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Old 25th March 2010, 10:00 PM   #121
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You are correct Salas. I can switch that. The potentiometer will be removed for fixed resistance once the actual value is established.

Did I get the phasing correct?
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Old 25th March 2010, 11:42 PM   #122
Salas is offline Salas  Greece
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Wasn't following from the start, but isn't the cathode coupled phase splitter connected to the wrong output valves? Like the first half must feed the lower KT88 when its reversed now? Look at this example.
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Old 26th March 2010, 12:24 AM   #123
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The FB signal applied to the voltage amplifier cathode must be in phase with the signal applied to the voltage amplifier grid. If the signal at the 8 Ω speaker tap of the O/P trafo is out of phase with the I/P signal, exchange either the O/P trafo primary wires or the lines coming from the phase splitter, but not both.
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Old 26th March 2010, 12:51 AM   #124
chrish is offline chrish  Australia
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Sorry, hope this is not too off topic, regarding a decent test tone generator, a cheap one can be had with this kit Altronics - Your One Stop Audio Visual & Electronics Supplier it is based on the design by Rod Elliot of Elliot Sound Products (I use his active crossover boards) Miniature Audio Oscillator

Altronics is an Australian electronics kit/parts distributor so don't know if they ship overseas. I have two of these, they are very cheap and produce a nice clean square wave and sine wave. The claimed output of 41Hz-18kHz was designed to stop speaker damage and can be changed with a few minor component changes.

Regarding the feedback, I use the dumb method. Wire it up leaving the feedback disconnected at one end. On the disconnected end, use a wire with alligator clips or similar. With the scope connected to the output and a signal at the input, connect the feedback circuit with the alligator clip. If the amplitude increases, your feedback is positive, if the amplitude decreases, the feedback is negative. If the feedback is negative, all is OK. If it is positive feedback, correct using the method Eli describes above.

Good luck with the project!

Chris
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Old 26th March 2010, 12:53 AM   #125
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Salas, Attached is the original Mullard 5-20 diagram. I think it is a matter of symantics as to how it is drawn in this case as long as they are in phase. I put a signal generator symbol with a positive wave form to indicate phase on the input and output.

I have come to the conclusion that my second channel on the osc scope is messed up worse than originally thought. Now I am not sure if I have the signal in phase or not. I ran a test signal through both channels, one instant they are in phase the next they are not. Thought I was seeing things so asked the wife to look and she said they looked the same???????? Oh Well. Good excuse to go to ebay, have a "new to me" scope coming next week, a Tek 2430A. I think that will do well for most analog stuff.

I am going to assume that I had it hooked up for positive feedback since when I lower the feedback resistor value, the gain increases.

To get NFB working, I need to identify where I am losing stability, something still is not quite right on the amp in regard to phasing and noise. With a 100k resistor for feedback hooked the other direction it can't even power it up without oscillating. (I don't feel like losing tubes)

My plans are to
1) Clean up my mess (Wife's Orders)
2) Install ferrite tomorrow
3) Install 2SK3563's instead of IRFBC20
4) Install "Wire Wound" anode load resistors for the LTP. MOX that are in now are VERY noisy.
5) Test each stage for gain, phase, and distortion.
6) Fix any problems and then try NFB again with certainty of phase.
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Old 26th March 2010, 12:59 AM   #126
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Sounds like a plan.
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Old 26th March 2010, 01:12 AM   #127
chrish is offline chrish  Australia
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Looks like you may have been typing while I was answering. The empirical method to determine feedback mentioned above only needs one channel of the scope and you don't have to think about phase.

On the other hand, if you need an excuse to justify to SWMBO the purchase of a new toy, I will play along

Cheers!

Chris
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Old 26th March 2010, 01:12 AM   #128
taj is offline taj
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Quote:
Originally Posted by SGregory View Post
My plans are to
1) Clean up my mess (Wife's Orders)
2) Install ferrite tomorrow
3) Install 2SK3563's instead of IRFBC20
4) Install "Wire Wound" anode load resistors for the LTP. MOX that are in now are VERY noisy.
5) Test each stage for gain, phase, and distortion.
6) Fix any problems and then try NFB again with certainty of phase.
Wow. It's almost the same as my list. Only the last 5 items are different.

..Todd
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Old 26th March 2010, 01:18 AM   #129
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Chrish, Please play along.....

taj, Me thinks I will be on #1 for a while...
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Old 26th March 2010, 01:24 AM   #130
Salas is offline Salas  Greece
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Do your transformers have some guide about their cable colors? I think that those old schematics were drawn differently on the phase splitter because their transformers were wired differently.
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