phase inverter

[geoff 1965]

hello,
i've been looking into LTP phase inverter application and attenuation and there is a couple of area's i'm still unsure about.i've established the reason some have unequal value plate resistors i.e. 100K & 82K is to balance the phase inverter on amps like the plexi's that use one half for signal and the other half for negative feedback.The tail resistor affects the amount of signal into the next stage i.e. if you use the 18W inverter with 6V6 tubes you have to lower the resistance to 22K and for EL34's lower to 10K so this must boost the signal.

18w lite III.pdf

but have a look at this lite3 schematic and notice the tail is still 56K but the 820R is reduced to 470R and the 470K's have been increased to 1meg.
so what do these changes in value actually do? do they relate to the attenuation?
food for thought!

[JMPGuitars]

If you look at Mark's Lite III layout, he used the standard 18watt values...but I think one or the other was unintentional? Who knows. the first link below.

Simply put, a lower value should increase the load on the PI, and decrease the power tube distortion. A higher value should reduce the load on the PI and increase the power tube distortion (and likely more noise/crossover distortion). These values need to be set for the tubes, and the tone you're seeking (preamp vs. power tube distortion).

these threads too:
viewtopic.php?p=75039#p75039
viewtopic.php?p=35068#p35068
viewtopic.php?f=12&t=7774
viewtopic.php?f=4&t=25180
viewtopic.php?f=4&t=24968
viewtopic.php?f=4&t=18126

[geoff 1965]

thank's Josh, this is interesting! so reducing the 820 cathode to 470 will give a bit more phase inverter distortion, if you have a PPIMV which is actually more like a post phase drive where you loose el84 breakup at low volume this is handy to know.
just the 470K to 1meg to work out now!
1meg with 470R is plexi superlead values and the same as the lite3 with the exception of the 10K tail for the EL34's. the higher voltage "spitfire" has 1 meg's so i wonder if this relates to higher phase inverter plate voltage?
just been on the aiken website to read up on the effect the grid resistors have in the phase inverter;they have little or no effect on gain for normal values,if they are too low in value they will attenuate the signal.
they do have an effect on frequency response,using a higher value will result in greater low frequency response.
so without an oscilloscope it will be hard to see the difference between 470K's and 1meg's.

[JMPGuitars]

The cathode resistor is for biasing the cathode (it does interact with the grid bias resistors also, but that's a different story).

I was referring to the grid leaks (grid bias - pins 2 & 7 on the ECC83) resistors. Yes, too low a signal attenuates, functionally similar to a fixed PPIMV.

This is all specific to how the 18W circuit functions. If you're looking at those values for different circuits I couldn't tell you what the effect might be. Possibly similar, but 18watters generally lean on power tube distortion, while the majority of other amp circuits are primarily using preamp distortion. Adjusting PI bias points for functionally dissimilar amps is not likely to have as much of an effect (which isn't always that drastic anyway).

As I'm sure you saw in some of those threads I linked, dropping below 470k to 330k or 220k is a tone + distortion change. 220K for vox chime, or 330K for a balance between the Vox + Marshall sounds (like in the EF86 Lite).

Thanks,
Josh

[geoff 1965]

yeah i think i see what he's done,he's used 220K grid leaks on the el84's for cleaner headroom and the changes in value in the phase inverter compensate for the loss of power tube distortion.
i realized after that you meant the phase grid leaks.
thank's Josh