Switching a preamp tube

[dkevin]

Is is possible to set up a preamp tube to have 1 of 2 possible plate resistors and to switch between them? I am trying to layout a Gibson GA-40 clone and I am wondering if one 5879 tube can be utilized by both the microphone channel and the instrument (tremelo) channel ...It would involve trading one plate resistor for another and adding 2 components to the cathode of the 5879. Thanks for your advice!

[Chuck H]

The cathode resistors for both channels use the same values. There are no other components that need to be switched. It's possible the other components you are looking at are only apparently connected to the cathode via a common ground connection on the schematic.?. As to plate resistors... The difference is much more profound than just "value" because one circuit accommodates the tremolo circuit and the other is supposed to be independent. That gets more complicated. I'm sorry, I don't see a good way to make the channels switchable with one tube serving double duty.

[eschertron]

The biggest difference I see between those two channels is the layout of the grid stoppers and leaks. the instrument channel is loaded down more than the mic channel. And then which filter network are you going to use? Between the grid and plate loading differences, the change in plate DC load line is inconsequential.

My 2cents would be to use a switch to bypass the filter bank, come up with a good compromise grid load, and forget about the plain jane input 1. (insert 'grain of salt' smiley thing here)

edit: d'oh! Chuck beat me to the punch!

[Enzo]

The upper input is loaded by about 411k in the high gain jack, the lower channel by 420k (neither jack is higher gain there). I don;t see that as much difference. the lower channel does have the signal reduction though.

I don't see a lot of point in the mod.

[Chuck H]

I figure he wants to use the one tube to save space, extra wiring, expense, transformer limitation, or ??? Totally not worth it. Notice that the trem channel has amendments to the plate load in that it's split and grounds some top end. They surely didn't design that in from the start. They discovered they needed to do it for function. And that would surely affect the "not trem" channel. So it seems too complicated to meet the criteria of both channels with just a switch.

When I first looked at this thread I thought I might see some slick split load and dual cap thing I could pull off and BAM! Hero! I didn't see it though once I read deeper.

[dkevin]

I'm afraid I had no high-minded purpose in asking the initial question. I have long wondered whether there is a way to switch plate resistors to a single tube given that most of the other loads (cathodes/cathode bypass caps) are (nearly) identical. The inputs could be configured as a high/low and the screen-resistor/cap to ground in the instrument channel could be switched in or out. My concern was with being able to select (on the fly, as it were) between the 220k of the microphone channel and the 510k of the instrument/tremolo channel with its r/c network. I probably would also add a bypass to the r/c network to regain some of the drive that was sacrificed in order to stabilize the tremolo section.

[Chuck H]

Well, one good thing about having actual "channels", as in two discrete input tubes, is that you can achieve channel switching with a common A/B pedal.

[eschertron]

I'm afraid I had no high-minded purpose in asking the initial question. I have long wondered whether there is a way to switch plate resistors to a single tube given that most of the other loads (cathodes/cathode bypass caps) are (nearly) identical. The inputs could be configured as a high/low and the screen-resistor/cap to ground in the instrument channel could be switched in or out. My concern was with being able to select (on the fly, as it were) between the 220k of the microphone channel and the 510k of the instrument/tremolo channel with its r/c network. I probably would also add a bypass to the r/c network to regain some of the drive that was sacrificed in order to stabilize the tremolo section.

While I don't like the idea of [break-before-make] switching between two plate resistors, I might jumper in a parallel resistor on switch - say two 470k resistors, either one alone or both in parallel to get ~220k - for a different load line. It probably makes a loud pop when it transitions, even at idle, but it's not quite as drastic as opening up the current path for a fraction of a second. May be good for experimenting, to get design ideas.

Another potentially neat thing about the circuit is the 220k resistor from the screen grid to ground on the instrument channel. Again, I'd think for experimental purposes only, switching that resistor in and out would be OK and allow some quick A/B testing. I haven't played with pentode input tubes and so would be interested to hear different variations and combinations on the test bench.

Keep us posted!

[Gregg]

Is is possible to set up a preamp tube to have 1 of 2 possible plate resistors and to switch between them?

Yes, it's possible and you can see it implemented in some Mesa amps like Roadking and Mark V.
What you need is a relay (if you're going to use one) that can handle the voltage.
However note that when switching between the resistors you'll hear a loud pop due to changing tube's operating point and the momentary cut of the supply voltage to it.
In the amps mentioned above that is somewhat avoided by implementing a mute circuit.

[KevinOConnor]

Hi Guys

Yes, you can switch the plate resistor value and several amp builders have done this.

There are two common approaches: The first places two Rs in series and one R is simply bypassed with the switch or left in play. The two anode loads are then either the sum of the Rs or just the unbypassed R.

In the second approach, a fixed high-value R ties the anode to the supply all the time and represents one of the plate load value options. A second R is switched in parallel to provide a lower plate load value that is a composite of the two Rs.

There are no special attributes required for the switch, although a good quality one should be used with a high insulation rating. The C&K 7000-series switches have a 1kV rating. The methods can be extended to more than two values using a SPDT-center-off switch, or to four or more using a rotary switch. The ultimate is to use a pot for infinitely-variable loading, although this will likely introduce the "scratchy pot" problem.

There will be DC transients when the switch is manipulated and this cannot be avoided unless you go high-tech and use active devices to switch or simulate the resistance values.

Similarly, you can switch the cathode R value.

Have fun