Observations and Questions VTA Board for ST70

Well I did the mods on my ST70 yesterday. I not only replaced the board, but also replaced the tube sockets. Probably easier that way.
I must say the sound is awesome! More detailed, fuller, more bass, tight not muddy, better soundstage and detailed highs. Incredible sound through my stacked Large Advents @ a 4 ohm load.

I have a couple of questions though. First I am using my PAS3 instead of my McIntosh MX110. Why in the hell would Dynaco put out a preamp that would not be suited to the ST70? I have ordered the 1uF caps to install. Second, I have biased the tubes to .4 vt each. However I get a reading of 33 mA instead of 40 mA. What gives? I have not set the AC balance yet because I do not have a scope. I will do that Tuesday. But I am trying to figure out why I do not have 40 mA on each tube. I installed the 10 ohm resistors tying pin 1 and 8 together to ground. I installed new 1K 1 watt resistors on each tube also. What am I missing? Can I set the bias on each tube for 40 mA?

myaddiction wrote:Well I did the mods on my ST70 yesterday. I not only replaced the board, but also replaced the tube sockets. Probably easier that way.
I must say the sound is awesome! More detailed, fuller, more bass, tight not muddy, better soundstage and detailed highs. Incredible sound through my stacked Large Advents @ a 4 ohm load.

I have a couple of questions though. First I am using my PAS3 instead of my McIntosh MX110. Why in the hell would Dynaco put out a preamp that would not be suited to the ST70? I have ordered the 1uF caps to install. Second, I have biased the tubes to .4 vt each. However I get a reading of 33 mA instead of 40 mA. What gives? I have not set the AC balance yet because I do not have a scope. I will do that Tuesday. But I am trying to figure out why I do not have 40 mA on each tube. I installed the 10 ohm resistors tying pin 1 and 8 together to ground. I installed new 1K 1 watt resistors on each tube also. What am I missing? Can I set the bias on each tube for 40 mA?

I experience the same thing that you do. I set my ST-70 amp with VTA driver board to give .400 VDC on the bias and then get about 34.4 milliamps when I switch to milliamps. I have a feeling that the inductance or the resistance of the meter may have something to do with the "off" reading. To really measure accurately that milliamp reading you would have to have an milliameter in series between pin 8 of the output tube and the 10 ohm bias resistor that is grounded to the chassis.

You don't need a scope to do the AC balance adjustment. You do need a signal or function generator and a dummy load to run across the output binding posts. If you set the 10K ohm AC balance controls on the VTA driver board to their mid position (about 5K ohms from the center pin of the AC bias pot to each of the two outer pins) you will be very close to the ideal position. The instructions to set the AC balance controls on a VTA driver board are below.

Bob

Setting AC Balance on the VTA driver board

Note – Do not attempt to set the AC balance on this amp unless you have the following ….
1. A Function generator or a signal generator capable of providing a 100 Hz signal to the amp’s input jacks
2. A Digital multimeter (DMM)
3. A .10 Mfd @ 100 volt or higher voltage capacitor
4. Two non inductive 8 ohm resistors 20 watts or higher

The AC balance controls are the small potentiometers that you soldered in on the BOTTOM of the VTA driver board (one for each channel). Setting each potentiometer as outlined in the driver board assembly manual when you assembled the board will give you an AC balance that is close to optimum. If you have the equipment listed above and you want to reset or peak the AC balance for each pair of output tubes then read below ….

Be sure your speakers aren't connected during this test, as they could be damaged. Also, I recommend you make up two DMM test leads with a 0.1uf 100 volt or higher rated capacitor in series with each lead. The leads plug into your DMM, and the caps block DC. This allows you to do the AC balance readings on the meter's AC ranges.

Allow the amp to warm up for at least 15 minutes and then first set the normal DC bias for each output tube with the DC bias potentiometers on the TOP of the driver board.
1. Connect an 8 ohm, 20 watt (or higher wattage) non-inductive resistor between terminals marked "G" and "8" IN PLACE OF THE SPEAKERS on BOTH channels. These may be purchased at Parts Express online.
2. LEFT CHANNEL - Set your DMM to AC volts, the 2 volt scale is usually fine. Connect one of your DMM leads to pin 8 on V2 and the other lead to pin 8 on V3 (OR connect one DMM lead to EACH of the DC bias measuring terminals on the power takeoff sockets on the front of the amp where you normally measure DC balance for each tube. Polarity is unimportant.) Using a standard interconnect, apply a 100 Hz test signal (at about .30 volt is good) from your function or signal generator to the LEFT channel input jack. To avoid overheating the 8 ohm non-inductive resistors, prolonged testing should be avoided.
3. Adjust the LEFT CHANNEL AC Balance control until the meter reads as close to zero as possible. It will move around some, you will not get it to zero out completely in most cases but that's normal. Just go for the lowest reading you can get. When you get the lowest reading you are at optimum AC balance for that channel.
4. Repeat steps 1, 2, and 3 for the RIGHT CHANNEL AC balance on V6 and V7
5. Your amp is now set for optimum AC balance on both channels