Bottlehead Forum
General Category => General Discussion => Topic started by: Doc B. on November 01, 2019, 06:13:46 PM
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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.
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REALLY nice!
And really apartment-friendly!
These would be my dream now
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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?
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More info please : what is the driver tube?
The driver tube is the 5670.
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.
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.
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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.
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Yes, you'd have to change 3 resistors to make it into a #45 amp.
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What are you guys going to name this kit?
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We may be open to suggestions on naming.
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Since there are two and it’s based on the Kaiju: Bi-ju or Kai-two. Or something better.
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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
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Name suggestion.
If the circuit is a refinement of the Paramour, Stereomour amps how about MonoAmore ...
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I was thinking along the same lines. Specifically, Monomour, a play on words on the French words "Mon Amour".
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(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.
(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.
(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.
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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.
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Cool. Thanks PB.
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
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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.
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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.
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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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PT-10 is maxed out at about 390 volts,if you are drawing 50mA.
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Many thanks PJ.
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Roughly when will these be available for order?
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We are working that out this week and I hope to have an answer soon.
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The amp will come with DC filaments straight away.
@PB - does it in fact come stock with a DC heater circuit? I can't spot it in your pics and the "In Depth" description on the purchase page makes no mention of it and states that the amp has a humpot - which would seem superfluous if the output tube is DC heated.
cheers and thanks, Derek
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Hum pots are still useful with DC heaters. The DC heater circuit is built above where the meter mounts. We would have a really tough time getting to a 250uV noise floor without it!
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Cool. Thanks.
We would have a really tough time getting to a 250uV noise floor without it!
Yeah, I guess i should have figured that. 1/4 mV is incredibly low. Does the 5670 contribute to that low figure?
cheers and thanks, Derek
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No, the 5670 doesn't contribute measurably to the noise floor, at least not that I am able to measure.
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I want to give a BIG thank you to kgoss and Raymond P for helping us decide on the name! Love it, love you guys, and love all of your support!
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Will it come wrapped in lace with scented candles? ... You know, on account of the name and all :D
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I know you would not recommend it, but is it possible to built these in mirror configuration without too much trouble? What would be the greatest challenge for mirror build?
Note that, I did mirror build for my Paramount. Its rather confusing but thank God nothing serious happened.
Thanks
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I don't see why you couldn't. The plates come powder coated with the top side being well finished and the bottom side not so well finished so the grounds can connect to the chassis properly. You would have to sand one down and powder coat the other side.
If you didn't have any issues building the mirrored Paramount, then building a mirrored MonAmour shouldn't be too tough. Things will change a fair bit in terms of 4 pin socket wiring, as you'll have to mirror the socket as well so the filament pins still face the hum pot (you probably did this for your Paramounts as well).
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Thanks Paul,
If the plate already finished then I might just use it rather then flip it and try to match the finish!
BTW these have universal power transformer, correct? Its 240VAC line in my country
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Yes, the power transformers are universal, and there is a true 240V nominal winding option (along with 220V and 230V).