SR45 amplifier

[braubeat]

I would love the boards for this project if they are available. I do not need any additional instructions.

Michael

[Doc B.]

To measure small levels of hum, I twist the meter leads together - it minimizes pickup of magnetic fields, typically from the power transformer but they are everywhere in a modern house. Ideally you would use coax.

We've been doing some empirical research in the lab with cables that past few days. The improvement in magnetic field rejection of coax vs. twisted pair is pretty amazing. We've been plugging various interconnect cables into a S.E.X. amp and listening thru headphones while moving the cable around very close to the power transformer. Some cable made with inexpensive Belden coax stomped everything else we tried for killing magnetically induced hum.

Before everyone runs out and changes over to cheapo Radio Shack coax interconnects (which worked great too), the downside is the coax we tried didn't sound as good as our hand made twisted pair in braided shield cables do. But I think it might be interesting to look into using coax for power supply wiring near a power transformer.

Perhaps the simple lesson from this is don't run your interconnects close to power transformers.

[Jim R.]

Put me down for a pair of the PT2 transformers as well.

Do we have a count yet and how would we go about making this happen?

If we need somebody to coordinate a group buy, I'll be3 glad to do it, but I probably should wait until we are moved into the new plac ce -- say late August sometime.

Thanks,

Jim

[debk]

i'm in no hurry, but would like to get a set of boards and power transformers

Deb

[Jim R.]

I actually want to build a pair of paramour IIs for 2a3s, so yes, would need an appropriate c4s/driver board for that amp as well.

This way I can leave the stereomour in the other system and not have to worry about moving it between systems.

-- Jim

[Jim R.]

Ok, taking the bull by the horns, I've started an official PT2 group buy thread here:

http://www.bottlehead.com/smf/index.php/topic,3069.0.html

Please read the rules before posting there and I will update that thread as information becomes available.

Thanks,

Jim

[Jim R.]

Quick question -- is the parafeed cap 3.3 or 10 uF for the SR-45 configuration?

Thanks,

Jim

[debk]

In the schematic that I have it is 10uf

Debra

[Paul Joppa]

Quick question -- is the parafeed cap 3.3 or 10 uF for the SR-45 configuration?

Thanks,

Jim

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!

[Grainger49]

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?

[Jim R.]

Thanks much Debra and Paul.

Grainger, the answer is "yes", but Paul has stated many times that one should experiment with parafeed caps in order to find the best match for your speaker load.

-- Jim

[Paul Joppa]

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?

I'll take the opportunity toexpand on this a little, since it's a problem that I've struggled with for a long time.

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.

[2wo]

I am using 6.8uf, with great success...John

[debk]

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

[Jim R.]

What Deb said...

Also, seriously considering changing my driver/shunt reg tube to the venerable 6dn7 -- wiill the soft-start board still work for this application as well as a pair of paramour IIs?

-- Jim