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    Bad ST-70 transformer?



    Posts : 44
    Join date : 2010-02-12
    Location : Alaska

    Bad ST-70 transformer?

    Post by polarflux on Thu Mar 04, 2010 6:23 pm

    I received a new ST-70 driver board from Roy, put it together, installed it, and I am still having trouble with my amp. This is an amp that Bob built that I bought second hand and been having a bunch of trouble with.

    During the initial start up I noticed that the AC filament current was very low (~200 mV) and the 12AU7s were not glowing (but they did get a bit warm). I used the 12AT7s that I have and they did glow? I went ahead and put in the rectifier and the left power tubes and attempted to bias. I noticed significant hum coming from the power transformer, which settled down after the rectifier had been on a few moments. The hum was still audible after a minute. With both bias pots in the center position the bias voltage for both tubes was very high. Turning both pots completely clockwise only reduced the voltage to ~0.8 volts. On the right side things were a little weirder. I started with both pots full clockwise, turned on the amp and the readings were ~0.3 volts for the front tube and ~0.09 volts for the rear tube. What was weird, was when I attempted to increase the bias voltage on the front tube (the pot at full clockwise) the voltage would go down, and when adjusting the bias on the rear tube (the pot in the full clockwise position) the voltage would go up, but only slightly over one volt, and so would the hum coming from the power transformer? Adjusting the rear bias seemed to draw much more current for some reason. I do need to check the tubes I was using, too.

    Another strange thing, the resistors in R7 and R8 are 6.8K, but when I measured them in the board they read 100 ohm? I pulled one out and sure enough they were 6.8K.

    Anyone have some insight on this?


    Posts : 44
    Join date : 2010-02-12
    Location : Alaska

    False alarm...

    Post by polarflux on Thu Mar 04, 2010 7:17 pm

    Well, the amp seems to be working the ways its supposed with the exception that the bias is high. One of the EL34's I was testing the amp with was probably malfunctioning and giving me weird readings. I plugged in the four kt88s I have and the biases for all the channels is in the 0.58v range. A bit high, but a few resistors should lower the voltages into an acceptable range. I am using a low gain board and with only the 12 AU7 plugged in during the start up I could not see any glow from them. I think that threw me off to begin with. Although not glowing, they are working.

    There is still an audible buzz in the right channel that has been there since I received the amp. I hope the output tranny is not slighly buggered. At least the amp is making music. Grounding issue? Also, when I place my hand close to the voltage amplifier tube a buzz becomes audible. Normal?

    Hey, what is the best way to check voltages while the amp is running? It hard to flip upside down with tube attached.

    Bob Latino

    Posts : 2455
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bad ST-70 transformer?

    Post by Bob Latino on Thu Mar 04, 2010 8:02 pm

    Hi Phil,

    You can run the amp upside down with no problem. Just place it on a couple of layers of towels so the heat of the tubes won't damage the surface that you are on.

    Do a full voltage check on the amp and check your results with what is below.

    GZ34 – Pin 2 to ground – 420 - 450 volts DC
    Pin 8 to ground – 420 - 450 volts DC
    Pin 4 to ground – 355 - 380 volts AC
    Pin 6 to ground – 355 - 380 volts AC

    Any EL34 – Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting)
    Across pins 2 and 7 – 6.0 – 6.5 volts AC
    Pin 3 to ground – 400 - 435 volts DC
    Pin 4 to ground – 400 - 435 volts DC (2 or 3 volts higher than pin 3)
    Pin 5 to ground – minus 30 to minus 40 volts DC
    Pin 6 to ground – minus 30 to minus 40 volts DC
    Pin 8 to ground – approx .400 volts DC (depends on bias setting)

    Note - the symbols mentioned below are for a STOCK quad cap. If you have another quad cap the symbols could be different ??
    Quad cap
    Section # 1 (half-circle symbol - one choke lead and wire to pin 8 of the rectifier tube) –------ 420 – 450 volts DC
    Section # 2 (no symbol - another choke lead and the two red wires from the two output transformers) --- 420 – 445 volts DC
    Section # 3 (triangle symbol - on right side of the amp) ------------- 420 – 445 volts DC
    Section # 4 (half-square symbol - faces the front of the amp) ------ 380 – 430 volts DC



    Posts : 44
    Join date : 2010-02-12
    Location : Alaska

    Thanks for the resistor trick

    Post by polarflux on Fri Mar 05, 2010 1:30 pm

    Thanks for the tip about adding the extra resistor to get th bias voltage into a controllable range. I was able to get the KT88s nicely biased at .5v each. The slight hum is balanced in both channels and is probably only audible since I am running Klipsch Fortes which are very efficient. That old board with all the extra capacitance definitely provided more low end whump, but at the expense of some sort of electromagnetic mayhem.

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