ST70 No Bias

I've recetly acquired a factory wired st70. When I got it I plugged it into power just to see if it lights up. All tubes were glowing, so I called it good enough and put it away since I knew I should replace the caps on it before I start using it. Couple months go by and I've ordered all new caps, a couple of resistors and a new can capacitor. I replaced the can capacitor as well as the 6 capacitors on the PCB and the two capacitors on the 7 hole rail. I've plugged all the tubes in and started it. Reading 500v at the capacitor can, but almost no voltages on the bias points. I read 0.02 on one and 0.002 on the other. Turning the bias pots makes no change at all. I've noticed there the selenium rectifier has been replaced, and there seems to be the solid state rectifier mod. Done as well. Could that be influencing the bias?

I'm attaching a couple of pictures. Hopefully you guys can give me some advice.

Isstatu1 wrote:I've recetly acquired a factory wired st70. When I got it I plugged it into power just to see if it lights up. All tubes were glowing, so I called it good enough and put it away since I knew I should replace the caps on it before I start using it. Couple months go by and I've ordered all new caps, a couple of resistors and a new can capacitor. I replaced the can capacitor as well as the 6 capacitors on the PCB and the two capacitors on the 7 hole rail. I've plugged all the tubes in and started it. Reading 500v at the capacitor can, but almost no voltages on the bias points. I read 0.02 on one and 0.002 on the other. Turning the bias pots makes no change at all. I've noticed there the selenium rectifier has been replaced, and there seems to be the solid state rectifier mod. Done as well. Could that be influencing the bias?

I'm attaching a couple of pictures. Hopefully you guys can give me some advice.

Did it work before you started to replace stuff ?
If you did not even test it, you are in deep water. Grab the manual and start fault diagnosing.

peterh wrote:
Did it work before you started to replace stuff ?
If you did not even test it, you are in deep water. Grab the manual and start fault diagnosing.

Did not test it before replacing stuff. I'll start measuring.

Bob Latino wrote:To see if the bias system is working, measure the negative DC voltage from pin # 5 on each of the four output tubes to chassis ground. You should get about -48 VDC to -23 VDC on each of the four output tubes. The exact value you get will be determined by the position of the bias pot.

Bob

Alright, reporting with my findings. Tested with the black lead on the chassis, VDC, all tubes populated, not connected to speakers.

V1 (Rectifier)
#1 508V
#2 426V
#3 0.03V
#4 Open Line (Diode)
#5 0.005V
#6 Open Line (Diode)
#7 0.002V
#8 426V

V2 (Left Rear Output Tube)
#1 0.022V
#2 10.4V
#3 524V
#4 524V
#5 -72V
#6 0.02V
#7 10.4V
#8 0.022V

V3 (Left Front Output Tube)
#1 0.018V
#2 10V
#3 525V
#4 525V
#5 -72V
#6 0.06V
#7 10.1V
#8 0.018V

V6 (Right Front Output Tube)
#1 0.004V
#2 5.05V
#3 523V
#4 523V
#5 -72V
#6 0.03V
#7 5.16V
#8 0.04V

V7 (Right Rear Output Tube)
#1 0.004V
#2 5.7V
#3 521V
#4 521V
#5 -72V
#6 0.016V
#7 5.54V
#8 0.004V

The quad cap:
Unmarked: 526V
Half Circle: 527V
Square: 366V
Triangle: 454V

I was starting to think about grabbing a new board and most of the components, but this is a factory wored one so I did not want to lose the rivets. I definitely will get a st70 kit in the future and wire it from scrath. Sounds like my kind of fun for a weekend or two.

So I've started looking at the diagrams and the way the bias pots are wired. Everything looked just like it should be. Then I went over it again and pulled on each wire ever so slightly. One grounding cable that goes to the 7 hole terminal was broken by the grounding lug. Stripped the end, soldered it back. Voltages are perfect, set the bias, plugged in some speakers and voila! It sounds beautiful! Can't wait to explore it more. Thank you Bob for your help!

Isstatu1 wrote:

Bob Latino wrote:To see if the bias system is working, measure the negative DC voltage from pin # 5 on each of the four output tubes to chassis ground. You should get about -48 VDC to -23 VDC on each of the four output tubes. The exact value you get will be determined by the position of the bias pot.

Bob

Alright, reporting with my findings. Tested with the black lead on the chassis, VDC, all tubes populated, not connected to speakers.

V1 (Rectifier)
#1 508V
#2 426V
#3 0.03V
#4 Open Line (Diode)
#5 0.005V
#6 Open Line (Diode)
#7 0.002V
#8 426V

V2 (Left Rear Output Tube)
#1 0.022V
#2 10.4V
#3 524V
#4 524V
#5 -72V
#6 0.02V
#7 10.4V
#8 0.022V

V3 (Left Front Output Tube)
#1 0.018V
#2 10V
#3 525V
#4 525V
#5 -72V
#6 0.06V
#7 10.1V
#8 0.018V

V6 (Right Front Output Tube)
#1 0.004V
#2 5.05V
#3 523V
#4 523V
#5 -72V
#6 0.03V
#7 5.16V
#8 0.04V

V7 (Right Rear Output Tube)
#1 0.004V
#2 5.7V
#3 521V
#4 521V
#5 -72V
#6 0.016V
#7 5.54V
#8 0.004V

The quad cap:
Unmarked: 526V
Half Circle: 527V
Square: 366V
Triangle: 454V

You have -72V on the grids, thus the tubes will not draw any current and the B+ is too high.

There is a fault in the bias circuit, fix that. ( missing ground ?)

Many of the voltages you listed are strange. Pin 1 on the rectifier (typically unconnected) is higher than rectifier outputs on pins 2&8. As already mentioned your first cap voltage is even higher. On the heaters of some output tubes you showed 10.3 V, and on others 5.7 V. I suggest you take out all tubes and start a thorough review of of your AC voltage on the power transformer, then only with you rectifier tube check your DC voltage correctly, especially the negative voltage for the bias. (Assume you replaced the selenium rectifier with a diode?

Thank you for your help. It turned out to be a loose ground wire. After soldering it back on it's back to business.

Glad to hear you got this sorted out. Hopefully you learned an important lesson: when acquiring a piece of vintage equipment, test it for functionality before doing any mods. That way you're only troubleshooting the piece in its original condition, not that plus your mods. If you have any doubts about electrolytic capacitors that may not have been energized for decades, hook it up to a Variac and ramp up the voltage slowly, over a period of a day or more. But you apparently fired it up at full voltage immediately, which sort of defeats the purpose of replacing the caps, without bothering to check whether it passed a signal. Those parts you put in are expensive and if you were unable to get the amp going, you've nonetheless put a ton more money into it.

The original caps are definitely worth replacing, after first determining what needs to be done to make the amp work and fixing it. The "Authenticaps" look original but they are built from modern electrolytics, which are far better and more reliable than they were in the 1960's. I would do it in stages, testing after each mod. You may or may not hear a difference, but you will definitely gain in reliability.

I would also second Bob's recommendation, now that you have it working, that you consider replacing the driver board.

Well done , but just double checking  ....

Did you check that all the filament voltages are correct ? Voltage across filament pins on EL34's should be ~6.3V AC .

This pin connection diagram IS NOT applicable to 5881 & KT77 tubes. In those tubes pin 1 is unconnected and the suppressor grid internally wired to pin 8.

spantou wrote:This pin connection diagram IS NOT applicable to 5881 & KT77 tubes. In those tubes pin 1 is unconnected and the suppressor grid internally wired to pin 8.

Yes , that is correct @Spantou , you oh so very observant man . Bravo ! . EL34 is a true pentode , and KT77 is a tetrode . .
Heaters/filaments  are still pins 2 and 7 .