Lundahl Output Transformer Selection for 45 (2A3?)

[junker]

I recently learned about the Stereomour and am getting very excited about upgrading from my Line Magnetic 3W EL-84 SET!

Even with 3W I have to run the pot down about 1/2 way and the source down between -3 ~ -30dB depending so I think the 45 tube configuration will work well with my 16 ohm Altec Flamenco (Valencia).

I'm planning on upgrading the 3.3 uF parafeed cap and the 0.1 uF interstage cap, as well as maybe bypassing the last power supply and cathode bypass cap with some 125ËšC 0.01uF Sprague Vitamin Qs that I have (if that value will do anything?). And I'm also planning on upgrading the potentiometer with an Alps Blue Velvet. Maybe sometime later I could look at the chokes...

When learning more about the 45 tube I came across the old RCA data sheet at:

frank.pocnet.net/sheets/049/4/45.pdf

This document lists 3 operating points with an 180, 250, and 275 plate voltage, with load resistances of 2700, 3900, and 4600 ohms, respectively. At what plate voltage is this kit configured to run a 45?

Also, if I were primarily going to try using this amp in the 45 configuration, possible using it with 2A3 as a back-up configuration what would be the ideal primary impedance of the output transformer. In this range Lundahl has: a 4.5kOhm into a 4-16 ohm secondary impedance, and more options in the 5.5 kOhm into 4-16 ohm secondaries.

4.5 k   4 Ω   20W   2.5 kg   LL1679   B
4.5 k   8 Ω   20W   2.5 kg   LL1679   C
4.5 k   16 Ω   20W   2.5 kg   LL1679   D
----------------------------------------------
5.0 k   8 Ω   8W   1.4 kg   LL1663   -
----------------------------------------------
5.5 k   4 Ω   60W   4.0 kg   LL1688   C
5.5 k   8 Ω   60W   4.0 kg   LL1688   D
5.5 k   16 Ω   60W   4.0 kg   LL1688   E

Would the LL1679, LL1663, or LL1688 make the most sense here? From what I've been reading here on this site it's likely I'd want the option of comparing the 8 and 16 ohm output. Currently, my 3W amp out of the 8ohm taps can get crazy loud with the Altecs... Would the LL1679 be best to use here?

lundahl.se/wp-content/uploads/2013/05/1679.pdf
lundahl.se/wp-content/uploads/2013/05/1688.pdf

And lastly, would I still use a 3.3 to 10 uF parafeed cap with either of these output trannies?

Thank you so much for any advise!

Junker

[fullheadofnothing]

The linked transformers are intended for series feed, not parafeed. They are also a fair amount larger than the stock iron—I'm pretty sure they won't fit on the chassis. If you want to use these transformers, why not use them in a series feed amp?

[junker]

Oh, didn't realize that was different and was looking just take care of it on the original build. Is there any other iron that is a candidate for upgrading... maybe just add chokes to the power supply and call it a day?

[Paul Birkeland]

Oh, didn't realize that was different and was looking just take care of it on the original build. Is there any other iron that is a candidate for upgrading... maybe just add chokes to the power supply and call it a day?

It is worth mentioning that our stock parallel feed iron will offer the same inductance as the large Lundahl 1769, with excellent bandwidth and more secondary impedance options (And you won't have to spend an extra $600).  You will also lose the ability to install the impedance switch kit down the road, which offers additional convenience and flexibility. 

You could also e-mail Magnequest and ask about a suitable full nickel output transformer as an upgrade.

-PB

[Paul Joppa]

In 45 mode, the design operating point is 275v plate to cathode, 35mA plate current, and 4000 ohms load impedance.

[junker]

Thanks Paul!

Hey, how easy is it to configure the kit to run it at conservatively at 250V if I were to get my hands on some special 45's? Or would this affect other design elements? Thanks!

In 45 mode, the design operating point is 275v plate to cathode, 35mA plate current, and 4000 ohms load impedance.

[Paul Joppa]

The design is optimized for the 2A3; to run a 45 the power supply voltage is dropped about 30 volts by increasing a power supply resistance. This resistor gets pretty hot! Significantly increasing the heat under the chassis is not recommended without changing the layout, and perhaps increasing the ventilation area - i.e. to do so reliably is not a minor alteration.

For conservative operation, I recommend running the 2A3 as if it were a 45 - much easier to do.

[junker]

Thanks for the reply Paul!

Assuming I used a 30W rated Caddock MP-930 (in the heat sink attached to the top plate) what value would achieve a 250V plate voltage?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.partsconnexion.com%2Fmedia%2Fproduct%2Faccessories%2FACCESS-62493.jpg&hash=def57385ff593fa9bdcfdca6a9deb86b9a01b081)

[Paul Joppa]

I would strongly recommend building the amp in stock form first, and measuring the voltages before modifying to change the voltages. The actual resistor value will depend on the actual voltages in your build and on your power line. That said, it will probably be in the range 1600 to 2500 ohms. That's for each channel, two for stereo.

Using the chassis plate as a heat sink is not practical - it requires a fairly large free area, typically 5" by 5", which can get quite hot - there is no suitable spot I can see on the chassis. The heat sink you illustrated is probably not adequate; the Aavid 5311 is right on the edge - it's a hefty 1.35x1.5x0.5 extrusion.

[junker]

Sounds good. Thanks for the input. Most likely will run at 275v since I'd probably need as much of that 1.75w as I can get. 

[jimiclow]

My Stereomour is running at 250 volts, 30ma.
I'm not having heat issues. I'm using 2.7k Mills MRA-12, dissipation is less than 30% (IIRC). All 85C caps are upgraded to 105C.
More info here:
http://bottlehead.com/smf/index.php?topic=3252.msg27830#msg27830