Carbsandcaps
Super Member
I had recently acquire this seeburg push pull 2a3 push pull amplifier from the bay. It originally used a single 56 as the phono preamp then parallel 56’s as the driver for the 2a3’s. It did have the full set of tubes including a superglued Chinese 2a3 .
I decided instead of trying to find another one I would convert this into a stereo single ended amplifier. And while I was at it why not make it switchable for a 2a3 and a 45 tube? Oh and I also just picked up these vu meters with built in control board and everyone loves vu meters so why not?
After some discussion on here I figured that a single 56 wouldn’t be enough for a 2a3 (still up for debate so don’t judge me for it) so I decided to use 2 56’s in series as drivers. The input socket just happened to be a 5 pin socket so putting another 56 in was no problem. A attempted a direct couple setup between the 56’s but just couldn’t get the voltages right so for time and sanity sake I decided to go capacitor coupled (crowd boos and hisses). Pretty straight forward design. I decided not to bypass the cathode resistors to keep gain down a little bit but even with that they may be a tad much for the 45’s but I’ll take the compromise for running the 2a3’s so well
I went with adjustable fixed bias by tapping into the main hv and using diodes to get the negative voltage, with an adjustment range of 0-76v using an L design. I put some old speaker terminals in backwards next to the adjustment pot to measure bias voltag from outside the chassis.
The power supply originally used a field coil speaker so with no other option I had to use 2 50 watt resistors, 1k and 470 ohm to ground to bring the plate voltage down to about 280v who’s is a little high for the 45’s but should be ok. Using the original choke in a clc setup worked perfectly.
The vu meters were a simple install using a 1-3/4 hole saw and lots of cutting oil. Then they wire right to the speaker outputs. They work without power if you don’t want the backlights but of course I do so I just cut up a 12v power supply I had laying around and wired it to the power switch. The backlighting is adjustable (barely) and also the sensitivity which is critical when you’re only putting out 2 watts.
Now the final pieces, the output transformers. I stole an amorphous core set from a 300b amp that I built at least for the time being. They have a 2500k and a 3500k primary tap. I used the 2500k for the 2a3. They also have a 4,8 and 16ohm tap so I just wired a separate set of positive terminals to the 16 ohm to give me a 5000k primary for the 45’s.
To switch from 2a3 to 45, just switch speaker terminals and set the bias from -45 to -56v and it’s ready to play. It sounds great! Even with the 45’s playing it has a surprising amount of power. The chassis cleaned up pretty well for being 85 years old. I’m pretty happy with how it came out.
I decided instead of trying to find another one I would convert this into a stereo single ended amplifier. And while I was at it why not make it switchable for a 2a3 and a 45 tube? Oh and I also just picked up these vu meters with built in control board and everyone loves vu meters so why not?
After some discussion on here I figured that a single 56 wouldn’t be enough for a 2a3 (still up for debate so don’t judge me for it) so I decided to use 2 56’s in series as drivers. The input socket just happened to be a 5 pin socket so putting another 56 in was no problem. A attempted a direct couple setup between the 56’s but just couldn’t get the voltages right so for time and sanity sake I decided to go capacitor coupled (crowd boos and hisses). Pretty straight forward design. I decided not to bypass the cathode resistors to keep gain down a little bit but even with that they may be a tad much for the 45’s but I’ll take the compromise for running the 2a3’s so well
I went with adjustable fixed bias by tapping into the main hv and using diodes to get the negative voltage, with an adjustment range of 0-76v using an L design. I put some old speaker terminals in backwards next to the adjustment pot to measure bias voltag from outside the chassis.
The power supply originally used a field coil speaker so with no other option I had to use 2 50 watt resistors, 1k and 470 ohm to ground to bring the plate voltage down to about 280v who’s is a little high for the 45’s but should be ok. Using the original choke in a clc setup worked perfectly.
The vu meters were a simple install using a 1-3/4 hole saw and lots of cutting oil. Then they wire right to the speaker outputs. They work without power if you don’t want the backlights but of course I do so I just cut up a 12v power supply I had laying around and wired it to the power switch. The backlighting is adjustable (barely) and also the sensitivity which is critical when you’re only putting out 2 watts.
Now the final pieces, the output transformers. I stole an amorphous core set from a 300b amp that I built at least for the time being. They have a 2500k and a 3500k primary tap. I used the 2500k for the 2a3. They also have a 4,8 and 16ohm tap so I just wired a separate set of positive terminals to the 16 ohm to give me a 5000k primary for the 45’s.
To switch from 2a3 to 45, just switch speaker terminals and set the bias from -45 to -56v and it’s ready to play. It sounds great! Even with the 45’s playing it has a surprising amount of power. The chassis cleaned up pretty well for being 85 years old. I’m pretty happy with how it came out.
