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 Post subject: 180hz Hum?
PostPosted: Sat, Oct 26, 2019 3:20pm 
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Hack

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I have a rackmount tube preamp with noticably more hum than its head counterpart. I've established that:

1) it's 180hz hum
2) it's affected by gain level (isn't really audible until high gain territory)
3) it's not related to any instrument plugged in
4) the tubes are fairly new
5) the moment I turn off the power, the hum goes away (even for the 5-10 seconds where signal is still coming from the preamp and it sounds normal)

What is this symptomatic of? Circuit design? Tube heaters? Capacitors going bad?

Most capacitor-caused hum I'm familiar with is the old power section 60hz hum when filter caps are bad.

Thanks.


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 Post subject: Re: 180hz Hum?
PostPosted: Sat, Oct 26, 2019 5:41pm 
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sadistik42 wrote:
I have a rackmount tube preamp with noticably more hum than its head counterpart. I've established that:

1) it's 180hz hum
2) it's affected by gain level (isn't really audible until high gain territory)
3) it's not related to any instrument plugged in
4) the tubes are fairly new
5) the moment I turn off the power, the hum goes away (even for the 5-10 seconds where signal is still coming from the preamp and it sounds normal)

What is this symptomatic of? Circuit design? Tube heaters? Capacitors going bad?

Most capacitor-caused hum I'm familiar with is the old power section 60hz hum when filter caps are bad.

Thanks.

I think you mean 120Hz, correct?

100 or 120Hz Hum
This is probably the most common hum in an amp. It can be caused by:

A bad tube so swap out the tubes for known, good tubes.

An input jack that doesn't ground out when nothing is plugged in. If the jack's shunt switch does not make good contact with the jack's tip connector you will get loud hum when no guitar is plugged in but the amp will sound fine when you plug in a guitar.

Worn out filter capacitors. Electrolytic filter caps have a typical lifespan of 15 to 20 years. You can test for bad filter caps by alligator clipping in a parallel cap. If the new cap reduces hum then replace the original cap. If your amp is fixed bias don't forget the fixed bias circuit filter cap(s).

Missing ground. Loud hum can be caused by components that should be grounded but aren't. Forgetting to solder a volume pot's ground wire or a cold solder joint on a cathode resistor's ground are two common causes of loud hum. I like to use an alligator clip wire and connect one end to the chassis and carefully probe all the amp's ground connections to see if I hear an improvement. Touch the ground probe to all volume pots' ground terminal, the ground side of preamp cathode resistors, etc. Pressing solder joints with a chopstick can also help find bad ground solder joints.

Loops in the ground circuit. If your input jacks are grounded to the chassis and you also run a ground wire between them then a ground loop is formed (chassis is one side, the wire is the other) which can act as an antenna and pick up RFI noise and hum. You will also form a ground loop if you use shielded cable inside the amp and ground both ends. You should only ground one end of shielded cable.

Lead dress (wires too close to one another causing capacitive coupling) Chopstick the amp's wires around with the amp on and the volume up to see if you can decrease the hum. Pay close attention to the tube grid and plate wires--keep them as far apart from one another as possible. My first amp build had a loud hum because the first preamp stage's grid and plate wires were sitting on top of one another. Separating the wires silenced the amp.


https://robrobinette.com/Tube_Guitar_Am ... m_and_Buzz


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 Post subject: Re: 180hz Hum?
PostPosted: Sat, Oct 26, 2019 8:28pm 
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Power supply filter caps.
Even though it's a rack preamp, there's still b+ to power the circuit, and that means filter caps for the power supply.
I'd look there first.

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 Post subject: Re: 180hz Hum?
PostPosted: Sun, Oct 27, 2019 1:54pm 
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+1 are you sure it's not 120Hz?

Filter capa if so. Easy swap out. I really like Sprague Atoms FWIW. F&T is a close second.

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 Post subject: Re: 180hz Hum?
PostPosted: Tue, Oct 29, 2019 1:05am 
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Thanks for the advice. It's definitely 180hz (checked with an EQ notch filter), which is clearly related to 60hz (and by extension 120hz) because it's the second harmonic overtone. Looks like it's due for a cap change.


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