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Friday, March 16, 2018

The 45 / 2A3 Single Ended Amplifier

Hi!

I haven't built any of those since a while, here is a new version of a single ended stereo amplifier which can use either a 45 or 2A3.




The amplifier comes in two chassis, one for the amp itself and a separate power supply. Here the amplifier section:




It is interesting how the requests for landscape style chassis have increased lately.




Here a top view of the amp:




The selector switch for the output tubes is hidden in the corner between the tubes and the transformers. The output tubes are AC heated. Hum can be nulled through hum bucker pots. This particular build has both balanced and RCA inputs selectable by the switch between the driver tubes (6N7), alternatively the 6J5 can be used as driver without any mods. The use of the 6J5 results in about 3dB less gain.




The bias current of the output tubes is monitored through vintage panel meters.




Check out these beauties, vintage meters made by Weston in perfect condition, hard to find these days.




The complete set with power supply:




The amplifier is transformer coupled with Tango transformers. Balanced to SE conversion from the XLR inputs is done through Lundahl input transformers.




The power supply uses 4 TV Damper tubes in a full wave bridge arrangement. A trusted concept which is used in most of my amps and preamps.




Multiple choke / capacitor filter stages with oil caps throughout. No electrolytic caps or silicon in this amplifier.




Best regards

Thomas




18 comments:

  1. Where do you get your meters? Those are GREAT!!! I also like the landscape orientation.

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    1. I collected some over the last 20 years. Didn't buy any since at least 5 years. Those were pretty much the nicest I have.

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    2. I have a collection of about 50 but those are beautiful!!

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  2. I miss the days you posted schematics of some of the designs you were doing.

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    1. There are so many schematics published on this blog, what are you missing? I am not the type who changes things every 6 months. Many of my amps are using the same concepts since years since they are proven and work. And another important point: It is difficult to earn enough to live if you give away everything for free ;-) Use the schematics on my blog as a starting point and develop your own concepts from that

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    2. Very nice looking amps! Thomas, you have probably covered this before, but you can tell us why you prefer to use a separate chassis for the power supplies as opposed to monoblocks with each amp on its own chassis? Microphonics maybe? Thanks.

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    3. Thanks! The decision about the split always depends on size and budget constraints. Of course there are different philosophies and opinions about this, but for me if budget and available size are limited, a separate power supply and stereo amp is the better choice. In this case monos blocks would require two more power transformers (B+ and filament) and two more chokes (the B+ has a LCLC filter inside). This disadvantage is the need for a umbilical. If you look at my recent 46/46 amps there I have built monos. The chassis of those allowed for the extra space needed.
      In any case I prefer a good power supply with healthy transformers shared between the channels over monos with wimsy PSUs.

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  3. Did you mean bias current or plate current for the metesr?

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    1. Yes plate current, but for me thats the same

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  4. Are the panel meter's frame/body plastic or metal based? Thanks

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  5. Hi Thomas.. What made you decide on AC filaments for this build? You usually go with well filtered choke input DC. Is it a customer request? BTW, the Westons look superb. Expensive even back in the day. I only have one large Weston AC meter for my vintage variac.

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    1. I build amps to customers size and budget constraints which did not allow for DC filaments ion this case

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    2. Hi Rick,

      I'm the actual owner of this amp. My intention with this project was to have flexibility to use the 46 tubes (w/ adapter) as well as 45/2A3 via switch. DC filaments would not work in this scenario so I opted to stick with AC filaments.

      Kevin

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  6. I see balanced inputs on this case, in the futture I will try this option with a pair of Lundahls LL7903, what input ratio did you use in the present ptoject?

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    1. For this project 1:1 was requested by the customer

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  7. Hi ,

    I’m the owner of this amp and wanted to say that this is a very special amp. I’m currently using this with a 46 tube via adaptor and driving this using the 6AH4 preamp ( also built by Thomas) and the sound is just breathtaking. It easily beat another previous 46 amp built by a pretty respectable DIY builder here in the US. The fact that I can also use 45 and 2A3 tubes is just icing on the cake. Those Tango interstage and output transformers are something special. I want to thank Thomas for making my audiophile dreams come true. This will be a lifetime investment that I will never sell.

    Kevin

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    1. I also want to add that my journey to this point involved a lot of heartaches and disappointments along the way but its one where I've also learned a lot about the industry and what designs/designers are worth investing your money into. I arrived at Thomas after concluding that he is one of (if not the greatest) practitioners of transformer coupling in the world which I felt was essential for my dream final system. And so far, my amp and preamp which are more works of art then your regular stereo hasn't disappointed at all. One of the greatest audio minds/tube engineer of the last quarter century so take advantage while Thomas still has the love and desire to do what he does. Perhaps the greatest decision I've made in this hobby and one which will anchor my system for eternity.

      Kevin

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