• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

812A Build

Ok, I've finally gotten around to looking into upgrading my current home amp. Right now I have a 6146SE (6C4 driver) and plan to move/swap/upgrade to some 812As driven by 6SN7s. Here's the circuit I'm going to use.

I've already scratch-built it and it sounds great (Fostex 166ENs in a BIB)

[IMGHTTPDEAD]http://168.8.96.77/~pheaton/812A-audio-amp.jpg[/IMGHTTPDEAD]
 
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The scratch-built circuit. 80.7mA plate current.

I build on these copper-clad boards that I would take from trash cans when I worked for a defense contractor.

[IMGHTTPDEAD]http://168.8.96.77/~pheaton/812A-scratch-built.JPG[/IMGHTTPDEAD]

The power transformer I'm going to use is from a Kenwood TS-520. Nice nice. Its been running for a few hours and the core is barely warm.
 
Ok, I've finally gotten around to looking into upgrading my current home amp. Right now I have a 6146SE (6C4 driver) and plan to move/swap/upgrade to some 812As driven by 6SN7s. Here's the circuit I'm going to use.

I once thought about doing a SET with 812's, but couldn't locate a decent OPT. Based on that preliminary design, there seems to be something funky about that schemo. Zero bias and capacitor coupled? You can bias an RF amp like that, but it doesn't work for audio.

How do you get enough plate current without positive grid bias? I had to have Vgk= +5Vdc to pull the necessary Q-Point bias. (Loadline, attached) It is, after all, a rather high-u VT, even if not nearly so high-u as the similarly rated 811A.
 

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G'morn. Thanks for the comments. To be truthful, I don't know why this thing is working as well as it is! I was looking into biasing the grid and the darn thing is playing very well and is stable between different tubes (both in the drive and final stages).

I think I ran it about four hours yesterday with various types of music (that we listen to) and I'm amazed at how well it sounds and that was using a power transformer (Pri 125V / Sec 16V) that I nabbed from a bad UPS power supply as the OPT! Like I said, it is scratch-built right now. I measured the impedance of that thing once and it was around 1600 ohms @ 1kHz. Inefficient as heck though. haha

I like the idea of the FET follower in case of disaster.

I've never worked with an 812A before. It seems a bit temperamental compared to its cousin the 811A.
 

45

Member
2008-12-18 2:29 am
UK
One thing, as the cathode rises above ground, then the grid goes sinking below ground.

This config is putting in the neighborhood of -5V on the grid. The plate is barely glowing with no room lights on and it sounds really sweet.

But you are basically wasting more than 90 VA to get 0.5-1W in the best case?? Try to crank up the volume and see what happens....
 
But you are basically wasting more than 90 VA to get 0.5-1W in the best case?? Try to crank up the volume and see what happens....

Yes, it does get a tad nasty when driven hard. :)

I'm gonna add the C follower to the dummy test and see how it does.

I saw another circuit on here for an 811A that was basically a 6SN7, into a 6V6 (CF), then into the 811A. I may look into that config as well.

Here it is
 
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Yes, it does get a tad nasty when driven hard. :)

I'm gonna add the C follower to the dummy test and see how it does.

I saw another circuit on here for an 811A that was basically a 6SN7, into a 6V6 (CF), then into the 811A. I may look into that config as well.

Here it is

You have to look at the plate curves first. The 811A and 812A bias up at different points. Just use a FET follower and you will be able to drive the grid anywhere you want it.
 

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