SR45 amplifier

[Paul Joppa]

Now that we have the PT2 transformer group buy, is it possible to get the boards for the SR45.
I may be mistaken but from the schematic in the SR45 upgrade manual, the board looks like it is the same board from the extended FPIII.  Of course the component values are different.

Debra

Hey Deb - remind me next week - I'm out of town again all this week and my notebooks are at home. The original SR45 board preceeded the current "soft-start" board, and it would be expensive to make just a few - plus the newest version (4.4) is better in several ways. I'll just have to make sure what the wiring changes might be.

[debk]

Paul

Thanks, will do
enjoy your time out of town

Deb

[2wo]

Didn't  PB come up with with a dedicated shunt board, a few years ago? maybe ha still has the file.

There were a couple of errors in the original instructions/ schematic, I will dig thru my notes...John   

[2wo]

I think this is it, from the old fourm...John

http://db.audioasylum.com/mhtml/m.html?forum=bottlehead&n=138093&highlight=sr45&r=&search_url=%2Fcgi%2Fsearch.mpl%3F

[braubeat]

By the way, I think the Dowdy Plate chokes would be a good fit for this amp (sr45). 125H and 40ma sounds like a good thing to me. Of course they cost about 2.5 times as much as the BCP-15. You might have to adjust for the dcr of the unit which is probably different (it is not listed on Mike's website). Probably would not change things by more than a few volts though. Then get the nickel outputs and you would really have something.

Michael

[Grainger49]

By the way, I think the Dowdy Plate chokes would be a good fit for this amp (sr45).  .  .  . 

 
Well, they are not pretty, but I wouldn't call them dowdy.  (I know Jim)  Mike will still probably build them for this group.  And I think nickel was awesome on Paully's Paramounts with 45 output tubes!

[xcortes]

I'm indeed building my pair with Dowdy chokes in M3 (courtesy of Keto) and TL404 autoformers.

[Jim R.]

Regarding the Dowdy chokes,  I have to ask exactly what will this buy you?  I'm not challenging you as I'm sure you know far more about these things than I do, but I thought I remembered PJ stating more than once that there is a optimum ratio of inductance between the plate choke and OPT, and that because of the gapless nature of the pf OPTs, their inductance was so much larger than the plate choke, that it really wasn't of much concern to maximize the inductance of the plate choke.  Of course I could be totally imagining this, in which case I apologize, but I'm trying to learn here and without thinning my wallet too much.

-- Jim

[Grainger49]

BTW, I'm lurking on this thread because some of the best sounding amps I have ever heard are 45 output tubed amps.  I'm hoping my last generation Paramour (1) amps might be useful for such an amplifier. 

Paully's 45/MQ all Ni Paramounts were awfully sweet sounding.

[johnsonad]

Jim,

It's my understanding that the more inductance you have (quality inductance) the better.  Both VoltSecond and Mikey did some research in this area and it's posted on the MQ site (http://www.magnequest.com/tech_article2_voltsecond.htm, http://www.magnequest.com/tech2.htm) The Dowdy was designed for the 45 tube and still maintains a fairly low DCR.  If there is room on the chassis I intend to use the Dowdy in my build also.

Aaron

(edited for accuracy

[xcortes]

Hi Jim,

The rule of thumb I remember gives minimum inductance. In any case I'm using the Dowdys because I already have them.

[Paul Joppa]

IIRC, the Dowdy choke is quite large and unlikely to fit on the Paramour II chassis.

To clarify, increasing the already high) OPT inductance has little effect. Bigger is always better for the plate choke.

Note that you would probably want a larger parafeed cap; I get 28uF (14 to 56uF range).

[braubeat]

Just to keep the discussion flowing, I also have a tl-404 looking for a home. I have been wanting to mate it with a 71-a tube with battery powered filaments(as the tube was originally designed for). I bet this sr-45 circuit would be killer for this application. It might even be possible to do shunt regulation and ccs on the power tube with the more modest voltage and current requirements of the 71-a. What does everybody think? Crazy?

Michael

[xcortes]

I've always thought about doing a SR 71A with TL404s. In my case I'd use the extra voltage to do a direct coupled one.

[Paul Joppa]

With 380-400v available, I see no reason you couldn't do a shunt regulated, C4S loaded, direct coupled 71A amp. You'd want to trim the driver bias to get the voltages right, and re-adjust every so often - but if a half watt is enough for you and you're comfortable with battery filaments then you're already crazy enough that this will be no bother! What do you see for a driver? 01A perhaps?