Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: guitarhvac on March 07, 2011, 03:35:30 pm

Title: Problem with vibrolux bias
Post by: guitarhvac on March 07, 2011, 03:35:30 pm

I just got done building a Blackface AB763 vibrolux from the original fender Schematic.   The problem that i'm having is the amp is breaking up before it gets very loud at all.  Not a lot of break up, but definitely more than it should have. What should the bias reading be on each 6L6?  Now,  on each 6L6 the lowest bias reading that a can get is 42mv the max is around 59.  Now, when the bias is from 42 to 50mv there is a squeal.  After about 50mv the squeal goes away but there's still early breakup!!   Is the bias problem causing the squeal?  PLEASE HELP!!!!!!!!!!!!
THANKS!

Title: Re: Problem with vibrolux bias
Post by: phsyconoodler on March 07, 2011, 03:49:47 pm

Ok it souds like you are using 1 ohm resistors to check the bias and if thats right,then you are biasing very hot.Try 35-40mv maximum depending on your plate voltage.
  The average Vibrolux is around 450v.
The bias should be 35-40ma current for 450v.
 Hot bias = early breakup.

Title: Re: Problem with vibrolux bias
Post by: guitarhvac on March 08, 2011, 07:05:19 am

THANKS for that info!!  That clears the bias question up.  But i can't get the bias reading below 42mv. I need to install that 1 ohm resistor because i'm not using it now.   Any ideas about the squealing at 42 to 50mv?

THANKS A LOT!!!!!

Title: Re: Problem with vibrolux bias
Post by: sluckey on March 08, 2011, 07:14:41 am

Changing the 22K resistor on the bias pot to 27K should allow you to decrease the idle current even more.

Temporarily disconnect the NFB wire. Does the squeal go away? If so, reverse the OT plate leads.

Title: Re: Problem with vibrolux bias
Post by: phsyconoodler on March 08, 2011, 10:21:51 am

Just one question: how are you checking the bias if you don't have the one ohm resistor installed?Where are those readings coming from?

Title: Re: Problem with vibrolux bias
Post by: guitarhvac on March 08, 2011, 10:58:45 am

Hey guys thanks for the help you're giving me!  I'm new to working on amps so if i sound like i don't know what i'm doing, then that's pretty much the case.  I've got two of gerald weber's videos that i've been watching and i can read a schematic, so i'm trying! LOL! 

Sorry, back to the point now.  my question for phsconoodler-  I'm not sitting in front of my amp but i think i was measuring mV from pin 5 to ground.  Is that correct?

My question for sluckey- exactly which wire is the Negative feedback wire?

THANKS SO MUCH GUYS!!!!

Title: Re: Problem with vibrolux bias
Post by: phsyconoodler on March 08, 2011, 11:04:50 am

No.Thats the negative bias VOLTAGE and it's in DC volts,not millivolts.
  Those VOLTAGE readings are in the ballpark and you DON'T need to change the range resistor.You DO however,need to know what the current actually is.The 1 ohm resistors from pin 8 to ground will tell you in 'millivolts' which translates directly into 'milliamps'.
  You have an issue if the amp is breaking up early at those VOLTAGE settings on pin 5.
You have some homework to do finding out what those differences are.Read everything in Hoffman's 'library of information' and look at all the Gerald Weber stuff again and get a better grasp of what you are doing and then we can help you more.

Title: Re: Problem with vibrolux bias
Post by: guitarhvac on March 08, 2011, 11:32:40 am

Thanks physconoodle!  I'll do that!

Title: Re: Problem with vibrolux bias
Post by: sluckey on March 08, 2011, 11:37:52 am

Quote

which wire is the Negative feedback wire?

It's a wire connected between the speaker jack (maybe ext. speaker jack) and an 820 ohm resistor on the board.