EMI RS124

[Ó Draighnáin]

I used oil for the drilling and punching (hole & tool). Some sweat and blood too ! DIY for the win !

I always seem to leave a bit of blood on the chassis. Could contribute to mojo along with the magic smoke that needs containing.

I am building guitars now and it is no different.

I have built 3 x RS124s now, all to original spec. I like the meticulous way you plan out your chassis punching.

 

[thomasdf]

Thanks I am a huge fan of Illustrator, I have to admit ... I draw everything there : back panel, front panel, VU meter graphics and I do simulation with fonts, placement, control placement, knobs types, etc... It helps me figuring out things from a certain perspective and then having vector files for CNC cutting / engraving and / or printing, etc. Plus I can use pictures as templates, and steal (read : get inspiration) some designs, redraw some fonts ...
I am working on the VU meters as I type this. Pics to follow !


Edit :

Prototyping with this pair of Simpsons 1mA DC meters

 

[Rob Flinn]

Thanks I am a huge fan of Illustrator, I have to admit ... I draw everything there : back panel, front panel, VU meter graphics and I do simulation with fonts, placement, control placement, knobs types, etc... It helps me figuring out things from a certain perspective and then having vector files for CNC cutting / engraving and / or printing, etc. Plus I can use pictures as templates, and steal (read : get inspiration) some designs, redraw some fonts ...
I am working on the VU meters as I type this. Pics to follow !


Edit :

Prototyping with this pair of Simpsons 1mA DC meters

I believe the original meters were 200uA. Therefore to use this 1mA meter will require some fudging.

 

[thomasdf]

PRR wrote here that a 1mA shunt with a 220R should work so I’ll give it a shot, but I suppose it will affect the threshold
https://groupdiy.com/threads/altec-436c-vu-meter.33698/post-412284
Here again : fingers crossed. I am finishing drilling the 2nd rack enclosure. Next up is hardware mounting and figuring out where I’ll put the terminal strips and then onto some drilling again before soldering

 

[Rob Flinn]

PRR wrote here that a 1mA shunt with a 220R should work so I’ll give it a shot, but I suppose it will affect the threshold
https://groupdiy.com/threads/altec-436c-vu-meter.33698/post-412284
Here again : fingers crossed. I am finishing drilling the 2nd rack enclosure. Next up is hardware mounting and figuring out where I’ll put the terminal strips and then onto some drilling again before soldering

Hmm. For FSD you need 5 times more current through a 1mA meter than you would through a 200uA meter. I've not come across any shunts so far that increase the current through a circuit.

 

[thomasdf]

I have found this :
Post in thread 'Altec 436C build COMPLETED! with questions'
https://groupdiy.com/threads/altec-436c-build-completed-with-questions.52204/post-665397
Forum archeology !

It has to do with current idle in the 6BC8. I’ll do some testing and report back. But I’m not there yet !

 

[Rob Flinn]

I have found this :
Post in thread 'Altec 436C build COMPLETED! with questions'
https://groupdiy.com/threads/altec-436c-build-completed-with-questions.52204/post-665397
Forum archeology !

It has to do with current idle in the 6BC8. I’ll do some testing and report back. But I’m not there yet !

Yes, I've done that in the past myself you just need to be a little careful that it doesn't screw with the bias too much because there is not a great deal of resistance there (180 ohms per valve half) plus the resistor that you will be increasing to get your meter to work.....

 

[thomasdf]

That’s good to know ! Thanks
I can always try to fit some other motor in my meters, or change the meters.

 

[thomasdf]

Coming along nicely. I haven’t completely figured the component layout inside but I have quite some room for mistak… modifications Some holes are not perfectly aligned, I swear I wasn’t drunk when I made the dimples…!
I am happy how it turns out so far, but I can’t wait to start soldering. Metal work is not the funniest part

Let me know if you think I am photobombing the board !

 

[thomasdf]

I am in the building process and have an existential question :
I aimed at 180VAC mains transformer because I see 255VDC after the rectifier… but I see that under 20dBs of compression the B+ rises up to 265V. How is that possible with my transformer delivering 180VAC tops. Did I miss something ? The original rectifier was selenium so maybe my 1.4 ratio formula for diode bridge is wrong ? I couldn’t find the answer online yet.

Other bad news is I bought a JJ dual cap 100/100uF 500V… but it had a common negative pole whereas the schematics calls for different negative pole on C7 / C8a … rookie mistake. Maybe I could try to open it and see if I can separate the negatives…

 

[beatnik]

The original used two capacitors in series with resistors to be able to use capacitors with a lower voltage rating.
Since you have a 500V rated cap you could just parallel the two sections to make a 200uF 500V reservoir.

With regards to B+ voltage, a 10V difference shouldn't be too much of a problem.
I always drill the holes for transformers at last, so I can make sure the iron I chosen is up to specs, and orienting it for lowest hum pickup

 

[Ó Draighnáin]

Quoting abbey rd d'enfer from another RS124 thread:

"Don't worry so much about the absolute voltages. Remember that tube equipment most of the times was built for quite variable mains voltage, sometimes with 20% variations.

The important thing is making sure the voltages are balanced where they need to be (push-pull stages in particular)."

Have you read the thread entitled:

"Lowering HT voltage on RS124 clone, or not?"

​

 

[thomasdf]

I think i wasn’t clear. I am not worried about a 10VDC difference, I know tubes don’t care much. I am just wondering where the extra 10VDC come from under compression, and I wonder if my power transformer will supply / handle the draw.
Also wondering if a selenium rectifier has the same 1.4 ball park ratio between VAC and VDC than diode bridge. Not that I plan on using one, but I am being curious about the power transformer being used on the original unit.
I’d like to have the B+ close to the original to have the softer / rounder sound and lower ratio.

Re the electrolytic I want to be as close as possible to the original design first, so I do not use one side of the dual cap and added a pair of paralleled 47uF to replace it and have a proper negative pole.

 

[Murdock]

As far as I understand the current consumption goes down under compression so the voltage rises as the power supply is completely passive.

 

[Walrus]

Transformer regulation needs to be taken into account too.

 

[thomasdf]

It’s alive ! No smoke and worked straight away. B+ is 235V, it sounds absolutely gorgeous, round and grabby, I am very happy. It seems dead silent on my workshop speakers.
I followed the schematics 1:1 except for C8A cap (see above), I created a center tap (2x100R 3W) for the filament heaters and the timings where I got rid of pos 6 in favor of a new faster pos 1 (220K). Pos 2 is orig pos 1, and so on.
Compared to previous pics I decided to elevate the terminals strips on nylon standoffs because I am always scared of arcing, better safe than sorry

Now onto cleaning things a bit, making the second unit and see how the behave when linked and the big piece of work is the steel front panel…! I wanted it engraved but I cannot find someone able to do that, so I think I will silkscreen them with a friend. To be continued ! Thanks all for the help, info and inspiration

 

[JMan]

Nice work! I'm impressed with how minimalist the guts of this thing are. I know everyone says it's a simple circuit, but wow. I've built a couple of P2P LA2As and Pultecs (nothing crazy, but my only P2P experience), and by comparison it's wild how much less you have to stuff in this box. Makes it hard not to want to try one's hand at it.

 

[thomasdf]

I know ! I was surprised myself, I checked everything three times. The 3U rack feels very empty

 

[Rob Flinn]

I know ! I was surprised myself, I checked everything three times. The 3U rack feels very empty

Do you have any of the problems where you need to implement all the things you have been worrying about over the last few weeks ?

 

[thomasdf]

Overall ... no problem ! Except that old stock Hammond case ... very sturdy, VERY hard ! Drilling the hole for the VU meter is going to be a long and exhausting job haha