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I bought this little amplifier for a very reasonable price off of eBay. It came from a 1960s stereo console and uses one 12AX7 and two 50C5 tubes. Basically, I wanted into convert it into a small guitar amp, as my Killovolt is VERY heavy and not fun to move around.
Unfortunately, I was not able to find any sort of schematic online for it, so I reverse-engineered it and came up with a schematic. All I can say is that this is one weird design! Some of you who may be familiar with tube designs from this era might criticize some parts of my schematic, saying that they are incorrect. However, I assure you that I have double and even triple-checked this schematic to the amplifier itself.
I have NO idea why the filaments would be placed AFTER the semiconductor diode, unless the designers intended them to run off of somewhat filtered DC to decrease noise. I also can't figure out why they used two 160 ohm cathode resistors, one for each 50C5 output tube, only to couple them together. I can kind of understand how the treble and bass controls work, although they are very odd indeed. I have never seen such a setup, nor do I have any idea as to how well they actually work against a more conventional design, such as a Baxandall tone stack.
Unfortunately, I was not able to find any sort of schematic online for it, so I reverse-engineered it and came up with a schematic. All I can say is that this is one weird design! Some of you who may be familiar with tube designs from this era might criticize some parts of my schematic, saying that they are incorrect. However, I assure you that I have double and even triple-checked this schematic to the amplifier itself.
I have NO idea why the filaments would be placed AFTER the semiconductor diode, unless the designers intended them to run off of somewhat filtered DC to decrease noise. I also can't figure out why they used two 160 ohm cathode resistors, one for each 50C5 output tube, only to couple them together. I can kind of understand how the treble and bass controls work, although they are very odd indeed. I have never seen such a setup, nor do I have any idea as to how well they actually work against a more conventional design, such as a Baxandall tone stack.
Category All / Miscellaneous
Species Unspecified / Any
Size 1280 x 806px
File Size 93.3 kB
I agree, it is rather odd, but I wonder if it was a project to use up some extra parts left in inventory? Perhaps RCA asked their engineers to design something to use them up?
Putting DC on the heaters would help reduce hum, and as solid state diodes can handle the current, why not? Though, I agree, it was never standard procedure.
The tone controls are a puzzle... wow!
I puzzled long and hard on a strange push-pull but stereo amplifier which used a 12AX7 and a pair of 50C5s. Turned out it was a special circuit designed by CBS called the "Simplex".
It used sum and difference phase arrangements to pull it off. Though, I'm sure stereo separation wasn't best.
Putting DC on the heaters would help reduce hum, and as solid state diodes can handle the current, why not? Though, I agree, it was never standard procedure.
The tone controls are a puzzle... wow!
I puzzled long and hard on a strange push-pull but stereo amplifier which used a 12AX7 and a pair of 50C5s. Turned out it was a special circuit designed by CBS called the "Simplex".
It used sum and difference phase arrangements to pull it off. Though, I'm sure stereo separation wasn't best.
If the 120 isn't transformer isolated, I'd worry about that.
The diode sets up a ground level so that there is something of ground to work with when referencing the input signal, though running all the filaments in a loop like that is very odd indeed, but does permit one to get away from needing a separate filament transformer.
The two 160's are for balancing the overall loading of the output, so the two tubes have the same gain, if one tries to get hot (audio level wise), the other one is going to pull it down.
The diode sets up a ground level so that there is something of ground to work with when referencing the input signal, though running all the filaments in a loop like that is very odd indeed, but does permit one to get away from needing a separate filament transformer.
The two 160's are for balancing the overall loading of the output, so the two tubes have the same gain, if one tries to get hot (audio level wise), the other one is going to pull it down.
Thank you very much for your reply! To address all points that you've made:
The amp is not isolated. However, while it can be dangerous, both tube amps and radios have been running non-isolated designs for about years or so, so it's not that big of a deal.
Having all the tubes in a loop like that is very reminiscent of the All-American Five radio design, which was a very popular design for years and never did have a filament transformer. The ground is set up through one side of the line in, again very similar to the AA5 radio design, as well as many older amplifiers.
I have seen the technique of using a single resistor to keep both tubes more-or-less balanced, but in my experience, of both tubes are exactly the same, the it can cause problems. My question is why they used two 160 ohm resistors, only to connect them together to make 80 ohms?
The amp is not isolated. However, while it can be dangerous, both tube amps and radios have been running non-isolated designs for about years or so, so it's not that big of a deal.
Having all the tubes in a loop like that is very reminiscent of the All-American Five radio design, which was a very popular design for years and never did have a filament transformer. The ground is set up through one side of the line in, again very similar to the AA5 radio design, as well as many older amplifiers.
I have seen the technique of using a single resistor to keep both tubes more-or-less balanced, but in my experience, of both tubes are exactly the same, the it can cause problems. My question is why they used two 160 ohm resistors, only to connect them together to make 80 ohms?
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