If I wanted to build an SR45...

[Jim R.]

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.

[Jim R.]

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!

[Jim R.]

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!

[Jim R.]

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.