Charlie,
All signal cables will pick up to some extent, however if the cable is being "driven" then it tends to hold it down. Caps will pick up because they are ineffect an open circuit. to prevent pick up you can wrap tin foil around them and then wrap a wire arround that then cover with tape. Gnd the wire. The tape will insulate, the wire will Gnd the foil like a a screen.
All signal cables will pick up to some extent, however if the cable is being "driven" then it tends to hold it down. Caps will pick up because they are ineffect an open circuit. to prevent pick up you can wrap tin foil around them and then wrap a wire arround that then cover with tape. Gnd the wire. The tape will insulate, the wire will Gnd the foil like a a screen.
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When you pass a signal through a resistor you "lift" it away from source or Gnd and it then becomes susceptible to noise. So your pot track is a resistor when the wiper is in the middle you have resistance to signal and earth. If the metal case is not earthed you have a non screened "lifted" signal.
So the smaller the signal the more chance of pick up. This can be AC, RF, EMI, and transient. Any cable that has a current running through it will have a magnetic field arround it. This magnetic field "if it changes" due to AC or signal can "induce another signal in any cable that the magnetic field cuts through. If you screen and earth you can shield the cable from this field. This is never perfect some may get through, however it will be reduced.
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Yes, thanks for all your help.
Ive grounded the boby of the pot, and the 'buzz/hum' is still there, but not as much. So ill pick up some shielded cable after work tomorrow and run that to the driver circuitry. A first I thought it was my wiring, but im not sure it is any more. There is definitely a scratchy sound when the pot is moved, so it might be worth replacing the pot too...
Charlie
You need to screen signal cables, And use "distance" to minimise pick up from power, heater etc.
If you earth your screened cables at both ends you will get Gnd loops. Also if you are using a non metal case you wll have to take even more care! Due to no RF protection. I am refering to radio.
If you earth your screened cables at both ends you will get Gnd loops. Also if you are using a non metal case you wll have to take even more care! Due to no RF protection. I am refering to radio.
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The input signal is probably coming from a fairly low source impedance, so this needs screening less than the pot output (which has a smaller signal and a higher impedance). If you are short of screened cable then screen the grid lead, and use twisted pair for the input lead.
A noisy pot may simply be due to wear (replace the pot) or it could come from DC through the pot or slider. Connecting the slider straight to a grid, as many do, often causes noise as the valve grid current has to go through the pot and the slider contact. May be OK for a rough and ready guitar amp, but poor engineering for hi-fi. You need a coupling capacitor and a grid bias resistor to ground.
A noisy pot may simply be due to wear (replace the pot) or it could come from DC through the pot or slider. Connecting the slider straight to a grid, as many do, often causes noise as the valve grid current has to go through the pot and the slider contact. May be OK for a rough and ready guitar amp, but poor engineering for hi-fi. You need a coupling capacitor and a grid bias resistor to ground.
Hi Charlie !
Despite the hum problem,how is the sound?
And if you have noise problems now with the amp, wait until you build the preamp.....
But with the help of the forum you will get there.....
Best Regards
If that 15K is the only fixed grid to ground you have you should try changing it to 470K. Most amps have a high input impedence to the drivers. Your grid may be a too close to chassis currents.
Hi Charlie !
Despite the hum problem,how is the sound?
And if you have noise problems now with the amp, wait until you build the preamp.....
But with the help of the forum you will get there.....
Best Regards
Before I was using halogen light heaters as my output transformers, yes I know.... but.. it was a project to see how cheap an amp can be made, and still sound good. Which it did. Im very happy with the new transformers, the top end is a lot clearer and so is the bass for that matter. The price was great too.... ill fork out for some 'proper' components in my next build.
Yes, looking forward to future builds and getting your help on them
Cheers
Charlie
DF96, could you just explain that to me so I can follow this conversation. ALso, is 15K generally too low?
Thanks
Charlie
There is a school that believes there are local currents in a chassis. If that is the case the closer the grid is to the chassis potential the higher is the likelyhood of some induced hum. Just a theory...
As a grid leak resistor. Yes it is very low. a 470K is more common. There is not a lot of current. Every part has multiple funtions. That resistor also acts like a voltage devider with the lower, or grounded half of the pot. A low value like 15K throughs away a lot of the signal. With more signal and same noise S/N goes up
Also the pot is a volume control but there is capasitance "everywhere", in the input able, the tube, the hookup wire,... In combination with the pot you have and RC filter and "R" changes as you adjust volume. RC might be right for filtering out buzz and radio stations or not at different volume settings. People use fixed resistors in series with grid and large greak leak resistors and you can have "enough" RC at all volume settings.
Designing the resister network that feeds the first grid is an art of trade off. Lower values give lower noise but load up the source. Low or high values grid stop resistors block either radio or hum.
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