RCA 96-A from scratch
- Thread starter DaveP
- Start date
Help Support GroupDIY Audio Forum:
DaveP said:Well Covid-19 or not, the beat goes on!
Self-isolation is ideal for getting on with projects!
I have finished the Power Supply today with the exception of two dropper resistors that will have to be determined on test.
There is a 50VA TX for the DC heater supply at the top, then two 110V TX's in series for the HT/B+ and finally a smaller TX for the elevated supply. My chances of finding one TX to do all those jobs are zilch.
They all fit in nicely on the right hand side panel, this mounts them vertically and avoids any hum pickup.
The next job is to fit the major components on the main amp panel and settle the layout.
DaveP
Wow ....thats some meaty power supply
)On Daves builds , i can always see the PSU is isolating on its own !
This forum is taking forever to load lately!?! ???
After a day of trial designs and a day of cutting holes, I have ended up with this layout:-
The type 89 output tubes are six pin types and the 5651 regulator is the B7A, all the rest are octal.
The interstage is a Hammond 124F, it has sufficient inductance to give a comparable frequency response to the original.
Now the fun begins......
DaveP
After a day of trial designs and a day of cutting holes, I have ended up with this layout:-
The type 89 output tubes are six pin types and the 5651 regulator is the B7A, all the rest are octal.
The interstage is a Hammond 124F, it has sufficient inductance to give a comparable frequency response to the original.
Now the fun begins......
DaveP
I've never had much success measuring Inductance but I ran a frequency response in my initial tests.
The 15k primary was driven by the 6K7's and the secondary was tested with 10K and 15K loads.
As you can see, the 10K load was better:-
The original 96-A spec from their catalogue states 30-10K +/- 1dB so the 124F can meet that spec OK as you can see.
DaveP
The 15k primary was driven by the 6K7's and the secondary was tested with 10K and 15K loads.
As you can see, the 10K load was better:-
The original 96-A spec from their catalogue states 30-10K +/- 1dB so the 124F can meet that spec OK as you can see.
DaveP
With this type of one-off project, there are no plans as it's a prototype, so it's best to provide plenty of wiring possibilities as you decide what goes where. From experience, a line of tags either side of a tube base usually gets the job done with point-to-point.
I've also started installing the busbar. The pin 1 for metal tubes makes a handy connection, just have to remember that the glass bottles have something else there! The OPT has bare connectors so I've placed it upside down within the chassis for safety.
The next job is to wire up the heaters, which is easier to do before the top is fixed to the chassis permanently.
DaveP
I've also started installing the busbar. The pin 1 for metal tubes makes a handy connection, just have to remember that the glass bottles have something else there! The OPT has bare connectors so I've placed it upside down within the chassis for safety.
The next job is to wire up the heaters, which is easier to do before the top is fixed to the chassis permanently.
DaveP
I've started adding the components, a work in progress.
This involves a lot of anticipation so that wires are put in place ready for later connection. it all looks a mess at this stage, but I update the schematic with the colour code of the wiring as I go.
All the tubes are put in place to run them up on 6.3V DC. This involves a tricky test to determine the correct value of dropper resistor for the heater supply. You can't use croc clips for this as their resistance is too high when 3.1 Amps is passing. Trial and error brought me to a 0.47 ohm metal clad 25W resistor.
Here, the separate power supply panel is jury rigged to test the heater supply, the dropper resistor has to be selected and put in place before this panel can be fitted to the main chassis.
I decided to use double wiring for the heater supply due to the amperage.
DaveP
This involves a lot of anticipation so that wires are put in place ready for later connection. it all looks a mess at this stage, but I update the schematic with the colour code of the wiring as I go.
All the tubes are put in place to run them up on 6.3V DC. This involves a tricky test to determine the correct value of dropper resistor for the heater supply. You can't use croc clips for this as their resistance is too high when 3.1 Amps is passing. Trial and error brought me to a 0.47 ohm metal clad 25W resistor.
Here, the separate power supply panel is jury rigged to test the heater supply, the dropper resistor has to be selected and put in place before this panel can be fitted to the main chassis.
I decided to use double wiring for the heater supply due to the amperage.
DaveP
Wise choices. Looks great as always!
Thanks,
The power supply is now complete and has been tested under a dummy load. The regulated supply gives a steady 250V DC at 73mA.
Before I could fit the power supply panel to the main chassis, I had to assemble the chrome grills on both sides. There were 40 screws to fit.
The polished panel has been partly exposed where the masking tape was pulled off. This will be where the lettering is placed later on. I'm quite pleased with the overall effect.
DaveP
The power supply is now complete and has been tested under a dummy load. The regulated supply gives a steady 250V DC at 73mA.
Before I could fit the power supply panel to the main chassis, I had to assemble the chrome grills on both sides. There were 40 screws to fit.
The polished panel has been partly exposed where the masking tape was pulled off. This will be where the lettering is placed later on. I'm quite pleased with the overall effect.
DaveP
Ricardus
WILL SOLDER FOR FOOD
Did you make those chrome grills or did you sacrifice some old microphones?
Ricardus
WILL SOLDER FOR FOOD
DaveP said:
Oh wow. Fantastic!
It is a thing of beauty, Dave! But may I ask why, having gone so far to replicate the original look, you now moved the meatball logo to top left instead of center and used a square, not round meter? I know the original meter "hood" is unobtainable, but round meters are plentiful.
The customer likes 4 in meters, simple as that and to be honest, that hood meter is quite ugly IMO.
Its almost impossible to make exact copies, where would I get the transformers for example?
What I can do is use original tubes and keep the audio circuit the same and hopefully the sound will be pretty close too.
DaveP
Its almost impossible to make exact copies, where would I get the transformers for example?
What I can do is use original tubes and keep the audio circuit the same and hopefully the sound will be pretty close too.
DaveP
Ricardus
WILL SOLDER FOR FOOD
DaveP said:
Oh, this isn't for you?
