Hello
I'm a little new to refurbishing old Radios so I'd appreciate any help out there. I have an old Coronado Model 803 Wood Cabinet Radio which can run on 120v AC or 6V DC Battery, depending how the outer power plug is wired. Mine is the 120V version and I have been going through the capacitor replacements and have found something that doesn't follow the Rider Schematic.
The schematic shows a .005uf (I think) capacitor in parallel with the Primary of the Output Transformer, which, in effect, connects between the Plate and the 2nd Grid of the output tube (6G6G). In the radio, this capacitor is connected between pins 3 & 8 of the 6G6G output tube, which I believe is the Plate (3) and the Cathode (8). I've also found some other issues like a tube replacement that shouldn't have been, and the ground wire from the Antenna Coil has been cut.
Does it sound viable having a capacitor connected between the Plate and the Cathode of the Power Output tube? So, is this an error in the Capacitor placement on the chassis itself or the schematic?

Thanks and any insight would help alot.
Joe

:Hello
:I'm a little new to refurbishing old Radios so I'd appreciate any help out there. I have an old Coronado Model 803 Wood Cabinet Radio which can run on 120v AC or 6V DC Battery, depending how the outer power plug is wired. Mine is the 120V version and I have been going through the capacitor replacements and have found something that doesn't follow the Rider Schematic.
:The schematic shows a .005uf (I think) capacitor in parallel with the Primary of the Output Transformer, which, in effect, connects between the Plate and the 2nd Grid of the output tube (6G6G). In the radio, this capacitor is connected between pins 3 & 8 of the 6G6G output tube, which I believe is the Plate (3) and the Cathode (8). I've also found some other issues like a tube replacement that shouldn't have been, and the ground wire from the Antenna Coil has been cut.
:Does it sound viable having a capacitor connected between the Plate and the Cathode of the Power Output tube? So, is this an error in the Capacitor placement on the chassis itself or the schematic?
:
:Thanks and any insight would help alot.
:Joe

Joe:
It is not at all uncommon to find a cap connected across the primary of the power transformer, I think it is to prevent parasitic oscillations in the output stage. (Never wondered about why azza matter of fack). I would wire the radio according to the schematic.
Lewis
:

::Hello
::I'm a little new to refurbishing old Radios so I'd appreciate any help out there. I have an old Coronado Model 803 Wood Cabinet Radio which can run on 120v AC or 6V DC Battery, depending how the outer power plug is wired. Mine is the 120V version and I have been going through the capacitor replacements and have found something that doesn't follow the Rider Schematic.
::The schematic shows a .005uf (I think) capacitor in parallel with the Primary of the Output Transformer, which, in effect, connects between the Plate and the 2nd Grid of the output tube (6G6G). In the radio, this capacitor is connected between pins 3 & 8 of the 6G6G output tube, which I believe is the Plate (3) and the Cathode (8). I've also found some other issues like a tube replacement that shouldn't have been, and the ground wire from the Antenna Coil has been cut.
::Does it sound viable having a capacitor connected between the Plate and the Cathode of the Power Output tube? So, is this an error in the Capacitor placement on the chassis itself or the schematic?
::
::Thanks and any insight would help alot.
::Joe
:
:Joe:
:It is not at all uncommon to find a cap connected across the primary of the power transformer, I think it is to prevent parasitic oscillations in the output stage. (Never wondered about why azza matter of fack). I would wire the radio according to the schematic.
:Lewis
::
:
:
Thanks for the reply and just to be sure, can you think of any reason to put a capacitor from the plate to the cathode in a power output tube, as that seems to be the way it is currently wired?
Joe

The cap in question is a high freq. roll-off. It's there for tone compensation. Normally the cap connects to the plate of the audio output tube, then to B- or ground.

Yep, the higher the frequency of the signal through it, the lower impedance the capacitor is. So a 0.005 uF cap from plate to ground will tend to damp out high audio frequencies, which is desirable in an AM radio since anything above 9KHz or so is an artifact, not an actual broadcast signal.

Sometimes (particularly in Euro tube radios) you will instead see pF-range caps installed from plate to ground at the output tubes. These sets use degenerative feedback to manage tone and so a relatively low-value shunt capacitor is unnecessary. However a tiny cap is still needed to bypass RF artifacts, which are of course inaudible but which can degrade output stage efficiency if present.

Quite frequently this cap will short to ground and apply full B+ to ground through the output transformer primary. This is usually catastrophic to Euro sets, which seem to have less robust output transformers than do most USA-built sets. It is one of the first things I check and replace when I repair any set.

:Yep, the higher the frequency of the signal through it, the lower impedance the capacitor is. So a 0.005 uF cap from plate to ground will tend to damp out high audio frequencies, which is desirable in an AM radio since anything above 9KHz or so is an artifact, not an actual broadcast signal.
:
:Sometimes (particularly in Euro tube radios) you will instead see pF-range caps installed from plate to ground at the output tubes. These sets use degenerative feedback to manage tone and so a relatively low-value shunt capacitor is unnecessary. However a tiny cap is still needed to bypass RF artifacts, which are of course inaudible but which can degrade output stage efficiency if present.
:
:Quite frequently this cap will short to ground and apply full B+ to ground through the output transformer primary. This is usually catastrophic to Euro sets, which seem to have less robust output transformers than do most USA-built sets. It is one of the first things I check and replace when I repair any set.
:

Thanks for all of the information from you guys and I realize that there should be a Cap in parallel with the Primary of the output transformer, which the schematic shows, but my BIG question is, the way this chassis is configured, that capacitor is going between the Plate and the Cathode of the Power Output tube, and is not going to ground on either end, and that connection isn't shown in the schematic.

What would happen if I powered it ON with this capacitor going between the Plate & the Cathode of the Power Output Tube, which is the way it was wired when I got it, and NOT in parallel with the Primary of the Output transformer.
I'm starting to think that this non-schematic connection, along with the 6K8 tube in place of where the 6D8G tube should be, and also the ground connection of the Antenna Coil being cut, maybe this radio was used as a teaching tool of some kind for Tech's to find out what is wrong with the radio in a school or something.
Who knows but I appreciate your help.

Thanks.
Joe

What would happen

"What would happen if I powered it ON with this capacitor going between the Plate & the Cathode of the Power Output Tube, which is the way it was wired when I got it, and NOT in parallel with the Primary of the Output transformer."

If the cap isn't shorted... nothing dramatic. You can remove the cap and try it with/without from plate to cathode, or shunting the transformer primary to see what audible effects are- this won't damage anything.