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Old 18th March 2012, 11:37 AM   #11
DF96 is offline DF96  England
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Quote:
Originally Posted by scitizen17
The very minimal feedback clearly restores the low frequency component.
Feedback operates below about 8Hz, so will do little but shift LF phase up to about 80Hz. I can't see the point in this, apart from perhaps making up for the lack of any LF rolloff at the input of the amp.
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Old 18th March 2012, 11:59 AM   #12
SY is offline SY  United States
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Quote:
Originally Posted by scitizen17 View Post
Nothing forgotten. The very minimal feedback clearly restores the low frequency component. Nothing more needed or intended.
Then you don't need the compensation network or the cap across the feedback resistor. All of those components are there in a normal design to ensure closed-loop HF stability.

You might add some reactance to your dummy load (or use a speaker) to see what happens to those square waves.
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Old 18th March 2012, 01:02 PM   #13
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Thanks all. That bypass cap is a problem. I see that now. I'll clip out the cap and try a few modifications today.
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Old 18th March 2012, 01:10 PM   #14
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Originally Posted by Globulator View Post
What are the diodes in series with the tube rectifier for?
I'm running the 5AR4 pretty close to max here. It has been recommended by several folks here to add the diodes for prevention of arcing in the rectifier at turn-on. This is a common mod over on the Dynaco forum.
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Old 18th March 2012, 01:28 PM   #15
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With the SS-diodes in place you could tie the anodes of the 5AR4 together. That would halve the voltage drop over the 5AR4 (both rectifiers in the 5AR4 conducting every cycle).
Just a thought..

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Old 18th March 2012, 01:33 PM   #16
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Originally Posted by Wavebourn View Post
Looks like he forgot to draw one more resistor in series.
Can I lift the ground end of the RC cathode circuit, insert say a 100 ohm resistor to ground, and then apply the feedback at the junction of the RC circuit and the 100 ohm resistor?
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Old 18th March 2012, 01:49 PM   #17
DF96 is offline DF96  England
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Yes. You would of course have to change the value of the feedback resistor so you get whatever feedback you thought you had before.
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Old 18th March 2012, 01:54 PM   #18
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Originally Posted by artosalo View Post
The grid voltage of cathodyne ( 65 V ) is not optimum. Should be some 80...85 V when Ub = 300 V.
When I calculated the load line for the 6922EH, 70V across the plate and cathode resistors indicated a point for equal voltage swing across the inverter. If I change the grid voltage wouldn't I also have to change the plate and cathode resistor value?
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Old 18th March 2012, 01:58 PM   #19
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Originally Posted by DF96 View Post
Yes. You would of course have to change the value of the feedback resistor so you get whatever feedback you thought you had before.
Can you recommend a starting point for the value? I don't believe I need a large amount of feedback. The amp sounds great just like it is. Maybe no feedback at all would be better?
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Old 18th March 2012, 02:06 PM   #20
DF96 is offline DF96  England
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You may need some feedback to reduce output impedance. Otherwise you are likely to get a boost at the speaker bass resonance. Feedback should be designed, not merely plastered on at the end.
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