2a3 45 conversion paramore

[najo49]

I would like to convert my paramore with upgraded x former to 45 operation. I am thinking that several  resistor values must change for the 45. Could some kindly builder walk me thru this with explanations  for the various steps. Help on this to double check my understanding would be greatly appreciated. Jan O

 

[Grainger49]

If it is a Paramour II the instructions are in the manual.  If it is an original Paramour, like mine, the cathode resistor changes in each amp.  In the case of my amp, probably yours, the output transformer is not optimized for a 45, it is for a 2A3 but will do for a 45. 

My problem is that I don't know what the new cathode resistor needs to be.  I'm watching because I have a pair of 45 I want to try in my amps.  ;D

 

[najo49]

Thanks Grainger, I guess we have to wait until paul weighs in on this for the answer.

 

[Paul Joppa]

The stock and official upgrade transformers are 4K impedance and just fine for the 45 as operated in Paramour. (It's the 3K BH-5 that is less than ideal - but it's such a good transformer, I'm ok with that.)

For the older Paramour, here are the changes:
Cathode bias resistor
The cathode bias resistor is changed from 1200 ohms to 1600 ohms.
High voltage power supply
In all cases, it is worthwhile to connect the signal ground to chassis ground. Run a short piece of wire from B1, B2, or B3 to the center tab of the 5-tab terminal strip at the driver socket

 

[Natural Sound]

The stock and official upgrade transformers are 4K impedance and just fine for the 45 as operated in Paramour. (It's the 3K BH-5 that is less than ideal - but it's such a good transformer, I'm ok with that.)

Just to be clear, you are saying that the BH-5 is "less than ideal" for 45 use? If so is it because the 45 likes to see a higher impedance? Are any of the other MagneQuest output transformer options a better match for a Paramour II operating 45's?

PS: I understand there may some holes to drill due to different form factors.

 

[Paul Joppa]

...
Just to be clear, you are saying that the BH-5 is "less than ideal" for 45 use? If so is it because the 45 likes to see a higher impedance? Are any of the other MagneQuest output transformer options a better match for a Paramour II operating 45's?

Yes, the BH-5 impedance is low for a 45. Specifically, you cannot optimize for maximum power, and the damping factor is low. Other than that, the BH-5 and the similar TFA-2004Jr are excellent sounding transformers with a 45. The only Magnequest alternative is the EXO-45 or -46, which have 5K primary impedance. These will give a better damping factor, and in a Paramour I with the adjusted resistors you should get close to the maximum available power. To my ear, the BH-5 is superior sounding, especially if you want a nickel core - partly that's because the core is bigger and has more magnetic headroom. The resistors I chose optimized the design for a 4K primary impedance.

All of the above are relatively minor differences; triodes are quite tolerant of load impedance variations. Probably the most audible would be the effect of damping factor on deep bass, but that's so strongly affected by room acoustics that what effect you actually hear becomes quite unpredictable.