V1 weird voltages in a vintage Ampeg SVT

[boroman]

I have a weird problem. I have an old SVT which I kind-of-rescued (this is my 4th or 5th SVT now!). I like challenges, but this one is a tough one! While I replaced some burnt resistors, old caps, tubes, grounding, etc etc. and got to the point the amp is almost dead quiet on idle (in a good way) but the problem is, amp overdrives at lower volumes. I'd say close to 50W but not 300 as planned... Problem lays somehwere in the main chassis, because pluggin instrument into the power amp directly (avoiding all the upper part/preamp) is exactly the same.

If of course did complete tube swap - no change.

I did voltage measurements and eveyrthing is up to specs +/- other than V1 (12DW7) which seems to have abnormal voltages
pin 1 (cathode) is almost 500V -- should be close to 280v
pin 3 (plate) is almost 500v too -- should be close to 95v (!)
pin 6 is like 50V higher too.

I checked all the resistors nearby - visually the look ok, measurements are close, but did not unsoldered them from circuit.
- I found 15k (power amp side balance) pot shot (does resistance reading only turned to one side) but checked my other SVT and even totally unbalanced, it sounds nice of close to full RMS, for dure 2-3 times louder than this one.

I will replace the pot today, but any other thoughts, on that voltages?

[loudthud]

Pin 1 is the Plate, pin 3 is the cathode. Pin 6 is also a Plate.

What does this mean : pin 6 is like 50V higher too.

[Chuck H]

Something is wrong with all this. Please re check your work and information. Pin allocations are wrong and voltages indicated are inconsistent with the schematic. Further, I was unable to view the link because the page wouldn't open. I looked up the 1977 service manual elsewhere. I'm not linking it because the site indicated it was not secure.

[Helmholtz]

I did voltage measurements and eveyrthing is up to specs +/- other than V1

So all supply node voltages measure good?
What is the voltage at node C?

[stokes]

Like Chuck said, recheck your work, stated/expected voltages and pins dont match the schem. If voltages are extremely hi I would say that is overdriving V1 and causing it to distort so early.

[boroman]

Sorry I mislabeled pins - their descriptions (plate,cathode) are wrong, but pointa of tests (pins) were right. Sorry about that.
What I was writing about the balance pot, it shorts to ground after approx 10ohm reading (so most of the pot is gone). So VERY bad. That was probably cause of everything here. As I measured with the wrong, pot, voltages go up a lot in V1 when pot reaches bad spot. I didn't have time to put new one today, will check everything tomorrow.

I also ordered 100R pots. What I learned in those old ampegs, the 100R resistors to ground (insetad of a 100R pot) is olny a temporary way. I hate all sorts of hum, so let's have it done like it should be...


Will come back for update here in few days.

Thank you!

[TomCarlos]

See if this works.

When posting pdf files, I guess our site does not like blank spaces between characters. So you need to use underscores to fill in blank spaces.

SVT_Manual_5_1_1977.pdf

[Helmholtz]

The link works fine for me as is.

[J M Fahey]

Sorry I mislabeled pins - their descriptions (plate,cathode) are wrong, but pointa of tests (pins) were right. Sorry about that.

Please spare us the mental gymnastics

Post small table showing voltage at *all* 12DW7 pins.

[Helmholtz]

...and don't forget the supply voltage at node C.