Understanding pentode/triode switch

[cc.]

I have found several pentode/triode switching circuits floating about the internet, they all seem to be something like this:



I understand that the idea is to connect the plate and the screen via a resistor to get 'triode' operation. What I don't understand is why the circuit also leaves the resistor in when the screen is connected to the screen supply for pentode operation. Why not put the resistor in the other side of the switch (between the plate and the switch)? Then the resistor is in circuit during triode operation, but during pentode operation the screen is just connected directly to the screen supply.

Any ideas?

[Tubis]

If you watch some schematics of pentode operation you can se that the resistor is there in that operation as well.

[cc.]

That's interesting, do you know what they are there for?

The amp I am thinking of modding does not have those resistors. There is a 1.5K resistor to generate the screen voltage (and a 50uF cap to smooth it), and then both of the screens are connected straight to that.

[Tubis]

Its a current limiter.

[Enzo]

The scren grid is just a tiny little wire in the electron beam. If it absorbs too much current, it melts. The resistor prevents that.

[booj]

Only problem is some folks think it soundes a little mushy.

[Enzo]

Not half as mushy as a burnt out tube. Many guys report that the best tone their amp ever produced was when the bias was way hot and the tubes were melting down. SOunded great for a few minutes. The purpose of the screen resistor is to prevent tube meltdown, even while the tube might sound "better" as it dies.

[mooreamps]

Only problem is some folks think it soundes a little mushy.

It's not supposed to be that way. A proper value of SCR is supposed to add the correct amount of increased "touch responce" to the tone.

-g

[Chuck H]

Doesn't the capacitance within a pentode increase in triode operation? That could be interpreted as "mushy" since some top end snap or sizzle is lost. Having tried it myself that's what I read it for.

Chuck

[booj]

SCR?

[Enzo]

SCreen Resistor - or something similar.

[Steve Conner]

I love the American habit of making up new acronyms on the spur of the moment and expecting everyone else in the world to guess what they stand for.

In this case, SCR is already taken, it stands for Silicon Controlled Rectifier. If you meant screen resistor, it's commonly shortened to "Rg2"

BTW, yes, in triode operation there is more Miller effect between the control grid and screen grid, since there's more signal on the screen grid. (I wonder if anyone ever tried bypassing their screen resistors to ground with small capacitors, maybe 1uF or 0.1, to get triode bass and pentode high end...)

[Enzo]

It is worse than that. We like to take some simple term, then restate it in more fancy official sounding words, until we can get it up to several syllables and THEN we abbreviate it. SO Shell Shock becomes post traumatic stress disorder, which we can then knowingly refer to as PTSD.

My wife has started an organization to fight this. It is called SUSAN, for Stop Using Stupid Acronyms Now.


SO does ATM stand for Automated Teller Machine or asynchonous tranfer mode?

[Chuck H]

(I wonder if anyone ever tried bypassing their screen resistors to ground with small capacitors, maybe 1uF or 0.1, to get triode bass and pentode high end...)

I thought of that...But I haven't tried it yet.