Fender Amp Loud AC noise

[tautech]

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.

I 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.

Very good point. 
Yes, tag strips from before the days of Phenolic materials could absorb moisture.

[SeanB]

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.

[john_p_wi]

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.

[gadget73]

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.

[cncjerry]

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.

+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.

[Seekonk]

Film capacitors are probably the farthest component from an ideal model. I've analyzed a lot of
capacitors 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.

[gadget73]

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.

[Seekonk]

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

[dfmischler]

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.

[wguibas]

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.