What does the triode/pentode switch do?

[Handsome Devil]

Hi guys.

I have a Blackheart Handsome Devil half stack and I have old stock tubes in it. It have a 15/7 watt switch and I wonder how that work. Does it cut halves of the 4 tubes or cut 2 out of 4 tubes? Or does it just reduce the voltage instead?

I was wondering if I will wear out the halves of the tubes using 7 watts exclusively.

Thanks guys.

[MWJB]

A triode is a tube with 3 components: a control grid (signal goes in here), a plate (signal comes out here) & a cathode (much of the time a resistor goes from this to ground to bias the tube, other times the cathode is just grounded and a negative voltage applied to the grid to bias the tube).

A Pentode has 2 more components, a screen grid & a surpressor grid...a pentode can be more efficient & make more power.

Your switch just selects these 2 modes probably by tieing the plate & screen grid supplies together for "triode" mode, that's where the power drop comes from, all tubes are still functioning, they will normally be biased the same (within reason), it shouldn't wear out your tubes any quicker.

[Handsome Devil]

So it's really a voltage drop for lower wattage. Good to know, I was worried that I'll wear the tubes unevenly.

Thanks.

[MWJB]

Not a voltage drop, a power drop by putting the power tubes into a less efficient mode of operation...but yes, it won't wear the tubes unevenly, nor lead to their early demise.

[Chuck H]

Unless I'm way off I think that triode mode could actually extend tube life. With the possible exception that the screens are at a higher than ideal voltage and the affect from this. Otherwise, IIRC, triode mode would ideally use a lower primary Z. So with the same OT and speaker load you would be presenting a higher than ideal primary Z which typically means less tube wear but harder on the amp components. Which shouldn't be any problem at the lower wattage. So other than the increased capacitance (assumed from my percieved top end loss in triode mode) it's a win/win.

[Handsome Devil]

In case it matters, it's a class A push/pull.

This amp is great and as to what the maker says it's overbuilt so it's pretty heavy duty. That was the selling point for me. No onboard FXs either so it's sweet for my pedals.

[stormbringer]

It switches the amp from pentode to triode and back. Ok, just ignore this post.