ioginy wrote:I did some quick drop in's a little bit ago. Threw in a 22uf cap with the first stage cathode resistor and put a 100pf cap on the NFB resistor. Not sure how I did it but now the amp motorboats and doesn't have the volume it used to. With the info you've given me, I'm going to set the first stage gain and then move the cathode resistor switch to the second stage. I think that will be a more effective adjustment. Sadly it's necessary or I would take it right out.
I also noticed, looking at the bandmaster 135 and 100 schematics that the PI has different values from the PI in single PP amps. The 1M resistors are 330K. I am also going to re-do my bias control, as I think I am just barely getting -40vdc and I would like to have a bit more adjustment.
I sadly don't have a scope, though getting one is on my list of things to do. Properly biasing (though I am sure someone will shoot this down) is easier with a scope. The 1 ohm cathode resistor is handy, though not very accurate.
Adding the cap to the NFB network basically feedbacks only low frequencies below the cutoff point of that cap. That could have caused your motorboat problem. Disconnect the cap to see of course. It is a fallacy to believe that since you are amplifying bass, you only need bass frequencies so don't go too overboard on trying to just amplify bass. The more highs you have, the more present the bass will sound to a point.
I forget what your cathode resistor value was on the first stage. Keep in mind that the cap's effect on the sound changes with the size of the resistor. Fender used a 25uf in conjunction with a 1k5 resistor. Go up in the size of the resistor and you don't need as large of a cap to have the same effect. In bass amps, Sunn used a 250uf with a 1k5, and some Marshalls used a 330uf with a 1k5. So lets assume you have a 1k5 and a 22uf on the first stage with around 220v on the plates. The next stage you may have 230v on the plate, and a 4k7 with a 10uf on the cathode. A coupling cap between the two might be a .022uf with the coupling cap sizes getting progressively larger as you go through the amp to where the PI might have a pair of .22uf or .1uf leading to the power tubes. Making the second stage have a 4k7 will allow it to have a larger input signal before it distorts and having a higher voltage (within reason) on the plates will allow that stage to be able to swing larger signals, which it needs for bass.
The 330k resistors instead of 1M aren't a choice I would make for a guitar amp. For bass it might work ok. The lower value makes it harder for the driving stage to supply current and reduces gain. Having a high voltage on the cathodyne PI will allow it to swing more voltage and to get more power out of the amp, and stay clean longer up the volume range. Putting the 330k resistors in there makes the PI work harder.
Biasing with a scope is very inaccurate as your result will change every time you do it. That method also tends to bias cold. The scope is more useful for troubleshooting and for setting the gain structure throughout the amp. You can see what the signal does as you adjust parts basically. Very useful for that. The 1 ohm resistor in the cathode is a great way to bias. It includes the screen current as well as the plate current, but thats ok. It ends up biasing a little on the safe side as a result, and with that method of monitoring the voltages, it allows you to set the bias where you want it within the typical 60% to 70% dissipation range.
Greg