Basket case Ampeg SVT2 pro

[Climberkalen]

@ g1 I have a normal ground probe for reading across the filimant pins, but usually an alligator clip tip to the silver chassis screw that's the main ground for the ac mains board.

[Climberkalen]

So to follow up on voltages, I did read the pinout backwards. I installed the au7s and single ax7 on the poweramp to see if that was throwing things off. These are the voltages on powertube V1.

Pin1 is 0
pin2 is 7.25Vac (i also spaced and forgot to swap my meter back to AC on thr first reading of the filimants)
Pin3 overloads the meter, so assuming 660Vdc
pin4 is negative 10mVdc with a little fluctuating
pin5 is negative 95.5 Vdc
pin 6 is nothing but when I wiggle the probe around in the socket It fluctuates slightly and goes back to 0
pin 7 is the 7.25ish Vdc
pin 8 is nothing but will fluctuate low mvs and go back to 0 when I wiggle the probe.

to keep it simple im gonna focus on 1 power tube socket at a time trying to upload the photos, but its not letting me. I'll try to upload them in another post.

again thanks for the assistance so far.

edit:it will not let me upload photos, says the photo is 2.7mb but only have 978kb?

[The Dude]

You must have something plugged in wrong, ground reference is off, etc. Voltages still funky. Pins 2&7 should read about 3.2VAC to ground or 6.3VAC across the pins. You have no screen voltage on pin 4. Pin 5 is now at least negative, but about double what is expected. I guess I'd start by tracing screen voltage and see why it's not there. You could have one or more open screen resistors or a power supply problem. What do you have for voltage on both sides of R58 in the power supply?

[Climberkalen]

The Dude I don't have any of the 6550s installed, it reads 7.25 from pin 7 to 2. Could the tubes not installed let the voltages be higher that 6.3?

[The Dude]

Yes. 7.25 is fine for filaments with no tubes installed. I'd concentrate on why there's no screen voltage as I said above. That's the thing that seems to be off the most. If you have no screen voltage on just this tube, it's likely an open screen resistor. If you have no screen voltage on any tubes, I'd first check voltages at R58.

[Climberkalen]

The Dude Voltage reading on R58 is 538Vdc on one leg and 527/528Vdc on the other.

[The Dude]

OK. That node should go to all of the 22 ohm resistors that feed the tube screens. See the amp portion of the schematic I posted. You have voltage at the power supply resistor, but no voltage at the screens. So, either the voltage isn't making it to the output tube board, or it is and your screen resistors are open. You'll have to figure out which it is. Check both sides of the screen resistors (22 ohms) and see if the voltage is on either side. It's also a good idea to just quickly check those six 22 ohm resistors and make sure none are open.

[Climberkalen]

The Dude, I thought that the screen grids on the power tube board were upped to 220ohm and the diode in parallel gets removed? I got negative 15v on one side and negative 14 on the other of each R2,R4,R6,R7,R9,and R11

[The Dude]

I'm not sure where negative voltage would come from at that point. I think you have something plugged in wrong. Follow the screen resistors to the top left corner on the power amp page of the schematic. They are supposed to connect to the 345V node. In post #21 you measured 527V on that node, yet it is not getting to the tubes. You have a connection issue. Trace your connections and find out why screen voltage is not getting to the screen resistors.

[Helmholtz]

Screen voltage is lower than screen supply voltage.
So when you connect the red meter lead to the screen and the black lead to the supply you will get a negative reading.
Reverse meter leads to get a positive reading.

[The Dude]

If he measured correctly, there is no screen supply voltage getting to the board at all. I'm not even concerned about the voltage drop of the screen resistors at this point. That all assumes measurements were taken correctly.

[Climberkalen]

Sorry just got home from work. The Dude and Helmholtz for measuring the voltages at the tubes I was grounding the black lead on the AC mains board, the main ground lug with an aligator clip. The on that has the goofy aluminum bracket around it. Positive lead straight in the tube sockets. Measuring filimants across pin 2 and 7. Im trying to upload pictures still. It's not letting me add them as an attachment

Also am I wrong about the power tube boards screen grids being bumped up to 220ohm and the diode in parallel removed?

[g1]

Also am I wrong about the power tube boards screen grids being bumped up to 220ohm and the diode in parallel removed?

You are correct, see attached

[Climberkalen]

Thank you g1. I did update it to 220ohm. I did all the funky solder joints and that update when I when I added the tiny little cap on IC2 to prevent the false triggering issue. I did a ton of lurking on the forums before I made my post. So I have to give you all a huge thank you. G1, the dude, nevetslab and the many others helping in the other posts about SVT's got me this far.

I also very much appreciate not giving me the usual "take it to a tech". So just to put a little ease in the minds of you all. My neighbor was a radio tech for the navy back when they still used tubes for everything. He taught me a little about circuit topology, reading schematics and soldering. He passed before we moved up to the oscilloscopes. He was a cool old man. I also worked with my pops as an electrician before I moved. Sorry to go down memory lane a minute. just wanted you all to know I'm not some bone head poking my finger around in a live amp.

Once i eat dinner ill let it warm up and do some probing. Ill try to backtrack from the screen grids to the power node that has the appropriate voltage.

But one thing thats bugging the hell out of me. It won't let me upload photos. Is it a newer account restriction thing or is the file size limit 970 somthing kb. Pictures im wanting to upload are like 2.5-2.7mb.

[The Dude]

I believe there is a new user upload limit, though not entirely positive. That said, I always crop, compress, edit, etc. photos before uploading. I've always been able to get decent pics at less than around 200K. We're not selling professional family photos here. Pics are just for reference, so don't need to be high res (for the most part). Plus, it takes up less server space and they load faster.