How to fix Fender Tweed DeLuxe 1955 Leaking Caps

Leaking non-electrolytic capacitors are a common problem on older amps. Here's how to diagnose faulty ones.

[@davidsimpson3380]

Thanks for the video Stuart! and thanks for the troubleshooting tips!

[@vintageguitaramp_guitarteacher]

Lovely old amp Stuart and another great video. Thank you. Hope you are well.

[@stuartukguitarampguy5830]

Thanks very much. Yes all well here.

[@peterday7077]

Good job Stuart. And a new Stuart video is always welcome. 🙂

[@stuartukguitarampguy5830]

Another one coming soon!

[@thesloaneranger1]

Another interesting video... you are literally the only person I know who can make amp repairs fun as well as educational

[@stuartukguitarampguy5830]

Thanks very much!!

[@edwardhannigan6324]

Great to see those old legends Stuart..Interesting fix and well done on the outcome..Certainly a prize amp to own..Thanks for sharing and look forward to the next relic..Ed..uk..😄

[@stuartukguitarampguy5830]

Thanks Ed it was a nice little amp.

[@DANWRIGHTITIS]

Great Video Stuart!! WOW - what a great looking amp - don´t see many Brits taking on a PROPER Fender Repair ! I´m a big fan of your Ópposite Number´ Uncle Doug across the pond, too . He´d´uv spilt his coffee checking out the condition of this beauty!! 🤠Cheers from Madrid

[@stuartukguitarampguy5830]

Yes I love uncle Doug's channel also Terry over at D-lab.

[@wadehicks9270]

If you can do a video on the second amp. I like how you go into great detail while explaining what your doing. A fella can learn a lot by watching your videos. Great video as always be safe.

[@stuartukguitarampguy5830]

Yes that's next on my list it's a 1959 tweed.

[@keiranbradley3238]

Always an education watching you work Struat. It's great to see what these old amps look like inside, it's crazy money that they go for, I think there's a lot of "magic dust" inside folks ears, not the circuits!. Haha.

[@stuartukguitarampguy5830]

Yes agreed!

[@patreidcocolditzcastle632]

ive a kit version of this.all f&t and Sprague caps .a jensen speaker.its one of my best amps.just great sound for dirty tones and super easy to work on.i much prefer these and plexis compared to jvms,or dsl type amps.there just a head ache and for very little tone.you can make these old tweeds roar like a lion if you have the right hands..loved this video it helped me a lot.your so onto it Stu..ps because mines a kit version i can put all the modern caps in and i bet it sounds better then old amps with old caps.even if i had the old version id still want new caps and keep the old caps in a bag for resale as im a player and gigging guy

[@stuartukguitarampguy5830]

Thanks Pat

[@patreidcocolditzcastle632]

no problem love your work and im learning a lot@@stuartukguitarampguy5830

[@petercornell2002]

Great vid Stuart, many thanks

[@stuartukguitarampguy5830]

Glad you enjoyed it.

[@electrolytics]

Thanks Mr. Stuart. Rock and Roll.

[@stuartukguitarampguy5830]

Cheers

[@user-qm7nw7vd5s]

My favorite new KZbin tech channel! 👍👍

[@stuartukguitarampguy5830]

You should get out more! :)

[@5barkerstreet]

only thing is that he cabs things together

[@michaelevans3852]

Wow! Amazing litte amp. It's always nice to see an example of a vintage tube amp such as this. As always, I'm amazed at how these parts hold up for so many years. Thank you for sharing your expertise with us again. I love the way you checked for leakage while in circuit. What brand of cap did you use on this amp?

[@stuartukguitarampguy5830]

Hi Michael thanks for the feedback. can't recall the cap make, sorry. It's often several months before I make the video after doing the work.

[@nicksregor4208]

Those electrolytic caps were absolutely stuffed with more modern replacements

[@edwardhannigan6324]

Love your work by the way and the info attached for us newbies..It's a pity that there are one or two comments here from armchair electronic ghosts trying to give advice to you on your content and methods without having anything to show for themselves..! I happen to be a test pilot with my own F-16 and a I am also brain surgeon on the weekends..Sorry I have no content to show though..🤣🤣🤣 Keep on doing a great job Stuart top channel..Ed..Uk..😃

[@stuartukguitarampguy5830]

Goes with the teritory I'm afraid. ANYTHING you put up on line will attract negative comments. "I saved a dolphin from drowning today." "Good for you, but what about all the turtles that need rescuing? Too busy saving dolphoins to care about those I guess."

[@edwardhannigan6324]

@@stuartukguitarampguy5830 Yes I agree, just save what you can Stuart; including old amps..😊 Ed

[@sjgreaves]

I beleive the grounding scheme used on the original 5E3 was a bit iffy and likely contributed to hum. Using a 12AX7 for V1 is very common in these, even Fender did it with sone of the re-issues.

[@goodun2974]

The main reason for the audible hum is because the circuit has an unbalanced heater supply, where the chassis is used as the return for the AC current that heats the tubes. Notice that one green wire from the power transformer is soldered to the chassis, and the other green wire loops to the pilot light and then to all of the tube sockets, to just one heater pin of each tube, and just one side of the lamp socket. The other heater pin of the tubes, and one side of the pilot lamp socket, are connected to the chassis instead of being wired back to the transformer. This means that the audio ground becomes modulated by the 50 Hz hum of the AC voltage that heats the tubes. Later Fender amplifiers used twisted *pairs* of wire jumping to all the tube sockets and to the pilot light, and also had a pair of resistors connected from the heater circuit to the chassis to function as an artificial centertap. This lowers hum significantly and also protects the power transformer in case of tube short circuits because the resistors in the artificial center tap (typically two 100 ohm 1/2 watt carbon film) would act like fuses and burn open. It's a royal pain to rewire an amp like this to the more "modern" twisted pair, centertapped heater circuit, but it really pays off in improved clarity and sound quality, and a slight increase in headroom and power output, because the amp doesn't waste power attempting to amplify a 50 Hz hum signal (50 Hz in the UK; in America it would be 60 Hz).

[@jtbracknell]

Nice job Stuart. Question about the megger, can you test the caps in circuit?

[@stuartukguitarampguy5830]

No unfortunately as there is often a DC path to ground somewhere else in the circuit which spoils the measurement. E.g. think of a cap with say a 1M resistor in parallel with it. I you tried to mesaure the cap you'd get a 1M resistance reading.

[@bjornstromberg7975]

Hair-dryer test was new for me. Thanks. I've never used a "Megger" to test Caps with , but OK. BUT don't try a Megger (Isolation tester) on caps rated under 500V or you may hear a loud "POOF"... ;) (Sure you know, but maybe not all viewers)

[@WolfgangGrimm-xm4nw]

Hi Stuart, on the grid of the phase inverter tube there MUST be a DC offset, otherwise it would not work. The 1 MEG resistor has no virtual earth but it's on cathode potential.

[@stuartukguitarampguy5830]

Yes true. All those caps were bad though.

[@jothanankrogh]

On suspect leaky signal caps, I will unsolder and lift the 0vdc lead off the board and retest for dc. On many Fenders I've worked on the small dc voltage from a suspicious cap was actually board conductivity, and the dcv error was coming through the board from the nearby high voltage eyelets.

[@stuartukguitarampguy5830]

Ah useful thanks. I never try to measure cap leakage in circuit. I always remove one end like you say.

[@benwright6330]

..70 and sharp as a whip

[@stuartukguitarampguy5830]

SWISH... CRACK!!!

[@tjsogmc]

Nice old amp. I just picked up a 1966 Ampeg B15n with all original caps. And would you know it? Every cap tested excellent. The electrolytics and the film caps both were just fine. I replaced them anyway because there's just something I don't trust about a 57 year old capacitor, no matter how it tests.

[@stuartukguitarampguy5830]

Yes it's interesting how one amp can have almost every cap faulty, and another amp (just as old) can have all caps good.

[@jothanankrogh]

Same here, my 66 B15NF has perfect signal capacitors, altho I did go ahead and replace all electrolytics among many other things as it was a heavy restoration.

[@jimirox1928]

Excellent work Stuart! Do you have any sure fire way to tell a 60hz hum apart from 120hz hum? If I hear them together or one directly after the other I have no problem telling which is which but If I just plug in a amp that hums I usually end up inserting different style caps in different locations to see if I can get it to stop humming or at least get it to quiet down. My oscilloscope is often helpful but not always. Thanks again for your time and for sharing your vast knowledge. Rob.

[@goodun2974]

The main ripple and hum measurable/visible by oscilloscope in an amp with a full-wave rectifier (such as is used in this tube-rectified Fender, or even a later higher-powere Fender with solid-state diode rectifiers), or from a solid-state bridge rectifier, will be 120 Hz, double the mains AC frequency here in America (50 Hz mains in the UK generates 100 Hz ripple). In this case, however, the amp has a significant amount of 50 Hz hum *not* related o rectifier and filter caps but instead because one leg of the 6 volt AC heater-circuit wiring is grounded to the chassis, which functions as the return for the heater circuit. Look at this schematic and you'll see the grounded, unbalanced heater wiring, and then look at the amplifier in the video and see that one green wire from the power transformer is soldered to the chassis and the other wire loops to the pilot lamp and then over to the tubes. This means that the AC current to heat the tubes can modulate the audio signal ground which is also passing through the chassis. Later Fenders, and especially their larger, more powerful amps, used twisted pair wiring to all the tube sockets as well as to the pilot lamp, and a pair of 100 ohm half-watt resistors were used to form an artificial center tap from the 2 wires of the heater circuit to the chassis, which also afforded protection for the transformer in case a tube short-circuited. It's a royal pain to rewire an amp like this to a "modern" twisted-pair, balanced heater circuit with artificial center tap, but it pays off with greatly reduced noise, increased clarity, and perhaps even a little more headroom and power, because the amplifier is no longer working hard trying to reproduce a 50 Hz noise signal.

[@jimirox1928]

@@goodun2974 Thanks very much Goodun for the very helpful information.

[@goodun2974]

@@jimirox1928 , you're welcome; and note that I edited my comments for the sake of clarity and fixed some grammatical errors and oopsies added by autocorrect, so my comment will make more sense if you reread it now.

[@stuartukguitarampguy5830]

Good 'un below has answered this very well. Actually, I missed that when I was working on this amp otherwise I might have considered rewiring it.

[@ricobass0253]

Interesting to see that Fender used a single triode phase splitter in this amp. Rather than the usual differential pair, this produces an in phase signal at the cathode and an inverted signal at the anode. The equal 56k cathode and anode resistors ensure the signals are equal in amplitude (but not source impedance ). N.B the d.c. voltage at the grid will not be zero in this case, but will sit at around a third of the h.t.

[@stuartukguitarampguy5830]

Thanks. Yes that's quite a rare configuration. I missed that grid voltage! No matter as I changed all the signal caps in any case.

[@jothanankrogh]

This also is used in the later Princeton models

[@itsonlyme9938]

Hi I have purchased of ebay an ESR/capacitance meter and checked the capacitance of one 2200 MFD cap which was showing 3300 MFD its for a Farnell bench PSU I decided to change it.

[@stuartukguitarampguy5830]

Okay great. I find those meters are not good at the very high microfarad caps so I suspect your original cap was good.

[@motokev2727]

Wow, 1955 amp. Would love to own that one.

[@stuartukguitarampguy5830]

Yes sweet!

[@SIXSTRING63]

Surprised Bonamassa hasn’t snatched it up. I think he owns 75% of vintage tweed Fenders in America.😝😝

[@stuartukguitarampguy5830]

Ha ha!@@SIXSTRING63

[@rjprivate]

Ow please a '59 champ needs a video. You have to show that

[@stuartukguitarampguy5830]

It's my next vid!

[@rjprivate]

@@stuartukguitarampguy5830 awesome, looking forward

[@motokev2727]

When using your insulator meter for capacitance and current leakage. Does it matter how you hook up the alligator clips to the capacitor, i.e. the polarity ? Thanks

[@stuartukguitarampguy5830]

Hi, no it doesn;t matter which way around you do it as the caps are non-polarised. The meter is actually measuring resistance. A capacitor shoud have zero DV resistance. If it shows any resistance it is leaking and needs to be changed. The only difference between this meter and a normal bench multimeter is that this meter measures with a high (say 500V) DV voltage across the cap to force any potential leakage (i.e. to see if the cap breaks down at high voltages).

[@waynegram8907]

STUART, why does applying heat or a hair dry make the leaky capacity worse? Those coupling capacitors aren't using electrolytes. The heat of the capacitors dielectric material gets more leaky with heat

[@stuartukguitarampguy5830]

Hi, you know that's quite a good question and I don't really know the answer! Anyone?

[@goodun2974]

I will hazard a guess that either the heat makes the overlapping foil layers in the capacitor expand and get closer to each other, reducing the insulation factor between them, or perhaps it further vaporizes and activates the moist air that has infiltrated the capacitors over the decades. Note that the Baker light show of many vintage capacitors is actually hydroscopic, and not resistant to moisture infiltration; If and if the capacitor leads are manipulated a lot in the process of installing them it might crack the seal where they enter the body of the capacitor and allow away from moist air to more easily get inside. Every time you use the amp and the capacitor heats up, and then subsequently cools when the amp is turned off, you will create a vacuum inside the capacitor that tends to draw air inside of it.

[@guitfidle]

How did that noisy pot sound again? 😁

[@stuartukguitarampguy5830]

SSchhh...schhh...schhh....schhhh. From memory.

[@guitfidle]

🤩

[@goodun2974]

Stuart, it's not surprising that the amp still hums: it has an unbalanced heater circuit and so the audio ground is being modulated by the (UK standard) 50 HZ AC that heats the tubes. You might not hear a lot of 50 Hz because the small speaker and small output transformer will limit it somewhat, but the 50 Hz hum is still there and the amp is forced to waste power attempting to amplify it. Although it's a royal pain to rewire such amplifiers to a more "modern" twisted pair heater circuit with a pair of resistors as the artificial centertap, it pays off in increased clarity, reduced hum/noise, and possibly a little more headroom or power output ( less intermodulation distortion, and the amp isnt being forced to waste power amplifying 50 Hz ripple). Also, If a tube short circuits, the artificial center tap resistors will burn open and act like fuses to protect the power transformer. I call that a win-win situation!

[@stuartukguitarampguy5830]

Yes thanks for that, I maybe should have done that on this amp. Tbh I completely forgot about that cause for the hum, forgetting I was working on such an old amp.

[@YeatzeeGuitar]

Filter caps look stuffed

[@stuartukguitarampguy5830]

You can't tell that that just by looking. Surprisingly these checked out fine. The amp isn;t gig critical and so these can always be changed at a later date if they go bad. The customer wanted it left as original as possible.

[@YeatzeeGuitar]

@@stuartukguitarampguy5830 looks like the new cap is bulging out of the top and bottom, can see the silver where there wouldn't be any with original astrons (looking at a set right now). Either way great vid, enjoyed it.

[@gingercat777]

Surprised you didn't replace the filter caps, they will fail sooner than later .

[@goodun2974]

Yes, they will fail, and quite possibly blow up the power transformer as well; also, the heater wiring is unbalanced and doesn't have a pair of resistors as an artificial centertap ---- the resistors tend to burn open like fuses if a tube short circuits.

[@themaninthesuitcase]

I’m not convinced they are original. Iirc those old minimite caps have orange caps, and those look to have a metal ones. I think these are modern caps stuffed into the old cases.

[@stuartukguitarampguy5830]

Normally I would, but as I said in the video I was under instructions from the customer to do the bare minimum. I work for the customer and do what they want, it's their call.