Increase Bias Supply -Voltage

I was able to set my xf2's idle current comfortably at 45-50ma with bias pots at about middle range. I had a 5k resistor on the terminal strip between 3 and 4 with a 10k between 1 and 2. I just installed a matched quad of NOS 6n3c-e and initially couldn't bias each of them lower than 52ma. I started lowering the resistor values to the point where I have 3k between 3 and 4 and 2k between 1 and 2. The pots are at roughly a quarter turn up clockwise with tubes idling at 45ma each so I can live with that. My question is this. Has anyone experienced such low negative voltage at the input of the power supply that much lower resistance is required to properly set bias at the tubes? With factory resistors being 10k each I'm at about 25% of that. I overhauled my ST-70 with a factory replacement board because I have 3 sets of strong 7199's (I'm aware of the virtues of VTA designs) 2 strong quads of Mullard xf2's and wanted to first hear the amp in its original design. I've replaced the quad can (all points test to spec) replaced the selenium rectifier with a N4007 diode included in the Dynakitparts bias supply kit and installed a bias balance kit. I'm using a strong near NOS Mullard GZ34 and all voltage test points are in acceptable range. I built a bucking transformer that feeds the amp with 116-118vac and have a CL-90 in the amp after the fuse so internal AC is always 115-117. Any thoughts?
Thanks

I am looking at the bias circuit differently than you are. Most of the Dynaco amps use a single wire tap off of the power supply transformer. The circuit then goes thru the diode (SR on the original design) creating DC. Then a resistor to a pot (or pots for the ST70) and a resistor from pot to ground. If you add up the original resistance and divide into the DC voltage at the output of the diode you will see all the Dynacos have around 2mA of load. When you lower both resistors to try to make the tap voltage work out you are now lowering the total resistance which raises the total current on that transformer winding. Maybe it works but you should ask your self why do that. Dynaco chose 2 mA of load in all their amps for a reason. Probably for ease of transformer specification and fabrication. Adding a large current draw to that winding may have long term consequences.

My suggestion is to draw the circuit and label DC voltages at nodes. Then look at what you are trying to do with the pot tap voltages to the output tubes. If you need more negative bias then lower the resistor value from diode to pot but then add that difference to the resistor that goes from pot to ground. That way the total load (2 mA current) will stay roughly the same. What this does is effectively move the pot higher toward the diode but keeps the circuit similar to the original design. Or visa versa if you need less bias voltage.

If this does not make sense for you send me a PM and I will go thru it more. I can send circuit and calculation info.

Here is the original Dynaco schematic , so you can see what resistor values should be .

Because the 6P3S-E (that's English for the Cyrillic 6n3c-e) , has a different biasing to EL34 you require a higher bias voltage . 6P3S-E is essentially a copy of the 6L6G or 6L6GT . Check datasheet 6L6G 6L6G datasheet and compare to EL34 datasheet .

Simplest way is to increase value of 10k resistor between pot and ground to 22k . This will give the pot a smaller range , but raise available negative voltage . If changing the value to 22k is too much to bias the EL34's high enough , decrease the the value to .... say .... 16.5k (2x 33k in parallel).

To understand how the negative DC supply works : If the total negative supply is -60VDC , measured from ground to the point between the 10k resistor and diode . Then the 10k resistor , 10k pot and next 10k resistor form a voltage divider . Each with 20V across them - 60/3 = 20V . The pot will then have a minimum of -20V and a maximum of -40VDC available at the centre(wiper) pin . If the pot is set to centre position (5k either side) then voltage at centre pin will -30V (divided exactly in half) .

Measured from Ground you have 10k resistor + 10k pot + 10k resistor = 30k  

60V/30k = 2V/kilo-ohm

With 22k resistor between pot and ground the minimum voltage available to pot(@60V total negative supply) will be -31.5VDC  and maximum of -46VDC.

Measured from Ground you have 22k resistor + 10k pot + 10k resistor = 42k    ...or..  -31.5VDC(22k x 1.43V) and -46VDC(32k x 1.43V) . 32k=22k resistor + 10k pot

60V/42k = 1.43V/kilo-ohm

-60V is a theoretical value .





I hope that helps .

Fredeb - Three are two pots in parallel. Total of 5k ohms not 10k ohms.

WLT wrote:Fredeb - Three are two pots in parallel. Total of 5k ohms not 10k ohms.

Thank you @WLT , I missed that.


Another solution to bias for 6L6G/6P3S-E would be to increase the cathode resistor value and up wattage rating .

After some back and forth with different resistor values in the bias supply, I've found the sweet spot to use both 6n3c-e and el34 with adequate adjustment range. With 10k after the diode and 15k between pot and ground I got optimum results. I tried at least 6 combinations including the suggestions given. First I went with 5k and 5k which total resistance wise would be the same but didn't give me the range I needed for both sets of tubes. Now with the 6n3c-e my pots are up about 1/3 full and with el34's about 2/3. I'm leaving well enough alone, it's the best I could dial it in. Thanks for the excellent responses, I truly appreciate your efforts to help.

TubesGlo wrote:After some back and forth with different resistor values in the bias supply, I've found the sweet spot to use both 6n3c-e and el34 with adequate adjustment range. With 10k after the diode and 15k between pot and ground I got optimum results. I tried at least 6 combinations including the suggestions given. First I went with 5k and 5k which total resistance wise would be the same but didn't give me the range I needed for both sets of tubes. Now with the 6n3c-e my pots are up about 1/3 full and with el34's about 2/3. I'm leaving well enough alone, it's the best I could dial it in. Thanks for the excellent responses, I truly appreciate your efforts to help.

Well done for sorting it - enjoy ! I really love the 6P3S-E , I use them in my highly modified ST70 and almost stock Quad ll monoblocks , and others . They were dirt cheap a few years ago , so stock up now !

The only change made to the Quad ll's was increasing cathode bias resistor from 180e to 220e with large value (3300uF) bypass . The bypass cap improves bass response in the Quad ll's .