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Finished Another DIY Build - 801A SET in A2 Operation

L0rdGwyn

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Just showing off a build I finished today, idea for this amplifier started over a year ago, it has been a long development...

At a high level, it is a 6F5 input with CCS load, cap coupled to a 6BX7 cathode follower, which is direct-coupled to the grid of the 801A output. The 801A is biased at near 0V on the grid, so the grid swings both positive and negative, A2 operation with the grid drawing current, provided by the cathode follower (traditional SET designs are A1 operation only, typically, without grid current). There is a modest amount of negative feedback from the plate of the 801A to the cathode of the 6F5, it is buffered by a PMOS. NFB is necessary to get an acceptable output impedance from the 801A, which has a relatively high plate resistance.

The mains and output transformers were custom-made for this design by Sowter UK. I was going for a "vintage" look, something like an old Hickok TV-7D/U tester. Sockets are vintage Amphenol, knob is vintage Kurz-Kasch. The chassis was designed by me in CAD software, then machined and powder coated by Dave at Landfall Systems, he does all of my DIY chassis, great guy and highly recommended.

This is a 7W into 8ohm amplifier, around 0.3% THD at 1W output, output impedance 1.7ohm.

Here is a schematic of the amplifier circuit. The power supply is pretty complex, so no simple schematic to post, but features one Maida regulator per channel in the B+ supply, one TL431 shunt regulator per channel in the B- supply, and Rod Coleman filament regulators.

801A_SET_Final (1).png


Here is the circuit.

PXL_20210522_172442612 (1).jpg


And the amplifier itself.

PXL_20210522_183301863.jpg


PXL_20210522_183314667.jpg


PXL_20210522_183322334.jpg


PXL_20210522_183334690.jpg


This powers my restored Snell J/II. Of the amplifiers I have built, this one has by far the best sound! Incredible bass dynamics, clarity and staging. Really makes the speakers disappear.

Thanks for checking it out!
 
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L0rdGwyn

L0rdGwyn

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I did one last thing - sleeved the 6F5 grid cap leads with paracord today, gives the amp a little pizzazz.

PXL_20210523_191300850-2.jpg


Here how it looks with some graphite plate RCA 801A.

PXL_20210523_191213763-2.jpg
 

Jim Matthews

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Snazzy.

8 watts pushing 90 dB/watt sensitivity?
 
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L0rdGwyn

L0rdGwyn

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7W into 8ohms pushing 92dB/W sensitivity. Louder than I would ever listen at 1o'clock position ;)
 

Jim Matthews

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Corner loaded, I suppose?

For those of our gentle readers that haven't built things with wire, this is very tidy with beautiful solder joints (and there are many).

Kudos
 
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L0rdGwyn

L0rdGwyn

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Not exactly corner loaded, in an ideal world they would be, but my listening room is...my living room, so spouse-acceptance-factor plays a large role in what I can and cannot do, HA! Fortunately my tube gear is acceptable, and dare I say it, welcome :)
 

horias2000

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Beautiful build. I'm now working on building a tube amp myself. First one for me. I've built plenty of solid state ones but no tube amps so far. I stumbled upon the amplifier built by Bob Cordell, using the Kt88 in a push pull configuration. I'm at the stage of simulating the circuit in ltspice and I was wondering where did you get the tube spice models? I found a library online, called ZZZ. Like all things online, you can't really be sure of the quality. And I'm also looking at buying the output and power transformers and I'm in a bit of a pickle. I also started a thread on the forum but with no answers yet. Should I go for toroidal output transformers or the classical one like you used? I can find very good toroidal output transformers at the same price of the classical ones. The company is called Toroidy and I bought several power transformers from them for my other projects. The quality is very good. I'm not sure if there are any downsides to using a toroidal instead of the EI type. Looking at the specs of the toroidal, they specify a very wide working frequency, so that is good.
 
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L0rdGwyn

L0rdGwyn

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@horias2000 below attached is the Ayumi Spice model library, there are KT88 tetrode and triode models. This is probably the most widely used library of tube Spice models out there.

You can find the library via Google search, but here is a diyAudio thread where members share Spice models: https://www.diyaudio.com/community/threads/vacuum-tube-spice-models.243950/

I haven't used toroidal output transformers personally, a friend of mine however is planning to use them in an upcoming single-ended project, I see no reason not to use them if they meet your specifications. Also you'd be better served asking questions on diyAudio if you have an account there.
 

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L0rdGwyn

L0rdGwyn

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I am in the opposite position - I only build tube amps, but I have the parts to build a solid state amplifier, although I don't know when I'll do it :) I have a nice set of THF-51S VFETs sitting waiting to be used.
 

horias2000

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@horias2000 below attached is the Ayumi Spice model library, there are KT88 tetrode and triode models. This is probably the most widely used library of tube Spice models out there.

You can find the library via Google search, but here is a diyAudio thread where members share Spice models: https://www.diyaudio.com/community/threads/vacuum-tube-spice-models.243950/

I haven't used toroidal output transformers personally, a friend of mine however is planning to use them in an upcoming single-ended project, I see no reason not to use them if they meet your specifications. Also you'd be better served asking questions on diyAudio if you have an account there.
wow, thanks for the library. I will most probably post on the diyAudio forum as well.
 

tomchr

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Very nice build. Neat trick with the para cord too.

What are the HV connectors on the wires for the anode caps? They look like RCA connectors, but I gotta believe they're not. :)

Tom
 
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L0rdGwyn

L0rdGwyn

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Very nice build. Neat trick with the para cord too.

What are the HV connectors on the wires for the anode caps? They look like RCA connectors, but I gotta believe they're not. :)

Tom

Thanks! Fear not, they are grid caps with RCA connectors :)
 
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