Author Topic: Fender Amp Loud AC noise  (Read 12805 times)

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Offline wguibas

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Fender Amp Loud AC noise
« on: December 19, 2015, 09:05:19 pm »
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.


















« Last Edit: December 19, 2015, 09:15:21 pm by wguibas »
 

Offline Johnny10

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Re: Fender Amp Loud AC noise
« Reply #1 on: December 19, 2015, 09:20:24 pm »
How old is this amp?
Tektronix TDS7104, DMM4050, HP 3561A, HP 35665, Tek 2465A, HP8903B, DSA602A, Tek 7854, 7834, HP3457A, Tek 575, 576, 577 Curve Tracers, Datron 4000, Datron 4000A, uTracer, HP5335A, EIP534B 20GHz Frequency Counter, TrueTime Rubidium, Sencore LC102, Tek TG506, TG501, SG503, HP 8568B
 

Offline Johnny10

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Re: Fender Amp Loud AC noise
« Reply #2 on: December 19, 2015, 09:24:15 pm »
You may want to take a look at Uncle Doug's you tube videos on vintage tube amps.

Tektronix TDS7104, DMM4050, HP 3561A, HP 35665, Tek 2465A, HP8903B, DSA602A, Tek 7854, 7834, HP3457A, Tek 575, 576, 577 Curve Tracers, Datron 4000, Datron 4000A, uTracer, HP5335A, EIP534B 20GHz Frequency Counter, TrueTime Rubidium, Sencore LC102, Tek TG506, TG501, SG503, HP 8568B
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #3 on: December 19, 2015, 09:31:38 pm »
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.



« Last Edit: December 19, 2015, 09:39:29 pm by wguibas »
 

Online tautech

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Re: Fender Amp Loud AC noise
« Reply #4 on: December 19, 2015, 09:36:45 pm »
I don't know how old it is.  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.

Note these are paper covered Electrolytics from yesteryear, very common before plastics were mainstream.
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Offline Johnny10

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Re: Fender Amp Loud AC noise
« Reply #5 on: December 19, 2015, 09:40:08 pm »
Cool
That was a quick repair!
Tektronix TDS7104, DMM4050, HP 3561A, HP 35665, Tek 2465A, HP8903B, DSA602A, Tek 7854, 7834, HP3457A, Tek 575, 576, 577 Curve Tracers, Datron 4000, Datron 4000A, uTracer, HP5335A, EIP534B 20GHz Frequency Counter, TrueTime Rubidium, Sencore LC102, Tek TG506, TG501, SG503, HP 8568B
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #6 on: December 19, 2015, 09:47:14 pm »
Got Lucky! 1960's circuitry looks like a bodge job, guess I don't have to be too neat with my repair.
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #7 on: December 20, 2015, 11:26:41 am »
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.
« Last Edit: December 20, 2015, 11:42:42 am by john_p_wi »
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #8 on: December 20, 2015, 12:52:36 pm »
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)? 
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #9 on: December 20, 2015, 01:04:37 pm »
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.
 

Offline jitter

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Re: Fender Amp Loud AC noise
« Reply #10 on: December 20, 2015, 01:10:44 pm »
Got Lucky! 1960's circuitry looks like a bodge job, guess I don't have to be too neat with my repair.

I 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...
 

Offline Seekonk

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Re: Fender Amp Loud AC noise
« Reply #11 on: December 20, 2015, 01:53:16 pm »
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.
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #12 on: December 20, 2015, 02:07:07 pm »
Keep listening to all this advice and it will be improved to the point that it doesn't work anymore.

Care to elaborate? Nothing has been suggested beyond standard maintenance and safety update procedures?
 

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Re: Fender Amp Loud AC noise
« Reply #13 on: December 20, 2015, 08:27:28 pm »
Got Lucky! 1960's circuitry looks like a bodge job, guess I don't have to be too neat with my repair.

I believe the correct term for that "bodge job" would be "point to point construction;)...
+1
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Offline Brumby

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Re: Fender Amp Loud AC noise
« Reply #14 on: December 21, 2015, 12:27:46 am »
Keep listening to all this advice and it will be improved to the point that it doesn't work anymore.

Care to elaborate? Nothing has been suggested beyond standard maintenance and safety update procedures?

Fundamental 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.
« Last Edit: December 21, 2015, 12:32:34 am by Brumby »
 

Offline ciccio

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Re: Fender Amp Loud AC noise
« Reply #15 on: December 21, 2015, 02:03:41 pm »
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.
I 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

Ciccio

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Offline Seekonk

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Re: Fender Amp Loud AC noise
« Reply #16 on: December 21, 2015, 03:22:00 pm »
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.
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #17 on: December 21, 2015, 08:43:03 pm »
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. 
« Last Edit: December 21, 2015, 09:29:50 pm by wguibas »
 

Offline ciccio

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Re: Fender Amp Loud AC noise
« Reply #18 on: December 21, 2015, 09:04:48 pm »

Hope you guys had sense of humors in your time. 

Sorry if my comment was misunderstood: I was not criticizing your competence...

Best regards
Ciccio

Strenua Nos Exercet Inertia
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #19 on: December 21, 2015, 09:12:39 pm »
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.
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #20 on: December 21, 2015, 09:25:00 pm »
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.
« Last Edit: December 21, 2015, 09:30:34 pm by wguibas »
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #21 on: December 21, 2015, 09:35:59 pm »
ok,  thanks I'll keep the blue ones, I read that elsewhere also.  Most of the sites I read about these amps like vintagefenderrepair.com sell kits to replace the 7 black 25uF/25v caps in addition to the paper ones.   That's all I planned on doing, are they ok to change?  You can get carried away with what you read on the internet.

I 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.
 

Offline wguibas

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Re: Fender Amp Loud AC noise
« Reply #22 on: December 21, 2015, 09:41:17 pm »
Sounds good, I just ordered the 6 power/bias caps to replace the paper ones.   I'll let you know how it turns out.
 

Offline john_p_wi

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Re: Fender Amp Loud AC noise
« Reply #23 on: December 21, 2015, 09:47:30 pm »
Good luck.

Again, watch the polarity of the bias cap (the one with the broken lead).  Cap + lead is grounded.
 

Offline Radio Tech

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Re: Fender Amp Loud AC noise
« Reply #24 on: December 22, 2015, 12:32:37 pm »
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.


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