Positive/Negative Feedback - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th June 2010, 01:13 AM   #1
cbj591 is offline cbj591  United States
diyAudio Member
 
cbj591's Avatar
 
Join Date: May 2010
Default Positive/Negative Feedback

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
Attached Images
File Type: png 6SL7-6V6-Push-Pull-Tube-Amp-Schematic-pinouts.PNG (54.9 KB, 262 views)
  Reply With Quote
Old 28th June 2010, 02:54 AM   #2
diyAudio Member
 
atmasphere's Avatar
 
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??
  Reply With Quote
Old 28th June 2010, 03:21 AM   #3
cbj591 is offline cbj591  United States
diyAudio Member
 
cbj591's Avatar
 
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
  Reply With Quote
Old 28th June 2010, 09:01 AM   #4
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
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.
__________________
Hobby-Horse
  Reply With Quote
Old 28th June 2010, 10:19 AM   #5
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
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.
__________________
Hobby-Horse
  Reply With Quote
Old 29th June 2010, 11:30 AM   #6
Ian444 is offline Ian444  Australia
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.
  Reply With Quote
Old 29th June 2010, 03:48 PM   #7
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
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.
__________________
Hobby-Horse
  Reply With Quote
Old 11th July 2010, 02:47 AM   #8
Ian444 is offline Ian444  Australia
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.
  Reply With Quote
Old 11th July 2010, 09:45 PM   #9
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
Join Date: Sep 2004
Location: Madrid
OK, thanks for the feedback. Gingertube posted a schematic I might try someday.
__________________
Hobby-Horse
  Reply With Quote
Old 12th July 2010, 07:49 AM   #10
Ian444 is offline Ian444  Australia
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Positive regulator to produce negative voltage? stef1777 Parts 45 8th February 2008 07:46 AM
Onix OA21s - speaker post positive touched negative siongkim Solid State 0 30th April 2007 04:33 AM
Tends TA10 negative or positive grounded emperor Class D 10 19th April 2007 12:39 PM
Small voltage deviation at positive and negative rail 2Bak Chip Amps 1 14th December 2003 06:42 PM
Positive regulators for negative supplies? JohnG Solid State 7 4th February 2003 12:28 PM


New To Site? Need Help?

All times are GMT. The time now is 09:24 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2