ST 70 High B+ voltage

MZH000 wrote:I upgraded my ST 70 rectifier to a new GZ34 from a 5U4 and replaced the can capacitor to 40-80-30-20 uf from Dynakitparts.com. I wired the capacitors as recommended (40 at rectifier, then 80 after choke, then 30, then 20). Line voltage is 122 volts. I get 500 volts at the rectifier cathode/40 uf lug after the tubes stabilize. The original Dynaco spec says it should be 435 volts. The original I assume had the line voltage at 117 volts. The combination of higher line voltage plus larger filter caps gets the B+ to 500, I assume. I'm running 45ma through the output tubes (KT77).

Question: Is there any harm in keeping the voltage so high. My output tubes are KT77, so plate voltage can go to 800 volts. The filter caps are rated at 550 volts, except one which is at 525 volts. So I am below the max. Bias, as I said, is set for 45ma through each output tube.

Also, my driver board is the classic VTA ST 70 board with CCS, running 3 12AU7's. The B+ going to the board is 445 volts, 25ma. It should be 375 volts. Any harm running at this level?

MZH000 wrote:I installed the VTA board on an original ST 70 (other than KT77 output tubes) per instructions. The B+ going to the VTA board comes off a 2.2k resistor on the last two lugs of the capacitor can, as specified in the VTA board instructions. There is a voltage drop of 55 volts across the 2.2k resistor, so 25ma is going through it, which seems right on.

The power transformer secondary measures 380 volts AC relative to ground. So that's about 5.5% higher than the 360 volts specified in the ST 70 manual. 122 volts is 5.5% above 115 volts, so that's consistent. The ST 70 specs say the voltage at the cathode should be 435 volts. 500 volts is 15% more, not 5.5%. So that is the mystery.

MZH000 wrote:Upon further research: The 380 volts AC on the PT secondary I have is an RMS value. The peak voltage is sqrt(2) * RMS Voltage or 1.41*380 = 535 volts. In an ideal world where the DC voltage is fully filtered (no ripple), the DC voltage after rectification would 535 volts. BUT we need to subtract voltage loss of the tube, which for a GZ34 is about 10 volts. So the ideal fully filtered DC voltage would be 525 volts. Now we must subtract the ripple voltage, since in the real world we can't achieve 0 ripple. The ripple voltage can be calculated with the formula: Vripple = I/(f*C), where I is the current going through the circuit, f is the ripple frequency and C is the filter capacitor value in farads. So for my ST70: I = 205ma (4*45ma for the KT77s, 25ma for the VTA board), f = 120 and C = 40 uf (or 0.000040 farads). Plugging in the numbers we get Vripple = 42.7 volts. But this is peak Vripple. RMS Vripple would be 42.7/1.7 = 25 (we use 1.7 = sqrt(3), not sqrt(2), since the ripple wave is more sawtooth than a sine wave). So subtracting the ripple voltage from the ideal 525 volts = 525 - 25 = 500 volts. Voila!

See hyperphysics.phy-astr.gsu.edu/hbase/Electronic/rectct.html#c4 for details on the math.

tubes4hifi wrote:you never say (or I missed it) if that 500v reading (and 445v to the VTA board) are at start-up or after waiting one minute for all tubes to draw full current.

pichacker wrote:If the transformer had a 120/125v tapping then this would bring the secondary voltages back to levels where an external device was not necessary. Steve