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Old 4th September 2008, 12:25 AM   #71
Mike L. is offline Mike L.  United States
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Location: Upstate New York
Thanks once again. From the data sheet you so kindly provided, it looks like a 1N5305 is a suitable substitute. Now I can start collecting all the necessary bits and have a go at building it once the weather turns. (With the price of fuel, I may be spending a lot of time huddled around the amplifier this winter.)
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Old 6th September 2008, 04:46 PM   #72
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Location: Medford, MA
Have you seen this:


811A A2 simple amp

This is the same ckt topology as post #5 with substantially less
complexity and I will argue better performance.

My gripes with post #5 are as follows:
He is forced to use a semiconductor B+ regulator on his driver
stage because he uses a variable-resistor divider network to bias
the EL84 cathode follower at the grid! If, in fact, he used a choke
or resistor to bias the cathode of the EL84, he'd not need this complexity.

He's using feedback from the 811A plate to the 12AX7 cathode.
Gaaaah !!!!

His operating point is too low. At lower B+, the 811A grid is going to produce GOBS of grid current. He's biased the 811A hot at 33V.
The result is you'll get a lower Ra, but also less power and instead
have to dissipate alot of current from the 811A grid. Keep in mind
that grid current is not useable for audio power - it's simply a sunk
cost!

He's using DC filaments but no common mode chokes! I'd not
even bother and use straight AC with a humbucking pot in the
filament circuit to null out hum. If you're going to do DC filaments, do them right - use CMC's. Heck - you can get CMC's in the 6A range upwards of hundred of mH in the Digikey catalogue.

You're better off with slightly higher B+ (400V or alittle more). In
this regime, you'll get MORE plate current and LESS grid current.
You'll get more power. Your driver stage will be alot cleaner.

With my design, I run at 430V, which is a typical, attainable B+ for the kinds of off-the-shelf transformers for guitar amps or the Hammond catalog. The grid is happy at 22-24V (you can go higher if you want). You can use many different tubes on the cathode follower: 6{v,w,y,k}6GT, 6CL6, EL34, 6L6... Finally, since we need to swing 40V, you can use something from the 6SN7 family
as your gain tube. That includes stuff like the 5687, the 6CG7,
12BH7 and all that jazz. Since the gain stage only needs to
develop 40V, the amp is now 2Vp-p input sensitive. This means this
design DOESNT NEED a preamp - you can simply place a 100K stereo attenuator pot on the amp and drive this directly from a D/A converter.


-- Jim
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Old 11th September 2008, 08:59 PM   #73
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Quote:
Originally posted by tubelab.com
When I have the time to explore this further I will probably resume the sand state experiments, since I have that engineering mentality about choosing the best component for the job.

For those that want to follow the pure tube path, we need to find the schematic for the old Tektronix 504 scope. It used a power supply that derived all of the operating voltages off of a power oscillator operating at an ultrasonic frequency. If my fuzzy memory is correct it used a 6DQ6A. It's been 25 years since I worked on these.

Anyone out there got the diagram?
George,

I've got a soft copy of the full manual with schematics, dated 1962. The other tube in the supply is a 6BL8. Can't email it though, 'cause it's almost 20MB. Ping me offline.

-- josť k.
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Old 12th September 2008, 07:42 PM   #74
Mike L. is offline Mike L.  United States
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Jim,

Thanks very much for your post. I printed out the schematic and will give it a try during the cold months ahead.

I assume you have built this circuit. How would you characterize the sound? Also, the 811A's I have have a radioactivity warning label on the boxes. They were made in 1962. Do these things contain a radioactive isotope or do they generate x-rays in use?

Mike L.
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Old 12th September 2008, 08:20 PM   #75
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Quote:
Originally posted by Mike L.
Also, the 811A's I have have a radioactivity warning label on the boxes. They were made in 1962. Do these things contain a radioactive isotope or do they generate x-rays in use?
811s have W/Th filaments, and thorium is indeed radioactive. There really shouldn't be enough thorium in there for this to be a major concern. So far as X-Rays are concerned, the usual operating voltages aren't high enough for this to be a problem.
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Old 14th September 2008, 06:39 PM   #76
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Oh yes, I've built it and it sounds amazing. I was even surprised
at how little I can hear the background AC hum on 98dB eff
speakers. This amp has alot of power (10W) and can drive many
kinds of speakers - not just limited to horns or Klipschen.

As far as radioactivity, the filament is coated with trace amounts of Thorium which enhances the metallurgy and reliability of
the tungsten at hi temp. Natural thorium is weak alpha emitter,
similar to what's in your smoke detector. Thorium is also used
in Coleman lantern mantles, as a catalyst. All DHTs use thorium:
845s, 211s, 572 and the big RF tubes.

The radioactivity labelling on the boxes is more for the benefit
of military cargo handlers.... They like to know if cargo is supposed to be hot so that they dont have to worry if something
(1) got contaminated or (2) a stray nuclear weapons component
has somehow gotten into the regular parts depot shipment for the Signal Corps.

If you follow the news, last year, Minuteman ICBM nosecone parts were accidentally shipped to the Taiwanese army.. They
Taiwanese had ordered batteries for a helicopter, and due to mixups in warehouse, got something for more interesting to play with!



-- Jim
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Old 16th September 2008, 06:48 AM   #77
minhaj is offline minhaj  Bangladesh
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Jim,

I follow your work and I like it. I am thinking of building your 811a circuit. I am unclear about PS though. At idle what load current I should consider while modeling the PS?

I undestand 811/811A ra is high, how 5K OPT behabes then! I do not want the input tube because I will use my AIKIDO pre so how I can go for some feedback If I want to....!!

Pls need your help.

Regards
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Old 20th September 2008, 07:29 AM   #78
GK is offline GK  Australia
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Quote:
Originally posted by Miles Prower


811s have W/Th filaments, and thorium is indeed radioactive. There really shouldn't be enough thorium in there for this to be a major concern. So far as X-Rays are concerned, the usual operating voltages aren't high enough for this to be a problem.


I doubt that the radioactivity from such a tube is very high at all. It would only be an issue if the tube was broken and there was a possibility of ingesting / inhaling the stuff.
Iím a licensed handler and use quite potent radioactive sources (ones that would inflict amputation-requiring burns with a several hours of body contact) at work.

A guy where I work once drilled into a sealed source of several GBq in the lathe and sprayed caesium salts all over the place. I donít have any 811ís at the moment, but I could measure an 813 or two and some other tubes with the calibrated equipment I have access to if anyone is interested.

In fact, Iíd be interested my self just to see how easily (or otherwise) any radiation from such tubes can be detected.
I have a collection of old WWII military radios and Iíve been quite surprised at just how hot the glow in the dark radium paint on a few of the various dials was.

Hmmm.....
Something intersting to do on mondays lunch break.

Cheers,
Glen
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