SR45 amplifier

Hi Howie,

Ok, I didn't realize you had the pt2s, though I do believe PJ said that the soft-start boards are not suitable for the sr-45s, but I could be wrong about that too. :-)

I'm sure that whatever you build it will turn out really nice.  Even if you don't build an sr-45, the soft-start boards, pt2 and dowdy and exo45xls should make a nice sooped-up paramour II 45, which is what I may do with my other pair of pt2s.  And of course you can add a grid choke to either of these as well. Iron is good :-).

The dowdys are big so you'll probably want to build a somewhat larger chassis -- my plates are 8" x 11.5" OFC copper but I'll be putting the ac inlet and binding posts on the back of the frame instead of on top as is traditional BH style.  These will be a real labor of love for me, as I'm sure they will be for you as well.

-- Jim
 
Wouldn't the magnequest iron upgrade for the sex amp (BH-1 OT and BH-2 Plate choke) be ideal for the SR-45? IIRC the BH-1 was 4K.

Michael
 
I went for the Paramour upgrade iron BH-5 full Ni and BH-6 plate choke.  Mike said he can get that to me faster. 

see Paul Joppa's post on the similarities of the BH iron to the magnequest iron for the SR-45
http://www.bottlehead.com/smf/index.php/topic,3000.msg36166.html#msg36166

Deb
 
Deb,

If I didn't go with the dowdys, that's the set I would have gone with myself.  With the dowdys in hand and the nickel exo-145s easily available, and the idea to use the tfa-2004s in the upcoming paramount 2a3s, the choice was easy.  Now I have a second pair of dowdys gapped for 60 mA on order to finish the paramounts.
Good luck with the project -- I think you're really going to like these, especially with the solo piano stuff!

-- Jim
 
thanks Jim

It will still be some time before I build them.  Mike said it will take 8 to 12 weeks to get the iron. 

In the mean time I will get the rest of the parts collected

Deb
 
I' finished the first amp a couple of days ago and have been troubleshooting it ever since. It works now but the SR voltages are way off. I'm getting B+reg of close to 400v and a 34v bias. I've looked at everything mNy times and can't find the problem. Of course I already tried a different 6cm7 as well as new sets of 316k (2x 158) and 2490k resistors. Any clues as what might be the problem?

Thanks
 
Now that I ordered the iron, now I have to decide what 0.1uf coupling  capacitor to use.
The two I am thinking of are the V-cap CUTF and the Duelund Cast-PIO-Cu.  The V-Cap is about half the price of the Duelund

Anyone have on opinions on which to use?

Thanks
Deb
 
Hi Deb,

I'd probably go with the v-cap CuTF -- they're probably a good deal smaller, but also should you ever want to run EML tubes in the amp, the warranty on the tubes will be void if you use a PIO couupling cap, no matter how good it is.

I've heard the CuTFs and they are incredible and although I have some 1 uF CAST PIOs, I've not installed them in anything yet -- thinking I'll probably put them in the s.e.x. 2.1.  And based on the generality of film caps having somewhat lower ESR than PIOs, my guess is that the dynamics and clarity of the v-caps would be a bit better, but how that would manifest in the real world is not something I've got any experience with in this amp.

Either one would probably sound amazing, but different.  I like to think of the CuTFs as "warmly invisible" if that makes any sense -- all the nice tone of good copper caps, but just completely out of the way sonically, even if that is somewhat of a paradox. :-).

-- Jim
 
Hi Xavier,
There were a couple of errors in the the Manual, I don't have what they were in front of me but you could search the old forum and find them but by looking closely at the photos and following the schematic, you should be OK.

First are the LEDs lit?, if so what do you read at pin 1 and 9 of the 6CM7? Pin 1 is the output and should be ~320, pin 9, the cathode should be ~13.5
 
Hi John,

First let me accept that I am very stupid. Because the amp worked perfectly for 10 minutes and when I put a probe carelessly to take a pic showing the sr voltage I shorted b reg and killed something. So I replaced the board components and now the amp works but the two voltages you refer to are now 3
400 v on pin 1 and 34v on pin 9. The leds do light. I've checked and rechecked everything and can't find a problem. Thus my desperation.

Btw, I checked the forums (old and new) and indeed all the manual errors are documented.

Thanks for any ideas,

Xavier
 
Ok, the voltage at the cathode Is too high,  you may have lost the 431.  Check the voltage at the the 2 set resistors R and R2B see if they make sense...John
 
Thanks John,

I replaced all the board components after the "accident". I'll check the voltages at the resistors and report back.

 
But now that you mention it, I checked the bias setting resistors for the 431 and they measure ok both in resistance and the voltages were ok according to the resistances (that is, the 400 v  drop was divided proportionally to the resistances). So the most obvious suspect is the 431. I'll go ahead and replace it although it would be weird to get a faulty one. Maybe damaged it when soldered it?
 
431's can be a little fussy.  You want to look for 2.5V at the junction of the 2.49K resistor and your Vset resistors (IIRC there are two in series for the SR-45).

Also, you want to see some voltage at Kreg, but not more than 32.  34 is above the allowable amount for the 431, so if it was't originally dead, it may be dead now.

One thing to triple check is that the grid of the regulator portion of the 6CM7 is grouded.  IIRC this is pin 8 (it's either 7 or 8).  If this pin floats (like from a broken carbon composition), you'll have some regulation issues. 

-PB
 
Thanks Paul,

The grid stopper is ok and the grid is grounded ok. Yes, the voltage setting resistors are 2x 158k in series for 316k. That's what gets the 320v at the plate. I guess I'll replace the 431 and test.
 
New 431. Old problem. Voltages are 394 at the plate of the 6cm7, 3.08  between the 316k and 2.49 k resistors and 46 v at the cathode of the 6cm7 (on top of the 431).

So the 431 is obviously not doing its job. The question is why. It's gotta be something deceivingly stupid.
 
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