Something new - 2A3 monoblocks

Doc B

Former President For Life
Staff member
PB, PJ and I have been working on a new project for the past couple months. This is our new premium 2A3 monoblock amplifier. It is designed to run a broad range of 2A3 variants up to and including the JJ 2A3-40 by virtue of a two way switchable operating point. The current draw of the 2A3 in a given operating mode is indicated by the meter. It uses a parallel feed output stage and a cap coupled, shunt regulated driver stage. It uses the big OT-5 output transformer that we use in the Kaiju. I'm going to let PB weigh in on the performance specs of various 2A3 tube types, as he is the one who has been playing with them. Sonically it is a little more towards neutral than a Kaiju (a but less lower midrange/upper bass weight) and thus has a little bit more resolved sounding vocals and a wee bit more top end shimmer. That said, with similar circuits and iron both amps are definitely from the same sonic family. Universal power transformer, and a very low noise floor similar to the old Paramount monoblocks. Kit price is estimated at $2250 for the pair of monoblocks. Yes, those shrouds and the black finish are how the production kit will look.
 

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That’s really exciting! Congrats, guys. I’d love to grab a pair someday - but I probably ought to finish the Kaiju and the Bee Pre first. And get better speakers. And a better turntable. And a good bankruptcy attorney.
 
Very intriguing...
More info please : what is the driver tube? how much power with JJ2A3-40? DC filament option? looks like 8X10 base size like Paramount?
 
triode said:
More info please : what is the driver tube?
The driver tube is the 5670.

triode said:
how much power with JJ2A3-40?
7W @ 10% THD with the switch on the higher current operating point.  The Sovtek 2A3 that we will send with the kit will make 6.5W or 5W depending on the switch position.  I suspect the big EML 2A3 will also be around 6-7W, but we don't have a pair on hand to try out.  The fun thing about this amp is that you can go from the low current operating point with an RCA 2A3 to the high current operating point with the JJ 2A3-40 and go from listening to a 2A3 to 300B amp very easily. 

triode said:
DC filament option? looks like 8X10 base size like Paramount?
The amp will come with DC filaments straight away.  The chassis is 10"x12".  The larger chassis helps fit the substantially larger iron, and it helps keep the amp running a bit cooler on the inside.  We are also providing the amp with film cathode bypass caps, and that requires a bit of extra space as well.
 
Great looking amps guys. And since you designed them I know they will sound as good as they look. They will definitely go on my wish list. I don’t really need them, but I want them! 😃

I’ll go ahead and ask if these can be modified to run 45 tubes, since its a standard forum question for a 2A3 amp.
 
Since there are two and it’s based on the Kaiju: Bi-ju or Kai-two. Or something better.
 
Very cool. More questions:

(1) Choke? The Kaiju OT is mentioned, but not the plate choke. Is is it the Kaiju PC?

(2) OT impedance - wired for 3K, 6K, or some other primary impedance?

(3) Switching operating points - when switching between high and low current, does the amp also switch between choke inductances?

cheers and thanks, Derek
 
Name suggestion.

If the circuit is a refinement of the Paramour, Stereomour amps how about MonoAmore ...
 
Deke609 said:
(1) Choke? The Kaiju OT is mentioned, but not the plate choke. Is is it the Kaiju PC?
Yes, but with the full winding, so a bit more inductance.
Deke609 said:
(2) OT impedance - wired for 3K, 6K, or some other primary impedance?
The Kaiju OT is 3K.  I wouldn't be surprised if the performance wan't optimal trying to reflect 6K.

Deke609 said:
(3) Switching operating points - when switching between high and low current, does the amp also switch between choke inductances?
No, these don't run as much HV current as a Kaiju does, so there's no need to go to the lower current tap on the plate choke.
 
kgoss said:
If the circuit is a refinement of the Paramour, Stereomour amps how about MonoAmore ...
The only relation these have to the Stereomour is that they both use 2A3s.  The iron, driver, operating point, DC filament supply, HV supply, etc. are all quite different.
 
Cool. Thanks PB.

Paul Birkeland said:
The Kaiju OT is 3K.  I wouldn't be surprised if the performance wan't optimal trying to reflect 6K.

I've noticed that PJ often specs iron so that it can be configured in multiple ways, and so figured the Kaiju would be no exception and would have at least one other primary impedance besides 3K. 

cheers, Derek
 
Deke609 said:
I've noticed that PJ often specs iron so that it can be configured in multiple ways, and so figured the Kaiju would be no exception and would have at least one other primary impedance besides 3K. 
That goes back to when we were using the Speco T-7010 in the old Paramour and SEX kit.  It had both 4K and 8K primary taps, but was discontinued rather suddenly.  Its replacement needed to fill both those roles, so that's where that came from.  If you go back and look at the Magnequest upgrade iron available, there was a specific output transformer for the SEX amp and a specific output transformer for the Paramour.

For our recent higher end products, we have stuck with 3K transformers optimized for 3K use.  Some of the older Bottlehead 2A3 amps from the last century did use 2.5K output transformers though.
 
As PB indicated, the Kaiju OPT was designed for exactly one impedance. I only played games with the plate choke, to get more inductance at lower current -  i always intended to use the combination with 2A3s, and it has finally come together.
 
Had I not already committed to new 70H plate chokes and new OPTs for the SII-45B rebuild, I'd definitely be your first customer. But I'm still interested.  Another question: can the PT-10 support a B+ of approx 500V and 50 mA draw? If not, how close could I get?

cheers and thanks, Derek
 
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