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Old 8th April 2005, 03:01 PM   #11
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Default 811

I have a book here that has all the valve type calculations I will
find the feedback calculation for you

I had to do this on my 807 push pull amp thats a beast of a thing !! the feedback was set for a 600 ohm out put inpedence
where as the out put tran I was using is at 8ohms so I had to change the feedback resistor I think it ended at about 2k

You are right it is better to calculate it rather then use the feed back of another circuit then you know that it is right.

It is not hard math really I will have a look



John807
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Old 8th April 2005, 03:20 PM   #12
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Default 811 feedback

Done it worked it out for you !!

you need a 1k8 resistor coming from your out put transformer

then you take the ground of your pre amp valve's cathode bias cap and resistor not the driver because it is copuled to the 811
solder the 1k8 to it and a 100ohem resistor to ground dead simple!!!
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Old 8th April 2005, 05:25 PM   #13
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Thanks. I'll try it this weekend and take some measurements. At the time I didn't have a scope.


Adam
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Old 11th April 2005, 12:44 AM   #14
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Quote:
Originally posted by astouffer
I kept all the resistor values the same but my plate supply was a different voltage. Not sure what it was (my parents use the amp at their house) maybe 280. 811 plate voltage is around 700 and the grid is around +14. The output transformers are 6.5k Hammonds.

Now that the amp is built I would like to add some feedback to improve the sound a bit. Anyone know the best way to go about it? Add a 100 ohm resistor to the 1k coming off the bottom cathode and make that the feedback point? Calculating this is beyond my skill.


Adam
Ack! +14 on the grid?

Seems a bit high... how much current at idle are you drawing?
It's a zero bias class B tube, iirc... are you trying to run in Class A??

Anyway, check out the Altec 1570b schematic. Removing the feedback network's cap and a few other minor mods, like a better output transformer and it is a very nice amp design...

_-_-bear

Oh... wait a sec, single ended 811? Hmmm... I'd expect that the B+ needs to be dropped some to keep the quiescent current down?

The two schematics show <500v and ~300v respectively...
interesting idea to run the cathode of the driver to the grid of the 811 with no ground reference at all...
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Old 11th April 2005, 03:43 AM   #15
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Just took some measurements last night. The 811 grid was +18 and exceeding the PD. I added a 150 ohm 5w resistor from the wiper on the hum pot to ground and this dropped the PD down to 40 watts. I must have forgotten that 65 watts PD is intermittent only, 45 is continuous.

The grid must have positive bias otherwise there would be no current draw. Its the reverse of most other tubes. There is also a resistor going bad. It causes some static after a while and seems to stop the second I reach for the meter. I hope it dies a firey death


Adam
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Old 15th August 2005, 12:18 AM   #16
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Default Those 811's dont have draw current on 6.3 !!!!

Hi All
The Amp is now all together as far as drilling is. heters use a battery charger transformer !! works but gets a bit hot !
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Old 15th August 2005, 11:01 AM   #17
jim is offline jim  Belgium
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Hi,

I once build the schematic in "astouffer's" (post #5).
Sounded very nice although a bit sharp at the top.
5 clean watts.

For a collection of 811a schematics :
my page

Greetz,
Jim
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Old 15th August 2005, 12:30 PM   #18
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Default Interesting

A very interesting circuit these are the ones that i am going to use to build doing so right now infact on the bench are you a radio ham ?

Jonathan Kelly
MW3KGQ
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Old 16th August 2005, 03:35 AM   #19
amperex is offline amperex  United States
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Default 811A or perhaps an 812 Tube

811A are zero bias tubes at 1200 volts or so. I would believe at a low 450 to 700 volts, the tubes would require a positive bias. How about an 812 tube that would perform with negative bias & low voltage?
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Old 16th August 2005, 03:48 AM   #20
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How about an 812 tube that would perform with negative bias & low voltage?

There may be an availability problem with the 812. It wasn't nearly as popular with hams as the 811A, having the same P(D) ratings, but less gain and needing more bias. Easy to scrounge up 811A's at "ham-ventions", 812's:
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