Positive/Negative Feedback

[cbj591]

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]

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]

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]

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/grommes/assets/pdf/littlejewelreview.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]

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]

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]

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]

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]

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

 

[Ian444]

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.