Changing Plate Resistor...

[BesaMoogie]

...on a Switch?
So is there any good solution without being dangerous with the high voltage applied to the switch?

I konw working with relays would be one way to do it. But I want to keep it simple if possible.
So I am thinking of maybe having the switch inside the amp chassis installed. That way, you have to unpower the amp get the chassis out to have access to the switch. The Switch would be well dimensioned in this situation to handle the Voltage/Current

Another idea is using a trim pot insalled directely to the PCB. Again, you have to disconnect the amp and pull the chassis out to make adjustments.

 

[FourT6and2]

A trim pot works. And a switch will also work. You could put the switch top-side, by V1. Or you could put it inside the chassis. Would even be fine with amp in standby as long your amp is wired such that no B+ is flowing. Or you could just shut amp off completely. But also make sure there is a "drain" resistor on the filter caps to discharge them to ground when amp is off. Most modern amps do.

If you use a trim pot, make sure it's rated appropriately.

That said, V1a plate resistor can affect the amp's noise floor. So a high-quality, 1-watt, thin-film or metal film resistor is best. Most trim pots are carbon trace, which will be noisier. But you can always try it to see.

In the end, goosing V1a plate resistor value is typically a first step to getting more gain, so it's worth a try. But another option is two different cathode arrangements on a switch to bias the stage hotter/colder. Something like 2K7/0.6uF and 10K/0.1uF. There will be a slight pop when switching. But it's a little better than switching the plate.

 

[RedPlated]

The safest way is putting it on a push pull pot. It’s better isolated than a mini toggle. This is how the “Pull Compression” feature works in the Hellion and it works great.

Set your plate to the higher value, then parallel another resistor over it via the switch on the push pull pot get the desired secondary value. For example, use a 330k as the primary resistor and parallel a 680k via the switch for 222k.

My resistors are both PCB mounted as I designed it this way. But otherwise, just run the secondary resistor across the switch lugs on the push pull pot. Run shielded cable from the common lugs of the pot to the primary resistor, across each side.

There’s very little, if any, switching noise done this way.

 

[fusedbrain]

The safest way is putting it on a push pull pot. It’s better isolated than a mini toggle. This is how the “Pull Compression” feature works in the Hellion and it works great.

Set your plate to the higher value, then parallel another resistor over it via the switch on the push pull pot get the desired secondary value. For example, use a 330k as the primary resistor and parallel a 680k via the switch for 222k.

My resistors are both PCB mounted as I designed it this way. But otherwise, just run the secondary resistor across the switch lugs on the push pull pot. Run shielded cable from the common lugs of the pot to the primary resistor, across each side.

There’s very little, if any, switching noise done this way.

Lots of good info in this thread!

The above quote is the ONLY way I would consider implementing this type of mod. Even if you use relays, you need to do it by switching in a parallel resistor so that the B+ to the tube is NEVER interrupted, even momentarily.
A properly rated pot is fine too, but would seem better suited for testing purposes. I think having 2 values on a switch would be a more useful mod.

Also, about switching cathode caps in and out..... I've had success eliminating any popping noise by keeping one end of the cap grounded, and putting the switch between the other end of the cap and the tube side of the cathode resistor. The voltages here are very low, and keeping the cap grounded prevents any charge building up on the cap and causing a pop when the switch is thrown.

If you are switching both the cap and the resistor value, use a double pole switch, one side for the caps and one side for the resistors.
Again, parallel resistors are the way to go. Leave the highest value resistor hardwired in and use the switch to parallel another resistor to get the lower value you want. There are lots of parallel resistor calculators on line to get the math right.
Keep one end of the cathode caps grounded at all times, and the switching should be pop-free.

 

[BesaMoogie]

Thanks for the good advice everybody!

 

[SpiderWars]

Look at old Komet schematics. They had a slow/fast switch (I think that’s what they called it) that did this. And that’s the very early Komets from 25+ years ago.