Favorite PI when it crunches?

[redelephant]

I'd like to add a PI as the last stage in a preamp section, which will feed the power amp PI thru a double track pot as a MV. The goal is to have PI dist before the MV, and even be able to drive both PIs into full dist when balls to the walls mayhemming.

So I need to make up my mind about which type of PI to add in the preamp...LTP type or paraphase type? The LTP gives a little more gain, but the paraphase is easier to get symmetrical output swing. I also save a triode in the latter, which I can use for some other wierd idea.

Any comments? I'll probably end up trying both, as a matter of fact I've been planning a preamp with several types of circuits, but time....time...I need more time.

[Steve Conner]

Well, probably not your hybrid design with the MPSA42s.

The paraphase also uses two triodes. Maybe you meant the cathodyne? The cathodyne only has half the output swing of the other types, so it's only suitable for driving smaller tubes like the 6V6, EL84 and so on.

I guess the paraphase theoretically makes more distortion, because it doesn't allow the even harmonics to cancel. One tube distorts the signal a bit, then the second one distorts it some more. It also has the highest gain of all the circuits. (the cathodyne has gain of 1, another reason why it's not the bargain it seems)

My old Toaster amp started out with a floating paraphase, but I modified it to the blackface Fender style LTPI. Didn't make much of a difference to the sound, to be honest.

[MWJB]

Theoretically the cathodyne might be seen as limited to smaller tubes, but it still finds it's way into bigger amps (5E8A Twin, Orange 120, countless Danelectros, Kendricks) driving pairs, or quartets of 6L6/EL34. The PI triode indeed has a gain of one (or a shade under) but in guitar amps it is always mated to a preceeding gain stage. Taken as a whole tube (triode A & B), voltage gain typically far exceeds that of an LTPI - whereas a 12AX7 LTPI might typically kick out 15-20VAC for every volt fed in on triode A's grid, a cathodyne will kick out x40-60VAC.

Paraphase PIs don't often find their way into fixed bias amps, but Vox AC50/100, Ampeg VT40 are notable examples.

[redelephant]

Thanks for replies guys. Yeah Steve I meant cathodyne and agree a hybrid PI wont be the best I didn't even think of the paraphase but will have to concider it now.
The PI in question will drive the PI of the power amp, so we are not talking large output swings here. Since the pwramp's PI will do the real phase inversion, the output of the preamp PI does not have to be symmetrical.
The idea is: two gainstages (with a gain control in between) - tone stack - preamp PI - dual MV - pwramp PI - PP 6V6. I will lose some signal in the tonestack, and a cathodyne will require a lot of swing to crunch - perhaps more than I will have coming out of the tonestack? Aha...that's where that spare triode comes in, like you tried to say MWJB (are u Russian since you have no vowels in ur name? How do u pronounce it?) I imagine a cathodyne feels a bit more compressive since it will have the voltage swing both ways around it for a larger delta-V than a LTP. ???

[kg]

well, it's not fair to compare a split load with the other plate loaded types, because with the latter you need two triode sections anyway.

while it's true you don't net the same voltage swing there should be more than enough to drive your outputs fully. just make sure your supply voltage is high enough.

my favorite for breakup would be a plate loaded gain stage direct coupled to the split load splitter. the split load's grid will pull current at the top of the swings, messing up the phase splitting (which is a good thing imho).

[redelephant]

DC coupling, plenty gain, and messed up phase splitting This is getting interesting.

[tubeswell]

The idea is: two gainstages (with a gain control in between) - tone stack - preamp PI - dual MV - pwramp PI - PP 6V6.

Can you post a schematic of that? - I am curious about how you plan to implement the 2 PIs.

[kg]

Can you post a schematic of that? - I am curious about how you plan to implement the 2 PIs.

i'm envisioning a differential amp, basically. aka long tailed pair, with inputs to both grids.

[hasserl]

Can you post a schematic of that? - I am curious about how you plan to implement the 2 PIs.

Yeah, I don't get the idea of 2 PI's. The first one splits the signal for the power amp, what is the 2nd one doing?

[redelephant]

Here is a schem of the preamp PI, cathodyne version. Following this is a transformer loaded LTP for interstage-transformer coupling into a fixed biased pair of 6V6s.

The idea is just as sound as having more gain stages then needed to drive the output to max. Using a PI in the preamp before the MV just gives the possibility of having PI distortion at any volume. The typical SE preamps we use give even and odd harmonics, while a symmetric PI output should give predominately odds. I just want to experiment and see (hear) how that works.

I will have to add a bypass, or an attenuating switch before the preamp PI so I can get the SE stages to distort before the PI stage, that way I cen still get typical preamp distortion, and switch in PI distortion.