Tweet
I have recently built the Slo lead channel into a SE KT88 power section, "my little metal amp". I really like it but I need help with bass response. I want to add some NFB and I was wondering if any more experienced builders have any ideas. I was just hooking the cathode resistor to the 8ohm tap but I dont hear any difference. I have been looking at the Fender champ schematics and they run a 27k from the cathode of the 12ax7 to the OT. Any good ideas would be appreciated.
-
-
If the preamp tube's cathode, to which you are appying the NFB, is bypassed by a capacitor, you are also bypassing the NFB voltage to ground too!
Remove the cathode bypass cap and try again, but for an 8 ohm OT, use a 47K to 56K resistor from the OT.
If you like the way that is working, try holding the preamp stage's cathode biasing resistor up from ground by about 47 to 100ohms.
No you can bypass the biasing resistor again only still hold the cap and resisitor up from ground by the 100 ohm resistor and and insert the NFB
inbetween the 1500 ohm resistor and the 47-100 ohm one... now use something like 2700 ohms to 4700 ohms from the OT.
OK that was a mouthful.... look at the way the NFB is used in a blackface Champ or Princeton is wired. -
Bruce,
I have done exactly what you are describing. I have added 47ohms to the cathode cap-resistor to ground and then my feedback resistor is 2700ohms to the 8ohm tap of the OT, voltage divider per say. I have the feedback resistor on a switch so I can go back and forth to see the difference. The only thing is I dont hear a difference. The cathode I use for the NFB injection is the 4th stage. Does it matter what stage I use and would a higher feedback resistor give me more fb?Comment
-
Chunk,
Are you trying to build a "resonance" circuit? Have you thought about changing some of the values of the cathode bypass caps in the preamp to enhance the bass response?
How are those KT88s treating you? I am thinking of building a poweramp for my preamp using them. How many watts is she dissipating?
~F~F
"Ruining good moments since 1975"Comment
-
I tried different values of caps and cathode resistors for more bass but it just doesnt sound as good. I know that the feedback loop is just what I need from previous projects but I am getting no results. I have the feedback resistor on a switch to see if there is a difference but nothing happens.
As far as the single KT88, I am very pleased. I like EL34's but the KT88 is loud and proud!Comment
-
Have you seen any of Peavey's resonance circuits?
Man, I am interested in those KT88s. I wonder if they have a more pronounced low end than 6L6s.
~F~F
"Ruining good moments since 1975"Comment
-
Does Peavey have any SingleEnded amps? And if so where is there a schematic?Comment
-
Blast! I didn't notice the SE part of your post
I only slept 2 hours last night if that is an excuse! I know you want to stay "all amp", but a nice solid state eq inserted after the preamp just might be the ticket.
~F~F
"Ruining good moments since 1975"Comment
-
Still no FB!!! I did just as Bruce said which is exactly how the early fender champs were built and I get no change. I even lifted the KT88's cathode resistor and went directly to the 4ohm tap and still no change. Help!Comment
-
Just a shot in the dark but did you ground your OT secondary?Comment
-
If everything is referenced to chassis ground, something must be wired wrong then because this works on every amp I've done it to.
Is there anywhere I can look at the schematic to see how this is laid out?Comment
-
I had another thought...
are you using one of the triodes of the push pull phase inverter after the tone stack to drive the SE power tube?Comment
-
The OT secondary is grounded.
Bruce here is the link to the schematic I used:
http://www.schematicheaven.com/newam...ano_slo100.pdf
Page 1 only
I am using V1b, V2a, V2b, V3b, V3a-minus the R24, into the tone stack(no fx) then MV
After the MV I was using a power amp from ax84's SEL:
http://ax84.com/static/corepoweramps..._Schematic.pdf
The OT is a Hammond 125FSE Pri-Blue-#3 pin on KT88 Brown-B+2
Thanks for any help, I am miffed!!Comment
-
OK in that schematic, where are you inserting the NFB?
Does the cathode follower (which is connected to the FCX loop in the schematic) drive your tone stack and then the MV pot's output directly into the grid of the power tube?Comment
-
That is correct. The cathode follower drives the tone stack (no fx) then goes into the MV which then goes to the 5.6k then into the power tube.
Where I was injecting the NFB was into the cathode of the last gain stage before the CF. After the 1.8k/1uf I have a 50ohm resistor like you previously said going to ground. The feedback resistor is then at that junction of the Rk/Ck and 50ohm and goes to the 8ohm tap.Comment
Comment