Positive/Negative Feedback

cbj591 · 28th June 2010, 01:13 AM

Good evening all,

During my quest to eleminate the hum from my 6V6 amp, I have some how went from negative feedback to positive feedback. I did have to pull alot of stuff apart and put it back together.

It looks like I switched the wires going to the .1uF caps from the pre-amp tubes(pin 2&5 on the 6SL7)). Also I may have done the same thing on the back side of the caps.

I assume this could be the problem? If it is, I need to sit back and take a break. I have moved so much stuff trying to eliminate the hum, I am starting to make things harder on myself...

Thanks in advanced....
Brian

atmasphere · 28th June 2010, 02:54 AM

If you got the 0.1 uf caps swapped it would become positive feedback.

On another note, is it just me, or is that cathode circuit of the 6SL7 missing a cathode resistor??

cbj591 · 28th June 2010, 03:21 AM

I did swap the leads to the caps by accident when putting it back together... Oh well, another night of correcting mistakes...

Yes, it is missing a resistor. We addressed this in my other thread. I was just curious about how the feedback changed.

I'm still learning.

Thanks for the heads up...
Brian

Cassiel · 28th June 2010, 09:01 AM

I don't know why they didn't tell you where that schematic came from. It is a variation of this amp:

http://www.grommesprecision.com/grom...ewelreview.PDF

There's no missing resistor but I don't understand why your value is 10k. Try 2.2k.

Excellent sounding amplifier. That is my fav phase splitter.

Cassiel · 28th June 2010, 10:19 AM

Earlier post a bit sloppy. I have time now to explain it better. The 2,2k resistor bias the driver tube and also sets the feedback ratio. They saved one resistor. That's why 10k didn't make sense. Now, since your plate resistors are 100k this is what I'll do:

490 ohms resistor bypassed biasing the 6SL7. A little more current sounds better. 2.2k feedback resistor. I have tried this and it sounds GREAT. You can trust me as I have experimented with this type of circuit.

Ian444 · 29th June 2010, 11:30 AM

Similar cct in in this pdf stamped 1956. I also like this type of splitter, am thinking of putting mosfet source followers between the coupling caps and the resistors that feed the 2nd half triode.

Cassiel · 29th June 2010, 03:48 PM

Interesting. " Low power high quality circuit which can be used to improve the quality of commercial amplifiers"

Less 1% IM distortion. They didn't use it, at least I didn't find it in a Dynaco amp.

About the mosfets, if you do it please report results. I'm interested.

Ian444 · 11th July 2010, 02:47 AM

I did try the mosfets, but not with the paraphase splitter, I changed to a cathodyne which was no improvement over the paraphase. Then I added the mosfets and fixed bias, which was a night and day difference, big increase in clarity. This is the 2nd PP amp I have fitted mosfet followers to, same result both times, excellent.

Cassiel · 11th July 2010, 09:45 PM

OK, thanks for the feedback. Gingertube posted a schematic I might try someday.

Ian444 · 12th July 2010, 07:49 AM

Yep, I used Gingertube's schematic, from the Baby Huey. I did one amp without the CCS and this amp with the CCS, both very good.

28th June 2010, 09:01 AM	#4
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	I don't know why they didn't tell you where that schematic came from. It is a variation of this amp: http://www.grommesprecision.com/grom...ewelreview.PDF There's no missing resistor but I don't understand why your value is 10k. Try 2.2k. Excellent sounding amplifier. That is my fav phase splitter. __________________ Good sound comes and goes.

28th June 2010, 10:19 AM	#5
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	Earlier post a bit sloppy. I have time now to explain it better. The 2,2k resistor bias the driver tube and also sets the feedback ratio. They saved one resistor. That's why 10k didn't make sense. Now, since your plate resistors are 100k this is what I'll do: 490 ohms resistor bypassed biasing the 6SL7. A little more current sounds better. 2.2k feedback resistor. I have tried this and it sounds GREAT. You can trust me as I have experimented with this type of circuit. __________________ Good sound comes and goes.

29th June 2010, 03:48 PM	#7
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	Interesting. " Low power high quality circuit which can be used to improve the quality of commercial amplifiers" Less 1% IM distortion. They didn't use it, at least I didn't find it in a Dynaco amp. About the mosfets, if you do it please report results. I'm interested. __________________ Good sound comes and goes.

11th July 2010, 09:45 PM	#9
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	OK, thanks for the feedback. Gingertube posted a schematic I might try someday. __________________ Good sound comes and goes.

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28th June 2010, 02:54 AM	#2
atmasphere diyAudio Member Join Date: May 2007	If you got the 0.1 uf caps swapped it would become positive feedback. On another note, is it just me, or is that cathode circuit of the 6SL7 missing a cathode resistor??

28th June 2010, 03:21 AM	#3
cbj591 diyAudio Member Join Date: May 2010	I did swap the leads to the caps by accident when putting it back together... Oh well, another night of correcting mistakes... Yes, it is missing a resistor. We addressed this in my other thread. I was just curious about how the feedback changed. I'm still learning. Thanks for the heads up... Brian

29th June 2010, 11:30 AM	#6
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	Similar cct in in this pdf stamped 1956. I also like this type of splitter, am thinking of putting mosfet source followers between the coupling caps and the resistors that feed the 2nd half triode.

11th July 2010, 02:47 AM	#8
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	I did try the mosfets, but not with the paraphase splitter, I changed to a cathodyne which was no improvement over the paraphase. Then I added the mosfets and fixed bias, which was a night and day difference, big increase in clarity. This is the 2nd PP amp I have fitted mosfet followers to, same result both times, excellent.

12th July 2010, 07:49 AM	#10
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	Yep, I used Gingertube's schematic, from the Baby Huey. I did one amp without the CCS and this amp with the CCS, both very good.

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