SR45 amplifier
- Thread starter debk
- Start date
Paul Joppa
Moderator
I've posted a few times on this, but it bears repeating as a reminder, and in case someone comes across this thread "out of context". The value is a range, in this case 4.5 to 18uF; I chose 10 as about the geometric mean and widely available. Experiment! You will be glad you did!Jim R. said:
Grainger49
New member
Does the Parafeed cap react with the speaker impedance curve? Or is it a match with the plate choke and/or the output transformer impedance?
Paul Joppa
Moderator
I'll take the opportunity toexpand on this a little, since it's a problem that I've struggled with for a long time.Grainger49 said:
I did a large amount of modelling with SPICE, some years ago. One conclusion was that EVERYTHING has an effect, including all the power supply capacitors, resistors, and chokes and the cathode bypass cap. Another conclusion was that you must include things like the choke DC resistance. Some effects are stronger than others, of course, but the minor players are not so minor that they can be safely ignored.
A further complication is that the output transformer inductance and equivalent series and shunt resistances are fairly strong functions of frequency and signal magnitude. And of course you must allow for a wide range of speaker impedances, which for a ported box will have two large peaks, one on each side of the low-frequency cutoff.
Given those difficulties, I conclude that it is impractical to use detailed modelling to choose the parafeed cap value. I then simplified the study to a resistive speaker load, perfect power supply, grounded cathode, and infinite inductance output transformer - in other words, just the plate choke, parafeed cap, load resistance, and tube plate resistance. This study showed that there was a broad range of capacitances that gave good results, and there is no value that produces seriously bad resonances. It's this range that I quote as suitable, and within which I suggest experimenting.
debk
New member
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
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
Paul Joppa
Moderator
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 said:
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
Michael
Grainger49
New member
braubeat said:
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!
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
-- Jim
Grainger49
New member
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.
Paully's 45/MQ all Ni Paramounts were awfully sweet sounding.
johnsonad
New member
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
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
