Fender Amp Loud AC noise - Page 1
Forum section: electronics / repair

author: wguibas
author_rank: Unknown
timestamp: December 19, 2015, 09:05:19 pm
content: A friend of mine asked me to look at his Fender amp, when you plug it in and turn it on after a few seconds it starts to buzz/hum and after a few more seconds gets annoyingly loud. Any ideas where to start, this thing is from before my time. Just noticed after removing the metal cover, it has paper caps, I'll check those first. Also, are the big tubes supposed to give off a lot of heat, like a portable heater?Here are some pics.

author: Johnny10
author_rank: Frequent Contributor
timestamp: December 19, 2015, 09:20:24 pm
content: How old is this amp?

author: Johnny10
author_rank: Frequent Contributor
timestamp: December 19, 2015, 09:24:15 pm
content: You may want to take a look at Uncle Doug's you tube videos on vintage tube amps.

author: wguibas
author_rank: Unknown
timestamp: December 19, 2015, 09:31:38 pm
content: According to wikipedia 1964?, "The AA763 (July 1963) and improved AB763 (March 1964) circuit is arguably considered the "best" circuit version produced for this amp by collectors and aficionados. The complex brownface "harmonic vibrato", however, was replaced by a simpler electro-optic oscillator. Power tubes were the even beefier 6L6GC. Amps retrofitted to this circuitry are described as having been "blackfaced," a reference to the black faceplate used on these amps during much of this era."I just found the problem. One of the legs from a paper cap was broken off its solder joint. I resoldered it and the noise is gone.

author: tautech
author_rank: Super Contributor
timestamp: December 19, 2015, 09:36:45 pm
content: Quote from: wguibas on December 19, 2015, 09:31:38 pmNote these are paper coveredElectrolyticsfrom yesteryear, very common before plastics were mainstream.

author: Johnny10
author_rank: Frequent Contributor
timestamp: December 19, 2015, 09:40:08 pm
content: CoolThat was a quick repair!

author: wguibas
author_rank: Unknown
timestamp: December 19, 2015, 09:47:14 pm
content: Got Lucky! 1960's circuitry looks like a bodge job, guess I don't have to be too neat with my repair.

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 20, 2015, 11:26:41 am
content: That's the bias cap, arguably the most important cap in the amp. Imho, it should be replaced, along with the other paper caps located under the dog house. Note the polarity of the bias cap.Additionally the ac "death cap" under the power switch should be removed and a proper 3 wire power cord should be installed with the ground wire bolted to the chassis for protective earth.

author: wguibas
author_rank: Unknown
timestamp: December 20, 2015, 12:52:36 pm
content: Thank you for the advice.. I cleaned the 1/4" jacks, they were pretty oxidized. Should I replace those also? Can I use modern day electrolytics from a reputable brand on mouser or do I have to/should buy from a place like angela.com and get what they are offering (sprague)?

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 20, 2015, 01:04:37 pm
content: Any cap from Mouser of the same or greater voltage and similar capacitance would be fine. That being said, suppliers such as antique (tubes and more) or angela will have the more appropriately sized axial elytic caps.The jacks are probably better than anything you can buy today, really.Try to leave as much of the amp origional, there is no nead to change any of the signal or coupling caps with modern caps if they are not electrically leaking.Updating the elytics and power cord should get another 50 years out of the amp. Nice amp for sure.

author: jitter
author_rank: Frequent Contributor
timestamp: December 20, 2015, 01:10:44 pm
content: Quote from: wguibas on December 19, 2015, 09:47:14 pmI believe the correct term for that "bodge job" would be "point to point construction"...Wow, an amp from the time when a musical foursome from some harbour town in the north of England was quickly becoming a world-wide phenomenon...

author: Seekonk
author_rank: Super Contributor
timestamp: December 20, 2015, 01:53:16 pm
content: Keep listening to all this advice and it will be improved to the point that it doesn't work anymore.That said, high voltage caps these days can be expensive. 470uF 200/250V caps as found in old PV power supplies are quite common and cheap. Two or more in series will add up to the voltage you need. Just be sure to parallel a 300K or so resistor on each one to balance up the voltage and use at least two or three times the capacitance.

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 20, 2015, 02:07:07 pm
content: Quote from: Seekonk on December 20, 2015, 01:53:16 pmCare to elaborate? Nothing has been suggested beyond standard maintenance and safety update procedures?

author: tautech
author_rank: Super Contributor
timestamp: December 20, 2015, 08:27:28 pm
content: Quote from: jitter on December 20, 2015, 01:10:44 pm+1

author: Brumby
author_rank: Unknown
timestamp: December 21, 2015, 12:27:46 am
content: Quote from: john_p_wi on December 20, 2015, 02:07:07 pmFundamental safety steps are something I would never discourage, but whether 'improvements' result in an amp that's no longer operational or not, there is another aspect of equal importance.....Many years ago I was treated to an education in guitar amplifiers - by a guitarist. If you talk to anyone who plays lead guitar with any seriousness, they will rave on about the 'sound' or 'tone' of a particular brand or even model of amp. It's similar to the valve -vs- solid state debate with audiophiles - but on an intensity that is an order of magnitude or two higher. Fender is this... Peavey is that... He was a Marshall man - through and through.What it comes down to is - the distortion. The distortion characteristics are what give different amps their signature 'sound'. Overdriven stages, supply voltage sag and less than ideal signal handling all add up to affecting the signal.'Upgrade' the wrong thing and you could change the sound - which is not likely to win you any new friends.

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 02:03:41 pm
content: Quote from: john_p_wi on December 20, 2015, 11:26:41 amI totally agree: if there is a 3 position power switch change it with a new one, a simple dual pole, dual position (on-off both live and neutral) and remove the capacitor that connect earth to line or phase.Line cord must be replaced.A friend of mine was hospitalized after being electrocuted by his guitar when the "noise killer" cap went short, and his guitar went live.The OP question about heat generated by tubes let me believe he has not a great experience with high voltage, tube amp electronics: please be careful.Best regards

author: Seekonk
author_rank: Super Contributor
timestamp: December 21, 2015, 03:22:00 pm
content: There are a lot of "capacitor changers" on this site. I suppose this gives them some sense of validation. The poster is a neophyte from all indications. He has found the problem and resolved it. To continue further seems unwarranted. Working with old equipment without experience can cause new problems. Oxidized solder connections require addition of new solder. Blobs of solder may short out. Attempting to remove old wire leads may break terminals. Tinning on leads may have oxidized making new solder not stick. There are a whole host of things, stuff happens. Replace a bunch of components and you will be responsible anything that happens to this in the future.

author: wguibas
author_rank: Unknown
timestamp: December 21, 2015, 08:43:03 pm
content: Looks like I'm gonna upgrade all the paper caps and the 7 caps in the preamp section. Upgrade the power connection and ground lead. Doing a lot of reading on the internet, it seems this amp has a master volume control install after its manufacture. This is apparent by the knob covering the words "twin" on the face plate. and the use of a non existent in the 60's cable tie holding its wires. I thank everyone for your suggestions and info.

author: ciccio
author_rank: Frequent Contributor
timestamp: December 21, 2015, 09:04:48 pm
content: Quote from: wguibas on December 21, 2015, 08:43:03 pmSorry if my comment was misunderstood: I was not criticizing your competence...Best regards

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 21, 2015, 09:12:39 pm
content: Please trust me on this one, I've been knee deep in amps for more than 20 years - DO NOT change the blue molded jupiter caps in the pre amp section unless they are electrically leaky or produce dc on a pot. There is absolutely no truth to "orange drops are better" or other snake oil caps that are spewed about on the internet. Do to RoHS, changes in polyester substrates, cap manufactures being bought and sold etc, caps today will not sound the same, they certainly are not made the same.Additionally, only change the plate load resistors / signal resistors if there is popping or other artifacts due to the CC resistors - they have drifted over time, ie aged, and replacing with new caps and resistors will change the tone of the amp - in effect, returning the amp to its' infant tone of 50 years ago.As I said previously, start with replacing the paper electrolytic bias cap and HV filter caps under the dog house - stop there and evaluate before continuing. Look at Hoffman amps for more info / parts, mojo, antique etc. My choice would be Hoffman or antique (tubes and more). Doug (Hoffman) is a damn good guy, and hosts quite an active amp forum.

author: wguibas
author_rank: Unknown
timestamp: December 21, 2015, 09:25:00 pm
content: ok, thanks I'll keep the blue ones, I read that elsewhere also. You can get carried away with what you read on the internet.

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 21, 2015, 09:35:59 pm
content: Quote from: wguibas on December 21, 2015, 09:25:00 pmI wouldn't bother with those caps, unless you know one or more are defective. Those caps work with the cathode bias resistor, in effect setting the amplification frequency of that particular stage. Changing these could make a dramatic change - not necessarily for the better. IF you must change, I have had good luck with Nichicon and Panasonic caps in this position. Again, don't fall for the snake oil caps. We use those (cathode bypass) caps along with the coupling caps (the blue molded jupiters) to tune amps to the player and their style, playing with those make a dramatic change in tone - be careful with changes.

author: wguibas
author_rank: Unknown
timestamp: December 21, 2015, 09:41:17 pm
content: Sounds good, I just ordered the 6 power/bias caps to replace the paper ones. I'll let you know how it turns out.

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 21, 2015, 09:47:30 pm
content: Good luck.Again, watch the polarity of the bias cap (the one with the broken lead). Cap + lead is grounded.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 22, 2015, 12:32:37 pm
content: Quote from: john_p_wi on December 21, 2015, 09:12:39 pmI really agree with john on this. I have been working on tube amps for 35 years.Also while you are at it you may as well do it right. Remove front panel controls and clean the mating surfaces. These corrode over time and cause all sorts of nasty noise.Also look over the turret boards. See if any are damaged. Note some turret boards over years build up with moister and causes other problems. Don't know how many I have had to replace due to this.Good luck on your project.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2015, 05:27:21 pm
content: Quote from: Radio Tech on December 22, 2015, 12:32:37 pmVery good point.Yes, tag strips from before the days of Phenolic materials could absorb moisture.

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2015, 05:40:52 pm
content: Uh, tag strips are almost always made from tinned steel/brass and SRBP, which is a phenolic resin bonded paper. Only ones that are not is the tag strips TEK used in the older scopes, which were ceramic with a silver flash to make the joining places.

author: john_p_wi
author_rank: Regular Contributor
timestamp: December 22, 2015, 06:11:44 pm
content: Fender amps of this era used "fish paper", basically densely compressed card board, for the circuit board material. They can become conductive if the amp is stored in humid conditions.

author: gadget73
author_rank: Regular Contributor
timestamp: December 22, 2015, 10:32:01 pm
content: Good practice to swap out the filter caps. I've got an amplifier here for a friend of mine that burned out it's power transformer when one of the B+ caps shorted. I was able to come up with a suitable transformer thanks to another friend, but its not always so easy. Likely more support for guitar amps than hifi stuff, but if you've ever smelled a baking power transformer, its not real nice. Most of my dabbling with electronics is on tube hifi amps and antique radios, and I've had my fair share of dead filter caps, and a couple of times I've had power transformers that were dead because of it. The fact that a lot of old gear had no fuses makes the problem much worse.

author: cncjerry
author_rank: Unknown
timestamp: December 23, 2015, 06:25:31 am
content: Quote from: Seekonk on December 21, 2015, 03:22:00 pm+1. Lots of cap changers on this site. Also, at some point in digging into this amp, if you go further than needed, something will break including your friendship as you try to explain, "...all I did was change a few old capacitors out, I didn't blow that transformer!" There is a lot of vibration though, instead of swapping the large paper caps out, maybe you can wire-tie them together.Reminds me a of a friend who's computer was running slow. I put more memory in it, cleaned up the c: drive which was about full, took out about a million temp files, emptied the trashcan, etc. How the hell was I supposed to know that he used the trashcan as a folder? Were's all my pictures? Actually stored things in there. I worked on the PC before it was released in '80-'81 and am still learning things.

author: Seekonk
author_rank: Super Contributor
timestamp: December 23, 2015, 01:52:52 pm
content: Film capacitors are probably the farthest component from an ideal model. I've analyzed a lot ofcapacitors on my ESI. I've looked at black cats, black beauties, VitiminQ, orange drops etc. looking for some quality that makes them special. I've sold hundreds of dollars of vintage caps on ebay, they were some of my best sellers. No problem taking their money, but I wouldn't put any of those caps in my stuff. Most capacitors are pretty crappy. When nobody would buy a polyester suit they said lets make capacitors with this stuff. It is hard to imagine how all this stuff works with the capacitors out there. Flavoring circuits with certain kinds of capacitors seems like something to avoid. Polypropylene MKP capacitors seem to have the best performance for audio in the reasonable price range. If I miss anything about the old days it is when capacitors still had metal in them.The TRASH folder is my catch all file. I never delete anything there.

author: gadget73
author_rank: Regular Contributor
timestamp: December 24, 2015, 04:04:42 am
content: I guess I am something of a "cap changer", but most of the gear I work on is at least 50 years old at this point. I do have some really old caps in service that test OK, but there are certain things I just won't trust to keep working. Wax-sealed paper capacitors, high voltage electrolytics, and any line to chassis capacitor falls squarely into that realm.

author: Seekonk
author_rank: Super Contributor
timestamp: December 24, 2015, 05:22:07 pm
content: For grins I looked up a big capacitor company most have never heard of. A company I used to work for had a lot of custom caps made by them. Nice people to deal with if you have a volume application. If I still had that connection I could get a bunch of free samples. I see they have gotten into a bunch of custom audio caps.http://www.bishopelectronics.com/audio_capacitors.php

author: dfmischler
author_rank: Frequent Contributor
timestamp: December 25, 2015, 01:21:08 pm
content: As a former owner of a similar Fender amp (wish I still had it, it sounded great) I want to repeat the advice to leave it alone as much as possible. Even the inability of the power supply to keep up when the amp is driven hard affects the output stage distortion that is a big part of the sound. "Upgrades" of parts that haven't failed are more likely to do harm than good.

author: wguibas
author_rank: Unknown
timestamp: January 09, 2016, 09:19:34 pm
content: So I finished replacing the paper caps, All is good. I changed the power cord to a 3 prong hooking the hot/black to the fuse, then on/off switch then transformer feed. The White/neutral to the other transformer feed and the green with a ring grounded to a transformer mounting bolt. Disconnected now are the ground switch and cap and the aux power socket.The amp has 1 blown 12" Fender Speaker and Blue unlabeled 12" Speaker. We are going to put a set of Ted Weber 12F150's.I also changed all the 1/4" jacks because they were very dirty and made a lot of noise.The only other issue I notice is the vibrato volume control is very dirty, popping and scratchy. I'm gonna order a new pot and swap it out.Thanks for all your help and suggestions.