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AA764 to 5F1 questions (kinda long)

12/10/1999 3:40 PM
Colin Brown
AA764 to 5F1 questions (kinda long)
Hi All:  
I just finished "converting" my 77 sf champ AA764 to 5F1 as a learning project and I have a couple of questions. The circuit is the same as the 5F1 except in the power supply. The original can cap 40x20x20 was shot so I replaced it with single caps on the circuit board like the layout diagram for the 5F1 circuit and used a 47uf, 20uf, 20uf in place of the 16uf, 8uf, 8uf then I used a 4.7k and 22k resistor in place of the 10k and 22k. I now get the following readings: 23.8v across the 470 ohm cathode res, 387vdc on pin 3 of the 6v6 (nos rca) and 376vdc on pin 4. I think this is still running hot but I've read some posts that say Fender did push the 6v6 in these circuits, so are these numbers in line? the amp sounds good. In the negative feedback section I would like to try a variable negative feedback in place of the 22k fixed resistor, so could I put a 50k pot in it's place or would I have to replace the 22k with a smaller value then put a pot in series with it? (I plan to put the pot where the bass control used to be.) Also in the 5F1 circuit there is no 25uf 25v cap across the 1500 ohm resistor on the 12ax7 could I put a couple of different values there with a switch to make a variable "gain" switch? maybe a 10uf and a 25uf to give a 0 - 10 - 25uf selection? I'm learning as I go and having a great time with this champ, any thoughts or suggestions appreciated as I have learned alot from being a lurker on this page.  
12/10/1999 4:14 PM

I'd say this is idling a bit too hot unless you are using a good ol' american 6V6 tube. My quick calcs show 50ma through the cathode resistor, and a total plate+screen dissipation of 18.2 watts.  
You might want to drop the screen grid voltage down a little, by putting the 10k resistor back in the spot where the 4.7k is now. Also, a 500 ohm cathode resistor may help. Bruce has a lot of experience with amps of this type, and may have some specifics to recommend.
12/10/1999 5:20 PM
Colin Brown
Hi Doc:I should hav...
Hi Doc:  
I should have mentioned that I put the 10k in at first but I had a huge difference, I think, between pins 3&4 it was something like 40 vdc and I thought the diference should be closer to 7 - 10 vdc. I could put it back in and recheck the readings.  
12/12/1999 3:23 PM
Colin Brown
10k back and 560 ohm cathode resistor
Hey Doc:  
Last night I put the 10k back in place of the 4.7k and I had to up the cathode resistor to 560 ohm to get the bias below 50ma, now I'm sitting with 45ma and the sound is there. I'm getting just over 400vdc on pin 3 and 376 on pin 4, the tube is a nos usa. Thanks for the suggestion.  

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