Hum in tube amp

[vcelkamaja]

Still proceeding with elimination noise / hum in this amp
So after reducing 150Hz hum cause by transformer coupling I'm hearing some noise - sounds like a white noise and it is audible from ~ 0.5 m away from the speaker.

I did one more experiment - so far I used the amp without a global negative feedback. What I did now is that I used feedback from the output terminal to the input tubes - per this circuit - page 8: http://analogmetric.com/download/6L6 SE Amplifier User Manual.pdf

What I can hear now is kind of repeating noise - not sure how to describe it in English something like tick tick tick… with the frequency of ~ 0.2 Hz or so. Any idea why is it happening and what should I try to do to reduce it? I also used it with 33k resistor and 100p capacitor as per schematic: http://analogmetric.com/download/6L6 SE Amplifier Schematic.pdf (actually I used 51k and 120p - can this be an issue - some instability?)

 

[ejp]

This is called motorboating, and it is caused by excess LF phase shift. As a first defence, review all your coupling capacitors: can they be smaller?

 

[vcelkamaja]

Thanks a lot - what I found out that there is an optional capacitor between grid and cathode on the PCB too. I soldered a small 33p cap there and the noise (motorboating is almost reduced). Can it be that this cap somehow compensates for the stability of the feedback?

is there anything wrong with adding this small capacitor of 33p between the grid and cathode of the input triode?

 

[nbtone]

Something to try,

Just disconnect the Gnd from the heater supply and put a 0.1uf polypropylene cap rated at 600v one end on the connection Ground and the other to the heater. See what happens.
Do the same to the other channel. You can remove any dividers for the HT/B+ lifting and just float the heaters with the cap...

Regards
M. Gregg

I know this is an old thread, but I’m working on a 6.3vac heater supply where there is no center tap for the heater supply. I tried using two of the .1uf polypropylene 600v capacitors going to ground from each side of the 6.3 vac supply. It eliminated the hum, but there is some kind of ringing at a very low volume in the background now. Do you have a suggestion for what might clean this up?

 

[TG]

Don't use reactive components (capacitors, inductors) where they aren't needed. Especially don't use capacitors and inductors together.
A pair of equal resitors in the 100-300 Ohm range will do just fine.

 

[kissabout2002]

No way capacitors I normally use two 100 ohm resistors and that's it any hum goes away...all the best

 

[steventrus]

I have been experimenting by building vacuum tube headphone amps. There are two types I have built. The first is a white cathode follower that uses 3 6922 tubes. I had a problem with a hum that had the following characteristics:
60 hz hum
hum disappeared instantly when power is shut off (even thought the music
continues to play)
volume does not affect hum - it is constant.
so in this amp I solved the hum problem by:
using dc for filament and grounding the negative
having a B+ circuit that uses a large cap, voltage dropping resistor, and
another large cap

my next project was a different design, based on a 6922 and a 6AS7 (would be interested to know any opinions on which design is better)

I have the same hum problem on this amp, but can't get rid of it doing all of the things mentioned above.

I don't know if this information helps, but I use a circular ground going from one part to the next forming a circle and the ground is connected to the wall outlet ground.

Any help in fixing this hum would be greatly appreciated.

 

[kissabout2002]

steventrus have you done what is in the picture? No matter you using dc or ac for the filament this simple circuit take the hum off.....all the best

 

[Chris Hornbeck]

I know this might sound basic, but the first thing I'd ask is: do you get the hum with the amplifier connected only to power and (floating) speakers?


YOS,
Chris

 

[kissabout2002]

Depends, you can get the hum with the volume close mostly 60Hz but you can get also sort of hum just with the volume open, in this case comes from the preamp, short circuit the input will help understand the source of the hum.

 

[Chris Hornbeck]

Well, I mean with *nothing* else connected - wires pulled out. No preamp, no nothing.


This might seem elemental, but AC mains ground loops should be ruled out first.


YOS,
Chris

 

[kissabout2002]

so yes you can get hum with nothing connected and volume close

 

[steventrus]

steventrus have you done what is in the picture? No matter you using dc or ac for the filament this simple circuit take the hum off.....all the best

Thanks for the suggestion. I will try your recommendation and let you know if it helps

 

[steventrus]

I know this might sound basic, but the first thing I'd ask is: do you get the hum with the amplifier connected only to power and (floating) speakers?


YOS,
Chris

The hum starts about 30 seconds after I turn the headphone amp on (after the tubes have warmed up). The hum is there even with without anything connected to the input RCA sockets. Also, if I am playing music and I turn off the power, the music continues to play for a few seconds and the hum is completely gone the instant the power is turned off. I built the amp to the exact specifications of the circuit. However my power supply is a little different from the circuit diagram, due to what parts I had available. Specifically, the circuit diagram calls for a 10H choke, but I didn't have one, so I used a voltage dropping resistor instead. This technique works on my other headhphone amp (white cathode follower), but does not work on this one. I have ordered a chock and when it arrives I will install it to see if that fixes the problem. This is the link to the circuit diagram for this headphone amp:

A Single-Ended OTL Amplifier for Dynamic Headphones. – HeadWize Memorial

for reference, this is the link to the circuit diagram for the white cathode follower headphone, that works with no hum:

The Morgan Jones Mini Tube Headphone Amplifier. – HeadWize Memorial