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 Post subject: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Mon 22, 2018 4:29 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Hi all,

A friend of mine has an amp that has an issue, I don't know how to advise him. Can anyone make a suggestions?
Issue: He has a Rickenbacker M10, a single ended 6L6 amp. He recapped it correctly, didn't change lead dress and checked all resistors for value. He is getting a pretty big hum (not just high end noise) as he turns up the tone control (I have not seen this kind of tone control before). ANyway, I recommend he check all his grounds, try a different 6SN7 possibly replace the wire feeding the two 200K resistors to the input grid with shielded wire. Anything else to try? Thanks!


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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Mon 22, 2018 5:18 pm 
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That's a standard tone control where the capacitor bleeds off higher tones so it doesn't get pass to the next stage. Some thoughts:

- This looks like a re-creation of another schematic; it's marked "unofficial." Is it possible this is a modification, experimental, or there's a mistake?
-- This type of tone control is sometimes seen after the final amplifiers not in that position; but theoretically, it should still work
-- 0.02mfd seem a bit high; given the position, this might be enough to pass 60hz from the ground/common into the audio stream; (~0.005mfd might be a better value)
-- "150K?" why is there a question mark? Perhaps there should be another fixed resistor in series?
-- In some circuits, there's a 0.01mfd between volume control and grid (pin 5) of amplifier tube (6L6), but there isn't one here
-- Also, in a common configuration, there would be a capacitor across the input to the speaker transformer (~0.015mfd), there isn't one here
-- Original wiring in that position may have been shielded, but there isn't one now

Path forward: suggest you to disconnect the tone control to be 100% sure that's the source of hum (should go away if you disconnect it). Then use the information above to see what you can do about it. Don't discount the possibility of a mistake or wrong component value...that's easy to miss even after multiple looks.


Last edited by AJJ on Oct Mon 22, 2018 5:33 pm, edited 2 times in total.

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Mon 22, 2018 5:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Hi

Schematic shows 6SN7 V1b has an open grid. Need something plugged into a jack to complete the circuit. Without it grid is floating and can cause hum.

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Mon 22, 2018 5:29 pm 
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....wait, this may be a modified guitar amp. In that case, you should rule out the possibility of a ground loop when other gear is connected.

The outfit is still in business. It might not hurt to sent them a note (or call):

http://www.rickenbacker.com/


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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Mon 22, 2018 11:12 pm 
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Update.

I stumbled upon this; you may find this interesting and useful:
https://courses.physics.illinois.edu/ph ... 10_Amp.pdf


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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Tue 23, 2018 2:49 am 
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Quote:
- This looks like a re-creation of another schematic; it's marked "unofficial." Is it possible this is a modification, experimental, or there's a mistake?

There are definitely at least two errors on the schematic: there is no grid resistor for V1A and the input jacks aren't grounded. The first will produce unintended distortion (which may be unpredictable or intermittent). The second will produce LOUD hum whenever the volume is turned up regardless of the tone control.

Is this a genuine Rickenbacker M10 or a "replica" that someone is trying to build. If this is a "reproduction" of the schematic, it would be better to get an original copy to look for other mistakes.

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Tue 23, 2018 4:23 am 
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The schematic is a digital reproduction, and omissions or mistakes might only exist there and not in the actual amp.

If suggested tube swap, tone control disconnection doesn't eliminate the hum, remove the extra input jack, not needed.
Check for cold solder joints, of course. Maybe one of the new caps is bad, not unusual these days.

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Tue 23, 2018 11:21 am 
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If you lift the ground prong of the cord with a three prong to two prong adapter (with all due care) does the hum go away?

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Tue 23, 2018 5:38 pm 
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First be sure there is a grid resistor on V1A.

Then consider a cathode to heater short in the 6L6 output.

When you say "tone turned up" do you mean high frequency on or cut? Als does the setting of the volume control affect it since as drawn the 6L6 grid is essentially grounded when the volume control is at 0.


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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Fri 26, 2018 7:52 am 
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Location: Powell River BC Canada
Input jacks mounted on insulated bushings. Then the sleeve wired back
to where the cathode resistor of V1B goes to chassis, or to the ground
tree which is not depicted in the schematic.

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 Post subject: Re: Amplifier hum with tone turned up ... suggestions?
PostPosted: Oct Fri 26, 2018 11:25 am 
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Joined: Mar Wed 16, 2011 4:17 pm
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Location: New York City, New York
If this is an original Rickenbacker M10- the heater wiring scheme is terrible. Like most guitar amps from the era they are 60hz hum machines due to the combination of high gain and poor heater wiring schemes.

When playing with a band you hardly notice it, but playing alone in your house it is objectionably loud for most people.

Converting the scheme to an artificial center tap with 2 resistors going to ground and using twisted pair wiring will instantly quiet the amplifier down. A friend uses a Rickenbacker M8 that I modified in that way and it is almost silent at idle even with the volume knob all the way up.

The link below has pretty much everything you ever wanted to know about tube heater wiring

http://www.valvewizard.co.uk/heater.html

Also the construction on the original amps are terrible- double check everything and make sure any ground connection(s) are wired/soldered properly.


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