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Reasonable Tube amplifier Hum / Noise
Reasonable Tube amplifier Hum / Noise
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Old 27th September 2020, 09:14 PM   #31
wiseoldtech is online now wiseoldtech  United States
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My own tube amp, originally a Magnavox 93 Series amp (now highly modified by me), is laid out sensibly, has a choke, and has absolutely no hum.
Of course, it also has a "hum pot" in the filament line.
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Old 27th September 2020, 10:14 PM   #32
oemcar is offline oemcar  United States
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Reasonable Tube amplifier Hum / Noise
Quote:
Originally Posted by 6A3sUMMER View Post
If there are no bleeder resistors on the B+, you need to install one (Safety 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!
4 takeaways-

1. Check if chassis is ferrous
2. Remote mount choke coil, then try grounding armature (B-) ...correct on the latter?
3. Use non ferrous spacers on PT & OT's if chassis is ferrous, to reduce/prevent noise
4. Report findings

Will update-
Thx,
Jim
ps. -and dont get shocked ;<)

Last edited by oemcar; 27th September 2020 at 10:16 PM.
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Old 27th September 2020, 10:26 PM   #33
oemcar is offline oemcar  United States
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Reasonable Tube amplifier Hum / Noise
Quote:
Originally Posted by wiseoldtech View Post
My own tube amp, originally a Magnavox 93 Series amp (now highly modified by me), is laid out sensibly, has a choke, and has absolutely no hum.
Of course, it also has a "hum pot" in the filament line.
Need to post pics of mods to my amp. Dynaco ST-35 PC board retrofit design in process. Like those Z565's.. they get much love here.
What's your B+? UL or no? Cathode or grid bias?
Thx,
Jim
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Old 27th September 2020, 11:14 PM   #34
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Quote:
Originally Posted by jjasniew View Post
The 6BQ5 grids all measure about 4mV DC, except one, which is 30mV.
Is this a property of the final tube? (relocate it and remeasure).
365V/300V are high supply voltages for type EL84, you might experience an insufficient cathode to heater isolation.
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Old 28th September 2020, 12:03 AM   #35
oemcar is offline oemcar  United States
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Reasonable Tube amplifier Hum / Noise
Not to hijack- although related...
Will post findings in new thread- apologies
Jim
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Old 28th September 2020, 12:34 AM   #36
wiseoldtech is online now wiseoldtech  United States
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Quote:
Originally Posted by oemcar View Post
Need to post pics of mods to my amp. Dynaco ST-35 PC board retrofit design in process. Like those Z565's.. they get much love here.
What's your B+? UL or no? Cathode or grid bias?
Thx,
Jim

My amp is wired UL, the bias is the novel David Gillespie design (a form of fixed bias regulated to track any PS fluctuations) and the B+ to the OPT's is 300V.
Overall, the amp performs like one of those $5000 "designer amps".
I get a solid 17W/ch @4 ohms out of her with those Z565's.


She sits on top of the equipment rack in the corner.
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Old 29th September 2020, 03:14 AM   #37
jjasniew is offline jjasniew  United States
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Well, I fixed it.

After a little hair tearing, verifying the output stages performed identically with both grids shorted (6A3sUMMER's suggestion) I became convinced it's something to do with the driver tube for that side. With the rectifier out of its socket, I was getting 0.3mV on each speaker output; 0.1 with the amp off.

After verifying any ground potential difference were unmeasurable, I was out of ideas. Then it struck me this tube is the LAST on the heater string. Because this is a console amp, there was an extra filament winding just sitting there, unused. So I cut the connections and clipleaded it to this winding. Bravo! 0.5mV! So I twisted up a pair of solid core and - being careful to wire it up in the same phase - back to 1.2 mV, while the other channel is 0.5...

Shoot, so I reversed the phase, 1.1mV. Then I remembered WOT said "Of course, it also has a "hum pot" in the filament line". I dug out a pair of 68 Ohm 1/2 watt resistors and experimented where to connect the center tap to chassis. Turns out it didnt matter, so I connected them to the same solder blob as the Gnd of the 3 wire AC cord.

Fully expecting the other channel to now go up to 1.2mV, I happily found that both are sitting at 0.5mV, with inputs shorted, loaded into 8 Ohms resistive. The guy has the amp in his possession and we'll see if he cant hear anything this time.

Why did I do it? He seems interested. When I last visited, it was evident he couldnt identify the output transformers, nor the output tubes, as he asked about them - what they were. We all began somewhere similar. Plus, when you're retired, it's good to use the brain for something a little challenging electronics wise, even if it's a little POS amp like this one. It was amusing to see they actually bothered to bring the grid and cathode resistor ground for that tube all the way over to the input jacks - instead of just soldering to the chassis nearby - available on the next tab. Someone actually thought about the layout and connections.

Thanks for all the help, I'll post what the owner thinks when I find out.
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Last edited by jjasniew; 29th September 2020 at 03:31 AM.
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Old 2nd October 2020, 04:49 AM   #38
jjasniew is offline jjasniew  United States
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He says he hears a hum difference between channels. I got the amp home and sure enough, one channel is at 0.5mV, the other at 1.1 (inputs shorted, 8 Ohm resistive load)

This thing is Haunted. I fail to understand how it can change like that; almost as if every time you turn it on, exhibits something different, just on that channel.

I tried powering the driver tube's heaters - on the bad channel - with a DC bench supply - even worse, going up to 2.5 mV. I havent tried a 6V battery yet...

The output tube G1 clip to ground test suggests it's coming from the screen / driver tube B+. Then why does the other channel perform so well - and consistently too?

I'm flabbergasted. I assume building a DC heater supply from that additional winding will do nothing for the issue. The driver tubes are 12AX7s with the heaters paralleled for 6V.

Any ideas? I'm fresh out -
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Old 2nd October 2020, 05:46 AM   #39
6A3sUMMER is offline 6A3sUMMER  United States
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Take a 1uf 400V cap, and connect it in parallel with the 150pF cap to ground (the grid of the concertina phase splitter to ground).
Then power up the amp, and see if there is hum.

When you grounded the output stages control grids, the hum went away, right?
Now, with the 1uF cap across the splitter grid, does the hum go away?
Depending on that answer, the cause is either the splitter, or the input stage and/or the B+ to that stage

Without going through all the postings in this thread, I will ask if you checked whether each output tube has the same cathode current.

If you did not check the current of each output tube, then put an individual 10 Ohm resistor from each cathode to the common self bias resistor and common bypass cap. With a DMM, measure the voltage across all four 10 Ohm cathode resistors.
Example, 0.4V across 10 Ohms = 40mA.

Even though the outputs are in pentode mode with a high plate resistance, if the currents are badly unbalanced, there will be some hum from the output stage.

I do not remember ever seeing any schematic for the power supply. That might be helpful for troubleshooting.

Last edited by 6A3sUMMER; 2nd October 2020 at 05:56 AM.
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Old 2nd October 2020, 06:19 AM   #40
wiseoldtech is online now wiseoldtech  United States
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A recent Harman Kardon Citation amp that I had on the bench had old crappy loose tube sockets for all the driver tubes.
Hum and pops all over the damn place.


The only sure-fire remedy was replacing all 6 of them.



Oh, and hums?.... the HK amp required its bottom cover to make it totally silent.


This guy's console amp has some crappy wooden board for a bottom?
Not gonna get rid of hum with that crap.
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