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 General Electric RC4671A console -- stubborn hum!!!

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classicaudiorepair

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PostSubject: General Electric RC4671A console -- stubborn hum!!!   Tue Oct 19, 2010 8:23 pm

In the previous forum, I was registered as Fred Longworth. Still am. I've re-registered under the name of my repair shop.

In any case, I've been working on this GE console. Have replaced the previously redplating EL84's. All coupling caps have been replaced with high-quality films. The main power supply electrolytics have been replaced. The 5Y3 rectifier tube has been replaced with a NOS.

There remains a pernicious hum!

Many users would consider this acceptable, but my customer is very picky. I've bridged additional capacitance across the main filters as a test, and this doesn't help. I've also subbed on a test basis OTHER output, driver and rectifier tubes -- to no avail.

Only the right channel was redplating, yet the hum is in both channels at this time. This suggests that slight damage due to redplating of one of the output transformers is not the cause of the hum in both channels.

I'm scratching my head on this one.

Fred Longworth

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Johnnysan



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Tue Oct 19, 2010 8:30 pm

What is your bias voltage and what model are you working on?
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dberman51



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Tue Oct 19, 2010 9:56 pm

You're sure its power supply hum and not hum pickup? Should be easy to tell on this set as P/S hum would be 120 Hz.

Does this set have a hum balance control across the heater winding? I've found that can make a huge difference.

Any chance of an H-K short?

I remember your posts on the old forum. If you can't solve this, is there any hope that anyone else can? Very Happy

-David
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Radiosmoker

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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Tue Oct 19, 2010 10:35 pm

Hmmm Interesting, I didn't know the term red-plating till I looked it up. It means that the tube is getting red hot (pate or cathode) from a short in the power supply or is being used beyond it's designed boundaries.

Use a scope and trace backwards from speaker.

Also check for transformer grounding to chassis, might have a loose screw, or connection. Some transformers have a shield, maybe bad or no longer connected.

Check wire routing and move to verify if Hum sound is effected..

Also check audio components for damage as some might be OK but if the audio tube was shorted then it could have effected the balance because it is probably push-pull, and they are designed to cancel spurious noise. In other words a resistor may no longer match its compliment, or tubes may need to be matched. You get the Idea.
---------------------------------------
Now I'm treading on dangerous territory, but I remember in my solid-state classes that LM type op-amps can be used for voltage regulation and will internally filter out noise and hum. Just my 2 cents. Wink
PS- check the rectifier (both) plate voltages as one lower or higher could cause problems (wild guess)
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Tom Bavis



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Wed Oct 20, 2010 6:40 am

Schematic: http://techpreservation.dyndns.org/schematics/64/64_General%20Electric_RC4671_55.djvu

Is hum present with volume at minimum? With 12AX7 pulled? Were new filter caps grounded at EXACT same spot as originals?
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Radiosmoker

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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Wed Oct 20, 2010 11:59 am

Tom!

Schematic file downloads ok, but page is blank.
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classicaudiorepair

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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 4:37 pm

I want to thank everyone for commenting.

This is a pair of class-A single-ended amplifiers.

In any case, what I did do was closely examine the amplitudes of the hum in the power supply vs. the hum at the speaker terminals using my oscilloscope.

The hum -- as it stands now -- is 5v p-p on the 310v main dc line at 120Hz. The maximum value of hum indicated in the service manual should be 4v p-p. The hum at the speaker terminals is correspondingly 7mV RMS at 120Hz.

The main filter cap (which we replaced) is 100uF. When I bridge an additional 40uF across the 100uF, I find that the hum on the 310v line drops to 3v p-p, which is within spec. At the same time, the hum measured at the speaker terminals drops from 7mV RMS to 5mV RMS.

I guess that will have to do.

Fred
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Tom Bavis



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 5:01 pm

The 100 uF capacitor on the 7189 screen (260V) should have even MORE effect on hum.

As for schematic - it opens fine for me..
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dberman51



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 6:43 pm

Tom Bavis wrote:
The 100 uF capacitor on the 7189 screen (260V) should have even MORE effect on hum.

I second this motion.

When you referred to main filter cap I assume you mean the first section. Yes? If so, isn't 100uF kind of a lot for a 5Y3? The schematic says 75uF which I think is still on the high side.

How's the 5.6k 2-watt resistor? If it's not happy it might cause hum on the screen grid line.

-David
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Aaron

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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 7:19 pm

Tom Bavis wrote:
The 100 uF capacitor on the 7189 screen (260V) should have even MORE effect on hum.

Wait...what? A higher capacitance can yield more hum? Why?

Aaron
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Tom Bavis



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 7:44 pm

Aaron wrote:
Tom Bavis wrote:
The 100 uF capacitor on the 7189 screen (260V) should have even MORE effect on hum.

Wait...what? A higher capacitance can yield more hum? Why?

Aaron

I meant that this filter is MORE important for reducing hum, since it feeds the screen (which has gain) and the preceding stage (also amplified in the output stage).

As far as using too large a value for the first filter cap... the high resistance (200 Ohm) transformer will keep peak current within reason.
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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 8:45 pm

The screen grid measured 252v. The value in the schematic is 260v. The 260v line is nearly hum free, according to the oscilloscope. Ripple in low millivolt range.

* * * * *

I have not been able to locate 75uF capacitors @ 400v. The same for high quality 82uF capacitors. 100uF was used for the 75uF, because that is the only value easily obtainable. This means that the total capacitance straight off the 5Y3 has been upped from 75uF to 140uF.

* * * * *

A hum balance circuit is not meaningful here insofar as this is not a push-pull amplifier.

* * * * *

Now I'm wondering about adding a choke between the output of the 5Y3 and the first electrolytic.

Fred
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OldWireBender



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 11:30 pm

While adding a choke between the 5Y3 and the first filter cap may reduce the hum, it will also reduce the B+ voltages throughout the circuit. I estimate that you'd only get about 230 volts at the input to the choke.

This is based on the amp drawing about 95mA. The output tubes are biased by the 82 ohm common cathode resistor and the schematic gives 7.4 volts on the cathodes of the 7189A's. 7.4v/82 = 90.24mA. The rest of the circuit (12AX7), draws about 3.5mA for a total of no more than 95mA.

Looking at the graphs of output voltage vs. load current for choke input and capacitor input filters on page 5 of the GE data sheet for the 5Y3-GT:

5Y3-GT Data

and guessing about where the 314VAC curves would be yields close to the 310v the schematic calls out for the capacitive input filter. Doing the same guestimating for the choke input filter curves, I get about 230v at the input to the choke.

John
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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Thu Oct 21, 2010 11:52 pm

Thanks, John. You're right, the choke would cut the supply voltage enough to matter.

What do you think? Does 5mV RMS at the speaker terminals (120 Hz) seem a little high?

I tried an alternative speaker, and the hum was less audible. I've got to wondering. Maybe the aging speaker has become more nonlinear, such that it's producing greater amounts of harmonics of the 120Hz. Maybe there's enough 240Hz, 360Hz and so on to add subjective "unpleasantness" to the sound.

The customer has never owned a tube console before, and he is very picky. I've hardly ever serviced a tube amp (Audio Research is a BIG exception to this with their kick-ass power supplies) that didn't have a faint hum.

Fred
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Tom Bavis



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PostSubject: Re: General Electric RC4671A console -- stubborn hum!!!   Fri Oct 22, 2010 6:25 am

One other thing I didn't mention because it affects the frequency response - the .02 uF capacitor in the feedback loop acts a a "bass boost". Not really a boost, but a reduction in negative feedback at low frequencies - so the gain is reduced at mid to high frequencies, but full gain at lower frequencies. Negative feedback can cancel hum (if it exists at 120 Hz...) If the cap is jumpered out, gain will be flat, but bass will be reduced (I assume it's boosted to match the cartridge, passive tone controls and speakers). Perhaps sound will be acceptable with this cap jumpered - or add a resistor (~22K or so) across it to reduce the low frequency "boost".
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