I have a question about the bias on one triode of a 12ay7

[catnine]

I built this amp a while back maybe 2005 , basically what it is  is a 6G2 princeton from the input jacks to the grid of the second gain stage after the tone stack spliced to the grid of V2 on a fixed bias 5E3 . I used the tube layout of the 5E3 so V1 is just the preamp and V2 is the second gain stage and the split load PI .

 What I did was just use on triode of the 12AY7 and instead of a 820 ohm cathode bias resister I used a 1.5K bias resister . I was told the bias resister should be higher and I have 151.5VDC on the 12ay7 plate and 2.37 VDC on the cathode. I was told to use a 2.7K bias resister but that does not make sense to me. The person used a tube chart and came up with that value , said it would bias the 12ay7 more centered. To me the amp sounds  fine It has plenty of headroom whcih is what I want and a great bottom end.

 I really don't understand tube charts and this is a EH 12ay7 tube the chart had to be for older tubes.

[Ed_Chambley]

I was told to use a 2.7K bias resister but that does not make sense to me.
[/quote]
What size is the cap?

[catnine]

I was told to use a 2.7K bias resister but that does not make sense to me.

What size is the cap?
[/quote]

25uf 50volt bypass cap

[Willabe]

I was told to use a 2.7K bias resister but that does not make sense to me. The person used a tube chart and came up with that value , said it would bias the 12ay7 more centered.

More 'centered' will give a little more output before the signal clips (distorts) on either the top half of the sine wave or bottom.

Only 1 way to find out if you like it better, try it.


                  Brad      

[HotBluePlates]

HBP has left the building.

[catnine]

HotBluePlate :

 No I have not tried the 2.7K .

 Well I can yet it's a PITA , if I had a switch I would try it and then I can compare , no room for a switch . Not that I don't trust him yet all tubes vary and right now the amp does not break up even at 7 on the vol and I never go that high . He knows a heck of a lot more than I ever will . Took me two hours to figure out the tube chart . I just build simple amps all fender circuits so I never designed an amp .

 I have seen some fender schematics that show a 12AX7 biased the same way if they tie the cathodes together they used a 820 ohm and a 25uf bypass cap.

 The entire thing began because I had this idea of replacing the split load PI with a LTPI . I had this bug to do this thinking it may be better and he was trying to help me out .

 So far this amp started out as a 5E3 but I didn't like the breakup and the chassis I used from a brunt out 77 MM bass amp only had 2 inputs and 1 tone and 1 vol so I seached through fender schematics and saw the 6G2 would do what I needed and it worked . I even went further and set it up with a pair of JJ 6L6's and a GZ-34 and already had the PT and OT from Allen Amps that could run either but it was far to much amp for here so I went back to the 6V6's. The split load PI worked fine with either set . The build kinda grew from a princeton to a BF deluxe due to the tranny's I had ,of course it didn't have the LTPI . Then it grew into a wide panel pro with 2 twelves only with adj/fixed bias. The 5G9 tremlolux came up and I thought maybe just leave the tremlo out of the lux.

[Willabe]

You know, in most cases the input signal at this stage won't get big enough to matter which resistor is used.

Good point.


              Brad