Paramour 1 rebuild

[ludddite]

      I have been spending some of my down time rebuilding my paramour 1’s and chose to use the parts I’ve collected over the years to save some money and a desire to put them to use.  Magnequest tfa 2004sr. Ni. Pgp  8.1 and a sowter 8982 plate choke.
 Power supply is now  100 uf 600 v nichicon 270k to ground. C7x 270 ohm 10h, 100uf  600v, 2, 620 ohm in series for 1240, 47uf 450v ( this will be replaced with a 600 v rated cap )  1.2kohm cathode resistor.  3k output trans. 2 mv. @speakers
     The issue I’m having is that the pgp8.1 has a 115v primary and my line voltage runs t about 122v.  My working voltages are as follows
     @ 115vac. ( from my variac )
       Pgp8.1  356/356 -737vac. 466vdc @ 1’st cap 448vdc  @2’nd cap 356vdc @ the plate. 60.6 @ cathode.  2.57 vac. And 6.7 vac  ( this all seems fine to me ) 50 ma. 14.89 w. dissipation
       @123vac
         388/388-775vac. 494 [email protected]  375 vdc@ plate 64.2vdc cathode. 2.73 vac  7.15 vac.   
    53.5 ma. 16.69 w dissipation  (too hot). 2mv. @speakers
    My question is what would be the best way to go about adjusting the amp for my line voltages?
Should I just use dropping resistors on the heaters & adjust the B+ or dropping resistor on the primary to drop 7 vac?  ( seems schetchy. ) I’d rather not stress to trans.by running it hot.
     I’ll be playing with the operating point a little to match the 3k o.t. Butt I’m limited to 50ma. Due to the plate choke.
        Any advice would be appreciated.   Thanks, mike

[Paul Birkeland]

The PGP 8.1 is just not the correct transformer for a cap coupled 2A3 amp.  We did use in the Paraglow and Afterglow 2A3 amps which were directly coupled.

I have an extra pair of our PT-2 power transformers that are the improved power transformer that came after the Hammond power transformers we originally used, and I'm happy to trade those straight across to you for the PGP 8.1's. 

Another option you have is to reconfigure the PGP8.1 filament windings to 5V and run 300Bs in the amps (this is the easiest option).

Converting them to directly coupled 2A3 amps isn't the most straightforward process in the world...

[ludddite]

 The option of running a 300b was a big part of my thought process.  But the closest I have at the moment is a 2a3/40. I was hoping to be able to convert between the two.  Thanks for the offer of the trade but I’m not giving up on it yet.    Mike

[Paul Birkeland]

Yes, just use the 2A3-40, that's a good idea.  I don't like the switch option, because if you plug a 2A3 in and you have the amp set to 300Bs, you will cause expensive problems. 

The problem here though is that you have the 8982 plate choke, and that's set for 50mA.  This is a GREAT choice for something like the Stereomour/Paramour circuit that runs a 2A3 at 300V p-k/50mA.  You are absolutely married to that circuit if you have to use these chokes.  From the numbers you posted, you're basically there, you just need to use the JJ 2A3-40 and call it good.  53mA isn't the end of the world for a choke rated for 50mA.  I also happen to have too many EXO-003 chokes, so another trade offer is available here to get you a 60mA rated part if you wanted it.

As far as the line voltage goes, you're 9% high on the 2A3 filament voltage, and that's not so great.  What I would do would be to buy four CL-120 thermistors.  Start with just one thermistor in series between the incoming AC hot and the PGP8.1.  Fire up the amp and let it run for 5 minutes, then see what your filament voltage is on the 2A3.  If it isn't low enough, add another CL-120 between incoming AC neutral and PGP8.1 neutral.  Two CL-120s should drop the AC voltage rather substantially, and they have the side benefit of a very soft start for your amp.

[Paul Joppa]

Here are the "good fit" operating points that I use:

250v 60mA 2500 ohms (RCA data sheet)

250v 50mA 3000 ohms

275v 55mA 3000 ohms (My personal amps from 1997 VALVE v.4 n.11)

300v 50mA 4000 ohms (Paramour/Stereomour)

300v/60mA 3000 ohms (MonAmour)

350v/70mA 3000 ohms (Parabee, Paramount, Kaiju)

As you can see, if your plate choke is limited to 50mA, I would not go over 250v plate to cathode. PGP8.1 wants to make 450v and is well suited to running a 300B at 70mA using the EXO-003 (that choke is very conservatively rated, according to Mike LaFevre) - this combination with the TFA-2004Ni was in Parabee.

The EXO-003 is a tight fit on the original chassis; if you can find a BH-7 choke from Magnequest it will fit under the TFA-2004 just fine and runs about 25 henries at 70mADC. That's about the same spec as the PC-2 that was in Paramount.

[ludddite]

 Cl-120. Great call !  I had thought about that before I started for slowing the inrush & totally forgot about it. Tunnel vision I guess.  I spent two day tracing down a 20 mv. 60 hz. Hum,Only to find out that it was the driver tube
   re: exo 003  I’ve been looking for those chokes for quite some time, I had wanted to build the m91 amp.but for now I’m committed to the sowters. I’ve had them sitting in my closet for a few years & I’m happy to finally put them to use.
      Just to clarify, I never intended this amp to be “switchable” between 2a3 & 300b. Just that easily converted to a 50 ma. 3k 300b.  I think the w.e. Data sheet lists it at 5 or 6 watts.
 Stereomour 2 convertible between 2a3 & 45. And a 2a3 convertible between 2a3 & 300b seems to cover all my bases....for now.
       One other thing I’ve been planning is the shunt reg. ( stereomoure 2). The pgp8.1 should give me enough to do that.   
                                 Thanks for the help.    Mike

[ludddite]

  Thanks for the operating point list P.J. I played with a online load line calculator & the 250 v p/k seemed ideal but I was hesitant to start out dropping that much voltage.  I was aware that I was going to have to drop a lot volts. I taking the dial it in approach, and see what happens.

[Paul Birkeland]

When you plug in the 2A3-40 with your 1.2K cathode resistor, what are the voltages you get on the 4 pin socket?

[ludddite]

I had to plug in the jj’s I’ve been testing with sovteks.
@115 vac.  302 p/k 360 on the plate 58 on the cathode 2.5 vac on the filaments.
@122vac  ( line voltage )  321 p/k  384 on the plate 62.7 on the cathode 2.74 on the filaments. 
  The cl-120 seems to be the right track to start dialing things from. 
   Thanks. Mike

[Paul Birkeland]

I would use the CL-120(s) and just run the 2A3-40.  The P-K voltage is a little too high for a 2A3, but completely OK for a 300B/2A3-40.  The 320V/50mA operating point would be lovely with a 5K transformer, but I bet it will sound quite nice with the iron you have, and it looks to have plenty of power available as well. 

The other option is to really go for it and directly couple the driver to the 2A3.  If you're feeling adventurous and you aren't out of play money to mess with this, let me know!

[Paul Joppa]

Some more information - here are the specs (from a Freed catalog) for the PGP 8:

Input 115v 50/60Hz - so at 60Hz it should work up to 138v input
Nominal HV 700vCT
Choke input filter max 260v/100mA
Cap input filter max 385v/70mA
Specified rectifier 5Y3 - so with UF4007s you can expect more voltage, maybe 30-40v more?
Filament #1 5v@2A (This is the winding that Mike modified into 2 windings of 2.5v each)
Filament #2 [email protected]
Temperature rise 45-50 degrees C

Opens up all kinds of interesting possibilities  :^)

PGP stood for Professional Grade Power. I can't find a date, but it cost $9.00 back then.