2A3 conversion

[Demsy]

I have downloaded and printed out the 'five simple steps' converting the 300B Paramount to 2A3. Done the steps 1 to 4, but step 5 is totally different than what I have in my amp. I also checked the version 1.1 manual and indeed it doesn't mention anything about the 9 pin socket lay out being different for 300B and 2A3.
Maybe Doc, PB or PJ can help me out with what I encountered during the conversion steps. I have done 1 amp and am going to do the second amp. But will wait firing them up until I've received any confirmation. I also tried searching about it, but can't find anything. Thanks

 

[Paul Birkeland]

Yeah, you have the adjustable bias TL-431 instead of the LED, so you can essentially leave the 9 pin socket alone.

You will, however, need to be sure the 5670 is biased as the manual requires, as this sets the bias for the 2A3. 

-PB

 

[Demsy]

Thanks PB for the instant reply! Will leave the the 9 pin socket as it is and will surely adjust the bias voltage accordingly, later.

 

[Demsy]

The conversion went very well especially without having to change anything on the 9 pin socket. Even installing the 47uF electrolytic cap with the huge Solen was also a breeze. All the resistances and voltages are within the spec on both amps.
The only problem encountered was adjusting the bias/plate voltage. On one amp the trimpot reaches the maximum/minimum end, I couldn't go lower to the calculated value. Calculated was 153.3V and 156.6V. I couldn't reach 153.3V, the lowest is around 158V. No problem with the other amp, it easily reaches 156.6v and can even go further.
Adjusting the hum balance went well, the lowest is 0.002V for both amps. Dead quiet on the 95db DTQWT loudspeakers.
Couldn't believe how much better the sound is, low is more extended with more punch, mid and high are superb. Clarity, soundstage and imaging is also much better. Of course the only new components are the 47uF bypass caps and the Sovtek 2A3's. They need some time to burn in, I wonder how much better the sound will be after that.
I hope for some advice on the bias adjustment, what to do about it or just leave it as is. Thanks!

 

[Paul Joppa]

I probably should have put some tolerance on that plate voltage, shouldn't I?  :^)

+/- 5%  on the driver plate voltage is nothing to worry about. The reason for the adjustment is that, without it, the variation is more like +/-30% and that really IS too much. The magic "1/3 of the high voltage supply" is itself an approximation, and it also depends on variation between individual 2A3s so there's no way to be precise anyhow. I would not really worry about even 10% drift - that's only 3% of the high voltage supply, after all.

Actually you can try adjusting to different voltages near the target and compare the sound; you might find something you like better with your speakers/room/musical tastes. The tradeoffs are subtle and complex, so there's no point in analysis - your ear is a better discriminator.

 

[Demsy]

Thanks for the prompt reply, Paul!
That's what I thought, the difference in percentage is actually minimal, that's why I use the amps further.
Another thing is, with the 300B, when  the driver tube is replaced, the voltage has to be re-adjusted to 175V. So now with the 2A3, it's 1/3 of the A2 voltage. Would the A2 voltage change when replacing the driver tube? Please excuse my limited knowledge of elecronic circuits

 

[Paul Joppa]

The A2 voltage won't change - it's mostly a function of the domestic power line voltage.

The driver plate voltage is really only critical with the 2A3.

 

[Demsy]

Thanks again Paul, less work rolling the driver tube now!