811a SET amp.

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Nick
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#256 Re: 811a SET amp.

Post by Nick »

Well, the old C4S would be a good example.
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#257 Re: 811a SET amp.

Post by IslandPink »

Looks like a CCS :D
...Constructed as a cascode of the DN2540 ( depletion-mode Mosfet ) devices.
Last I heard, the IXYS-based CCS (board) from K&K was regarded as a step up from the DN2540 ones, and easier to build as the device(s) are less sensitive to static than the 2540's.
It used to be the IXTP01N100D in Kevin Carter's circuit, that may have changed.
Obviously you can build the circuit on a board yourself.
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#258 Re: 811a SET amp.

Post by pre65 »

I've officially started to collect the bits I need for the 811A rebuild.

Now got two UX5 Amphenol British made bases on the way.(for the 76 triode)

I'm hoping to gut the old version and start again, it will need a new aluminium to plate at the very least.
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#259 Re: 811a SET amp.

Post by Cressy Snr »

pre65 wrote: Wed Nov 15, 2017 3:22 pm it will need a new aluminium to plate at the very least.
Thought you could only anodise aluminium :wink:

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pre65
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#260 Re: 811a SET amp.

Post by pre65 »

Yes Steve, should have read "top plate". :)

On this new amp there is grid bias, which is new to me on a power valve.

The circuit shows the 811a with a 50R 5W hum pot across the filament, and also two 30R 3W resistors to the hum pot output to ground (via 1R sensing resistor).

As I'm using Tent Lab heater modules (845 type) do I really need a hum pot as well ?
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Paul Barker
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#261 Re: 811a SET amp.

Post by Paul Barker »

pre65 wrote: Thu Dec 01, 2016 1:20 pm I was looking for Arabian Nicks capacitor list on the old computer, when I came across a circuit for 811a in A2.

Now, I never really got my version working as I would like so I studied this circuit with interest. It uses a 56 and a "powerdrive" mosfet to drive the 811a.

56 seem quite rare, but it seems 76 is the same but 6.3v filaments, and I do have a pair of 76 in the valve drawer. So, another project waiting in the wings. :wink:

I'll add a picture of the circuit soon. Can't seem to log into Photobucket at the moment. :(
I like 56/76 better sounding than 1/2 6sn7 less gain. More open, less falsly creamy, but if the mosfet drive wants some tone adjustment 6sn7 might be better for you. mesch plate 26 an option, may sound better still, but-a nat,s forskin in it. The mosfet would probably allow the valve character to show through, but I would use a dc cfas Im not into all the mosfet follies going o around here.
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pre65
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#262 Re: 811a SET amp.

Post by pre65 »

Thanks Paul, I value your thoughts.
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#263 Re: 811a SET amp.

Post by IslandPink »

Is there a circuit somewhere still on this thread, or has it fallen down the photobucket black hole ?
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#264 Re: 811a SET amp.

Post by pre65 »

See if this works. If you click on the second image it should show up the right way round.
Scan.jpg
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Scan.jpg
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andrew Ivimey
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#265 Re: 811a SET amp.

Post by andrew Ivimey »

If you get the 76 up and running you could also try the 37...even more primitive. I have a few, phew...
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#266 Re: 811a SET amp.

Post by pre65 »

As the 76 is 6.3v and indirectly heated I'll use AC heating to start with.

I can't remember why I purchased the 76 but I'm glad to find a use for them.

The amp PSU will be my dual mono supply, so that is already taken care of. :)
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#267 Re: 811a SET amp.

Post by IslandPink »

Thanks for circuit.
76 is a good valve - cylindrical anode - very good tone.
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#268 Re: 811a SET amp.

Post by IslandPink »

pre65 wrote: Thu Nov 16, 2017 5:43 pm See if this works. If you click on the second image it should show up the right way round.
It didn't, so try this :
Scan1.jpg
I don't think you need the cathode by-pass cap on the driver valve, since you have a very high resistance ( CCS ) anode load, and a mosfet to add 'beef' to the drive into the 811.
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#269 Re: 811a SET amp.

Post by Nick »

I would be tempted to argue that the cascade CCS is a bit questionable as the minimum load the the driver valve sees will always be in parallel with 200k, so always 200k or less.
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#270 Re: 811a SET amp.

Post by IslandPink »

Well spotted, but isn't it then a bit like a Gm amp ? ( separating current-setting and loading )
The CCS can set the 76 at a decent current ( 8 or 10ma ) so it's in a very linear region. With effective 200k load you can probably still avoid the cathode bypass cap.
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