another 2A3 Paramount question

[Jim R.]

Paul,

I'll second what Aaron said!  Thanks for this, and yes, this should be a sticky, but where to put it as it covers multiple amps --perhaps tech topics?

Looks like I'v got the right iron coming no matter which plan I end up following -- upgrade existing Paramour IIs, or go with paramount 1.1 with 2a3.

Thanks again,

Jim

[dw]

I'm planning 2a3 with softstart and BH-5 and BH-6 iron upgrade. As for the parafeed capacitor,
Auricap makes 400V 10uF or 7.2 uF 600V parts. Can I use the 400V capacitor?
Thanks.

Dave

[Paul Joppa]

No, you cannot use a capacitor of less than 600 volt rating.

Well, you can if you want, but you should expect it to fail rapidly, possibly immediately, and possibly take out some other expensive parts - like the Magnequest iron!

[johnsonad]

Dave, use two 5uF 600v Auricaps in parallel. That's what I'm planning on doing.

[dw]

Thanks Aaron and Paul.
I'll go with the two 5uF 600v capacitors as well.

Dave

[johnsonad]

Dave,

There are other great caps out there that you don't need to parallel you may want to consider. I have a pair in my Paramounts so it was an easy choice for me. Some feel that parallel caps in the signal path is not a good thing and others say as long as they are identical caps you are okay. I'm not experienced enough to say either way

Regards,

Aaron

[Grainger49]

Here are a couple of my favorite reasonably priced 10uF caps (all at 630V).  I have the $11.00 ones in my Paramours:

http://www.diyhifisupply.com/node/91

http://www.diyhifisupply.com/node/88

http://www.diyhifisupply.com/node/164

[dw]

Wow, thanks for the help Aaron and Grainger!
I'll check out those Obbligatos.

Dave

[Jim R.]

Paul J.,

Another question about another configuration -- I'm assuming that the nickel tfa-2004 jr and the bcp-15 gapped for 50 mA would not work very well with the paramount built as a 300b, but with a 2.5v filament and a high-power 2a3?  However, would it be possible to adjust the bias of the 2a3 down somewhat to get the plate current in the <50mA range, at the expense of a few watts?

Basically I'm wondering if the iron I have on order would be suitable for the higher power 2a3 config in the new paramounts.

Thanks,

Jim

[Paul Joppa]

Paul J.,

Another question about another configuration -- I'm assuming that the nickel tfa-2004 jr and the bcp-15 gapped for 50 mA would not work very well with the paramount built as a 300b, but with a 2.5v filament and a high-power 2a3?  However, would it be possible to adjust the bias of the 2a3 down somewhat to get the plate current in the <50mA range, at the expense of a few watts?

Basically I'm wondering if the iron I have on order would be suitable for the higher power 2a3 config in the new paramounts.

Thanks,

Jim

Changing the plate current without changing the plate to cathode voltage would require a different transformer impedance for good performance.

You can do it, at the price of increased tube distortion. You would have to increase the cathode resistor from 1000 ohms to 1500 ohms, approximately, and the 300B distortion at full power would probably increase from 3% to 8% or so (reading between the lines of the WE table of operating points).

The stock cathode resistor is made of two 2K/25watt resistors, in parallel for 300B operation (1K) and in series for 2A3 direct coupled operation (4K). Replacing them with 3K resistors in parallel would get 1.5K. A few people have made this switch, using them in series for 6K and running a type 45 tube. This is still a substantial deviation for the original design.  :^)

In my opinion, the better alternative would be to keep the stock plate choke for 300B operation, but use the TFA-2004Jr. Then swap in the BCP-15 for 2A3 operation. The optimum parafeed capacitor would be a bit smaller with the stock choke (5 to 8uF?) but close enough to use the same cap.

[Jim R.]

Paul, thanks again for the help.  I decided that since the paramours and the stereomour are very similar aaqnimals that I will fix the tube socket on the one paramour (properly this time) and sell them, and I just bought a pair of paramounts on audiogon, so will rebuild those as 2a3s with the soft-start upgrade and the iron I'm gettig from MQ.

Thanks again,

Jim