How would you go about getting another -10/-15 DCV of bias voltage in this Marshall circuit?

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harddriver

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I am running the standard Marshall bias circuit in my new amp the only difference is I used a 50KL bias pot which gave me a range of about -25 DCV to -55 DCV. The issue is I have some Ruby 6550A tubes that tend to run a little hot so I would like to get at least another -10 to -15 DCV in the colder side on the bias range so about -25 to -65 DCV.

Right now the amp is running the 220K feeding the IN4007 diode and then the 15K, the 47K resistors then to the 50KL bias pot which give me the bias range of -25 to -55DCV. I can drop the 220K value to 180K, 150K or 120K or is it recommended to drop the 47K?
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Lower the 220K resistor coming off the HT secondary before the diode for the negative voltage supply. If you don't want to permanently change it you can also parallel a 1M resistor across it. Start with 180k, maybe 150K.

This is a dropping resistor for the negative voltage supply. Lower value = more negative voltage allowed to pass.

50W models take the supply from one leg of the HT winding. 100W models use a dedicated winding.
 
Purgasound said:
Lower the 220K resistor coming off the HT secondary before the diode for the negative voltage supply. If you don't want to permanently change it you can also parallel a 1M resistor across it. Start with 180k, maybe 150K.

This is a dropping resistor for the negative voltage supply. Lower value = more negative voltage allowed to pass.

50W models take the supply from one leg of the HT winding. 100W models use a dedicated winding.
Click to expand...
Thanks! That is along the lines of what I was thinking but it's nice to get some educated opinions!:2thumbsup:
 
Last edited:
So I replaced the 220K resistor coming off the HT secondary before the diode for the bias negative voltage supply with a 150K and here is where the bias range shifted to.

The amp now has a range of -76.4 DCV to -41.1 DCV which gave me more than enough room to bias those hot 6550A's to 50% dissipation of 36 ma per tube with a B+ of 476 DCV and a bias voltage at pin 5 of -60 DCV.

I think a 180K is going to be the sweet spot for this amp but I will have to order some. The 180K should shift it about another ten volts so roughly -66 to -30 DCV which should be about perfect.

220K= -55 DCV to -25 DCV
150K= -76.4 DCV to -41.1 DCV

Thank You Mr Purgasound for the guidance and advice!:2thumbsup::rock:
 
Picking nits here but the bias supply is just AC voltage (which swings positive and negative). It's the orientation of the diode and bias filter caps that makes it a negative voltage.
 
I thought I would do a final update to this. I replaced the 150K bias dropping resistor before 1N4007 diode in the bias circuit as explained above with a 180K resistor to center my bias range where I wanted it to have enough cold range and hot range to bias cold or hot 6550's and EL34's.

The bias range with the 180K resistor with a 50K bias pot now gives me a range of -67 DCV to -35.5DCV.

So to recap earlier values:

220K= -55 DCV to -25 DCV
150K= -76.4 DCV to -41.1 DCV
180K= -67 DCV to -35.5 DCV

My hot Ruby 65550A now biased at 50% dissipation at -60.4 DCV with ma currents of 37.8 ma and 37.0 ma.

Thanks again to Mr Purgasound!:2thumbsup: Your suggestions were much appreciated and worked perfectly.
 
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