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Post subject: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 7:57 am
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Hey all, got this in a trade and the amp works but there is a weird hum when it is turned on and the hum goes down as the master volume goes up. I don't think it is tube-related (which would be the easiest fix). All the pots are clean and crackle free. So any guesses as to what I'm looking at?

Also, any way to tell the model year? I can't find the serial on this amp.

Here is a video of the noise: https://www.youtube.com/watch?v=d_dZrbOZnGI


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 8:10 am
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Location: Coastal Bend, Tx.
Watched the video a couple times, I didn't hear any "HUM".
Cannot help so far.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 8:21 am
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At the start of the video it is on Standby, but once I switch it off it is pretty loud. You may need to turn up your speakers/headphones though.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 11:48 am
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When was the last time the power stage filter capacitors were changed(under the dog house-cover)?
I'm guessing it's a new to you amp, so you may not know the answer. There are EIA dated codes on the capacitors, if they are original a new set should help w/ the minor hum you have.
The serial number should be on the back of the chassis on the right side if looking from the back, it will look sorta like A123456. Other dating criteria is by way of component codes, this site may help you with that.
http://www.superiormusic.com/page195.htm
Hope this helps, if not ask more ?'s and if you post pics you will receive more replies.
If you'r not comfortable working w/ high voltages inside your amp, DONT. It can be fatal.
A decent amp tech should only charge around $50-75 + parts to replace your power supply caps, maybe less-its not that difficult.
Does it have a three prong power cord, it should?


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 12:44 pm
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The electrolytics in the power supply may be ready for replacement but I'm betting it's a bad choke.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 1:27 pm
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I'd also check the idle bias, on the output tubes. I've seen tubes flash that blue glow, when the standby switch is flipped --- when the idle bias is not right (too hot or under-biased). Too hot biased tubes sometimes make the amp do that funky hum.

Overall checkup, by a good tech will prolly find the issue.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 1:32 pm
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Just found something so obvious I missed it. There is currently only ONE power tube in this! One slot totally empty. When I asked the previous owner about it he said:

"It’s out to keep the volume down. The gentleman that sold it to me explained how it was set up: He put in one Kt 88 and biased it and the way that the circuit is set up apparently he runs the left side as a clean in the right side as the distortion and when you leave that tube out and he maintains the ability to be played without a crazy volume problem. I never thought anything of it and when I talked to Eric About it he didn’t think there was anything untoward about it"

I went ahead an ordered a matched pair of tubes, I just need to learn how to bias this amp and hopefully that will take care of it. If it isn't this then it's got to be a power issue which I'm not competent to deal with.


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 2:21 pm
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TheMTJ wrote:
Just found something so obvious I missed it. There is currently only ONE power tube in this! One slot totally empty.


Call me crazy, but I don't think one side of your transformer appreciates the missing partner.

UN


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 3:25 pm
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upnorth2 wrote:
TheMTJ wrote:
Just found something so obvious I missed it. There is currently only ONE power tube in this! One slot totally empty.


Call me crazy, but I don't think one side of your transformer appreciates the missing partner.

UN


Not only that but the amp was designed for a pair of photogenic Hollywood debutantes, not a portly English schoolmarm.

:lol:
Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Mon Sep 30, 2019 6:31 pm
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Righto!

UN


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 1:51 am
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" There is currently only ONE power tube in this! One slot totally empty. When I asked the previous owner about it he said:

"It’s out to keep the volume down."

:lol:

A pair of tube is a must to keep the amp more quiet.
See a amp tech to re-biased the amp .


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 4:49 am
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TheMTJ wrote:
Just found something so obvious I missed it. There is currently only ONE power tube in this! One slot totally empty.


Well, this certainly helps to explain the hum. With one output tube, your amp has lost the hum cancelling of a matched pair of output tubes. Plus, the idle bias prolly wasn't reset for just one tube. Bias is out of whack.

Unless circuit mods were done, a matched pair is a must in your amp. The silverface Bassman has a balance/hum pot, only. Adjust to minimum hum and check the idle current draw using a bias tool. There is no way to adjust the idle current draw on a balance/hum pot only amp. But, the idle bias of a good pair of matched output tubes should be in the 18-22 watts per 6L6GC tube, at idle. I'd get a BIAS kit or meter that measures the plate voltage, too. This is the only way to really know the power (watts) the amp is idling at.

There are many forms and kits available. Google and read-up on how to use these kits and how to proceed.

https://www.google.com/search?q=guitar+ ... 9930320767

Rob Robinette has a good site on Setting Idle Bias:

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Like I said, there are MANY YouTube videos on how to bias a tube amplifier. Take some time to view a few of them. Until you get the basic idea of the how's and why's.




Good luck! Post any procedural questions!

:)


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 10:49 am
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BMW2002Ti wrote:
The silverface Bassman has a balance/hum pot, only.


My Bassman Ten schematic shows a traditional bias-supply pot, with 430 VAC to the plates and -45 VDC for the bias.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 11:14 am
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Retroverbial wrote:
BMW2002Ti wrote:
The silverface Bassman has a balance/hum pot, only.


My Bassman Ten schematic shows a traditional bias-supply pot, with 430 VAC to the plates and -45 VDC for the bias.

Arjay


-45 volts is the voltage send to the 6L6's originals tubes.
We must absolutely not assume that it will work with other 6L6's

We must read 6L6's cathode current


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Post subject: Re: Fender Bassman 1974 - Weird Hum
Posted: Tue Oct 01, 2019 6:46 pm
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TheMTJ wrote:
Just found something so obvious I missed it. There is currently only ONE power tube in this! One slot totally empty. When I asked the previous owner about it he said:

"It’s out to keep the volume down. The gentleman that sold it to me explained how it was set up: He put in one Kt 88 and biased it and the way that the circuit is set up apparently he runs the left side as a clean in the right side as the distortion and when you leave that tube out and he maintains the ability to be played without a crazy volume problem. I never thought anything of it and when I talked to Eric About it he didn’t think there was anything untoward about it"

If it isn't this then it's got to be a power issue which I'm not competent to deal with.



I think it's probably the latter.

Retroverbial wrote:
Leave the amp in the *unlocked* boot of your car.

Within a day or two your problem will be solved.

Arjay


JMO.


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