ST-70 Volume Control

[nate96z28]

I'm building an ST-70 and have added an ALPS 100k pot to control the input. I've read that the 100k pot can be used with 11k resistors on each channel, but am not clear on a) whether those resistors run in series with each channel between the pot and input jacks and b) whether after installing the 11k resistors do you remove the 470k resistors going across the input jacks. Can anyone clarify? Thanks!

 

[peterh]

You could post a schematic of the suggested part.

 

[jbrew73]

Is the haller circuit what you are referring to?

 

[wdecho]

The pot itself should be enough resistance to prevent any problems on the input. I would probably leave the resistor in as well though.

 

[gadget73]

Wired like above the input impedance of the amp is highly variable. Need to be able to drive a pretty low load for that to work well. If its wired as a normal setup where the grid is tied to the wiper, the input load is just whatever the value of the pot is.

shouldn't need any extra resistors here honestly, the pot will sub for the 470K across the input jack, and there isn't any extra resistance needed for stability tuning on these amps.

 

[Lavane]

Tubes4HiFi used to sell a volume control that fit between the front of the chassis and the PCB. There isn't much room there. The shaft of the pot fits in the stereo/mono switch hole. I added one of those volume controls to my VTA ST-70 amp kit when I built it. No need to add or delete anything. Just replace the stereo/mono switch with the volume control.

 

[nate96z28]

Thanks everyone. Yes, it is the haller circuit I am referring to. Exactly what jbrew73 posted above. Sounds like with a 100k pot I can remove the 470k resistors from the input jacks and possibly omit the 11k resistors in series?

Im using a source that can put out 2vrms.

 

[gadget73]

its less the voltage than the output impedance. The Haller setup is a shunt so the input loading is highly variable, min of 11K, max of 111K. Not everything can run that low. If its wired as a normal 100K pot, the loading is a flat 100K and most things will handle that.

 

[StepheK]

Sounds like with a 100k pot I can remove the 470k resistors from the input jacks..

Im using a source that can put out 2vrms.

I would always have a resistor from the grid to the ground in case the opt wiper goes open the tube is still biased. You can use whatever the max resistor the data sheet shows, on many small signal tubes it's 1Meg.

And it's not the source output, it's the source output impedance that is gonna determine if it can drive that 11K load with that other wiring you showed, which honestly I've never seen it wired like that before. BTW audio note makes a volume pot that is small enough to fit in the front of a ST70.

 

[Lavane]

I would always have a resistor from the grid to the ground in case the opt wiper goes open the tube is still biased. You can use whatever the max resistor the data sheet shows, on many small signal tubes it's 1Meg.

And it's not the source output, it's the source output impedance that is gonna determine if it can drive that 11K load with that other wiring you showed, which honestly I've never seen it wired like that before. BTW audio note makes a volume pot that is small enough to fit in the front of a ST70.

Its a tight space for sure. And a bit of a PITA to wire if you have fat fingers like mine .
The pot you posted looks to be of a better quality than what Tubes4hifi was selling. I'll keep that one in mind if I ever need one to fit a tight space.
The one from Tubes4hifi that I bought does have fine detents with plenty of steps though. It's been in my amp since around 2011 or so without any issues.