Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mats on June 09, 2014, 02:34:16 pm
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Hi
Still in my learning about tube amps. Im looking at Fxxxx Model 26 amp, schematic, and 6N7 PI.
Cathode Res. is 5k in that schematic, is that correct do you think ?
I have seen from about 1k to 3k, in other amp schematics with 6N7 tubes, but not as high as 5k.
/Mats
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> Cathode Res. is 5k in that schematic, is that correct
Can't be correct or incorrect without knowing the rest of the circuit.
> Im looking at Fxxxx Model 26 amp, schematic
Google and I have no idea how I can look at "Fxxxx Model 26 amp" schematic.
...wait.... do you mean FENDER? Is that a dirty word?
6N7 is a funny tube (Fender probably used it because it was going out of style and cheap).
Look at page 2 of this datasheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6N7.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6N7.pdf)
250V, 5V bias, gives 6mA current. So the cathode resistor is 5V/6mA= 833 Ohms.
But we do not want 6mA!! Guessing the preamp supply is like 240V, and that the 6N7 plates should be sitting ~~half of that, leaving 120V drop in 120K plate resistors, we want 1mA.
Decrease current by increasing cathode resistor, but the tube fights you. To get a 6X change of current you need much more than 6X change of resistor. Possibly 36 times bigger! However the plate voltage is about half of where the sheet's condition is, and there's two triodes on one resistor.
> do you think ?
Why think?? Copy, try. What is the worst can happen? You don't like the 5K condition and have to throw another 12 cents and two solder joints for a 3K resistor? Get a 10-pack of 1K resistors, series, then use a jumper to short-out more or less 1K units.
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Hi
Sorry for Fxxxxx
Yes its Fender Model 26
Yes its just to try it and find out
and try other values.
Thanks for reply.