There is not big difference between 6.6K and 5K in case of UL-connected amplifier.
Secondly, when you connect speakers to your amplifier, it will see load impedance varying, say from 3k to 15k in worst case and this is also containing reactance.
Especially this is the reason why a GOOD amplifier must háve negative feedback to handle such impedance range.
Secondly, when you connect speakers to your amplifier, it will see load impedance varying, say from 3k to 15k in worst case and this is also containing reactance.
Especially this is the reason why a GOOD amplifier must háve negative feedback to handle such impedance range.
roline already explained very well how to build the feedback look.
Please follow those instructions.
Use of global negative feed back
It is amaizing how well behaved the amp is with the global neg feedback from the output transformers. Some may say too well behaved.
I attached the schematic for the Compactron amp, as designed is 20watts/channel into 8ohms but after doing research on other compatable sweep tubes, can achieve 100watts/channel with the proper iron and appropriate B+. It has the global output feedback to tame it to be able to run with 4 to 8 ohm speakers with the cheap $22 8k-8ohm output transformers. At that output level with the speakers I have, it can drive you out of the room. With 4.3k Raa output transformers, sweep tubes have been tested at over 200watts/channel with 550V B+.
TO add GNFB to your amp just mimic the schematic, if properly hooked up the output will drop a little in volume, it it gets louder, its hooked up wrong on the transformer output.
I have 1650P 6.6k selected for my KT88 amp running at 550V. I selected them for their larger class A operation sacrificing a little power. I only desire 60watts/channel, where over 25 if it will be class A. Most KT88's and some 6550's can take this B+ application.
It is amaizing how well behaved the amp is with the global neg feedback from the output transformers. Some may say too well behaved.
I attached the schematic for the Compactron amp, as designed is 20watts/channel into 8ohms but after doing research on other compatable sweep tubes, can achieve 100watts/channel with the proper iron and appropriate B+. It has the global output feedback to tame it to be able to run with 4 to 8 ohm speakers with the cheap $22 8k-8ohm output transformers. At that output level with the speakers I have, it can drive you out of the room. With 4.3k Raa output transformers, sweep tubes have been tested at over 200watts/channel with 550V B+.
TO add GNFB to your amp just mimic the schematic, if properly hooked up the output will drop a little in volume, it it gets louder, its hooked up wrong on the transformer output.
I have 1650P 6.6k selected for my KT88 amp running at 550V. I selected them for their larger class A operation sacrificing a little power. I only desire 60watts/channel, where over 25 if it will be class A. Most KT88's and some 6550's can take this B+ application.
Attachments
Hi ! I will make the modifications in my amp this weekend(feedback and fixed negative bias)and will post the results! Anyway i dond need a lot of power,prefer to invest in quality of the sound!
EL156
Not yet (very busy at work)but will do that soon and will be fun doing that,and maybe i will change the coupling caps(at the moment i have BIANCHI 0,22uf,but now i got some RUBYCON PIO,wich i think are better quality).
EL156
Hi!
Today i made some experiments with feedback in my EL34 PP ,tested different values of resistors,and spent some hours listening some CDs,and to be honest,didnt decide yet if prefer with or without NFB.And found one problem- I cant connect the Black Wire of the OPT to Ground ,and use the yellow for 8 Ohms and feedback(through 22K resistor) because it makes PFB,so i have to connect the yellow to ground and use the black as 8 ohm for output and for NFG. Do you think that is a problem?
EL156
Today i made some experiments with feedback in my EL34 PP ,tested different values of resistors,and spent some hours listening some CDs,and to be honest,didnt decide yet if prefer with or without NFB.And found one problem- I cant connect the Black Wire of the OPT to Ground ,and use the yellow for 8 Ohms and feedback(through 22K resistor) because it makes PFB,so i have to connect the yellow to ground and use the black as 8 ohm for output and for NFG. Do you think that is a problem?
EL156
I don't know if that would make any difference, but could you switch the leads going to the input grids of the EL34s? That would be an easy fix. Or switch the inputs on the phase inverter.
Be much easier to switch the connections between finals at the primary of the OPT.
Might have consequences regarding stray capacitance, though whether or not that's a problem remains to be seen.
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