I just thought I would pass on my experiences with my C-8 restoration project.
I got this little gem from a friend of mine. He has 3 more of them so he passed one on to me. I didn't have a power supply or the proper connectors to connect this to my MC-60 amp right away so I busied myself replacing all the electrolytic caps. By the time I finished this I got the two octal plugs needed to connect the C-8 to the MC-60. First thing I discovered is that there is an error in the schematic in the Sams Photofact folder for the MC-60. The schematic shows a connection from Pin 5 of the input connector to the non grounded end of the gain pot. The C-8 is wired so pins 5 and 6 are to switched mains. This arrangement delivers 120VAC to the audio input of the MC-60. It's a good thing there is a capacitor in-between Pin 5 and the tube. I got a horrendous 60Hz hum. Once I had snipped the wire at Pin 5 I was able to turn on the C8.
First problem and this was a real pain. The filament voltage for V1 and V2 is 12.6VDC delivered from a 2 diode 2 capacitor voltage doubling power supply. This was not providing any output. V3 users 6.3 AC drawn from the input of this voltage doubler. V3 lit right up. I messed around with this for about 2 days. I actually removed all the parts and breadboarded them free form the C-8 chassis. Everything worked perfectly on the breadboard. When I put it back in the circuit, nothing. Today I went down to give it another try. I noticed the front panel lamp was not lit. I wiggled the bulb and it lit up. When the lamp is lit the voltage doubler starts to produce a nice clean 12.7 VDC to V1 and V2. Success! It turns out the lamp socket was not making good electrical contact between the solder tab and the base. I soldered the tab directly to the base and it is now working fine.
I have it set up in my living room now connected to my Thorens TD-160 turntable. The phono stage is kind of noisy. I have cleaned the pots and switches but that doesn't do it. I suspect some if not all the Bumble Bee caps have drifted out of spec. I will start replacing them tomorrow. I'll keep you all updated.
I got this little gem from a friend of mine. He has 3 more of them so he passed one on to me. I didn't have a power supply or the proper connectors to connect this to my MC-60 amp right away so I busied myself replacing all the electrolytic caps. By the time I finished this I got the two octal plugs needed to connect the C-8 to the MC-60. First thing I discovered is that there is an error in the schematic in the Sams Photofact folder for the MC-60. The schematic shows a connection from Pin 5 of the input connector to the non grounded end of the gain pot. The C-8 is wired so pins 5 and 6 are to switched mains. This arrangement delivers 120VAC to the audio input of the MC-60. It's a good thing there is a capacitor in-between Pin 5 and the tube. I got a horrendous 60Hz hum. Once I had snipped the wire at Pin 5 I was able to turn on the C8.
First problem and this was a real pain. The filament voltage for V1 and V2 is 12.6VDC delivered from a 2 diode 2 capacitor voltage doubling power supply. This was not providing any output. V3 users 6.3 AC drawn from the input of this voltage doubler. V3 lit right up. I messed around with this for about 2 days. I actually removed all the parts and breadboarded them free form the C-8 chassis. Everything worked perfectly on the breadboard. When I put it back in the circuit, nothing. Today I went down to give it another try. I noticed the front panel lamp was not lit. I wiggled the bulb and it lit up. When the lamp is lit the voltage doubler starts to produce a nice clean 12.7 VDC to V1 and V2. Success! It turns out the lamp socket was not making good electrical contact between the solder tab and the base. I soldered the tab directly to the base and it is now working fine.
I have it set up in my living room now connected to my Thorens TD-160 turntable. The phono stage is kind of noisy. I have cleaned the pots and switches but that doesn't do it. I suspect some if not all the Bumble Bee caps have drifted out of spec. I will start replacing them tomorrow. I'll keep you all updated.