RCA 96-A from scratch

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emrr

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core13 said:
do you think replacin the gz34 on a 176 with diode will change the knee an give an other flavor?
to use it as bus comp for exemple

Look for posts from Larrchild,  newyorkdave, and PRR on the subject.  They're gonna be 10 years back, or more.  Exhaustively commented upon. 
 

IBart

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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 !
 

DaveP

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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
 

emrr

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What inductance are you seeing with the 124F?  Curious to see if it can do full bottom response. 
 

DaveP

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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
 

DaveP

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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
 

emrr

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DaveP said:
The original 96-A spec from their catalogue states 30-10K +/- 1dB so the 124F can meet that spec OK as you can see.

FWIW, the unit I restored and measured came in at -1dB 40-20K7,  with the -0.5dB points being 46-20K, all relative to 1K.  +0.5dB at 17K1, no other positive bumps.
 

DaveP

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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


 

DaveP

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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

 

DaveP

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They are trimmed  chrome cupboard handles and thin  chrome "bow handles".  They just happen to look like SM 55's ;D

DaveP
 

DaveP

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The input and output controls are now in place and the Art Deco chrome stripes.



Now I have to put in place all the various switches and controls along the fronts, Attack, Release, Threshold etc.

DaveP
 

mjrippe

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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.
 

DaveP

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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
 

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