Another Altec 436c/SR124 p2p :p

[DaveP]

It looks ok, but remember all these vari-mu's are a feedback circuit, so there needs to be more than sufficient B+ smoothing or else it will motorboat.  You have also got a lot more circuitry than stock so layout will be  critical or it will oscillate.  You must put a bypass pot in parallel with  the meter or it will slam hard over and may be damaged.

Best
DaveP

 

[ding]

I've been thinking about the meter all last night. I was looking at the ua 175 schematic and I think I might try to go for something like that with the meter/pate balance/cathode balance circuitry its got going on. It looks like a standard VU meter but I'm not sure if that will integrate properly with this. It looks like it will. The first 2 tube stages look very similar.

http://www.waltzingbear.com/Schematics/Urei/UA_175.htm

As far as B+ smoothing what do you recomend? I was just going for the basic (OLD) schematic PS design. I can take any help I can get with PS since in that respect I'm not too sure where to go. I kept it with the old voltage doubler. I got to say that 40u cap is not that cheap.

 

[DaveP]

I can't see why the 175 circuit wouldn't work.  You can get away with a 1mA meter and put your own scale on it.

While your at it, the 175 regulator is a worthwhile addition as it allows a higher compression ratio than the 436.

Any power supply that provides the correct voltages will be ok as long as there is sufficient capacitance to absorb 1-2 Hz frequencies.  The regulator does this.  By the way, the 6CG7 is set up at optimum on 150V, so don't be tempted to go over that.

Best
DaveP

 

[ding]

DaveP

That image of the 175b schematic is very fuzzy specially on the power supply part. I wouldn't trust myself to take that on. I saw the schematic you posted for the modified 436 you built which looks much cleared and easier to understand but I am not sure how to integrate it into what I have. I imagine 250v would go to V1 plates/V3 threshold pot and 152v would go to output transformer CT but I'm not sure where the 107v would fit in.

Alain

 

[ding]

Here is where I am as far as the schematic. I think I'm not goint to mess with it any more....?

 

[ding]

Here is my initial idea for a layout trying to keep as much as possible in the turret board for easy troubleshooting.

 

[Piedwagtail]

That'll be very distorted there's no HT on V2 pin1.
Sticking a cap on a winding to shield the transformer from dc is something Kingston does on his PM670 but he injects the dc to the plates by two resistors. Parallel feed with inductors has the same basis.
Your circuit is a standard centre tapped push pull transformer, the dc from the psu has to reach both plates of V2.

So get rid of that 470uf.

R

 

[ding]

That'll be very distorted there's no HT on V2 pin1.
Sticking a cap on a winding to shield the transformer from dc is something Kingston does on his PM670 but he injects the dc to the plates by two resistors. Parallel feed with inductors has the same basis.
Your circuit is a standard centre tapped push pull transformer, the dc from the psu has to reach both plates of V2.

So get rid of that 470uf.

R

You are absolutely right. I guess I miss understood DaveP when he was talking about 470uf as final cap. But looking at it again putting that cap there like I did would block dc to v2-1 so I guess I'm still not sure what DaveP meant. I'll update my schematic. The only thing I might do different besides removing that cap would be to get a 240v tranny and just go full wave on the PS. Although I have been looking at tube rectifier psu's I might stick with the original and see how far that takes me. Any advice on my spaghetti turret layout?

Thanks!

 

[DaveP]

I meant the final power supply cap, so it would be C8-b1 in your diagram, this is the feed to the 6BC8 so it needs to have minimum hum, you could also have another for C8-C1 but it's not so critical there.

Amplifiers used to be made point to point when skilled labour was cheap, then they moved to turret boards to make more of a production line approach.  Turret boards look nice because all the components are neatly arranged, but the downside is you have lots of excessively long wires acting like hum loops and aerials.  All your sensitive attack/release wiring needs to be screened  (its on V1 grid!) and the switches themselves may need screening in a tin box as well.  You may need to change the orientation of the power TX to avoid hum radiation so make room for that in your layout.

Look at some of my "from scratch" projects to see how layouts progress.

Bon chance

DaveP

 

[ding]

Thank DaveP!!!!! I understand what you are saying. So in your opinion I should stick to using as little components on a turret as possible. I'll definitely check out some of your builds. I briefly checked out that thread you linked earlier and it looks like I need to sit down and really spend some time reading thru it. There is quite a bit of things to be learned there. I appreciate all your help.

 

[Piedwagtail]

Looking at your layout, you could do turret in a screening box for all stuff near the V1 and envelope switches. The simplicity of the output valve could all be done on the socket, hardly worth laying out. The 100k partial feedack resistors and HT balance isn't going to be especially sensitive. Turreting the power supply is fine.

Having a couple of Fluke 407DR power supplies I've become addicted to keeping the supply in a separate rack.
I'm fond of pi filters: cap-inductor-cap for cleaning HT, if you can find a low mA 5 or so Henry choke you can further isolate different uses of the HT. Remember these were built to a price, even in those days, so adding a choke from ebay could be a useful upgrade.
Pondering how those fabricators of the day finalized their layouts, bindings and lacing, I suppose playing dodgems with the transformer set whilst watching the output on a 'scope on mV is a "technique". ie. I'm hinting getting a working 70s dual trace oscilloscope with easy to reach service manual will give you useful inputs on making things, including watching a sine wave go in and out of your new build. If you're an fpga EE dabbling in tubetronics please excuse me, I'm on the dessert wine on Boxing day !


R

 

[tubegeek]

I'm getting very close to building a two channel 436C with some of the EMI and NY Dave mods for attack and release etc. I expect to be building on terminal strips inside a steel chassis, with a toroidal power transformer outside and the audio transformers inside. I am taking the advice about shielding the control wiring into account, with a cover of some sort over the back of the face plate. Thanks to DaveP for his excellent photos and howto on his other varimu from scratch project, much appreciated and very inspiring. I hope you don't cut yourself on the photos of my metalwork - soon to come!

Transformers: Lundahl 1540 input transformers and Edcor XSM10K/600 outputs. The Edcor outputs were so cheap: if they don’t sound/measure well enough, I can always use them in a guitar amp or something, they were too cheap not to try out. DaveP's recommendation of the other Edcors is filed away just in case. I very much doubt the Lundahls will be the weak link in this project. 

Reading NY Dave's valuable comments elsewhere it seems he had trouble with power supply stability limiting the useful range of the attack control, so I want to provide a good, stiff supply. Suggested above by DaveP: a large final cap. Another choice that would make sense to me in 2016 would be to split the 10K dropping resistor into 5K + 5K and make an additional filter section, and to do likewise with the 6K8 resistor on the other power supply node. I say "in 2016" because I can easily put 47 uF at every filter position with today's caps (size and price) and that seems to me that it would be more than enough filtering for ripple. The final cap: perhaps bump up to 100uF or 470 uF to lower the impedance at low frequencies just that much further. Which node? Both?

Also along the lines of power supply improvements: fast/soft recovery diodes, I've had good results with those before. I am using a full wave bridge at either 220V or 240V for the high voltage supply, instead of the original voltage doubler. My toroid is spec'ed well for the project I think: Antek  model #AS-05T240, rated 50 VA overall, 220 or 240 V secondary, two heater windings rated 6.3V @2A each. I'll probably start with twisted AC heaters and only move to DC if I find hum to be a problem.

My one concern: is there SUPPOSED TO BE any sort of mojo interaction through the power supply in this unit? Will any of the side chain behavior be disturbed by well-meaning power supply "improvements" ? I am not so bold as to think I understand this circuit fully at the margins.

Any comments on which of the changes I suggest would be most fruitful and which I ought to stay away from? Other suggestions? Kibitzing? Tractor recommendations for spring planting season? Thanks!

 

[ding]

According to the schematics floating around this forum the RS124 used a 240v PS with a full wave rectifier but kept the original caps. I think when I start building mine I will probably go with a full wave but increase the caps per DaveP's suggestions. Looking forward to hearing about your build. I just got too many things in the hopper before I can get this bad boy going so it might be several months. I'm glad someone else is going to be building something similar. Post your schematics when you got something. I should update my schematics but I just haven't had the time between music making and building other projects.

 

[ilfungo]

Hi
I'm at the end of costruction of diy RS124 P2P.
All is working well but I want to add a super fuse mode like Chandler.
I've read It's  set the time of release as attack.
It's just a recovery switch bypass?
Thanks!!!

 

[ilfungo]

Someone can help me ?
Thanks!!!

 

[ilfungo]

My RS 124 Is Ready!!!
I'm very happy!!!
Some mods done ( some like Chandler Superfuse)....

 

[ilfungo]

And inside...
Very DIY ( old Digi 192 case)....

 

[ml-project]

My RS 124 Is Ready!!!
I'm very happy!!!
Some mods done ( some like Chandler Superfuse)....

How did you realize Superfuse Mode?

Michael

 

[weiss]

sorry for posting on this old thread, but where did you guys get the 470uF film cap from? I can't find anything on the usual stores..  ???

 

[Rob Flinn]

sorry for posting on this old thread, but where did you guys get the 470uF film cap from? I can't find anything on the usual stores..  ???

I doubt they used a film cap of 470uF.  It would be too big to fit in the box.