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Initial power up procedure

PostPosted: Tue Oct 13, 2015 6:26 am
by SteveP
I am building the triode electronic version of the ST35 Rev E. I didn't find details pertaining to initial power up in the manual.

Assuming proper pre-turn on ohm measurements per the manual (I haven't done a final measurement after completely wired yet), what is the recommended power up procedure? Should the unit be powered up without tubes on a variac slowly (I have a variac) and the voltages confirmed close to correct prior to putting the tubes in and turning it on? Your assistance, recommendations and advice are appreciated with my thanks in advance.

Thank you,


Re: Initial power up procedure

PostPosted: Tue Oct 13, 2015 3:28 pm
by dcriner
Without tubes, most of the voltages will meaningless. I would install tubes and bring it up slowly on your variac, watching for smoke, etc. You can check B+ and heater voltages as you bring up the variac, but personally, I probably wouldn't bother.

Re: Initial power up procedure

PostPosted: Sat Oct 31, 2015 6:46 pm
by SteveP
Good Evening,

I powered up the ST35 two weeks ago, with no major problems. But, per the table in the manual, several measured voltages are off.

Referring to the voltage table in the manual:
Regarding V1 and V2, all 116 volt readings read around 100 volts and all 207 volt readings read around 220 volts.
Also the 780mv readings measure about 660mv.

The results are essentially the same with three different sets of 12ax7 / 12au7’s.
Line = 119VAC
320V = 313V
380V = 367V
Here are the actual readings:

V1 V2 V3 V4 V5 V6
Pin 1 100 222
Pin 2 -0.2V 99 15mv 15mv 15mv 15mv
Pin 3 664m 104 13 13 14 13
Pin 4 3.07 3.07 3.07 3.07 3.04 3.07
Pin 5 3.07 3.07 3.07 3.04 3.04 3.07
Pin 6 103 220
Pin 7 -0.1V103 358 359 358 358
Pin 8 650m107
Pin 9 3.07 3.07 363 363 363 362

Last weekend, I verified all part values and connections and found no errors. I reflowed and inspected all solder connections.
I also replaced R11 because it had a funny bulge even though the resistance measured right (33K).

I used the amp for a couple of nights, thinking maybe the voltage readings might be acceptable.
But honestly, the amp does not that good.

I used the 4 ohm output from the z565 output transformers for negative feedback at J3/J4 pin 3. The resistance at J3/J4 pin 3 is set to 20.3K.

Did I use the correct (4 ohm) output for feedback? Is the negative feedback set to the recommended value?

Notes on initial resistance measurements:

The secondary of the PA-774 transformer reads about 71 ohms rather than 94 ohms as indicated on page 4 of the manual, initial impedance measurements (with all transformer connections wire to the board).

The resistance readings of pins 1 and 2 of J2 - J5 read over 10M but drifted lower as I measured, as though a capacitor was charging as I made my measurements (PC board not connected to the transformers).

I am baffled. I found no errors with components values or connections, but I must be missing something. Please help.



Re: Initial power up procedure

PostPosted: Sat Oct 31, 2015 6:51 pm
by dcriner
How does it sound? If OK, then forget it and enjoy.

Re: Initial power up procedure

PostPosted: Sun Nov 01, 2015 11:35 am
by SteveP

Thanks for your responses. Yes it does sound good. I will do just that, listen and enjoy.

I just wanted to make sure that the bias voltages for the input tubes are within tolerances. The measured voltages are a little farther off than what I would have considered OK. I won't worry about it unless instructed on this board that they are too far off, and how to correct.

Have a great day and thanks again,