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Schematic for 2A3/300B SET OTL amp

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rdowdall

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Joined 2011
#1
Hello Folks, I am looking to replace my solid state amplifier with a tube amplifier. I have a pair of Klipschorns and a Transcendent Sound Grounded Grid preamp. I am wanting to build a 2A3 or 300B SET OTL amp if I can find a schematic that has a proven track record. I'd appreciate any suggestions and also would be open to other ideas for other tube amp projects. I don't need much power.

Thanks,

Rick
 
#5
Transcendent Sound

pageboy said:
Have you looked at Transcendent Sound's own 300B SE OTL? It's the only such circuit I know of. It uses four 300Bs per channel. Not cheap.
Click to expand...

It's the "Not Cheap" that's the problem. I have been considering the mini-beast but I'd rather try the 2A3 or 300B. The output transformers are pricey on a standard SET so I would like to find a OTL plan.

Thanks
 
#6
As far as SE output transformers being pricey, have you looked at Edcor's units? They're less expensive than most. I'm using a pair on an SE amp I recently finished and they seem to be decent, although I've never heard any others to compare to.

I always thought that as many tubes as it took to make an OTL, that the price added up to more than output transformers anyway. You'll pay money on either side of it. It's just one of the expensive parts of the hobby.

I've been tempted to try that Mini Beast sometime also, just because I'm curious to hear an OTL. I've never heard one. It's probably just about the least expensive OTL anywhere.

I wish I could have a pair of Klipschorns. My little SE amp makes about 1 1/2 watts per channel and I'd love to hear what it sounds like through them. But they will be a while for me.
 
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#7
OTL

pageboy said:
As far as SE output transformers being pricey, have you looked at Edcor's units? They're less expensive than most. I'm using a pair on an SE amp I recently finished and they seem to be decent, although I've never heard any others to compare to.

I always thought that as many tubes as it took to make an OTL, that the price added up to more than output transformers anyway. You'll pay money on either side of it. It's just one of the expensive parts of the hobby.

I've been tempted to try that Mini Beast sometime also, just because I'm curious to hear an OTL. I've never heard one. It's probably just about the least expensive OTL anywhere.

I wish I could have a pair of Klipschorns. My little SE amp makes about 1 1/2 watts per channel and I'd love to hear what it sounds like through them. But they will be a while for me.
Click to expand...

I'll check out those Edcor transformers. I don't like the chassis much on the TS products. I bought the Kipschorns in 1986 for just under $2k now they cost $12k. I still love listening to them. I play via a home built music server and dac. Much better quality than the McIntosh 7000 CD player that I used to use. Not to mention the 32bit DSD recordings that sound fantastic.
 
#8
hello ,

the OTL is PP or SE Class A , so différent performance to final end

me old proto OTL for test , power out to 2 x 700 mW ( for +/- 300 mA current )
 

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#10
rdowdall said:
I don't like the chassis much on the TS products.
Click to expand...

Then scratch build one. The Rozenblit 300B SE-OTL schematic is available for the price of his 'Tubes and Circuits' book.

You are not going to get a usable SE-OTL using 2A3s or 300Bs with only one output tube and multiple tubes will be a significant cost factor.

The Transcendent 300B SE-OTL is worth the effort and expense though, it is an amazingly good amplifier when partnered with the right speakers. Here's one of my Transcendent 300B SE-OTL scratch build amps that I use with Lowther horns. They're modified to use DC filament supplies, so really quiet.

An externally hosted image should be here but it was not working when we last tested it.
 
#12
OTL 300b

nautibuoy said:
Then scratch build one. The Rozenblit 300B SE-OTL schematic is available for the price of his 'Tubes and Circuits' book.

You are not going to get a usable SE-OTL using 2A3s or 300Bs with only one output tube and multiple tubes will be a significant cost factor.

The Transcendent 300B SE-OTL is worth the effort and expense though, it is an amazingly good amplifier when partnered with the right speakers. Here's one of my Transcendent 300B SE-OTL scratch build amps that I use with Lowther horns. They're modified to use DC filament supplies, so really quiet.

An externally hosted image should be here but it was not working when we last tested it.
Click to expand...


Beautiful Amp - I have a copy of Bruce's book that I bought a couple years ago and never got around to reading it. There it is right on page 235.

Where did you find your chassis, or is it custom?

Regards
 
#13
300B Tubes

nautibuoy said:
Then scratch build one. The Rozenblit 300B SE-OTL schematic is available for the price of his 'Tubes and Circuits' book.

You are not going to get a usable SE-OTL using 2A3s or 300Bs with only one output tube and multiple tubes will be a significant cost factor.

The Transcendent 300B SE-OTL is worth the effort and expense though, it is an amazingly good amplifier when partnered with the right speakers. Here's one of my Transcendent 300B SE-OTL scratch build amps that I use with Lowther horns. They're modified to use DC filament supplies, so really quiet.

DSCF0925_zpsq7fifmbu.jpg
Click to expand...

What brand tubes did you use?
 
#14
#16
Power Supplies

Hi Ray - I was looking at your photos, and noticed the power supplies. Then I read in the forum about "Andrew" who sells the pcb for the PS. Is he still selling the boards?

Looks like a good solution.

Thanks for the photos.

Rick
 
#18
Power Supply

nautibuoy said:
There's also a lot of useful info on the project build here;

300B OTL with DC - Transcendent Sound Forum

Ray
Click to expand...

Hello Again Ray - I hope you don't mind the questions. I've been reading the material in the book, on the forum and looking at the photos. In Bruce's book the power supplies and transformers are incorporated into the amplifier. His amp kit he sells has a separate box for the power supply and the photos of your build are the same. Does keeping the power supply away from the amplifiers result in less noise?

Another question I have is, should I use a/c for filament current or use d/c as you did. Looks like the a/c is an easier build, I come to that conclusion due to the amount of discussion regarding the dc supplies.

Regards,

Rick
 
#19
rdowdall said:
Hello Again Ray - I hope you don't mind the questions. I've been reading the material in the book, on the forum and looking at the photos. In Bruce's book the power supplies and transformers are incorporated into the amplifier. His amp kit he sells has a separate box for the power supply and the photos of your build are the same. Does keeping the power supply away from the amplifiers result in less noise?

Another question I have is, should I use a/c for filament current or use d/c as you did. Looks like the a/c is an easier build, I come to that conclusion due to the amount of discussion regarding the dc supplies.

Regards,

Rick
Click to expand...

You're Welcome Rick.

The third box in the Transcendent kit is a low frequency AC filament power supply for the eight 300Bs. With standard 50/60Hz filament supplies you will inevitably have some amount of hum, the LF supply is intended to move that hum out of the audible range.

In my build the third box is just for the toroidal transformers that are used in the B+ and filament supplies. During the prototype build measurements showed the benefit of separating the toroids. Incidentally, talking of the prototype build, I'm not responsible for this development and it was led by a fellow audio DIYer on your side of the pond - you may have noticed in the thread I pointed you to. I've made him aware of this thread so he may pitch in.

Anyway, each of my mono amps has a CLCLC B+ supply and four DC filament supplies within the amp chassis

As for AC vs DC for filament supplies (and LF AC vs 50/60Hz AC), I don't know which you'll prefer so it's your call, but the amp will be in the excellent category with any of them. I can tell you that both of us who have built the DC version think it is more excellent than the AC versions we used previously. It is possible to build an AC version and then change to the DC version but remember that the output stage described in BR's book is quite different to the output stage of the DC filament version, having an output/current sink arrangement per 300B instead of the one shared by all four 300Bs in the original AC version.

Ray
 
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