Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on October 12, 2013, 12:56:31 pm

Title: mesa mk IV bias issues
Post by: phsyconoodler on October 12, 2013, 12:56:31 pm

I have a mesa mk IV on the bench and it's giving me some nasty issues.
First it makes a nasty noise when first fired up.The typical crackly staticy noises one hears,so I investigated a bit.
I watched as two tubes redplated after a few minutes so I naturally suspected bias supply or screen resistors.
  The bias supply measures -60v at the bias supply itself but won't come any lower than -35v at the tube grids.So I replaced the bias caps.A slight change to -38v. So I pop out the driveline caps for new ones and it comes up to -48v.Well closer but it's drawing 75ma at that voltage.It has two resistors that look like 220 ohm(red/red/brown) on the board(not labelled) that on one side measure the full -59v and the other sides drop to -48v.They are not on my schematics either the MK IV or MK IV A.
  I am at a bit of a loss as to whats' drawing the bias voltage at this point.The bias supply does double duty by supplying the tubes and the EQ. Maybe something in the EQ? I have the EQ in the off position.
  Any direction you can point me in at this point would be appreciated!

Title: Re: mesa mk IV bias issues
Post by: HotBluePlates on October 12, 2013, 11:00:34 pm

Got a schematic? The one in the Library doesn't include the power supply.

Title: Re: mesa mk IV bias issues
Post by: stratele52 on October 13, 2013, 02:47:52 am

If your tube are redplating ;

1- Tubes are short  Or
2- Bias too hot . OR
3- Tubes's grid do not receive bias voltage. Check socket contacts.
4- Check resistor on pins 4


If your negative voltage is -60 VDC at tubes's grid you don't have to lower it . Bias will be worst... hot hot hot at -38  
 At -60 VDC  bias filter caps are good .

Shematic , no power supply;

www.webphix.com/schematic%20heaven/www.schematicheaven.com/boogieamps/boogie_mkiv.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/boogieamps/boogie_mkiv.pdf)

Title: Re: mesa mk IV bias issues
Post by: stratele52 on October 13, 2013, 03:15:08 am

Mark IV user manual , no schematics


www.mesaboogie.com/manuals/Mark%204.pdf (http://www.mesaboogie.com/manuals/Mark%204.pdf)


Mark V power supply ; same for mark IV  ????

www.el34world.com/Forum/index.php?topic=11865.0 (http://www.el34world.com/Forum/index.php?topic=11865.0)

Title: Re: mesa mk IV bias issues
Post by: phsyconoodler on October 16, 2013, 04:53:16 pm

I have the power supply schematic but no way to load it.It is very difficult to follow on the amp as many components aren't labelled on the circuit board.
  I'm leaning towards the EQ supply being the reason why it is sucking voltage from the power tube grids but it is definitely going to take some time tracing it out.
  Nasty way to build an amp! NOT impressed!