xfmr

I have a Magnecord PT6-J unit that I would like to remake into a Federal Limiter. The output transformer looks like it will work with a 10kct primary and a balanced 600 ohm tap on the output. I plan to use an Edcor XSM 600:10K on the input (its fits right in where the EQ network module was so I can use the shield and overcome some of the Edcor's shortcomings there. My main concern is the power transformer has way more voltage than necessary. The power transformer is 700vct and has a 5Y3 for a rectifier which is on the orignal circuit, but I only need half that voltage for the Federal. I plan on adding a Hammond 158L Choke for smoothing, but what is the best way for me to knock down the voltage and is it even necessary? Can I run the limiter at a higher voltage? It looks like all the tubes can handle it, so what problems will I face running the circuit with higher voltage? Is it just as simple as doing the math and knocking down the voltage with a big resistor if need be? Will this effect the limiting capability in a negative way?


Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #1 on: June 19, 2010, 02:15:51 PM »
Using a choke will already lower your final B+ by ~50V.   You can drop excess voltage resistively as well - often better to use a pair of 1/2 size( at say 3W) vs a single big one ( at 10W).

Those are general techniques that will work for most any tube PS.

If it were me I'd look at saving that 700V PT for a bigger project that needs that high a voltage.  A 350CT @ 30 ma is on the cheaper end of tube PTs - the Edcor XPWR024 looks right on the money @ $20.00.

http://www.edcorusa.com/Products/BrowseSeries.aspx?SeriesID=37  


EDIT:   The XPWRO24 only has a single 6.3V/2A - switching the rectifier to a 6X5GT looked good but will exceed the filament by only 100ma!   The XPWR012 will handle the 5Y3 and everything else as is but is a bit pricier ($43).  Hammond's equivalent is a little cheaper:  http://angela.com/search.aspx?find=269EX
« Last Edit: June 19, 2010, 02:36:43 PM by lassoharp »

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #2 on: June 20, 2010, 01:13:15 AM »
I'd really like to use the power transformer I have and figure out the best way to make it work. Plus, I didn't mention it before, but I wanted to fit a NYDave one-bottle preamp in the 9 pin socket on-board and feed it with that HUGE input transformer. So, I'll need the extra juice this power tranny provides for that. It also has an 18 volt tap that I want to use with a voltage doubler for phantom power. But I'm getting ahead of myself. One thing at a time. First I'd like to figure out if I should lower the voltage to where the original design would have it or just run it with higher voltage if it won't hurt. Wouldn't running it at higher voltage just give it more headroom and more gain? How would it effect limiting? Thanks for the suggestion, though. If I build a standalone Federal from new parts I will be sure to look at that Edcor power iron.

schmidlin

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #3 on: June 22, 2010, 12:09:15 AM »
I have experience with both units, but this is a PS question in this regard.

You want to go from 350 VDC to 200 VDC.  (I will ignore the one-bottle for now)  Get a proper choke and place it between the 5Y3 and the first cap, and it will get you in the ballpark.  You need one with low DC resistance (~40 ohms) and high henries (40? I need to  check).  I ran across this in some Hammond PSUs that do this and have a couple handy if you need them, but they are not small, maybe the size of a Champ PT.  But you are only driving pre-amp tubes so you should be able to get away with much less.  Poke around.

PS: why don't you use the stock IT?  They have nice specs and good to 10,000 feet haha.  No center tap?  I would consider a Sta-Level type input and run with it, JMHO.  (NOT a fan of unshielded Edcor ITs, another story)

Best of luck.
perception > reality

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #4 on: June 22, 2010, 09:55:58 AM »
Quote
PS: why don't you use the stock IT?  They have nice specs and good to 10,000 feet haha.  No center tap?  I would consider a Sta-Level type input and run with it, JMHO. 

Well, I had considered this in fact, but the input tranny is a 200:50K mic level piece so I figured it wasn't suitable for this. I want to be able to hit the input with some hot signals and despite the size of this original input, the Edcor XSM 600:10K is not only the right size, but will be able to handle more signal.

Quote
(NOT a fan of unshielded Edcor ITs, another story)

Ah, yes it will not be unshielded. I was vague about it before, but let me explain fully. The plug-in Tape EQ module comes apart easily and its metal case and mounting holes line up perfectly with the Edcor XSM body. It slips right over it like a glove, almost like it was made for it. So I not only get a shield, but my Edcor Iron is not stealth inside this original case!

The reason I want to put the one-bottle in there is so that I have a complete recording channel for overdubs. Much like the UAudio 6176 I want a preamp feeding a limiter, all tube. It seems to have everything there to do this.

So, now for my power supply, should I place a big resistor right after the 5Y3 (100-250 ohm, 10-15 watt)? Then> 20uf capacitor> choke (Hammond 158L: 15H, 75ma)> 40uf capacitor. At the 40uf split the supply and have one leg go to 10K resistor for the One Bottle circuit and the other leg go to another resistor (I'll have to figure out the size for this) for the Federal Circuit.

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #5 on: June 22, 2010, 10:49:44 AM »
You should post a schematic of the Federal and the existing Magnecorder.  There's lots of very good help around the forum that likely will not bother to participate without a schematic posted.

You can do both projects on a single supply though you may run into unwanted interactions between each circuit.  It's very hard to tell if that will happen until it gets built.

I would strongly advise building the Federal first - get it working properly, hum free, etc.  Then tie the one bottle into the supply.  Easier to troubleshoot and it will give you a clear picture of how running the one-bottle off the same supply affects the Federal should that become an issue.

The existing mic input iron can be used for line input work with a proper pad.  It will also make a nice front end on the one bottle - probably better than the unshielded Edcor. So I see where you're going with that.

If you reread schmidlin's post - the plan was to use a choke input filter - the choke goes between the 5Y3 and the first cap - No big resistor.

schmidlin

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #6 on: June 22, 2010, 02:21:09 PM »
I was off on the henries: mine reads ~3.  Check here: http://www.surplussales.com/Transformers/PowerChk-2.html  Couple good options there like the (CPW) C2991.

The power resistors work differently, you'll get more sag, but not sure if this will matter in a compressor.  You may be fine going that route.

And those shielding cans, yep I am hoarding a couple, nice vintage vibe.  But I'm pretty sure they are plain drawn steel, no cast mu-metal or anything, so you may have limited success.  Every time I wound up paying for a shielded UTC, FWIW.  A-10 & A-11 both worked flawlessly.
perception > reality

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #7 on: June 22, 2010, 04:25:19 PM »
Quote
You should post a schematic of the Federal and the existing Magnecorder.  There's lots of very good help around the forum that likely will not bother to participate without a schematic posted.

Yes, of course. I will attach both the PT6-J and AM-864/U Schematic.

Quote
You can do both projects on a single supply though you may run into unwanted interactions between each circuit.  It's very hard to tell if that will happen until it gets built.

I plan on using the two circuits together, much like the UA 6176, so interaction won't be too much of a bother. As far as cross talk goes should I be more worried about shared heater or B+ supplies? I was thinking instead of bothering with Phantom power I'd use the 18 volt PSU tap knocked down to 12V to give the OneBottle it's own DC heater supply. As for separating the B+ I was thinking an extra CR stage after the split would give me enough isolation between the two circuits.

Quote
I would strongly advise building the Federal first - get it working properly, hum free, etc.  Then tie the one bottle into the supply.  Easier to troubleshoot and it will give you a clear picture of how running the one-bottle off the same supply affects the Federal should that become an issue.

Good idea, that is exactly what I will do.

Quote
If you reread schmidlin's post - the plan was to use a choke input filter - the choke goes between the 5Y3 and the first cap - No big resistor.

As yes, indeed. So: 315VDC>15H 75mA choke>20uf cap>(X) resistor>40uf cap>split @ ~250VDC, gotta figure out X.

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #8 on: June 22, 2010, 04:27:04 PM »
Here is the Federal schematic.

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #9 on: June 22, 2010, 04:28:52 PM »
And just for good measure, NYDave's 12AV7 OneBottle Schematic.


fazer

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #10 on: June 28, 2010, 08:58:52 AM »
You will find a handy circuit for adjusting high Voltage  its the mosfet on the left of the picture.  I would post it but its not allowed from his site.

Goto : http://www.geofex.com/Article_Folders/sstremolo/sstremolo.htm   

This is a Mosfet 820 with a Zener string that allows you to set the voltage at the Zener value.   Very simple to use.  Just an idea to adjust your B+ without chokes and dropping resistors.

xfmr

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #11 on: June 29, 2010, 09:32:00 AM »
Quote
You will find a handy circuit for adjusting high Voltage  its the mosfet on the left of the picture.

That looks like a very interesting power supply circuit. It is unclear how many Zeners to use in the string. I have not used Zeners before, how much voltage do each drop?

This could be very cool to use on some future project but I think I'm going to try to keep this one simple and old school: 5Y3GT, Choke input and resistor to get to the voltage needed.

I am still curious what the effects of running this circuit at higher voltages (but still within the tube's tolerance of course) would be if anyone feels like discussing that topic.
« Last Edit: June 29, 2010, 09:37:33 AM by xfmr »

fazer

Re: DIY Federal Limiter AM-864/U using Magnecorder PT6-J as Donor
« Reply #12 on: June 29, 2010, 05:22:14 PM »
That Pass Element is rated to 500V .  In the tremolo circuit it shown in,  the voltage tap is at about 410 volts and it works fine with an IRF 820 rated to 500 volt 2 amps.   

The Zeners are 1/2 watt each.  I'm using 2 100v Zeners to drop 400volts to 200volts.   This is also the heart of the circuit used for power scaling but with an adjustable pot instead of zeners on London Powers guitar amp scalers.   


 

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