Ampeg SVT

[leydenjar]

I have 580 vdc on node B in the power supply of this svt I'm working on.

Node A is 685vdc and R49 reads fine on my meter. R47 and R48 also check ok.

Node C is within spec as well.

Do wirewound like R49 change value before they fail?

I should add nodes D and E are within spec as well.

[Jazz P Bass]

What does node B read after R47 on the preamp board?
Kind of strange that node C is not high also.

[Tom Phillips]

It's very odd that you find that nodes A&C are "within spec" but that the node B voltage is so high. Are you absolutely sure you are measuring at the correct point and that the amp you have is wired as shown in the schematic you attached?

[Bruce / Mission Amps]

Of topic but what a great power transformer!!
Full wave Bridge HI-V rectifier, separate high voltage B+ winding for power tube plates, separate screen winding with a full wave rectifier, full wave bias supply... etc., etc...

[g1]

The schematic notes mention that if the P2 connector to the preamp is disconnected, the voltage at node B will rise to 600V. Is there power to the preamp?

What does node B read after R47 on the preamp board?
Kind of strange that node C is not high also.

[soundguruman]

You never said what the amp is doing or not doing.

[soundguruman]

Look normal to me, Node B is 585 and you have 580

I have 580 vdc on node B in the power supply of this svt I'm working on.

Node A is 685vdc and R49 reads fine on my meter. R47 and R48 also check ok.

Node C is within spec as well.

Do wirewound like R49 change value before they fail?

I should add nodes D and E are within spec as well.

[J M Fahey]

Agree.
What's the problem, if any?

[Tom Phillips]

Look normal to me, Node B is 585 and you have 580

Per the posted schematic Node B nominal voltage is listed as 485V not 585V. So the oddity here is that the OP's reading is ~100V high at "B" but back to normal at "C" and subsequent nodes.

Also to answer the OPs other question "Do wirewound like R49 change value before they fail?" I would say no and even if R49 changed in value it would not explain the readings posted.

[leydenjar]

Ok. I am updating my voltages to reflect change in variac setting. It's now at 122vac.

I have 716 vdc at node A and that's what I have on ther plates of the power tubes as well.

I have 601 vdc on node B. I am measuring on the other side of R49. This is not a mistype. I then have 467vdc on node C.

Node D is measuring -186 vdc and I am measuring where the anodes of D12 and D13 meet on the circuit board. There are of course two circuit boards on these SVT amps which , to me, makes them a PITA to work on.

Measuring at the cathode of D14, Node E, I get 356vdc.

Another problem: I have 435vdc on the chassis of the preamp. I find this shocking (pun intended) and I almost found it to be so literally. How 435 vdc is getting on the chassis I don't know and I can't see anything touching the chassis from the preamp circuit that would cause this.

The amp does not pass signal though connected to speaker it does hum.

[Amp Kat]

Ok. I am updating my voltages to reflect change in variac setting. It's now at 122vac.

I have 716 vdc at node A and that's what I have on ther plates of the power tubes as well.

I have 601 vdc on node B. I am measuring on the other side of R49. This is not a mistype. I then have 467vdc on node C.

Node D is measuring -186 vdc and I am measuring where the anodes of D12 and D13 meet on the circuit board. There are of course two circuit boards on these SVT amps which , to me, makes them a PITA to work on.

Measuring at the cathode of D14, Node E, I get 356vdc.

Another problem: I have 435vdc on the chassis of the preamp. I find this shocking (pun intended) and I almost found it to be so literally. How 435 vdc is getting on the chassis I don't know and I can't see anything touching the chassis from the preamp circuit that would cause this.

The amp does not pass signal though connected to speaker it does hum.

Think I had this same problem one time and it was the 6C4 tube was bad but would try pulling those preamp tubes along that string of Node B voltages and see if that brings the voltage back down. Plate impedance changes sometimes when tubes go bad.

[g1]

I have 716 vdc at node A and that's what I have on ther plates of the power tubes as well.

I have 601 vdc on node B. I am measuring on the other side of R49. This is not a mistype. I then have 467vdc on node C.

Ok, this makes much more sense than what was earlier reported (that node C was ok). Node A is close enough, as are D and E.
Get your preamp ground sorted out and you may solve it. Pin 4 of the P2 connector grounds the preamp. It is the shield of the signal wire (pin1) and it may be broken.

[leydenjar]

Thanks very much g-one. You were spot on.

[leydenjar]

I have to revive this thread because this amp has a horrible hum after I put i new tubes. I think the old ones were so weak the hum wasn't bad but with new winged c sextet it's unbearable. It sounds like heater hum. LOUD heater hum. I've got 4.5 v ac at node A ?! I've got .2vac at node D but 1.8vac on pin 7 of the 12bh7.

Help!

[leydenjar]

I have to revive this thread because this amp has a horrible hum after I put in new tubes. I think the old ones were so weak the hum wasn't bad but with new winged c sextet it's unbearable. It sounds like heater hum. LOUD heater hum. I've got 4.5 v ac at node A ?! I've got .2vac at node D but 1.8vac on pin 7 of the 12bh7.

Help!

Forgot to mention. The amp sounds very good now after all the work I did to it. Except for the hum of course.

Also, it does NOT have a hum balance pot as shown in the schemo. It just has 2 x 100 ohm resistors to ground. I have them grounded on one of the can cap grounds.