RCA BA-6A from scratch
- Thread starter DaveP
- Start date
Help Support GroupDIY Audio Forum:
Hi Jon,
love your studio and mics!!
Life would be easier for sure with a corner folder but I get by without ok.
I buy these pre formed boxes in the UK:-
http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/256377.xml
I try to use them as modules and join them together but sometimes I buy the biggest box and jigsaw off sides and corners to get the shapes I need. Then I just pop-rivet or screw them together. I also buy plain sheets from RS to help too, all the metalwork is Aluminium, though sometimes I add a steel sheet to guide away mag fields. The 19" rack front panels are 3mm Al sheets. Its quite hard work sometimes, but character forming!
My spare time is divided between making gear and recording, I use all my own tube gear into a Yamaha 16 track digital recorder, you can hear one of my efforts here:-
http://soundcloud.com/delayed-action/the-train1
thanks for the feedback
best DaveP
love your studio and mics!!
Life would be easier for sure with a corner folder but I get by without ok.
I buy these pre formed boxes in the UK:-
http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/256377.xml
I try to use them as modules and join them together but sometimes I buy the biggest box and jigsaw off sides and corners to get the shapes I need. Then I just pop-rivet or screw them together. I also buy plain sheets from RS to help too, all the metalwork is Aluminium, though sometimes I add a steel sheet to guide away mag fields. The 19" rack front panels are 3mm Al sheets. Its quite hard work sometimes, but character forming!
My spare time is divided between making gear and recording, I use all my own tube gear into a Yamaha 16 track digital recorder, you can hear one of my efforts here:-
http://soundcloud.com/delayed-action/the-train1
thanks for the feedback
best DaveP
Jean Clochet
Well-known member
DaveP said:
Ha!! That's such a cool way to get what you need without reinventing everything! I had wondered how you did it. Thanks for the hint : - )
B.T.W. I decided to go ahead with ordering the Lundahl 1660S interstage to try out. It looks like I can buy it for 80 Euro + shipping/tax as opposed to the Sowter 9830 @ £124 + shipping/tax. I also like how Lundahl specify the DC current for 0.9 Tesla which makes working things out easier although I'm sure the Sowter is a great product.
Searching around online it seems that the Lundahl is OK with up to 15% inbalance so inductance should remain OK for decentish bass as the anodes move around,
Jean.
The front panel is now fitted with all the controls and switches.
I downloaded an RCA logo off the web to balance the neons on the front, I have yet to draw a BA-6A logo which will be stuck on next to it.
I copied this picture of an original meter (had 500uA on it which was useful to know).
I cleaned it up with coraldraw, (it seemed to take forever!) then laser printed it to the right size, cut it out and glued it to the metal scale with UHU solvent type adhesive. It took as long to reconstruct the meter as it did to make the entire front panel.
I fitted a modern Sifam 100uA meter inside a 1960's Sangamo Western case I had kicking around, with a 300 ohm shunt resistor it becomes a 500uA meter to match the original, this is critical to get all the balance readings to line up in the red segment as well as the GR to be correct.
Next week I have 5 consecutive evenings to wire up the front and finish the compressor, so next week hopefully the final instalment .
best DaveP
I downloaded an RCA logo off the web to balance the neons on the front, I have yet to draw a BA-6A logo which will be stuck on next to it.
I copied this picture of an original meter (had 500uA on it which was useful to know).
I cleaned it up with coraldraw, (it seemed to take forever!) then laser printed it to the right size, cut it out and glued it to the metal scale with UHU solvent type adhesive. It took as long to reconstruct the meter as it did to make the entire front panel.
I fitted a modern Sifam 100uA meter inside a 1960's Sangamo Western case I had kicking around, with a 300 ohm shunt resistor it becomes a 500uA meter to match the original, this is critical to get all the balance readings to line up in the red segment as well as the GR to be correct.
Next week I have 5 consecutive evenings to wire up the front and finish the compressor, so next week hopefully the final instalment .
best DaveP
Jean Clochet
Well-known member
DaveP said:
Hahaha ;D
But WHAT a meter it is! Nice!
Looking forward to the final chapters from you,
Jean.
p.s. forgot to thank you for posting your audio clip the other day, good for you
The back of the front panel is now wired up.
The meter has been tested and the correct shunt fitted. It reads 500uA fsd.
The manual has the DC heaters running at 6.5VDC and that lines up exactly with the heater mark on the scale. There was a means to adjust the heater voltage on the original, but I have not bothered with that facility. I don't know why they chose that over 6.3V but it checks out anyway, mine will run 6.3V.
The pads are all assembled. The input pad has 2 x 25k sections like the original pots and the output pad adds up to 600 ohms to load the output TX properly; it can take the full output in the event of being run disconnected.
All the connecting wires are now on the front panel or coming from the chassis, these must now be soldered and tied up neatly.
More soon
DaveP
The meter has been tested and the correct shunt fitted. It reads 500uA fsd.
The manual has the DC heaters running at 6.5VDC and that lines up exactly with the heater mark on the scale. There was a means to adjust the heater voltage on the original, but I have not bothered with that facility. I don't know why they chose that over 6.3V but it checks out anyway, mine will run 6.3V.
The pads are all assembled. The input pad has 2 x 25k sections like the original pots and the output pad adds up to 600 ohms to load the output TX properly; it can take the full output in the event of being run disconnected.
All the connecting wires are now on the front panel or coming from the chassis, these must now be soldered and tied up neatly.
More soon
DaveP
Iv'e finished the amp now, but there are issues with the meter circuits to sort out.
For example the dropper resistors specified don't give meter readings in the red zone so they will have to be changed.
I've tested the frequency response and its dead flat from 20Hz to 30kHz then it dips slightly before coming back up at 70kHz. The square wave has a slight ring on it so I may try and tame the very top end with some little mica caps.
It took about 100 hours of research to gather all the info, schematics, manual and understand what every part did. It has then taken about 120 hours to make the box and wire it up.
The balance adjustments work very well along with the 50/60Hz signal you have to minimize.
I shall finish all the adjustments and post the final schematics in due course.
If I knew when I started what I know now, I would not have had all those voltage measurements and wires to the meter, they are just a waste of time in my situation. It might have been worthwhile back in the day but I will never be putting the hours in like a 24/7 radio station, I shall never wear out these tubes, I'm sure, and all the components are much higher spec and tolerances now, so it is never going to need constant checking. If anything does play up I shall just whip the base off and stick a DMM on it.
best DaveP
For example the dropper resistors specified don't give meter readings in the red zone so they will have to be changed.
I've tested the frequency response and its dead flat from 20Hz to 30kHz then it dips slightly before coming back up at 70kHz. The square wave has a slight ring on it so I may try and tame the very top end with some little mica caps.
It took about 100 hours of research to gather all the info, schematics, manual and understand what every part did. It has then taken about 120 hours to make the box and wire it up.
The balance adjustments work very well along with the 50/60Hz signal you have to minimize.
I shall finish all the adjustments and post the final schematics in due course.
If I knew when I started what I know now, I would not have had all those voltage measurements and wires to the meter, they are just a waste of time in my situation. It might have been worthwhile back in the day but I will never be putting the hours in like a 24/7 radio station, I shall never wear out these tubes, I'm sure, and all the components are much higher spec and tolerances now, so it is never going to need constant checking. If anything does play up I shall just whip the base off and stick a DMM on it.
best DaveP
letterbeacon
Well-known member
It looks incredible Dave - nice work! Can't wait to hear some samples!
Jean Clochet
Well-known member
Woah! This is real DIY folks
That's really great Dave, I'm happy for you.
It's a lot of hours but I can see why. And it's good you have only small issues to fix and can put it to use quickly after all the work.
Also consider that your hard work will help others who build so, thanks for the story along the way!
Many congratulations
That's really great Dave, I'm happy for you.
It's a lot of hours but I can see why. And it's good you have only small issues to fix and can put it to use quickly after all the work.
Also consider that your hard work will help others who build so, thanks for the story along the way!
Many congratulations
leadbreath
Well-known member
so how does it sound???
leadbreath said:
Nice to see a bit of impatience!
I might find out tonight if I can sort out the niggles in time. When I screw down that box and rack it, I want that to be for the last time.
I want to fully test frequency response, noise, input/output levels and GR and post the results for reference. Testing the sound will take a bit longer as it has now got variable timings (not just the original two positions) which have to be calibrated for the labels on the knobs, but I'll get there.
best DaveP
Mike Cleaver
Well-known member
Very nice work!
Yeah, the metering was necessary back in the day when we used these for radio.
Every Sunday night, the stations would sign off at midnight for maintenance until 6am Monday.
We'd go over everything in the studios/control rooms, then drive to the transmitter to check things there.
Running through the metering for the tubes was a weekly check.
Hope it sounds as good as it looks!
Yeah, the metering was necessary back in the day when we used these for radio.
Every Sunday night, the stations would sign off at midnight for maintenance until 6am Monday.
We'd go over everything in the studios/control rooms, then drive to the transmitter to check things there.
Running through the metering for the tubes was a weekly check.
Hope it sounds as good as it looks!
Mike Cleaver said:
Thanks Mike, thats what I thought.
I had miswired a metering wire to the wrong side of the 16 ohm resistor, so now I can measure all the tubes. I've changed a few of the metering resistors to make the readings line up in the red check zone on the meter. 56k's changed to 68K's, 56 ohm's on V3/4 changed to 82 ohms.
The other problem I had was with the meter zero, there was not enough voltage drop across the 16 ohm resistor to make the meter zero, so I moved the wire to the junction of the 18k, 240 and wiper of 100 ohm pot position (like every other vari-mu!) and this sorted the problem.
So I could hear audio, I stuck a matching TX on the output and attached a speaker! It actually makes a reasonable practice amp, it has so much output! I made a start recording BV's last night but it will take a bit of getting used to all the controls and switches. It does compress ok though and the meter readings are very smooth, it will go as far as -25dB without problems, not that you'd ever want that much, my first impressions, one heck of a beast!
best DaveP
Jean Clochet
Well-known member
DaveP said:
Yes it does! Double duty is nice though
Thanks for the ammenments to your wiring for meter readings, I will update the copy for my files.
And thanks Mike for the memories of working at the radio station, I like to read these things,
Jean.
Mike Cleaver
Well-known member
Yes, these BA6A could feed up to +30 into a 600 ohm broadcast line.
That's more than enough to drive a speaker.
Many were located at the radio stations and could feed the telephone line directly to the transmitter many miles away.
Most however were at the transmitter sites and usually had to be padded down to +8, the standard broadcast level of the day, compared to +4 which is today's standard.
For studio use, a pad usually is required on the output of a standard BA6A.
That's more than enough to drive a speaker.
Many were located at the radio stations and could feed the telephone line directly to the transmitter many miles away.
Most however were at the transmitter sites and usually had to be padded down to +8, the standard broadcast level of the day, compared to +4 which is today's standard.
For studio use, a pad usually is required on the output of a standard BA6A.
bobschwenkler
Well-known member
This is something I've been thinking about some. Would it be more ideal to modify the gain of the output section or is a pad just fine?
