22-09-2018, 08:24 AM
In theory the grid leak resistor is not needed if the wiper of the pot directly feeds the grid. In practice it's worth having so that if there's a problem with the pot then the valve's grid won't be left floating. If you have a pot whose wiper is making intermittent contact the resulting noises won't be pleasant. Just make the grid leak resistor at least 10x the value of the pot.
Actually you can use this arrangement to simulate a log pot with a linear one. Log pots are generally pretty crude approximations. If you use a linear pot and the grid leak is smaller than the pot value you can get a good approximation to log. You can do the sums but somewhere between a half and 20% of the pot value is about right. you can just try it and see if the control action feels nice. The downside is that the loading on the previous stage will vary with volume control setting. Whether this matters depends on the design of that stage.
I don't like Trevor's idea of putting a pot between cathode and ground. DC flowing in a pot's track can often make the action sound rather scratchy. Also, as Trevor points out, if you turn the pot rapidly you're putting a low frequency thump into the next stage as the coupling cap charges to the new voltage.
Actually you can use this arrangement to simulate a log pot with a linear one. Log pots are generally pretty crude approximations. If you use a linear pot and the grid leak is smaller than the pot value you can get a good approximation to log. You can do the sums but somewhere between a half and 20% of the pot value is about right. you can just try it and see if the control action feels nice. The downside is that the loading on the previous stage will vary with volume control setting. Whether this matters depends on the design of that stage.
I don't like Trevor's idea of putting a pot between cathode and ground. DC flowing in a pot's track can often make the action sound rather scratchy. Also, as Trevor points out, if you turn the pot rapidly you're putting a low frequency thump into the next stage as the coupling cap charges to the new voltage.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
