Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: luthierwnc on November 03, 2011, 06:55:20 pm

Title: Low Bias Voltage on 5E6-A
Post by: luthierwnc on November 03, 2011, 06:55:20 pm

Hi guys,  I'm working on a friends '56 Bassman after he hit a wall.  The amp has a 5E6 tube chart but is either the later version or a transitional model.  My problem is that the power tubes are pulling 64ma each.  The bias voltage from the tap is 47 and on pin 5 it is -41.  The cap is new (I tested another to be sure) and the selenium rectifier still works (subbed in a UF4007 to be sure of that too).  47 to 41 is pretty efficient and is right where it should be on the schematic.  The amp is bone stock except for new filter caps.  The PI caps tested fine for both range and not passing DC.  I also subbed the RCAs in there for a pair of TADs.  They only pulled 60ma each.

One experiment I did was adding 20k to the bleed resistor.  I knew it wouldn't get me in the range but I did want to see if it made a difference.  It did add a volt to the bias supply and raised the rail about 5 volts as well.  Because of the heavy power tube draw, the volts on dead-on the schematic.  With modern wall voltage, they should be 20+ volts higher at B+.

So, any ideas how to get the power tubes into the 35ma range?  For laughs, I put the amp on really low gain into a dummy load and ran a 1k sine wave into the jack.  The signal looked great all the way through.  At least, I don't think there were any weird parasitics messing up the draw.  Those old RCA tubes are pretty tough but I didn't want to drive them any harder than I had to.

Thanks for any and all opinions, Skip

Title: Re: Low Bias Voltage on 5E6-A
Post by: Blind Lemon on November 03, 2011, 07:48:57 pm

Drop the neg Bias voltage down to around -50, looking at the schem. make the 56K riz, in the bias circuit, larger. try a 68k or 82K. Your running at higher wall voltage than they did back then. Whats the plate voltage?


BL

PS thats actually high bias voltage.

Title: Re: Low Bias Voltage on 5E6-A
Post by: stratele52 on November 04, 2011, 03:34:54 am

Blind Lemon is right , try higher resistor in place of the 56 K or a smaller  resistor in place of 6.8 K. I have a Fender Bassman 5E6 schematic at home and it has a note to replace the 6.8 k by a 3.3 K.