Fender 10k-22k Tail Resistors

[plexi50]

Fender Amplifiers:

Is there a difference in tone or gain associated with using a 10k or 22k resistor in the end tail of the phase inverter connected to the .1 cap/820 ohm feedback resistor?

[dartanion]

Which particular amp are you referring to? This makes a difference ya know...

[plexi50]

Any fender amp period that has the long tail phase inverter. Most every Fender,Marshall and 100 different amps all use use this layout. Theres no difference. I am speaking of vintage amps. 22k was the standard since before time began. But i see a 10k in later fender circuits

[AdmiralB]

Essentially it's a tradeoff between output (voltage swing) and balance. Bigger tail resistors have better balance, but less swing. Randall Aiken has an example on his site that uses 47K - at that value, you don't need the dissimilar plate load resistors to maintain balance, but you reduce output.

[KLB]

Originally Posted by AdmiralB

Essentially it's a tradeoff between output (voltage swing) and balance. Bigger tail resistors have better balance, but less swing. Randall Aiken has an example on his site that uses 47K - at that value, you don't need the dissimilar plate load resistors to maintain balance, but you reduce output.

Yep. You usually see the larger tail resistors on amps with EL84 and 6V6 tubes. Both types require very little signal on the grids to reach saturation. The Deluxe Reverb uses a 27K resistor. Most Vox amps use a 47K. Also, the lack of negative feedback on Vox amps more than makes up for any lost output due to the larger tail resistor. The Vox style PI has two inputs, so you can blend two channels or a channel input plus effects loop return.

[plexi50]

So using a 22k resistor instead of a 10k resistor will produce less output volume? Pic with circled 22k or 27k. Not to clean of a schematic

[AdmiralB]

Yes, all else equal. You'll get better balance at the expense of PI output.

Keep in mind that until fairly recently, all that the designers would care about is that the PI could provide enough swing to drive the power tubes to full output power while the preamp was still fairly clean. Overdriving the power tubes by design wasn't on their minds.

I agree with KLB, except for the comment regarding 6V6s. It's true that EL84s (and EL34s) are extremely sensitive tubes, high transconductance. But 6V6s are not particularly sensitive. It takes a LOT more signal to fully drive one.

[plexi50]

Ok thanks much all you guys. I have always used a 10k in that spot but was curious what a 22k would sound like and what its affect would be versus the 10k. I did put a 22k in my super reverb circuit this morning and it sounds great but seems a little tighter and less open if thats how i can describe it. More concentrated tone. Maybe what im hearing is more of an actual balance in the output. That can be a good thing to. But im putting the 10k back for now

[dartanion]

Brown and blonde era Fenders used 68K tail resistors. However, these used 12AX7s in the PI. The whole LTP circuit changed when they switched over to a 12AT7 in the PI. So, not quite since time began, but BF era on.

Marshall used 10ks pretty frequently, save for 18/20 watters which were primarily 47k.

[ChickenLover]

Brown and blonde era Fenders used 68K tail resistors. However, these used 12AX7s in the PI. The whole LTP circuit changed when they switched over to a 12AT7 in the PI. So, not quite since time began, but BF era on.

I think you mean 6.8K tail resistors in the Brown/Blondes...which is even less than 10K.

The cathode resistor also changes with these different designs too. It is the cathode resistor that sets the idle current and that current has to go through the tail resistor to ground...which of course creates a voltage across the tail resistor which 'lifts' the whole PI from ground. You may notice that Voxes had a 47K tail but they also had much higher cathode resistors (1.2K?) while Marshalls/Tweeds had the 10K tail with a 470 cathode resistor.

[dartanion]

OOPS! Forget my decimal point.