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Reasonable Tube amplifier Hum / Noise
Reasonable Tube amplifier Hum / Noise
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Old 25th September 2020, 03:38 AM   #21
wiseoldtech is offline wiseoldtech  United States
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This guy that is bothered by HUM, is in my opinion being too picky for using a damn old console amp and EXPECTING it to perform like some "high end" unit.
Maybe try putting a bottom metal cover on the thing to shield it better.
Mod the PS for better filtering... put shields on the input tubes....

Or just tell the louse to spend some money on a "high end" amp and shaddap.
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Old 25th September 2020, 04:47 AM   #22
Rick PA Stadel is offline Rick PA Stadel  United States
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Would measuring it with the 5U4 unplugged give an indication whether the power transformer is radiating EMI to one output transformer more than the other?

Congrats on the 15 dB improvement by adding the 134 ohm resistor!

Since the channels measured fairly similar on your bench -- before he tried it on his shelf, could there be an HVAC closet behind his stereo bookshelves?

Regards

Last edited by Rick PA Stadel; 25th September 2020 at 05:02 AM.
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Old 25th September 2020, 08:18 PM   #23
k35kk is offline k35kk  United States
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Default Be sure all grounds are solid

It is wise to be certain that all ground connections are well made. This can be done by reworking each soldered connection to the chassis with an iron which has sufficient heat capacity to assure a well-flowed joint.
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Old 25th September 2020, 08:57 PM   #24
Bfpca is offline Bfpca  Canada
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Reasonable Tube amplifier Hum / Noise
Can you duplicate the hum results in your own system? If you short the inputs do you still have the 2mv hum in one channel only? These tests will tell you if the owner of the amp could have a problem with a source related ground loop.
Does the hum sound like a 60hz sine wave or more like 120hz with a high freqency buzz? This can tell you if it is being picked up from the power supply or from induced 60hz hum pickup. Check the input wiring on the noisy channel. Is it properly shielded or twisted and run close to the chassis. Is the input wiring longer or running close to a power transformer or PS filter cap? Replacing a twisted pair with a shielded twisted pair should reduce hum pickup somewhat, and perhaps rerouting the wiring could also help.
It’s hard to guess what is going on without seeing it and hearing the problem.
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Old 25th September 2020, 10:31 PM   #25
Bigun is offline Bigun  Canada
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Reasonable Tube amplifier Hum / Noise
If the outputs are in pentode mode then it’s not going to nearly as sensitive to the B+ ripple on the anode as will be to the supply to the screen grids. Perhaps try additional decoupling of the screens, current is lower there so the job is easier.

edit: I see screens have their own 300v supply which may already be well filtered but no schematic of the psu
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Last edited by Bigun; 25th September 2020 at 10:34 PM.
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Old 25th September 2020, 10:47 PM   #26
audiowize is offline audiowize  United States
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Quote:
Originally Posted by jjasniew View Post

I'm just waiting to discover the 2mV channel is the one whose output transformer is in-line with the power transformer!
The coils are not aligned though. This should not be an issue.

I had that same amp for a very long time and never noticed this behavior. The grounding is pretty abhorrent though, dealing with that may quiet things down a bit more, as would paying attention to heater routing.

I do agree though that this is an old console amp that doesn't exactly represent the pinnacle of tube amp performance.
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Old 27th September 2020, 03:28 AM   #27
oemcar is offline oemcar  United States
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Reasonable Tube amplifier Hum / Noise
Quote:
Originally Posted by jjasniew View Post

I'm just waiting to discover the 2mV channel is the one whose output transformer is in-line with the power transformer!

Thanks!
I too, am concerned as you are about PT inducing hum into adjacent OT...
Per below chassis layout, L ch has audible no sig 'hum', R ch does not.

Applying different PCB designs to chassis layout-
Original was Chifi PP- condition existed,
Currently Audio Note- condition still exists,
Soon Dynaco ST-35 design new PCB...

If still persists,
Then will be looking into either shielding for PT, or reconfigure mounting position.
Process is fluid-
Jim
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Last edited by oemcar; 27th September 2020 at 03:30 AM.
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Old 27th September 2020, 05:44 AM   #28
6A3sUMMER is offline 6A3sUMMER  United States
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Is the B+ choke (if any) right below one of the output transformers?

Is the chassis magnetic?
(ordinary steel?)
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Old 27th September 2020, 04:08 PM   #29
oemcar is offline oemcar  United States
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Reasonable Tube amplifier Hum / Noise
Per below-
Yellow square is where choke coil is mounted between filter caps. "R" & "L" (should be reversed- oops!) denote OT locations top of stainless steel chassis. Choke coil replaced original 75 ohm chassis mount- "X".
Blue outlines depict 7mm(+/-) PC board extension needed for subsequent designs...thoughts?
Jim
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Last edited by oemcar; 27th September 2020 at 04:13 PM.
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Old 27th September 2020, 06:14 PM   #30
6A3sUMMER is offline 6A3sUMMER  United States
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Turn the amp off, remove the power cord.
Then use a DMM to measure the voltage on the B+ capacitors.
If there are no bleeder resistors on the B+, you need to install one (Safety First). Careful, you need to discharge the capacitors first.

Unsolder the Choke, and remove it from the amplifier.

Using wire that has very good high voltage insulation, wire the choke outside of the amplifier (2 wires).
Then mount a screw, nut, and ground lug on the choke mounting tab; and solder a wire from the ground lug to the amplifier ground connection (Safety First).
Or, if you do not connect the choke frame to ground, be sure to wrap the choke in plastic, and do not touch it (Safety First).

Now, turn on the amplifier, and see if the hum level is improved (lower).
That will test if the choke was introducing hum into the left channel output transformer.

If you want to put the choke back in the amplifier, at least turn it so that the orientation reduces the hum level as much as possible.
you need to use that screw, nut, ground lug and wire to keep the choke frame grounded when you are turning it for least hum.
You really should use Electrical Safety Gloves when doing this, or you may come into contact with some point of B+ in the amplifier (Safety First).

Please let me know if that fixes the hum.

There are more than one kind of "stainless" steel. Some are magnetic, some are not. If a magnet is attracted to the steel, then the magnetic fields of the power transformer and choke can be magnetically transmitted to the output transformers.
Most of my amplifiers have Aluminum chassis.
But I have one magnetic steel chassis. I use Aluminum spacers, and longer screws to mount the power transformer, choke, and output transformers about 1/4 inch away from the magnetic chassis.

Happy Listening!

Last edited by 6A3sUMMER; 27th September 2020 at 06:24 PM.
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