geoff 1965 wrote: ↑Sat 07/20/19 6:13 pm
... i think my next build will be el34.
typically looking at different schematics when using the 18W phase inverter the tail resistor is dropped to 22K for 6V6's and i'm sure i read somewhere zaphod phil recommending 10K for el34's.maybe that's for a plexi type power section?
There's hours of entertaining discussions to be had arguing the 'right' values for the LTP component values
Just to be clear, the EL34 datasheet I was talking about shows 20v RMS at the individual grid, ie. 40v grid to grid, so about twice the absolute voltage of the EL84s.
Now IMHO this is no great drama at all, as the circuit with the '18 watt' LTP values & preceding gain stage with very low loss tone stack means getting 20+v signal swing to the power tubes is no issue whatsoever, even with wimpy single coils...
It's useful to look at the following gain calculator for the LTP PI: -
https://www.ampbooks.com/mobile/amplifi ... iled-pair/
If you set the values to 18watt values you get gain of 28+ on both the inverted and non-inverted signals, i.e. you need less than 1v into the LTP to get our EL34s to full power.
You'll also notice that changing the tail resistor to 22K or even 10K doesn't increase that gain factor much at all. Indeed, what it does most is lessen the balance. This is why you'll see 82K and occasionally 91K used on the inverting anode load resistor.
What has far more impact is changing the power tube grid resistors.
If you drop them from the 18watt 470K to 220K with a 10K tail (The "Plexi" values) you drop the gain from around 28 to 25.
Ergo... IMHO (and in my experience) you don't need or want to change the tail resistor here...