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August 18, 2019, 02:37:24 PM

Author Topic: 2A3 conversion  (Read 565 times)

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Offline Bryon

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2A3 conversion
« on: December 03, 2017, 11:32:41 AM »
The 300B Paramounts sound amazing in my system, but the desire to mess with a good thing is something I can keep at bay for only so long.  I've decided to convert my Paramounts from 300B to 2A3 and want to make sure I have everything right - so a few questions for those among us who know so much more than I do.

I have the soft start Paramounts, so am I correct that to switch to 2A3 operation I just have to follow the first four steps of "Paramount 300B to Paramount 2A3 Conversion In five simple steps" and simply adjust the voltage with the trim potentiometer to 1/3 the A2 voltage in order to bias the 2A3 tube?

The Paramounts are fed from a BeePree into a Marchand XM26 active crossover with the high pass set at 180hz or 200hz [I'd have to open it up to be absolutely certain of the exact point] so I hope I can reduce the capacitance of the Parafeed caps so I can use the Vcaps [2uf 600V]  I took out of my Extended Foreplay III.  I know using the 2A3's means you should use larger caps, but does the fact that the amps are just seeing about 200hz at the low end change that sufficiently so I can put the Vcaps in without any problems?  If it makes any difference the Paramounts are going into very high efficiency speakers - Lowther DX4 drivers in 204hz Azura horns.  If 2uf is going to cause problems, I've also got a pair of Mundorf silver/gold 6.8uf caps I could put in, but they are even larger than the Vcaps.

I'll also be replacing the 47uf electrolytic caps with Solens [although I'm not sure how I'm going to fit both these and the bigger parafeed caps in]

thanks for your time and advice

Bryon

Offline triode

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Re: 2A3 conversion
« Reply #1 on: December 09, 2017, 03:53:35 PM »
Hi Bryon,

This might not much help. I also have Paramount (old version with no soft start) configure to 2A3. My speaker also Lowther (PM7A) with Azura 204 horn, no cross over (full range). I believe you could reduce the 47uF cathode bypass cap since you don't need bass. I dont know much how lower you can get away with.
I also have Stereomour configure as 45 amp (which also can use 2A3). I would say Paramount 2A3 is more clean and more "hi-fi", but something about 45 tube that makes music sound really beautiful. I listen more to my Stereomour. Maybe its an option you could consider too.

Regards

Offline Doc B.

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Re: 2A3 conversion
« Reply #2 on: December 09, 2017, 05:10:17 PM »
Let me start this by saying that I am not picking on you Bryon. It's a very popular subject and I just happen to be jumping in at this moment. If I had a nickel for every time we get asked if one can use a smaller parafeed cap... The answer is always yes. You're not going to break anything.

Now as to whether we can give you an answer with great authority as to whether your cap choice is a theoretical optimum - nope. The optimal values for a given situation can be calculated using a capacitance-reactance chart. But even that is an estimate because the numbers we plug in are often not that accurate. The parallel feed R-L-C circuit creates a resonance. If you place that resonance just right you can create a bump just below the shall we say "native" bass rolloff of the circuit, thus extending the bottom end a bit in the shape of a knee that is deeper and steeper than the native rolloff. If the cap value gets too big beyond the optimal value the bump moves down too low, and you get a dip between that deep bass bump and the native bass rolloff. If it gets too small you create a situation where the bump moves up higher and adds to the frequencies that are allowed to pass by the native rolloff, creating a bass hump. Are these things audible? Does any of this really matter? Depends on the system and the listener. That's why PJ always suggests that a range of values will work fine.

So is 2uF or 3uF of 4.7uF of 6.8uF or 10uF a problem? Not likely. Does it create an unlistenable amp? Not likely. Is it optimal? Not likely. How can you tell what value is optimal or how far away from the stock value you can deviate? Measure the response. That's what we do. And even then we may used a slightly compromised value from the exact optimum to reduce the complexity of our inventory if we deem the deviation too small to hear.

I know PJ and PB are always happy to give you guys an educated guess on this matter. The reality is that until you measure the response you're still just guessing, even if you have been designing amps for 99 years. With laptops and free measurement software so plentiful these days it can be a valuable learning experience to measure your own gear.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Paul Birkeland

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Re: 2A3 conversion
« Reply #3 on: December 09, 2017, 05:42:04 PM »
PJ gives the educated guesses. I just throw darts at post it notes on the wall.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man