Fender silverface / blackface bias question

[Bernardduur]

Hey all!

I have a Fender AA270 which is a Silverface Fender Twin. I love her.

Lately I've been reading on blackfacing this circuit thus I replaced the differences between the AA270 to AB736 specs. Works great and fine!

Last I wanted to change the bias to blackface specs. Now the question:

In the schematics I read of both units the PT is the same. Dunno if that is only in schematic land or also in the real deal (can't find a number on my PT). If they are the same, can I simply use the same connection for the bias?

I ask this as I changed the bias to blackface specs (470 ohm, 220k's, etc) and now I can't bias my powertubes over 7W (I used the 1 ohm bias method). I must say I don't know the age of the tubes, the tubes are ringing when the amp is on (also on standby) and when I compare the tubes they are way off of each other. A retube is I think appreciated!

[tubeswell]

Hey all!

I have a Fender AA270 which is a Silverface Fender Twin. I love her.

Lately I've been reading on blackfacing this circuit thus I replaced the differences between the AA270 to AB736 specs. Works great and fine!

Last I wanted to change the bias to blackface specs. Now the question:

In the schematics I read of both units the PT is the same. Dunno if that is only in schematic land or also in the real deal (can't find a number on my PT). If they are the same, can I simply use the same connection for the bias?

I ask this as I changed the bias to blackface specs (470 ohm, 220k's, etc) and now I can't bias my powertubes over 7W (I used the 1 ohm bias method). I must say I don't know the age of the tubes, the tubes are ringing when the amp is on (also on standby) and when I compare the tubes they are way off of each other.

(Wiring and soldering technique aside - did you check your work?) a range of things affect the bias point of the output tubes (besides the individual tubes themselves), the B+ voltage and size of the plate resistor (which forms the load), the bias (affected by either -ve grid voltage - or conversely +ve cathode voltage relative to grid), the screen grid voltage (affects the grid voltage), and the Zed through the OT to the speaker. Changing only one of those things doesn't necessarily get you where you want to go. Is your B+, plate resistor, screen voltage and OT the same as the BF as well?

[stokes]

Sometimes when changing the bias balance pot to a bias adjust pot,you need to adjust the 15k resistor you moved from the wiper to the end of the pot,try a 25k or so.

[metalangst]

Also, if your tubes are way off eachother (not ballanced), swap them. If the off-ballance moves with the tubes, it's time to retube, if not,recheck the wiring.

[Bernardduur]

Thanks all for the replies;

I checked my work and it is as it should be; B+, grid resistors, all stayed the same. OT is the same between the models.

I wired the potmeter up as seen on the blackface twin schematic. The silverface potmeter has another lug on the opposite side of the potmeter. WHen I measure the potmeter I can get up to 7.5k instead of the 10k it is suppose to do........ will replace her today. I also wired the 27k on the other side of it and added another filter stage a la "Viva Analog" suggestions.

The off balance moves with the tubes; new tubes are ordered. The tubes also make noise when I turn on the amp (they "ring")

[stokes]

When you re-wired the pot you should have taken the connections from that "extra" leg on the back of the SF pot and connected them to the outer leg,opposite where the 15k ended up.If you didnt,then that is the problem.

[Bernardduur]

Hmmm, don't know how that pot should work then.

I swapped her for a normal 10k pot and still the same problem remains. I swapped the 470 ohm resistor on the input of the bias for a 1k to drop the voltage some more and now my bias is up to 15W

I'll see what happens when my new tubes arrive!

[MWJB]

"WHen I measure the potmeter I can get up to 7.5k instead of the 10k it is suppose to do........ will replace her today. I also wired the 27k on the other side of it" By doing this you have rebiased the amp colder. Reduce the value of the 27K. Try 19K or 22K. Varying the size of the 470ohm/1K you have in there now, is not the most practical method of rebiasing.

[Bernardduur]

Thanks!

It was the first thing that popped in my mind

I'll try to lower the 27k resistor.