Triode connection in ST-70

[Bob01605]

-----------------I wouldn't, but, Bob Latino would say otherwise...

Most of us will throw in a switch that isn't rated for the voltage it is seeing.
That is the main reason I do not recommend flipping the switch when the amp is running. That and the possibility of arching the contacts and the nasty pop thru the speakers.

If Bob wants to flip his switch let him but I surely wouldn't recommend to amyone to do it.

Just to clarify something > The switches that I use cause no transient spike when switched. There is no transient "pop" when the switch is thrown. I do caution customers to lower the volume level when switching from pentode to triode or vice versa but with the switches that I use there is no need to shut the amp off when making the switch (in either direction).

I also want to disagree with you on the bias voltage when switching from pentode to triode mode (or vice versa). What dcgillespie says above is correct. When you go from pentode to triode (at least on a Dynaco amp) the bias voltage WILL drop very slightly and insignificantly. On an amp with a triode/pentode switch if you are at 55 milliamps in pentode and switch to triode the bias voltage will drop about 2 millivolts to maybe 53 milliamps.

Bob Latino

[dcgillespie]

I hope this idea of using a diode in the screen grid circuit to allow it to be positively energized but also "preventing" any AC back flow from it (and therefore any supposed power loss) is not getting any credible support here. It is nothing short of audio mumbo jumbo.

Current flow in an electron tube is like that of any other electronic circuit: It flows from negative to positive. Therefore, it flows from the cathode of a tube, to any other element in the tube that is more positive than it is. One of these is the screen grid.

The complete, but simple circuit of the screen grid in a pentode has the negative side of the power supply connected to the cathode of the tube. The positive of the supply is connected to the screen grid. Current flows from the negative side of the power supply to the cathode, from the cathode through the tube to the screen grid, and completes the circuit back to the positive side of the power supply. Inserting a silicon diode between the screen grid and the positive side of the supply so that it will allow DC current to flow, will also still allow any AC current to flow in this circuit as well.

The idea attempting to be flown here is that a diode is a rectifying device, and therefore prevents any AC screen current from backflowing to, and therefore being lost through, the power supply. Well true, a diode can be a rectifier -- if the circuit and wave form it operates with causes it to be biased into nonconduction for a portion of the cycle like it does for example in a rectifier circuit where a large cap holds the cathode end of the diode positive while the AC waveform applied to the anode goes negative for a period of time -- but that is not what is happening here. In this case, the anode of the diode is ALWAYS more positive than it's cathode -- the anode being connected to the positive side of a DC power supply -- so therefore it is always conducting. As a result, any variations in screen current will flow right through the diode back to the power supply -- as if the diode were never there to begin with.

The installation of a diode in the screen grid circuit of a tube as suggested changes nothing with respect to how the screen interacts with the remainder of the circuit -- whether pentode or triode connected. Audio Snake Oil like this is all too common, but I hope it will be seen here for what it is -- and what it's worth.

Dave

[mesherm]

All opinions are welcome. I know one fact for sure. My triode connected PP amp was giving me 15 watts output until I added a 15 cent UF4007 in series with each screen grid resistor. Afterward its measured output was 18.2 watts. An extra 3.2 watts for 60 cents worth of parts is audio snake oil I can live with, unlike Golden Goddess 'Super Effect' Speaker Bullets for $4200.