PAS-2 Voltage Woes after Z-Mod and Upgraded Power Supply Board

Hi all,

I'm still very much a noob in terms of tube amps, so please bear with me.  My father recently handed down his PAS-2 preamp and ST70 amp, both in dire need of repair.  After reading and re-reading several forums, websites, etc, I decided to take the plunge and start with the PAS by upgrading the Phono and Line PCBs with TubeNirvana's z-mod PCBs and threw in the upgraded power supply board for good measure.  I soldered all connections as indicated and triple checked that all capacitors/resistors/diodes were correctly placed/oriented before supplying any power to the unit.  I also ran continuity checks after each new soldered joint to ensure no breaks or ground faults.  For test purposes, I plugged in my two old 12AX7s and two new 12AU7s along with the 12X4 rectifier tube.  As I don't yet have a working amp (or know anyone local who does), I'm left to judge the success of my work based only on the voltages that I can measure.  All tubes glow faintly orange.  According to the voltages on the original schematic, I'm way off base.  Here is what I am measuring.  Note that I recorded any measured voltage be it AC or DC...can anyone with experience with these boards let me know where these voltages should be?:

Wall: 122 VAC
Power Supply Board:
Transformer (blue wires): 10.5 VAC
B+P (PC6-16): 146 VDC
B+L (PC5-16): 205 VDC
B+IN PIN7 12X4: 392 VDC
Line F+ to Line F-: 23.9V

12X4
1: 363 VAC
2: 0
3 and 4: 10.5 VAC
5: 0
6: 362 VAC
7: 392 VDC

PC6
Tube1
1: 76VDC
2: 0
3: .45VDC
4: 66.7VAC/31.2VDC
5: 19.5VDC
6: 67.8VDC
7: 0
8: .47VDC
9: 53.9VAC

Tube2
1: 72VAC
2: 0
3: .45VDC
4: 66.5VAC/31.2VDC
5: 19.82VDC (intermittent)
6: 78VDC
7: 0
8: 0
9: 0

PC5
Tube 1
1: 92.3VDC
2: 0
3: 4.4VAC
4: 0
5: 90.6VAC
6: 91VDC
7: 0
8: 2.45VDC
9: 36.7VDC

Tube 2
1: 187VDC
2: 93VDC
3: 0
4: 0
5: 65.5VAC
6: 187VDC
7: 91.6VDC
8: 0
9: 25.1 VDC

Thanks in advance for any insights,

Jeff

deathman68 wrote:Hi all,

I'm still very much a noob in terms of tube amps, so please bear with me.  My father recently handed down his PAS-2 preamp and ST70 amp, both in dire need of repair.  After reading and re-reading several forums, websites, etc, I decided to take the plunge and start with the PAS by upgrading the Phono and Line PCBs with TubeNirvana's z-mod PCBs and threw in the upgraded power supply board for good measure.  I soldered all connections as indicated and triple checked that all capacitors/resistors/diodes were correctly placed/oriented before supplying any power to the unit.  I also ran continuity checks after each new soldered joint to ensure no breaks or ground faults.  For test purposes, I plugged in my two old 12AX7s and two new 12AU7s along with the 12X4 rectifier tube.  As I don't yet have a working amp (or know anyone local who does), I'm left to judge the success of my work based only on the voltages that I can measure.  All tubes glow faintly orange.  According to the voltages on the original schematic, I'm way off base.  Here is what I am measuring.  Note that I recorded any measured voltage be it AC or DC...can anyone with experience with these boards let me know where these voltages should be?:

Wall: 122 VAC
Power Supply Board:
Transformer (blue wires): 10.5 VAC
B+P (PC6-16): 146 VDC
B+L (PC5-16): 205 VDC
B+IN PIN7 12X4: 392 VDC
Line F+ to Line F-: 23.9V

12X4
1: 363 VAC
2: 0
3 and 4: 10.5 VAC
5: 0
6: 362 VAC
7: 392 VDC

PC6
Tube1
1: 76VDC
2: 0
3: .45VDC
4: 66.7VAC/31.2VDC
5: 19.5VDC
6: 67.8VDC
7: 0
8: .47VDC
9: 53.9VAC

Tube2
1: 72VAC
2: 0
3: .45VDC
4: 66.5VAC/31.2VDC
5: 19.82VDC (intermittent)
6: 78VDC
7: 0
8: 0
9: 0

PC5
Tube 1
1: 92.3VDC
2: 0
3: 4.4VAC
4: 0
5: 90.6VAC
6: 91VDC
7: 0
8: 2.45VDC
9: 36.7VDC

Tube 2
1: 187VDC
2: 93VDC
3: 0
4: 0
5: 65.5VAC
6: 187VDC
7: 91.6VDC
8: 0
9: 25.1 VDC

Thanks in advance for any insights,

Jeff

both PC5 and PC6 boards should have 12ax7 / ecc83 tubes, not 12au7. Wrong tubes
will screw up voltages ( and sound). Go get some 12ax7 !

Whoops, i now realize that this is not a PAS, it's a tube nirvana variant. Thus tube & voltage description from
the Dynaco PAS manual does not apply.

Hey everyone, Holger from TubeNirvana has been helping me to directly troubleshoot this problem and has already made some helpful suggestions to point me in the right direction. Nice to see such support in this day and age! I will report back in a few days with any progress.

I know that the original post is over a year old, but I am having the exact same issues with my recently modded PAS2 (with new Z-mod boards).  My voltages are nearly identical to the OP's, and I cannot figure it out.

deathman68--are you still out there?  Curious as to what the solution ended up being for you.

Hi mdye,

I'm happy to report that once I swapped the main transformer the problem went away and I was able to immediately dial in the bias voltages. I lost a lot of time and sleep over this one for a fix that in the end was so simple. Keep us apprised of your progress and shoot me a message if you need me to check/confirm anything since it would seem that very few ppl are running this particular setup.

Thanks for the quick reply, deathman! Was your transformer shot, or is it that the stock transformer just isn't up to the job when matched with the Z-mod boards? Did you just get a new transformer from dynakitparts or similar place?

Thanks again.

I guess none of you guys read my sticky post from a couple year ago.

https://dynacotubeaudio.forumotion.com/t1728-fully-modified-pas3-or-a-new-sp12

After trying everything I can think of and going over the boards too many times to count, I finally gave up and purchased a new PAS power transformer from Dynakitparts.  Unfortunately, I am still getting the exact same voltages out of the power supply even with the new PT installed--about 215V to the line stage and 172V to the phono.  Anyone out there have any ideas?

Really appreciate it!