Pilot 240

[oldman55]

Downloaded manual and schematics and they appear to match the unit other than an RC circuit that I cant find on paper.
It goes from the 16ohm speaker tap to:
1)an opt
2)pin 3 of a 12ax7 through another RC circuit.

Not that it really matters as I am just recapping and hoping it runs fine. Just curious as I dont even see an 18 ohm resistor on the parts list.
Also wondering about the "simpli-matic" test panel and how it functions.
Oldman

 

[tubeactive]

OK, that RC ckt sounds like a feedback network. However, some of the console units and swanky record players also had "tone networks" across the voice coil taps which were possibly interconnected within the actual feedback networks. These "tone" R-C networks are important for "voicing" the console or rp box with their sometimes small or non-isolated speakers. When restoring, choose a good quality tone cap...

Pilot was actually way ahead of their time concerning capacitor choice. In their deluxe preamps, models 210, 215, 216, 216A, plus probably some amps, they used different types of caps in certain positions. So, you might see a Black Pyramid Imp coupling cap in one stage and a wax coated Good-All cap in the next stage. While this was not only for a unique voicing, I believe they actually attempted to avoid the same distortion product (inherent in any gain or tone stage) from occurring again and again. When I rebuild or retrofit, their smart thinking has inspired me for decades....

 

[oldman55]

OK, that RC ckt sounds like a feedback network. However, some of the console units and swanky record players also had "tone networks" across the voice coil taps which were possibly interconnected within the actual feedback networks. These "tone" R-C networks are important for "voicing" the console or rp box with their sometimes small or non-isolated speakers. When restoring, choose a good quality tone cap...

Pilot was actually way ahead of their time concerning capacitor choice. In their deluxe preamps, models 210, 215, 216, 216A, plus probably some amps, they used different types of caps in certain positions. So, you might see a Black Pyramid Imp coupling cap in one stage and a wax coated Good-All cap in the next stage. While this was not only for a unique voicing, I believe they actually attempted to avoid the same distortion product (inherent in any gain or tone stage) from occurring again and again. When I rebuild or retrofit, their smart thinking has inspired me for decades....

Thanks for the thorough explanation Tube,
Though a component unit, it does have a couple of record changer sockets. Since not using them, then this circuit should somewhat meaningless?

 

[tubeactive]

Well, I do not have a schematic of the 240 or 110 Pilots. Parts for self-contained amps are rarely meaningless. So, what resistor is from pin 3 (pin 3 of 12AX7 or AU7 is a cathode) of the AF amp to ground ? Is there a cap across this (possible bias) resistor in question ?

Every R-C network has a time constant, measured in uSec. The lower the time constant, the higher the frequency. R x C = TC. Then, the mathematical constant of 159,155 divided by the TC yields the -3 db point re:frequency. 18 x .25 = 4.5 uSec. 159,155/4.5 = 35,367.78 Hz. Apparently, there is an intentional low pass filter (high freq cut) of -3 db at 35,368 Hz applied to the earlier stage for either stability of the amp or for a "tweeter saver" function. It is probably not meaningless. Since the pin 3 is probably at a very low DC potential and the voice coil connection blocks DC, this tone cap is possibly still electrically good. My guess is this tone network ties into another network, the feedback from that 16 or another speaker tap, also going to pin 3 of that same AF tube. Or, the actual feedback network goes to another early AF stage and this tone RC network (18 Ohms//.25 uf) simply limits bandwidth. It might actually be that pin 3 of the AF stage you indicate is actually grounded. Then, it would clearly be applied as a "tweeter saver" network. Whatever its' function, leave that network in place, for starters....

In the pic, toward the bottom, I see a different Pyramid Imp black cap which shows a waxy compound has physically leaked. That cap needs replacement.

 

[oldman55]

Well, I do not have a schematic of the 240 or 110 Pilots. Parts for self-contained amps are rarely meaningless. So, what resistor is from pin 3 (pin 3 of 12AX7 or AU7 is a cathode) of the AF amp to ground ? Is there a cap across this (possible bias) resistor in question ?

Every R-C network has a time constant, measured in uSec. The lower the time constant, the higher the frequency. R x C = TC. Then, the mathematical constant of 159,155 divided by the TC yields the -3 db point re:frequency. 18 x .25 = 4.5 uSec. 159,155/4.5 = 35,367.78 Hz. Apparently, there is an intentional low pass filter (high freq cut) of -3 db at 35,368 Hz applied to the earlier stage for either stability of the amp or for a "tweeter saver" function. It is probably not meaningless. Since the pin 3 is probably at a very low DC potential and the voice coil connection blocks DC, this tone cap is possibly still electrically good. My guess is this tone network ties into another network, the feedback from that 16 or another speaker tap, also going to pin 3 of that same AF tube. Or, the actual feedback network goes to another early AF stage and this tone RC network (18 Ohms//.25 uf) simply limits bandwidth. It might actually be that pin 3 of the AF stage you indicate is actually grounded. Then, it would clearly be applied as a "tweeter saver" network. Whatever its' function, leave that network in place, for starters....

In the pic, toward the bottom, I see a different Pyramid Imp black cap which shows a waxy compound has physically leaked. That cap needs replacement.

Guess my schematic doesnt match up with my unit after all.
There is a 330kohm to ground at pin 3. Schematic shows the 1000ohm to ground.
Replacing all the caps.
oldman

 

[oldman55]

Edit: deleted as further parts replacement solved issues.

Sounds great but cant find any manuals with instructions/explanation on the test panel on the back: