ELS
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Pentodes are awesome for tremolo oscillators, only problem is that it's tricky getting the waveform linear, you have to bias the pentode cooler (high cathode resistor, bypassed), but then you loose voltage swing.
But the benefits are:
Incredibly tolerant to tube aging
Incredibly tolerant to undervoltage, can run it at 100V B+ no problem
Possible to get a much higher frequency range
Longer lasting tremolo tube since you aren't stressing the tube with 450V just to get it to reliably oscillate
Here's an example design, the 2Meg is the speed pot, but you can go up to like 10Meg as well as low as 50k and it still works.
To tune it:
Increasing the cathode resistor makes it more linear but reduces voltage swing, increasing it leads to a spiky waveform.
Changing the screen resistor mostly just shifts the plate voltage, so you can balance the cathode resistor and screen resistor to get the DC plate voltage at around half B+, and maximum AC voltage swing.
I usually just leave the plate resistor at around 100k.
You can change the 820k to 330k, and add another 330k to ground, that way the 1uF cap only needs to be rated at half B+.
You can probably also use the screen supply to lift the heater center tap, only if you use the divider tho.
That's if you're into splitting hairs
Finally a use for triode-pentode TV tubes!
But the benefits are:
Incredibly tolerant to tube aging
Incredibly tolerant to undervoltage, can run it at 100V B+ no problem
Possible to get a much higher frequency range
Longer lasting tremolo tube since you aren't stressing the tube with 450V just to get it to reliably oscillate
Here's an example design, the 2Meg is the speed pot, but you can go up to like 10Meg as well as low as 50k and it still works.
To tune it:
Increasing the cathode resistor makes it more linear but reduces voltage swing, increasing it leads to a spiky waveform.
Changing the screen resistor mostly just shifts the plate voltage, so you can balance the cathode resistor and screen resistor to get the DC plate voltage at around half B+, and maximum AC voltage swing.
I usually just leave the plate resistor at around 100k.
You can change the 820k to 330k, and add another 330k to ground, that way the 1uF cap only needs to be rated at half B+.
You can probably also use the screen supply to lift the heater center tap, only if you use the divider tho.
That's if you're into splitting hairs
Finally a use for triode-pentode TV tubes!
