If I wanted to build an SR45...

[jrebman]

Would I have to first find a pair of Paramour IIs?  Or, would it be possible to get the iron a la carte, the schematics, and any other special parts?  I am fully capable of making my own chassis and top plates.

I've been trying some 45s in my Fi 2a3 monos and am somewhat smitten, but really want an amp optimized for the 45, and lots and lots of folks say the SR45s are fantastic amps.

Thank,

Jim

 

[Doc B]

We don't have power transformers for the Paramour anymore, so it might be easiest to find some used ones.

 

[dbishopbliss]

I should have my stock output transformers for my Paramour I around somewhere.  I never used them and went straight to the upgraded iron.  If I recall, the chokes used in the original iron were repurposed for the upgrade.  Let me know and I will dig around for them.

 

[jrebman]

Thanks, David, but after posting this I took a look on agon and saw a freshly listed pair of IIs and jumped on them.  They are all paid for and will be on the way to me soon.

Dan, so what's next?

Actually, I'm going to listen to these as they are for a while until the stereomour is finished (on the cornwalls) and then swap the stereomour in this system and start the conversion.

I'm psyched that I have a set of Paramours coming!

-- Jim

 

[2wo]

The SR-45 is a fantastic amp. I built al la Stereomour with a common PS. You are going to want the iron upgrade for sure...John

 

[Paul Joppa]

For some time I have been looking at a new version SR45. There are a few things I would like to change in the design, based on our experience with the original. Stay in touch; when you are ready to make the conversion I may have some refinements to the operating points, and I hope a new PC board with a heftier heat sink.

The original used the Paramour upgrade iron from Magnequest, with a 3K primary impedance. The Stereomour uses a 4K impedance, which is a bit better suited to the 45, but is not Magnequest quality. I don't know whether your Paramour IIs have the upgraded iron or not, but in either case it will be an interesting comparison with the Stereomour!

 

[jrebman]

Paul,

As far as I know these are stock, and I'm not even sure the seller built them as he doesn't have the manual.  So, among other things I'll need to get a manual and whatever instructions there were for the SR45 conversion.

Any other transformers that will work -- exo 45, etc.?

I will stay tuned to whatever you have brewing, but for now, as I said, I really just want to get the flavor of the amps as they are, and then maybe do the basic 45 conversion next, and then maybe doc's grid choke mod.

John, thanks for the note, and actually a lot of this was due to the comments you've made here and elsewhere about your SR 45s.

All this and my cornwall rebuild should keep me busy and happy with projects for a while to come.

Guess I'm finally going to have to put the Carina on the block though -- great amp, just not a triode or DHT and I'm spoiled now, especially since I have speakers that can really bring the best out of them.

-- Jim

 

[nullspace]

For some time I have been looking at a new version SR45. There are a few things I would like to change in the design, based on our experience with the original.

Hi Paul,

Would you mind divulging the changes you have in mind? I'm getting around to putting a SR45-inspired amp together as well.

I say SR45-inspired because while I have the SR45 kit I don't have the Paramour II's. For iron, I'm using MQ nickel TFA-2004 Jr. outputs and BCP-15 (40mA/ 50H) plate chokes. I'll be following the schematic included with the kit, with the following exceptions:
* I got Heyboer to make me some power transformers. The PS will be 6AU4GT full-wave -> LC w/ MQ VanOfMonks choke and Panasonic caps.
* I'm planning on increasing the current by about 4mA through the SR.
* Swapping out the grid-leak resistor on the input 6CM7 for a Altec 42292A (2k:10k) step-up transformer  + 10k pot.

Thanks. We can take this offline if you'd prefer.

John

 

[Mikey]

Hey Jim,

You might want to check out this writeup I did a while back:

http://www.bottlehead.com/smf/index.php/topic,361.0.html

It's been a couple of years since I did the conversion, but if
you have any questions just ask.  I'll try to help if I can!

Mike

 

[2wo]

Hi Paul,

I too would be very interested in any updates. My SR-45 was planed as a 2A3 amp using the BH upgrade Iron, I would not mind changeing it back and rebuilding the 45. Like I need an excuse to build something...John

 

[Paul Joppa]

OK guys, I printed out my notes and will review them for the differences - hopefully, on the ferry tomorrow. Quick look, the main differences are a bigger heat sink for the C4S that feeds the shunt reg, and DC filament power for the 45.

At the time of the original circuit, I came up with a variety of operating points for various output transformers in the range of 3K to 5K impedance; no reason I shouldn't gather those into a table and post it.

 

[ssssly]

DC filament for the 45? You definitely just got my attention.  Is that with the stereomour tranny or the PT2?

 

[Paul Joppa]

No, none of the various Paramour/Stereomour/etc. transformers are suitable. I'll keep the details under my hat until I know whether this will work in the real world!

 

[jrebman]

Hi Mike,

Thanks for the link.  I did read that yesterday after I knew that I bought the amps -- along with anything else I could find on the SR45.  I appreciate the offer to help, and I may well be taking you up on that, once I figure out what I need to get in terms of parts and instructions on how to make the conversion.

Thanks much,

Jim

 

[Paul Joppa]

OK, I went over my notes, and here are some comments:

* The original bag'o'parts (not a kit, no manual, "for experienced eXperimenters only") version was based on the upgraded Paramour II; specifically designed to use the Magnequest BH-5 transformer (3K impedance) and BH-6 plate choke. This is the amp Doc B uses on his Raal tweeters. It was designed to the constraints of the available power supply. The four limitations it had were 1) low shunt reg current, requiring the BH-6 to provide enough inductance for that to work; 2) AC filament power, 3) marginal heatsink on the C4S feeding the shunt regulator - it is not tolerant of high power line voltages, and 4) relatively low gain.

* I don't plan to do anything about the gain (item 4) above) - suck it up and use a preamp, guys!  :^)

* At various times I suggested three different operating points for the 45, depending on the output transformer impedance:

3K impedance: 250v at 36mA (standard), 1300 ohm cathode resistor
4K impedance: 250v at 31mA, 1550 ohm cathode resistor
5K impedance: 250v at 28mA, 1830 ohm cathode resistor

These had inconsistent estimates of the available power which I won't repeat, but they were generally in the range of 1.5 to 2 watts maximum.

The lower 45 current allows greater shunt reg current, but that happens at higher impedances where more demands are placed on the plate choke inductance, so I don't think varying the load impedance this way affects limitation 1) above to any useful degree.

* The new design I am working on uses a different power transformer with greater current capability, which does address the shunt reg current question, item 1) above. It also (I hope) will allow a DC heater supply, item 2) above. I am going to use a much larger heat sink and a new PC board, which addresses item 3) - my target is to provide for reliable operation between 110v and 130v from the power line. The filament power is not regulated, however.

* The new transformer also provides a bit more high voltage, allowing the 45 to operate at 265v instead of 250v, plate to cathode. I'll run it at 35mA into a 4K impedance transformer.

* The original used a 6CM7 shunt reg/driver tube. This is sort of a 9-pin version of the 6DN7, and the Paramour II power transformer is (just) able to heat a 6DN7 as well. The 6CS7 looks better than the 6CM7 on paper, but with a hair less gain. Until I get to hear them all in this application, I won't have any guidance on which is preferred.

 

[jrebman]

Paul,

Thanks for all this.

Let's suppose I just want to work with what I have/am getting (and at this point I don't know if the iron has been upgraded or not as the seller wasn't sure), what would you recommend as potential suitable transformers?  Stereomour? EXO 45 or 50? RH PF, or if I wanted to go custom, what would the specs be for inductance and RDC?  Also, what about the plate choke?  I have no idea what the specs on the original Paramour iron upgrade pieces are.

Also, as my speakers are 95/96 dB, I'd probably be fine with AC heaters, and if at a later date I wanted to change that, I can always build an external filament supply either based on the FC1, or maybe the diyhifi filament supply modules.

Funny, I've also been considering an extended FP III, so this may be coming sooner than I had originally planned.

I had also wondered about the 6dn7 as a possible driver/shunt reg tube, so glad to see you mention that.

Thanks again,

Jim

 

[Paul Joppa]

... what would you recommend as potential suitable transformers?  Stereomour? EXO 45 or 50? RH PF, or if I wanted to go custom, what would the specs be for inductance and RDC?  Also, what about the plate choke? ...

Instead of a recommendation, I'll offer some assessment - theoretical of course since I haven't compared these myself in an SR45 circuit:

The best sounding transformer would be a TFA-2004 Junior with nickel laminations. The OT-2 (Paramour II and Paramount upgrade) is very similar, just different construction details. Unfortunately, at 3K primary impedance it also has the lowest turns ratio and with the 45 will have the lowest damping factor. Steel (M6 or M4) laminations are less costly and provide greater power handling (which you don't need for a 45), but they do not sound quite as sweet to my ears.

The Stereomour output (OT-3) is not currently offered by itself. At 4K it has a higher turns ratio so the damping would be a bit better. It's not made with a nickel core. I tried to get as close to the TFA-2004 performance as I could, in a smaller package with conventional materials and workmanship.

The EXO-45 has a 5K impedance and would have the highest damping factor. It's available with a nickel core, but also has the least power handling capability, so I would not use the nickel core option unless you were not going to push it in the deep bass.

The Robin Hood is also 5K; it's kind of an EXO-45 with a growth hormone problem. :^) Power handling is very high - if you can get Mike to make one with nickel laminations it will still be effortless in the deepest bass. It is the largest of them all, and might not fit inside the chassis.

For plate chokes, the Paramour II upgrade package included the BH-6 plate choke, 40 henries at 50mA. Even better for a 45 would be the SEX upgrade BH2, 50 henries at 40mA (different air gap, same number of turns. Failing that, the BCP-15 is very similar, and available with either air gap. Because plate chokes have many more turns, the core material distortion is attenuated and I would not bother with nickel, which would also seriously de-rate the current capability.

Hope that helps!

 

[2wo]

Mine is close to Johns above. I use the BH-5 with a pair of EC-99's and I keep forgetting about the BCP-16 grid chokes. Gee we all can't seem to just follow instructions now can we

I replaced the input stage of the 6CM7 with a 12AT7 in an effort to get a bit more gain. I'm not sure it was worth it, there is more gain but I  still can use a preamp in my somewhat low gain system and the 6CM7 is a pretty good sounding tube...John

 

[jrebman]

Hi Paul,

Yes, thank yu!  This helps a lot.  I don't see the TFA-2004 jr. on the MQ products page, but the other TFA-2004s look to be 3k , so is the jr. also 3k?

How deep do you mean by deep bass?  My speaks do about 40 Hz and I don't use a sub. Thinking if exo-45 in pinstripe might work.  Failing that, I'll most likely just go with the stock paramour II upgrade OPT.  Also, my main source has a fairly low output Z and 2.25v rms output, so will I still need a preamp with my 95/96 dB speakers

If I wanted to raid my s.e.x. plate choke, and leave the nickel OPT, can I go back to the original s.e.x. plate choke, and what may the consequences be?

John, are you running yours full range?

Last night I found the paramour II product page and I didn't realize that these were the 8 x 10 format chassis.  Can't wait to see if these have the upgrades or not.

-- Jim

 

[elcraigo]

For what it's worth; I have Paramour I, with the SR45 upgrade PCB; but with the newer Power Transformer. This is the power transformer Doc does not have or can get anymore. I've use the BCP-15 for the Plate Choke, and Pin-stripped (nickle/M6) EXO45's for outputs. I works well for me. I use with DX3's in a large MLTL, and measured 101 dbSPL @1W.
As Elieen the Queen put it one time; I've squeeze all the performance out of an old Paramour I.
I'm glad to provide any notes I have.