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Old 23rd July 2011, 11:58 AM   #1
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Default Scott 222B Hum

I need some help please. This Scott was in nice physical condition before I started working on it, but it had weak output, with the left channel being very low. Since it's of 1961 vintage, I decided to replace ALL the caps, whereas I used Nichicons for the power electrolytics (bypassing the cans) and Russian K40-9Y's in the audio path. The tubes tested good, but I installed a nos rectifier tube for good measure. I replaced 2 scratchy trimpots and AC and DC biases were peaked to spec. Every inch of the chassis was inspected for bad solder joints and good grounding. Pots and switches were Deox'd several times. The amp smoothly powered up and produced superb quality sound, which continued to improve over time. (~2 weeks). Then it began to hum. It was noticable at first with low or no input, but continued to get louder, being strongest on the phono input selection. The hum is even on both channels. I retested all the caps for esr, shorting and correct value and they were all fine. Now before I begin guessing on expensive parts replacement, how can I trace the source of this hum? Could it be the main xformer?
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Old 23rd July 2011, 12:29 PM   #2
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Did you replace the bridge rectifier which powers the filaments in the phono section
(and produces the bias voltage for the output tubes)?
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Old 23rd July 2011, 01:42 PM   #3
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No I did not Frank. I have a schematic (see pics) and parts list, but didn't see anything resembling a bridge, except for the 5AR4 tube. Can you tell me where it's located in the chassis? And if not a rectifier, what else might be causing this annoying hum?
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Old 23rd July 2011, 02:56 PM   #4
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My mistake. I just downloaded the original HH Scott schematic for the 222B.
It does not have a filament/bias bridge.
I own an HH Scott 299C and wrongly assumed that the 222 had a low voltage bridge like most of the HH Scott amplifiers.
When you replaced the original power supply caps, did you remove the originals?
What is the frequency of the hum? 60Hz or 120Hz?
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Old 23rd July 2011, 05:25 PM   #5
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Yes Frank. Most were removed from the chassis. But I left the canned caps disconnected and bypassed them as shown in the pic. The freqeuncy is 60hz and dirty. (see pic)


Ken
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Old 23rd July 2011, 06:07 PM   #6
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60Hz. Could be a filament/cathode short on one of the tubes.
On my 299C, I installed a 1N4007 diode in series with each anode of the rectifier.
The diodes prevent damage if the 5AR4 develops a short.
I also added a 75 ohm resistor from each end of the 6.3 volt filament transformer winding to chassis ground. The resistors reduced the noise in my amplifier.
Are the B+ voltages normal?
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Old 23rd July 2011, 06:43 PM   #7
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Your preventative measures sound good and I may take your example. However, damage may have already been done. The B+ voltage is 363V at the rectifier. Spec is 380V, so I believe we're OK there. The service manual states that if a hum develops and increases, the power tubes may be gassy and should be replaced. Aside from the rectifier, are the power tubes the 12AX7's? I don't have any spares or quick access to a tube tester (hey, I'm a microelectronics guy) so I want to explore other potential causes before replacing them. Please note that this hum is negligable at normal listening levels on all but the phono selector.
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Old 23rd July 2011, 07:17 PM   #8
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The old HH Scott gear was very well designed. I listen to my 299C every day.
I'm a bit concerned that your B+ voltage is 363. The original spec was 380 with 117 volts on the transformer primary. If your line voltage is closer to 125 volts (as is normal today), the B+ voltage should be somewhat higher than 380 volts.
Something may be drawing too much current and causing the hum.
Fortunately, 6BQ5 tubes aren't too expensive. Antique Electronic Supply is a good place to start.
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Old 23rd July 2011, 08:23 PM   #9
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So, it's likely a 6BQ5 tube?
Can I look at the B+ without these tubes plugged in?
My line voltage is 120.
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Old 23rd July 2011, 08:43 PM   #10
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If you check the voltage with the tubes removed, it will be higher than normal.
The published specs are with the tubes in and operating properly.

My 5AR4 mod is easy to make. Pins 3 and 7 are not used. Move the transformer wire from pin 4 to pin 3. Install a diode between pin 4 and pin 3 with the cathode of the diode connected to pin 4. Move the transformer wire from pin 6 to pin 7. Install a diode between pin 6 and pin 7 with the cathode of the diode connected to pin 6.
This will ease the load on the rectifier tube as it is only passing d.c. It will extend the life of the tube many times over.

If you are hearing the 60Hz hum in both channels, it may or may not be the tubes.
I would expect 120Hz hum. With low B+ voltage and 60Hz hum, I would suspect a bad 5AR4 tube. Perhaps one of the rectifier sections isn't doing it's job.
You can temporarily replace the tube with a 5U4, 5V4, 5Y3 to see if the hum goes away.
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