Spit load on plates and caps to ground...

[EFK]

Looking at the first channel here, I understand that what I'm seeing are split load plate resistors, correct? As I interpret it, the .001 cap hanging off the plate feed to ground was probably stuck there to stop hf oscillation and maybe to roll off some treble (it's supposed to be the "bass" channel after all), what what I am having a hard time wrapping my brain around is why the big .05 cap to ground at the junction of the two 100K plate resistors? These are used throughout the amp in fact. Im sure they are somehow bleeding off signal, but I can't seem to grasp the effect of differing values here. For example, instead of .05, what if it was .1? Or how about smaller at .01? I sense that this cap might be affecting treble/bass balance right off the bat, but I'm not sure how... I'm really learning a lot with this old tangle in here!

[Merlinb]

This has caught people out before. Those 100k/0.05uF combinations are not really plate loads, they are the power supply filter stages! By using large dropping resistors, quite small 0.05uF smoothing capacitors can be used instead of the usual 16uF-100uF jobbies that you generally see. Neat, no?

[EFK]

WHHHHHAAAAAAAAATTTTT???????

I'm ready for a beer after that one.

[guitarmike2107]

Ok so it makes sense, But why do it that way. just to save space with smaller caps .. or does it increase the effect the power supply has on the pre amp sag etc

[Enzo]

The preamp stages are class A circuits, there is no sag there.

The first couple filter caps in the B+ are just that, filter caps. They smooth the DC. By the screens, the DC should be pretty smooth. The filter caps for the preamp stages are more about decoupling the stages than they are actually filtering out ripple.

If your screen node cap was working well at 20uf, there is no point in sticking a 2200uf cap in there. So by extension, if the small cap does the job, so be it.

[guitarmike2107]

So as long as you can afford the additional voltage drop due to the bigger resistance its all good and its probably cheaper and saves space too.

I was aware that pre amp stages are Class A, but for some reason I had the thinking that bigger caps in the pre amp would make the pre amp more “resilient” to power amp sag. Don’t ask me why. I have many flawed thinking patterns.

[EFK]

Since the subject of resistance is touched upon - would this system work equally well if one were to employ it in a more "typical" circuit? Say, for example, that one were to split a common 100K plate resistor into two 50K resistors and run either a .05 or .1 cap from the junction to ground? Or, is the effectiveness here dependent upon running high B+ into the PI/preamp (which this particular amp does) then using these large value split plate resistors?

I guess this is not really a split load then, as the signal is not being taken from the junction of the resistors.

[Old Tele man]

...it's called a "side-loaded" phase inverter.

...the output signal from first triode is scaled down by the voltage divider (270K & 12K) that is across its output to upper power tube (V5), and then fed to the other triode whose output drives the lower power tube (V6).

...Q: why two triodes? A: to get 180-degree signal inversion.

[Merlinb]

Since the subject of resistance is touched upon - would this system work equally well if one were to employ it in a more "typical" circuit?

If you wanted to stay 'typical' then you would have a perfectly ordinary, 100k loaded gain stage. All you would do is use an unusually large dropping resistor in the PSU, and a correspondingly small smoothing cap.

Notice that in the OP's schem, all three of the most sensitive triodes have their own individual dropping resistor, each supplied from one 'big' smoothing cap. (Like a single star earth, only on the positive side of the PSU!) This avoids the usually progressive voltage drop that you get with a 'chain' type power supply. Since the supply is close to 400V, you can easily afford to lose 100V across a dropping resistor and still be left with a useful voltage to run the input stage/s on.

[jrfrond]

Hey, is it my imagination, or do I see the business end of the upper tone control pot hung right off the plate with no blocking cap???

NOT COOL!!! :O

Sure, there's a cap blocking DC from ground on the OTHER end, but I DO hope that the schematic was drawn incorrectly! THAT is a recipe for possible disaster!!!

[EFK]

Not your imagination! Accurate! At least, it was....

[Enzo]

Should be simple enough to swap places between cap and control.