SR45 amplifier

[Jim R.]

Paul,

Great!  And maybe also the 6j5 too.  Now if I could just come up with some nickel TFA 2004s I should be well on the way to a really nice pair of 2a3 monos to be swapped with the SR-45s in my main system as mood and music dictate.

-- Jim

[Paul Joppa]

TWO 6J5s, that would be ... shunt reg, you know...  :^)

[Jim R.]

Oh, indeed -- forgot about that extra bit of goodness over the stock P2s.  I recall how much I liked them in more or less stock form and I can only imagine how nice this pair will be.

-- Jim

[xcortes]

"Xavier, a plate load dropping 20v would allow about 10vRMS swing, allowing 6v minimum compliance. That's about 0.020 watts in a 5K load"

Thanks Paul. It looks like I still have lots to learn!

You can drop the plate current for a 5K load; I'd go with 30mA at 250-275v. It leaves more current for the shunt reg which is a little starved due to the PT-2 limitations."

Fantastic. I assume the 250-275 is plate to cathode. Right? That's a bias of 50-55 so I need 300-330v at the plate, 315-345 on top of the plate choke. So the design Breg at 320 is fine.

To get a 50v bias the cathode resistor would be 1.66k.

Good for approximately 175mW and approx 6.8% THD.

Now, my big question: would I need to change anything at the C4Ss to account for the 6 mA less I'm running through the 45? Or does the shunt tube just shunts them?

Thanks a lot!

[Paul Joppa]

The shunt tube does the work.

[xcortes]

Awesome!

Thanks Paul.

[Jim R.]

Resurecting this discussion to run some ideas past the group.

I'm considering using my new PT2s to build a whole second SR-45 amp, but with a few changes...

First, I'm considering building both amps in one chassis -- like John did, and going with a slightly larger plate -- maybe something like 12" x 18", all film cap psu, and also looking into the possibility of using separate filament transformers and the FC1 to supply the 45 filaments.  Aside from the obvious reason, I'm also thinking this will take a bit of the load off the PT2 so I can run a 6dn7 slightly more comfortably.

I have a pair of Dowdys on order with Mike, and those come with endbells so I'll most likely mount them above the TFA2004 jrs, which should free up a slot for placing the fc1 in a magnetic null formerly occupied by the plate choke.  Also, the orientation will be rotated so that the opt and plate choke are at the rear of each half of the amp, the 6dn7s near the back and the 45s up front and keeping the pt more or less in the center of each side of the amp.

So, I guess what I'm asking is if my thinking on the lightnening of the pt's load for the 6dn7 makes any sense, and two if a separate filament tx and the fc1 have any issues associated with them -- so sort of like a hybrid sr-45 and paramount, and using the paramount driver/shunt reg board with the 6dn7.

This will allow me to leave the current sr-45s pretty much as-is and only steal the tfa-2004s when the time comes.

Thoughts?

-- Jim

[Paul Joppa]

One of the unique features of the PT-2 is that the two halves of the 6.3v winding surround the 2.5v winding, shielding it from line noise and rectifier spikes of the high voltage and primary windings. If you use another source of filament power, you might want to try it both ways to see if you hear a difference - I never did that experiment, just designed in the shielding from the first.

The PT-2 runs at 10-11kGauss, much lower than the usual transformers (16kG; 13.5 for 50Hz rated but operated at 60Hz). This reduces vibration as well as radiated magnetic field. Consider getting transformers with dual primaries so you can wire for 240v, run at 120 - if you want to preserve this advantage. Be aware that split-bobbin transformers, which are very common these days, do have large external electrical fields unless you have a grounded shell or end bells. The split bobbin does reduce capacitive coupling between primary and secondary, providing a similar benefit to a shielded winding at a lower cost.

The PT-2 will run a bit cooler with a lighter load, about 10% less than a standard Paramour II.

[Jim R.]

Hi Paul,

thanks much.  I'm not sure I completely follow the part about 240 v primaries, but that's probably due to my lack of transformer winding schedule knowledge.

I also wondered if I may need to load the 2.5v secondary some to keep voltages from going too high -- or maybe that doesn't matter so much with the shunt regulation -- except for the 6.3v filament, but that can be adjusted other ways.

Still, most of this will depend on what I experience once the sr-45s are in the system -- which of course means the system has to get put together first.  I'm working on it... :-)

-- Jim

[Jim R.]

Question for PJ -- are any of the new shunt reg/c4s borads going to be useful in the SR-45?  A related question would also be if there is any progress on a new driver board for the sr-45s, or would y9ou recommend that people use the paramount soft-start board?

Getting ready to start planning the new layout for the stretch model sr-45 -- 11-1/2 deep instead of 10" but still 8 wide, portrait orientation, and monoblocks is what I've settled on -- just works much better with the rest of my system to keep them as monoblocks.

Dowdys will go on top over the OPT, grid chokes will go where the plate chokes used to be, and the space between the driver and 45 will probably be mostly eaten up with a 27-33 uF PF cap (the downside of that huge 125H Dowdy.

I'm also going to go with AC filaments and most likely 6dn7 driver/shunt reg tubes.

Thanks,

Jim

[Paul Joppa]

Jim-

Sorry I missed your Sept 4 post; the forum software is not perfect (!) To answer (sort of) - because all transformer secondary voltages depend on the primary voltage and to a lesser extent on the other voltages and currents, there is no answer beyond "try it and see what you get" for filament voltages. Resistive losses in the wire from transformer to 45 filament are not going to be zero, as well! Best bet is to make a "true rms" measurement if you can, or can borrow a suitable meter; line voltage is far from a pure sine wave and the rectified high voltage makes the other windings even worse. Plan on possibly slipping in a suitable resistor...

As for the C4S boards, the latest version (v.4.4) is still what's in the Paramount "soft-start" kit. It has an adjustable bias trimpot for the driver section. As I think I've said before, this board does not have a really heavy duty heat sink for the C4S feeding the shunt reg; if you line voltage is over 120v that can be a problem. It's one I still plan to address in a new design, but that has not progressed in the last few months - the BeePre has taken most of our time lately.

I hope this is helpful; I've kind of lost track of this thread and don't remember all the things going on....

[Jim R.]

Hi Paul,

Yes, I too have lost track of all the details -- and yes you did say that the SS board in the paramount is not really up to the job -- I just forgot about that.

I haven't measured the line voltage in the new place yet, so will do that tomorrow.

Now I'll have to look at what the range of options are for that monster pf cap :-)

Do let us know when you're ready to look into a new driver board though.

Thanks,

Jim

[johnsonad]

I must be lost in this post.  PJ, were you going to redesign the board or stick with the design from the old board?  If there is no planned change on the board, could we put together a small group by on a run of boards?

[johnsonad]

PJ/PB,

It seems I missed out on the PT-2 order.  I intend to use different heater transformers. Would the PT-7 be the next best transformer to build around?

Thanks,

Aaron

[Paul Joppa]

I will have a new design - it may incorporate some of the Crack Speedball and/or BeePre boards, not decided yet. The old board has some issues I am not at all happy with.

It will be based on the PT-7 and include DC filament power. The circuit is pretty close to done, but the parts and boards are not selected yet. Right now the BeePre is still taking up a lot of time, getting all the parts in-house and tweaking the layout for the manual.