Brown Concert bias mod?

[Mars-Hall]

Just acquired a 1962 Fender Concert Amp.  The amp does not have a bias pot. With the tubes currently installed (GT 5881C), they are idling on the cold side. I remember seeing a nice sketch for adding a bias pot. However, none I see in the "Bias Mod" section of the library exactly match this configuration.  Please advise...   

[sluckey]

I would simply replace the 56K resistor with a 22K in series with a 50K pot. The pot would operate as a variable resistor only using two terminals (wiper and one end lug). If you use the small bias pot that Hoffman sells you could just mount on the rectifier/bias board. No other circuit changes necessary.

[Mars-Hall]

Thank you for the quick response.  To clarify, I could put one of the outside lugs (doesn't matter which), of the pot, to the bottom right terminal of the small board, where I replaced the 56K resistor with a 22K. Then take the middle lug of the pot and wire it the large board where the two 220K resistors meet.  Does that sound right? 

[stratele52]

I would simply replace the 56K resistor with a 22K in series with a 50K pot. The pot would operate as a variable resistor only using two terminals (wiper and one end lug). If you use the small bias pot that Hoffman sells you could just mount on the rectifier/bias board. No other circuit changes necessary.

+1

[sluckey]

Does that sound right?

No. The 22K is in series with the 50K pot. These two components replace the single 56K on the board. See pic...

[HotBluePlates]

"Why add the 22kΩ resistor when the 50kΩ pot is so close to the original 56kΩ?"

Cause that keeps you from turning it down to 0Ω and burning up your tubes due to no bias voltage. 22kΩ + 50kΩ also gives you a little range above the original 56kΩ to idle cooler than stock, if necessary.

[Mars-Hall]

Thanks for the diagram sluckey.  A picture is worth a thousand words...

[Willabe]

Please let us know how you think/like the vibrato in that amp and which version it is, 2 tube or 2 1/2 tube.

Looks like it's in pretty darn good shape.


             Brad    


 

[Mars-Hall]

Please let us know how you think/like the vibrato in that amp and which version it is, 2 tube or 2 1/2 tube.

Looks like it's in pretty darn good shape.


             Brad    

It's the 6G12-A circuit that has the true pitch shifting vibrato, utilizing 2 1/2 12AX7's.  There are a few thing that have been changed.  Has what I think are rare alnico Oxford speakers with the brown magnet covers. 2 of which, have been reconed. All the tubes and electrolytic caps have already been replaced. Just need to idle up the bias a little.

Cosmetically, the tolex is good, minus a cigarette burn and 2 holes where someone added a BF era handle, at one time. Also, the grillcloth has been changed.

The PT has been replaced, which peeved me a little upon inspection, after I got it home. What fooled me was someone used the original bell covers on the new PT. Will not make that mistake again and at least the OT and choke are original. Even at that, I'm still happy with the purchase. I called the seller on the PT, he swears he didn't know about it. I would like to believe him however, his gut shot photos conviently left out the PT. We live, we learn... Like I said, still happy even at that.



 

[Mars-Hall]

Here's a shot of the alnico Oxfords.  I personally have never seen anything like these before.  I've only ever seen ceramic mags in these amps. 

[1rebmem]

"Why add the 22kΩ resistor when the 50kΩ pot is so close to the original 56kΩ?"

Cause that keeps you from turning it down to 0Ω and burning up your tubes due to no bias voltage. 22kΩ + 50kΩ also gives you a little range above the original 56kΩ to idle cooler than stock, if necessary.

There sure are some darn smart people on here!! This guy's one of them. No BS, just fact every time.
I appreciate the education....Thanks.

[HotBluePlates]

Thanks, but I also learned a lot of it from smart folks on this forum who came before me.

[eleventeen]

When my niece was about 9, the neighbors gave her a brown Concert. Looks identical. It was absurd thinking of her actually moving that amp. Come to think of it, it's fairly absurd thinking of *me* moving such an amp.