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Post subject: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Sat Dec 28, 2019 7:50 pm
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Hey guys, so I have a Silverface 1972 Fender Bassman 50 (aa371 I believe) that has a noticeable hum and I can't quite figure out what is causing it. The hum gets louder when either ch. 1 or ch. 2 volume is increased, even if no guitar is plugged in. The hum also gets louder as you turn up the treble on each channel.

-All electrolytic and filter caps have been replaced
-I've tried different cables, speakers, and outlets
-All tube sockets have been cleaned
-All jacks and pots have been cleaned and tightened
-Tubes have been tested and known good power and preamp tubes have been substituted
-I chopsticked around with the amp on and could not find anything causing the hum (also moved lead dressing around a bit with the chopstick to see if that was causing the hum)
-I pulled preamp tubes and the amp was quiet when V4 was removed
The amp was also quiet when V2 was removed, but after a little while the amp started "motor boating" with very loud low frequency oscillations when nothing was plugged in and all pots were at 0. I quickly turned off the amp, put V2 back in, and the amp is working, but the hum still persists.

Any advice would be greatly appreciated!

Thank you!


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Sat Dec 28, 2019 9:20 pm
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Make sure the grounding switch on the input jacks is clean and tensioned for a good ground.
In this amp, the DC voltage on the plates of V1, V2, and V3 are mixed without coupling caps. I'm not surprised it motorboated


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Mon Dec 30, 2019 2:45 am
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http://hotroddiy.com/Replacing-Damaged-Jacks.pdf


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Mon Dec 30, 2019 6:13 pm
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Thanks for the info. The jacks are tight and working properly. I did notice that a .01 ceramic cap is really noisy when it is hit with a chop stick. One of the 6L6 470ohm grid resistors has gone way out of spec so I’ll be replacing the ceramic cap and resistor next.

I also want to test the coupling caps. I’ve heard you can take one end out of circuit and measure for DC voltage. Can anyone shed any light on this—which lead to detach from the chassis and how little DC voltage is acceptable, etc.?

Thanks!


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Mon Dec 30, 2019 6:57 pm
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fenderguy81 wrote:
Thanks for the info. The jacks are tight and working properly. I did notice that a .01 ceramic cap is really noisy when it is hit with a chop stick. One of the 6L6 470ohm grid resistors has gone way out of spec so I’ll be replacing the ceramic cap and resistor next.

An excellent first step. After doing that, adjust the bias balance for minimum hum.
Then maybe look at coupling caps, if necessary.


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 2:25 am
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fenderguy81 wrote:
Thanks for the info. The jacks are tight and working properly. I did notice that a .01 ceramic cap is really noisy when it is hit with a chop stick. One of the 6L6 470ohm grid resistors has gone way out of spec so I’ll be replacing the ceramic cap and resistor next.


Thanks!


I don't you knowledge in electronics. What you write is not enough to convince me your work are ok.

Test with no guitar plugged;

1- Connect Tip ( the positive) from each jack to amp ground with alligator clip jumper.
No more noise mean jacks ground / switch may not working .
2- You may need to remove rust on front pannel where are the jacks. Thigntened them may not be enough.
3-You need to clean the ground contact switch on each jack . This is not a easy job if you don't have the right tool. You may need to have jacks in you hand


Last edited by stratele52 on Tue Dec 31, 2019 2:34 am, edited 3 times in total.

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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 2:29 am
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Testing coupling caps;

1- Read voltage on both end, amp ON
2- Amp OFF ; disconnect the side where you read no voltage or less voltage.
Leave the other side connect in the amp
3- Power amp ON ; you must read 0 volts at the side you disconnect
Reading some volts = cap is leaking, replace


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 6:59 am
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fenderguy81 wrote:
I did notice that a .01 ceramic cap is really noisy when it is hit with a chop stick.


Those ceramic disc caps are microphonic quite often when you tap them. Doesn't always mean they should be replaced.

UN


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 7:47 am
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I would take a voltage measurement on the control grid of the next gain stage.
This is the easiest way imo to determine coupling cap leakage. If the leakage is excessive it will have an impact on the signal and the noise level in general of the tube. I would say anything less than .250 volts is probably ok. Obviously the closer to zero volts is best and voltage leaked to the output stage is noisier and more critical to output signal and could affect bias and operation or blow fuses.


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 3:17 pm
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As someone pointed out, the .01 ceramic disc cap is microphonic. I replaced it with a new .01 630v Mallory cap and it is also microphonic and did not help the noise issue. A few 220k and a nearby 470k resistor are out of tolerance so I am going to replace those resistors along with the 6L6 470k grid resistor and then retest the amp. I will try temporarily grounding the tip of jacks with alligator clips as well to see if that resolves the noise issue. Thanks for the input and the info on testing coupling caps. I will check them if the things listed above do not resolve the problem.

Thanks again!


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 4:41 pm
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About those microphonic parts... if they are in nominal range, I do this little trick. Apply a dab of heat resistant silicone sealer on top of the part. Sometimes, this helps. Blu-tack works on parts which don't heat up or are not close to heat.

https://www.amazon.com/Blu-Tack-S050Q-Reusable-Adhesive-75g/dp/B001FGLX72/ref=sxin_2_ac_d_pm?ac_md=2-1-QmV0d2VlbiAkNSBhbmQgJDEw-ac_d_pm&cv_ct_cx=blu-tack&keywords=blu-tack&pd_rd_i=B001FGLX72&pd_rd_r=a734f0e9-751f-4b51-bb76-277e113535f6&pd_rd_w=2gmHz&pd_rd_wg=9HP9E&pf_rd_p=709d2064-e546-4799-9e66-b352ea89951f&pf_rd_r=X62RTPBZJ8Y9ZKGV2CTG&psc=1&qid=1577835610


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Tue Dec 31, 2019 5:31 pm
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Sometimes an amp can sound "great" because of out of spec resistors. I've worked on amps that just had "that tone" because they weren't perfect according to the schematic. If I were you I'd search out the problem of your hum while keeping in place the parts that aren't the problemo. But that's just me.

UN


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Post subject: Re: 1972 Fender Bassman 50 Hum Noise HELP
Posted: Wed Jan 01, 2020 2:36 am
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upnorth2 wrote:
Sometimes an amp can sound "great" because of out of spec resistors. I've worked on amps that just had "that tone" because they weren't perfect according to the schematic. If I were you I'd search out the problem of your hum while keeping in place the parts that aren't the problemo. But that's just me.

UN


100% right

Anyone can replace parts.
Knowing which ones ( and why ) to replace or not is another story.

Many vintage amps have lost their quality in the hands of well-intentioned amateurs due to lack of knowledge.


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