but they look promising. Has anyone used them?

Wherever possible I use radials. Panasonic makes good stuff. Here's an axial to radial conversion board I made. The caps are in series so you get voltage ratings as high as 700v. You just half the cap value. PCB is the same size as a Sprague 40uF 500v cap. I'm going to start selling these soon.

 

[CJ]

 

[Alva Goldbook]

I typically rewire the ground switch in the back for stock/altered/no GNF. It’s a hole/switch thats already there and becomes obsolete at its old job with a 3 pronged power cord.

I used to do this, but found it kinda useless. NFB for me is a set it and forget it thing. One very handy thing to do is use the switch (have to replace it with a DPDT) is have a fixed bias/cathode bias select switch. You can even bias one to be lower output if that's your thing. The cathode bias has a different tone, especially if you use a lower value cathode bypass cap that's more tweedy in sound. Plus, if you have an issue with your fixed bias causing the tubes to run away into red plating, you can save your tubes by switching to cathode bias and it'll get your through a gig safely until you get a chance to service the amp.