And now for something COMPLETELY different! 300B PP Integrated Amp prototype!

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I am not sure if I posted much about this product here, so here are the photos and an explanation!

We have a phenomenal 8W SET 300B HP/Integrated amp, the HA 300. At Capital Audio Fest 2018 in DC, we showed with Fritz Speakers and had GREAT sound with only 8W of SET 300B and the Fritz Carrera Be monitors! Quite an unlikely pairing with their stated 87dB efficiency rating, but it worked!

While the sound was fantastic, several people asked if we could make a more powerful dedicated Integrated version of this amp. One gentleman, B. Klein, said that if I would build such a unit, he would buy the prototype! Well, we talked about it and the idea intrigued me. I said that with parallel SE 300B, we could probably reach 15-20W of power. This appealed to both of us....Him because he wanted a more powerful 300B amp and me because I just LOVE making cool new amps!

Well, the cliff notes version is as follows:
I told Mr. Klein to wait and let me see what I could do. A few months later, I told him OK, I've got the design concept worked out and I am confident we can do it. He sent me a deposit and the hard work began. I had NO IDEA how hard this design was going to be! 300B + PP + 25W + Zero Feedback is a TALL order! But, it can be done and done WELL! A parallel SE 300B amp would have been easy. But, I decided that I wanted a true 25W and this could only be achieved with P-P design. I also decided that it would have ZERO negative feedback!

Jack Elliano of Electra Print Audio wound the custom power, output transformers and power choke and I went to work. My lead tech basically scratch-built the enclosure and face plate (Nice Work Fred!) and the 'prototype' ended up being quite a beautiful piece! Flat black for the chassis, satin black face plate with anodized AL FAT knobs and Illusion Malbec powder coat for the transformer end bells. It sounds as good as it looks!

Ambrose Ai 325 Tube Integrated:
25W Push-Pull, 300B integrated amp.
Weight: 49lb.
Dim's: 17"W x 13"D x 3"H (Chassis only)
Tube complement: (4)300B, (4)5687, (2)6DE4 damper diodes (for soft-start and warmup).
Class AB, with the first 10W in pure Class A.
Zero feedback!
Frequency Response: 20Hz - 50Khz.

The pictures say the rest!

Regretfully, while it sounded BEAUTIFUL before I spent a fortune over-nighting this amp to Axpona for the show, UPS handled it a bit roughly it would seem... :scratch: It did not sound right when I set it up in Chicago. So, it was only a static display at the for Axpona Chigago. It really was right down to the wire. I have spent COUNTLESS hours getting this amp ready. So many different design iterations before settling on the final circuit. Mr. Klein was very understanding and patient and MAN does he have a great amp to look forward to!

I will work out the UPS damage and finalize everything in the prototype when I get it back next week.

What started out as a concept prototype build, really a commissioned project, IS going to market in full production! There is still work to be done and a final enclosure to be designed, etc. However, every dealer that saw it at the show said they wanted one of the Ai 325, 300B PP integrated amps AND the new KWH 225i Hybrid integrated when ready for prime time!

This is probably the hardest I have worked, these past few months, getting the Ai 325 and KWH 225i ready for the show. It was all worth it as both products are simply outstanding and I can't wait to get them into customers' hands!

Intro price: $8495 (Likely to be more than this when I finally reach production!)

Thanks and let me know what you think as well as if this is something that you would be interested in. It is capable of driving 87dB+ efficient speakers, assuming a min. 6 ohm impedance and relatively flat impedance curve. It won't drive Magnepans or Electrostats, but drives my 87dB Studio Electric M4 monitors VERY WELL and they were too much of a load for the 8W SET 300B HA 300!

Dan Wright



Very nice amp, the extra power is its appeal. That said, does the push pull sacrifice anything "musically" versus single ended 300B?  Also what 300Bs are you using?


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What a Beauty!
And using my beloved 5687 tubes to boot!
Question: What's wrong with a tasteful amount of negative feedback in a P-P circuit?

Paul Galli


Hi, first of all, I hear NO sonic loss in P-P vs. SE. It is a different presentation, I would say, but the P-P topology gives the amp a lower output impedance and better control of the speakers as well as power. There is a bit of a romantic character with SET's that is perhaps replaced with a bit more controlled sound in PP.

However, when combining the beautiful tone of the 300B with 25W of real tube power, you get the best of both worlds. There is still a breathless and openness beautiful tonality AND bass response and control that only P-P can provide!

Most P-P designs use a little or A LOT of feedback. Most P-P pentode amps use A LOT of FB! This gives great bandwidth and lower distortion. It also detracts significantly from sonics in my experience! I considered adding a 1-2dB FB loop in this amp, but in the end I just loved the sound without FB too much! Our A30 mono blocks, parallel pentode EL34 amps also originally included a small amount of FB. During final testing and extensive listening one day, I cut the feedback wire and inserted a jumper, so that I could listen with and without FB.  When FB was removed, the sound stage became HUGE and everything simply opened up! It was not subtle!

This amp achieves 20Hz-50Khz (-1dB) WITHOUT FB and also 25W of 300B power!

Even our SS amps use zero global feedback!

The 5687 was chosen for its unique qualities that are required to make this design work! It could not have been 6dj8, 6SN7 or 12AT7. Maybe 6H30, but I prefer the sound of the 5687.




Dan, thanks for providing your sense of the difference between the push pull and single ended 300B. One follow-up , which 300B tube have you selected? txs

Jon L

Ooooh, reminds me of my old amp VAC Renaissance 30/30 300B push-pull amp, with 6SN7 instead and adjustable negative feedback.  Now THAT was a beast of a powerful amp for 300B.  Much like this one..


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Is this an ultralinear circuit?

Paul Galli


I used EHGold 300Bs and Tung-Sol 5687 tubes.

The circuit is not ultra linear. These are triodes in P-P, not pentodes, so it is different.

Indeed the VAC 30/30 was a beast of an amp! I don’t know if anyone else has really built a powerful 300B P-P Integrated amp like this, so I am excited!



Haven't heard it, but Audio Space has a 22wpc 300B integrated - Audio Space Reference 3.1 300B

Rusty Jefferson

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Excellent time to introduce this model with the (FINALLY) reintroduced Western Electric 300b tubes.

I too prefer P-P to SET 300b and 845 amplifiers, and the 5687s do make great drivers. I recently changed my P-P el-84 amp (Pentode) from 6h30 driver to 5687 to great results.

What is the actual output impedance?  Single tap?


Hi, the transformers are wound with one tap, 6 ohms. Extra taps reduces BW and 6 ohms is a good setting for most speakers!

Yes, it is a good time for a 300B amp, but that is not why I did it. I designed and built it because it is the amp that I WANT for myself!