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Old 19th January 2013, 04:27 PM   #1
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Default schematic question? 805 feedback

Looking at the feedback to the 805 cathode, it goes to a 100ohm resistor. Is this a funny way of saying 50ohm on each side?

Has anyone here tried this type of feedback on a large triode? Was it with good results? How will this type of feedback effect impedance?

Last edited by scutterflux; 19th January 2013 at 05:54 PM.
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Old 19th January 2013, 04:31 PM   #2
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It would be nice to actually have the schemo in question.
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Old 19th January 2013, 05:51 PM   #3
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lol haha, ya,
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Old 19th January 2013, 06:15 PM   #4
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I want to implement that feedback in this circuit, so I would remove the ground and 1ohm then run the correct phase off the transformer as opposed to ground. The circut in question shows 135mA so I suppose I'd want to aim aprox. for that if I need to (I'm assuming this is a current feedback method), give or take, use a couple variable resistor to tune it in.

Also is my logic flawed??? I'm more on the experemental end of things, only knowing some theory and poor at calculations even though I'm studying Geophysics... I get by on thoery and really want to know amplifiers better.
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Old 19th January 2013, 09:49 PM   #5
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Quote:
Originally Posted by scutterflux View Post
Looking at the feedback to the 805 cathode, it goes to a 100ohm resistor. Is this a funny way of saying 50ohm on each side?
Pretty much. Since it's a floating DC supply, you don't really need a 100R variable in that position. That's used with AC heating for hum balancing.

Quote:
Has anyone here tried this type of feedback on a large triode? Was it with good results? How will this type of feedback effect impedance?
It's just your basic lNFB: cathode feedback. The 805 is a fairly high-u tube with a highish plate resistance, so lNFB will help get the effective plate resistance down for better woofer damping.

I've used lNFB, though not cathode, with pentode 807s with good results in cleaning up the pentode nastiness.
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Old 19th January 2013, 11:09 PM   #6
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What kind of feedback did you use or find is best?

I'd like to keep it a short loop after realising the improvement in resolution I had with removing gnfb from the circuit.

I questioned running off the anode of the 805 but to where and how to implement it is hard to find online?

Any help or suggestions are greatly appreciated.

I can post my schematics if you like.
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Old 20th January 2013, 01:51 AM   #7
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Quote:
Originally Posted by scutterflux View Post
What kind of feedback did you use or find is best?

I'd like to keep it a short loop after realising the improvement in resolution I had with removing gnfb from the circuit.
With 807s, I used parallel lNFB, from the plates back to the second voltage amp. It's a bit more flexible in that you can choose your feedback. With cathode feedback, you take what you get.

Quote:
I questioned running off the anode of the 805 but to where and how to implement it is hard to find online?

Any help or suggestions are greatly appreciated.

I can post my schematics if you like.
Taking parallel lNFB from the anodes of the 805s is going to be a bit more problematic than with 807s. 805s can run up to 1.0KV+, whereas 807s run ~365V. Even that requires at least 600V coupling capacitors since the AC is imposed on the DC, and rises well above the DC rail. With 805s, you're pretty much stuck with cathode feedback, either taken off the secondary or a special tertiary winding on the OPT for that purpose.
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Old 20th January 2013, 02:59 AM   #8
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Shisedo uses 750-800 volts for 40-50 watts using 805's. Why would you want to use 1000volts?
Shisedo uses 750-800 volts so you can draw a lot of mills without overloading the 805.
Whats wrong with cathode feedback?
Phil
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Old 20th January 2013, 01:43 PM   #9
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by scutterflux View Post
Looking at the feedback to the 805 cathode, it goes to a 100ohm resistor. Is this a funny way of saying 50ohm on each side?

Has anyone here tried this type of feedback on a large triode? Was it with good results? How will this type of feedback effect impedance?
I have an 838 SE amp which is almost same characteristics of 805 except for plate dissipation. Cathord feed back circuit is suited for this kind of high myu triode as the cathord feed back changes characteristics of tube from 838 to 845/ 211.

838 35W SE AMP 2006
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Old 20th January 2013, 06:44 PM   #10
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Nothing wrong with cathode feedback that I know of, just keeping my options open and trying to learn of the various ways. Is there anything you could think of that would be problematic with it? I'm all ears.

So it seems I will have to reverse phase of the transformer.
Then I suppose it really wouldn't matter what tap I run off of? (probaly 8ohm???)
I could then simply run feedback to the cathode and the two 47ohm resistors automaticaly attenuate the feedback?
I'll also have to lift the ground.

Off hand if anyone could tell me if I'm wrong about the above statments I'd sure appreciate it.

I may try this afternoon.
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