Question about the 36W PI...

[Breogan]

I am thinking about building a 36W Super Lite taking the 18W SuperLite schematic and doubling the power amp. I've been checking the Rev 5 schematic from this web and I've found out that the PI it isn't completely balanced. I mean, the cathode resistors have different values so each halve of the tube has a different votage, is this correct? If so, which is the purpouse?

Thanks!

[markh]

I think you mean the *plate* resistors are different? This is done to compensate for the inherent imbalance in the PI design, and is fairly standard on all single-input PIs of this type. The exception is the oddball cases (like the stock 18watt) that uses both sides of the PI as input.

I'd suggest using the standard 82k/100k pair if you're doing a "36 lite" with only one stage of gain before the PI.

--mark h

[Breogan]

Thanks Markh!!!

Yes, sorry, I've meant the plate resistors... Then, I am missing something, I've followed this schematic to build my 18W SuperLitw, and the PI has both plate resistor value at 100KOhm. So is this imbalanced?

http://i4.photobucket.com/albums/y101/b ... /breo2.gif

Sorry if this question is very basic...

[stoo]

If the tail resistor is large the PI has a more balanced output.
Have a look at the tech sections here (advanced)
http://www.aikenamps.com/
Stew

[Breogan]

If the tail resistor is large the PI has a more balanced output.
Have a look at the tech sections here (advanced)
http://www.aikenamps.com/
Stew

Thanks Stoo!

Which article are you refering to? I cant find any about the PI in that section...

[stoo]

tech info-->Advanced----->designing long tailed pairs...
near the bottom..

"Following is a schematic of the completed phase inverter design. Note that due to the better balance provided by the large "tail" resistor, which acts more like a constant current source, the in-phase plate load resistor is made the same size as the out-of-phase load resistor. It is not necessary to make it larger, as is the case in the "standard" Fender/Marshall type inverter, which uses a much smaller "tail" resistor."
Stew

[markh]

The imbalance isn't much, and with 10% resistors as used back in the day, it was a bit of wishful thinking to even try.

The larger the tail resistor, the better balanced it is, but you need high voltages to do this well, ideally a negative voltage for the tail. Aiken and some of the vacuum tube books have the equations so you can figure it out if you really want to.

The imbalance produces subjectively "good sounding" distortion, which is why some use 100k/100k even on the lite versions.

--mark h