KT66 bias settings on ST70

Just replaced what I believe were the original EL 34s in my ST 70 with a new set of Golden Lion KT 66.  With the bias pots all the way down I’m reading about 2.8 Vdc.  I’ve read on the forum that these should be around .645 vdc. This unit has 7199 driver tubes/ original pcb.  Thank you.

sneadhearn wrote:Just replaced what I believe were the original EL 34s in my ST 70 with a new set of Golden Lion KT 66.  With the bias pots all the way down I’m reading about 2.8 Vdc.  I’ve read on the forum that these should be around .645 vdc. This unit has 7199 driver tubes/ original pcb.  Thank you.

if you have a dynaco ST70 the bias should be 1.56Volt
2.8 is way to much, you need to adjust the bias circuitry to allow more neg voltage.
You also should change the 270k resistors to 100k, this prevents KT66 from
thermal runaway ( the ones between eyelet 21 and 22 in the manual. These should be
of matched )

Thanks Peter.... so I would also need to change the resistor between eyelet 6 and 2 right?? I downloaded the manual from this site ... it don’t see any step for 21 to 22 but I see four 270k in the circuit schematic ( change all four or just the 2 that go to the bias pots) i should add that the With the Old EL 34s in the bias set easily at 1.56 v

sneadhearn wrote:Thanks Peter.... so I would also need to change the resistor between eyelet 6 and 2  right?? I downloaded the manual from this site ... it don’t see any step for 21 to 22 but I see four 270k in the circuit schematic  ( change all four or just the 2 that go to the bias  pots)  i should add that the  With the Old EL 34s in the bias set easily at 1.56 v  

You are correct, change all 4 270k resistors.
Eyelets involved is 1-6-2 for one channel and 22-21-23 for the other.

2.8 volts at the bias measuring locations are far too much, it will kill your kt66. Start with
fixing this issue.
Replacing the Se rectifier to a Si diode ( 1n4004 will do just fine) will stabilize
the DC . The bias adjust pots might not be enough to dial in correct bias,
there is 2 10k resistors surrounding the pots that could need change.
A suggestion is to change the 10k located between the electrolytic caps to 15k if you
have trouble to set correct bias.

In order to adjust the polarization with KT66 valves, I placed a 15k resistor in parallel to the existing 10k resistor between the two electrolytes, the 10k resistance that derives to ground remains the same.
This worked for me.
I use a 1N4007 rectifier.

Ok I changed out 270 k resistors... that dropPed the voltage test point with pots all the way down to 2.6 vdc.  So putting a 15k ohm in parallel with both 10ks on the two electrolytes would be the next step?

sneadhearn wrote:Ok I changed out 270 k resistors... that dropPed the voltage test point with pots all the way down to 2.6 vdc.  So putting a 15k ohm in parallel with both 10ks on the two electrolytes would be the next step?

I suggest you start with changing the 10k between the caps to 5k, and if that does not
allow you to adjust bias , replace the other 10k resistor ( the one connected between
the pots and ground) to 15k.


While you are inside the amp, do check that the cathode resistors, one at each side,
shared between the two tubes, still measures 15.6 ohm. Wrong value here will
make bias measuring wrong.

.

Ok... so I changed out the Se  rectifier to a diode.... didn’t have a 5k resistor... so I placed a 15k in parallel with the 10k ... that brings the test point voltage down to 1.6 vdc with the pots full down.... so I’m thinking maybe replacing the 10k between the electrolytes with 5 k might do the trick and give some adjustMent  back to the pots . The 15.6 ohm resistors measure 16.2 ohms.