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Old 5th June 2013, 07:13 PM   #11
famousmockingbird is offline famousmockingbird  United States
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I haven't messed with the 5AR4 and maybe I should but it has it's own dedicated 5V winding.

The PT has two 6.3v windings, one for each channel. My first thought was heater hum induced into tubes but I tried new tubes and that didn't help.
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Old 5th June 2013, 08:14 PM   #12
famousmockingbird is offline famousmockingbird  United States
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Just tried a new 5AR4 in that channel and the same noise persists.

I plugged her in and have 6.6VAC on the heaters with zero DC volts when checked to ground. So heaters are left floating, maybe builder forget to create artificial center tap? But if this was a design flaw it would have been present from day one.
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Old 5th June 2013, 08:32 PM   #13
MelB is offline MelB  Canada
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How old is the amp? (How many hours on it?) Could be a leaky power supply cap.(?)
Did you swap the 12ax7 and 12au7 from side to side?
What happens if you just short the inputs?
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Old 5th June 2013, 08:36 PM   #14
20to20 is offline 20to20  United States
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Does this amp have a built-in volume control or are you using a preamp with RCA cables?

Do you have any input shorting plugs you can use at the input? If not just pull the stereo cables from the source device and use a paperclip or something to short the signal in. Then tell us if the hum is still there, better or worse.
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Old 5th June 2013, 08:49 PM   #15
famousmockingbird is offline famousmockingbird  United States
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I have the amp on my coffee table with the input jacks shorted, and there is no volume control on this amp.

The amp is only 7 years old. I didn't swap tubes from one side to the other but I tried other tubes I know that are good and the same noise is present.

Ok so I just used a known good 47uf 500v cap to parallel with the filter caps and bias supply caps and the noise is still present. I might try to ground that center tap and see what happens.
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Old 5th June 2013, 09:17 PM   #16
DF96 is offline DF96  England
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I haven't messed with the 5AR4 and maybe I should but it has it's own dedicated 5V winding.
Of course, I forgot that.
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Old 5th June 2013, 10:02 PM   #17
famousmockingbird is offline famousmockingbird  United States
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Well I got side tracked and now I have to get ready for band rehearsal. I will look into it later tonight or tomorrow. If anyone can think of something I am overlooking that would be great, you know what they say two heads are better than one. What I forgot to do is run the amp with no phase inverter and see if the noise is present.

I was positive a cap went bad but after paralleling all the electrolytics on that side to no avail I am stumped. It's not like it's not listenable, you can barely notice the noise but I am anal and it will bother me. My Scott 299 is dead quiet through these speakers which is what I expect this custom amp to be (and it was).

Last edited by famousmockingbird; 5th June 2013 at 10:12 PM.
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Old 5th June 2013, 11:51 PM   #18
nigelwright7557 is online now nigelwright7557  United Kingdom
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In valve amplifiers its vital power supply ground and audio ground are kept separate then just joined once at a star ground point.
PCBCAD51 pcb design software. 2018 version out now with lower prices >> http://www.murtonpikesystems.co.uk
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Old 6th June 2013, 12:16 AM   #19
bobrown14 is offline bobrown14  United States
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Did you try moving around your speaker wires? You can pick up hum if one channel wire is too close to AC line/outlet. I'd be checking to be sure it's not environmental, I.E. wiring issue/light/light switches (dimmers) etc. Your speakers are fairly sensitive and can reproduce easily AC induced hum, maybe you moved something around recently and now the AC is too close to speaker wires... took the cover off the computer or plugged in the computer power supply charger to the same circuit.. .there's soooo many ways to get AC hum from what ya got laying around and you don't even think about it. I'd unplug everything else on the house circuit you are using for your system except the audio gear and be sure it's not anything in the room or on the circuit your audio is plugged into.

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Old 6th June 2013, 04:27 AM   #20
rfengineer2013 is offline rfengineer2013
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I'm with bobrown14 on this one. From what I've read, if I am reading it correctly, the OP has an amplifier that has been quiet for 7 years and now has a slight but noticeable hum in one channel.

The first question is " what has changed". If not in the setup, then in the environment.

Before digging "under the hood" and making circuit modifications or changes, it would make sense to pull the components apart - unplug everything from everything - and start plugging things back together one thing at a time.

Check the cable routings, make sure everything plugs into the same wall socket (or use a common power strip), untangle all the cables (if they're tangled ), etc.

OK, once you've done all that if the problem is still there then it's time to start digging into the amp.

DF makes a good point about the floating 6.3V heater - it is generally a good idea to bias the heaters using a voltage divider off the B+ (HT) voltage.

In older designs, and most commercial stuff sold during the Age of Tubes, the heaters were left floating, or RF-bypassed using a small capacitor (the 0.022uF cap in your circuit is probably there for this purpose).

In modern amps, especially with sensitive speakers, the 60Hz heater voltage can couple to the cathode of the tubes and come out the speakers as hum.

To make the bias circuit, all you need is two resistors and a bypass cap, 0.47uF or so, rated at >50V for a bias voltage to 30-40V. If you have a bigger electrolytic, it's OK to use it - the more the merrier - but you don't need excessive filtering on the bias network to achieve your purpose (the heater will still have 6.3VAC on it anyway...)

The heaters will draw no current from this bias network, so you can use a 150k-ish first resistor (off the B+), and calculate something in the 30k-ish range for the lower resistor (to ground) using Ohm's Law to get the desired bias voltage from the actual B+ voltage in your amp.

Bypass the lower resistor to power-ground using the bypass cap.

Connect the bias voltage to the 6.3V center tap or tack it to one side of the heater at a tube socket - whichever is easier. Usually, it's easiest to do this back in the power supply - whatever you do, be sure to use the B+ and not the plate of one of your tubes to tap the high voltage...

In this type of circuit, the heaters do NOT return to system ground through a DC connection - so in effect the heaters now "float" at a controlled voltage but still get all of their current from the 6.3VAC transformer.

Finally, be sure to check the 0.022uF cap that is presently on your 6.3VCT - make sure the voltage rating is higher than your bias voltage. They come in many flavors, but the rating should be not less than your bias voltage (if it's rated higher, that's OK) for this. If it is not rated high enough, remove it. Otherwise, leave it alone (it won't hurt anything as long as the voltage rating is higher than your bias voltage)

As always, this comes with the standard disclaimer that if you feel uncomfortable or unsafe working with high-voltage equipment - please ask the Ham next door for help! These amplifiers can give a nasty shock at best, and can be lethal at worst, to inexperienced people. We were all inexperienced at one time, and survived (some would question that...) by learning from mentors or other interested people - so if you start fooling around inside the amplifier you've been warned

Good luck

-- Just an average Man of Bronze --

Last edited by rfengineer2013; 6th June 2013 at 04:32 AM.
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