Sr45 build, finally...

[ssssly]

So after 10 years of hording MQ iron, I'm finally going to build my SR45s. Just want to do a sanity check and make sure that I'm not missing something since its been so long.

Iron:

Power, PT-2
Output, TFA-2004

And now for the sanity check. Chokes.

Also in the magic Magnequest box are BCM-11 (12H, 100mA, 145 ohm), EXO-03 (40H, 60mA, 340ohm) and BCP-15 (50H, 40mA, 525ohm) chokes.

If I remember what I was thinking 10 years ago, I got the BCM-11s for the power supply filter and the EXO-03s for the plate chokes because I wasn't planning on reusing parts from my Paramours.

Just want to make sure that that makes sense.

Also wanted to make sure that there haven't been any improvements made in the past ten years that should be incorporated. I remember talk about DC heaters and soft start at some point. Did those or anything else ever flush out?

Thanks as always.

 

[Paul Birkeland]

You would want to use the BCB-15 for the plate choke. You could use the BCM-11 as power supply chokes, but that's a little overkill for a shunt regulated amp.  DC heaters would be an option with a different power transformer.

I wouldn't consider the soft-start to be necessary since the kit isn't directly coupled. 

PJ may have some thoughts of the TFA-2004 vs. using our OT-2 instead. 

[Paul Joppa]

I looked up my original notes, and saw that the BCP-15 (40H/50mA version) and TFA2004Jr (3Kohms) were chosen for the prototype only because they were the standard Paramour upgrade items at the time, and thus in stock. At the time I was working on the PC3 and OT2 (4Kohms), which I thought would eventually be better choices and are now used in the Stereomour and S3X amps.

But you already have the EXO-003 and TFA2004 which are good substitutes for the original design, though both are bigger than necessary. The main issue is the low primary impedance which will provide a lowish damping factor with a 45. The original design, labelled Rev 3.4, May 9 2007, is, I assume, the one you are working from. It compensated somewhat for this; using a different output transformer would call for a redesign.

If the amps are used with highly efficient speakers, it's possible there will be audible hum, which would benefit from DC filament power - but I would suggest listening to it with AC heating first. For unknown reasons, 45s often have less hum than 2A3s at the same heater voltage. Just leave a bit of space for an addon later in case you need it.

Be aware that the main C4S MJE5731A will get pretty hot, dissipating 5 watts. It will want a big heat sink. Also the 75K R2 should be rated at least 5 watts.

[Paul Birkeland]

To add a bit to what PJ said, when I recreated my own SR-45s recently with our current iron, I bumped up the regulated B+ a bit which is far kinder to Q2 in the shunt reg section, then I bumped up the value of the cathode resistor to account for the higher available B+.  This comes with the downside of pushing the limits of compliance for the shunt regulator, so if you end up feeding low mains voltage into the 120V primary, you may not be able to provide enough raw B+ for the regulator to work.

[ssssly]

After mounting another expedition into the audio parts closet I found another box of transformers.

Bringing the total transformers on hand to:

(4) PT-2
(2)PT-3
(2)PT-7
(2)BH-1
(2)BH-2
(2)PC-2
(2)C7-X
(2)BCP-15
(2)EXO-03
(2)BCM-11
(2) TFA 2004
(4)155J
(4)T7010
(1)360AX


Did I mention I'm a parts horder? You never know when something will come in handy.

So whichever combination of those transformers will work best is fine with me. They've been in boxes for ten years, no sense in them remaining there.

I can identify which ones will fry if I put them in the wrong place, but not which is best for which application in the circuit save power and output.

The SR45 schematic and build manual I am working from is labeled Rev 3.5, June 26, 2007. Is there a newer one?

I will be using the power supply PCBs Paul Birkeland made for me in 2010. Labeled SR45 PSU B which matches that circuit diagram. Thanks again PB.

They will primarily be powering a pair of JBL 2445s actively crossed over as a replacement for the 807s in a pair of Altec Magnificents. (Altec 19s made house pretty). Or Fostex 126 based Frugelhorns. I'll give them a listen to see if the hum is apparent with the AC heaters.

I have some large heat sinks for the 31As. Is there a known suitable replacement transistor that would be happier with the heat dissipation without redesigning the circuit?

Would any of the transformers on hand work as a grid choke? Was thinking the PC-2s might work. But again, my electrical engineering knowledge is limited to static loads. I can read and build off schematics or recalculate resistances, capacitance, voltage and amperage. Specing transformers in a dynamic circuit is just a bit out of my reach though.

PB, if anything my mains run hot. Generally around 126v.

[Paul Birkeland]

If you're using my boards, then that would just go with the original circuit design with the 3K iron.   If you had the OT-1s that went with the PC-2s, you could use those in place of the TFA-2004.

The BH-2 or BCP-15 would be appropriate plate chokes.  None of those items looks appropriate for use as a grid choke.

If you weren't using my boards, I'd use the PT-7s with the BH-1 operating at 4K and the BCP-15/BH-2 plate choke with a different operating point.  That would get you DC heaters in the process, but it would be a ground-up design.

[ssssly]

Well, I probably have enough parts to build both.

What did the OT-1 come in? I have a stock SEX, a MQ iron upraded SEX and a Seduction that I haven't switched on in years since I built my Paramount's and Eros.

[Paul Joppa]

I agree with PB that the 4Kohm OT-2 would be an improvement (better damping factor) and on his changes to more volts and less current to match its greater impedance. The OT-2 can be wired for 2, 4, 8, or 16 ohm speakers. Change the parafeed cap to ~5.6uF for 40H-50H plate chokes.

According to some old notes I dug up:

PT-2 is what I would use for power, because of its 2.5v winding which is shielded from the input and high voltage windings. It can be rectified to make 2.5vDC, as done in the Stereomour upgrade.

PT-3 is the predecessor of the PT-7. Neither has a 2.5v winding, but the PT-7's hefty 6.3vCT winding can be used to make 2.5vDC (we do that in MonAmour) and still power the driver/regulator.

BH-1 is nominally 8K:8ohms, so it will only work for 4 ohm speakers to get the target 4K load. Derived from a Freed design.

BH-2 is a modified BCP-15, 50H at 40mA (stock BCP-15 is 40H, 50mA). This would be perfect for a 45 - the more inductance the better.

PC-2 is 24H, 75mA from the Paramount. (OT-1 was the Paramount output, 3K ohms.)

C7X is 10H, 90mA; we used it a lot in the early days. If it's old enough to have paper layer insulation instead of a plastic bobbin, it's especially good (low capacitance).

EXO-03 and TFA2004 are from the old ParaBee; excellent iron for a 300B. I believe they would fit on a Kaiju and possibly on a MonAmour - I'd save them for something like that.

[ssssly]

Ok.

So what I'm gathering is as follows.

For an SR-45 using PB's PS Board and using the Rev 3.5 schematic with the iron I have available:

PT-2 for the power supply.
OT-2 for the output transformer.
BH-2 for the plate choke.
C7X (mine are indeed the paper insulation ones) for the power supply filter.


And then increase the B+ and Cathode resistor value compared to the original circuit as per PBs recent build.

What should I shoot for on the B+ voltage?

[Paul Birkeland]

Did I sell you a blank board or a populated board?  Can you post a photo of what you have?

[ssssly]

Populated.

[Paul Birkeland]

I'll confess that I just tweaked my SR45s to be more like the Stereomour in 45 mode and called it good. 

You'll have to change the two resistors I've pointed out on the boards to 180K 0.5W (1W is OK too, PR-01 would probably fit). This will set the B+ to 360V.

The cathode resistor should be increased to 1.6K 6+W rated.

I also attached a picture of my (kind of ugly) SR-45 insides built on some recycled copper plates.  I happened to have a 360K/1W resistor in my inventory so I used that rather than the 2x180K resistors.

[ssssly]

Sweet.

Thanks.

Is the OT-2 oriented at that angle for space or noise rejection?

[Paul Birkeland]

It's oriented where the existing holes were  

Those plates were quasi-Paraglow amps that ended up getting parted out.

[tsingle999]

Nice looking amp Paul! I love the copper plate. Looks like you have some nice mq iron in there too! But the nicest thing is the metal power switch;)