Low voltage Orange

[drewl]

Got a Rockerverb50 that only has like 380v on the plates and only about 15w of output power, so I checked the HV AC and it's only like 650vac.
It has a switch on the back for either 100v or 120v ac input, changing it to 100v input bumped up the power tranny's output to about 750vac and 450volts on the plates.
It's supposed to be about 500v dc.
Anyone know if they have a tendency for the power tranny's to go bad?
I'll have to check the wiring and ask the owner if it ever had more output, but it sounds like something funny with the PT.

[Steve Conner]

What heater voltage do you measure?

[drewl]

6v AC for the output tubes and phase inverter.
The other preamp tubes are running DC....I'll double check those.

[spy]

Don't forget to measure the voltage of your wall socket!
Otherwise the amp has low voltages, check the primary wiring. Something is going on over there. Does the PT become hot? If not it's ok....

[drewl]

Thanks for the help, but I'm repairing it, I'm not the user....unless the voltage for the entire eastern seaboard grid has dropped.

[Enzo]

Drewl, you have 650VAC on the rectifier, that's 325 per side, right?

325VAC rectifies and filters to about 459VDC.

Flip your meter to AC and measure that 380v plate supply. How much ripple you got? CLip a fresh filter cap on the first filter stage. Does that help?

[drewl]

according to the schem each ac high leg should be 290vac which they are, so that seems right.
Ripple is good, about 1.6vac clipped another cap in and it was the same.
Damn, I thought you'd nailed it Enzo.
Something is loading down the DC, I'll start taking things apart until I find out what.

Edit:
Removing output tubes, output tranny or choke from circuit yeilds no change.
Correct me if I'm wrong, but there should be ~500vdc on one side of the standby switch when it's in standby right?
That removes the first cap, so the diodes may be flaky (they measure okay).

http://users.telenet.be/orangefg/OFG...50C_sheet3.jpg

[drewl]

Just double checked, only 290vdc on standby switch in standby mode....

Another thing, when you turn it off that DC just slams to zero volts, no slow drain just BAM as soon as power is off right to zero.....Think I'll start with the diodes.
Thanks all.

[Enzo]

How about the center tap connection to ground, how is that. I really doubt your diodes are bad. A tube rectifier can get weak, but semiconductor diodes pretty much are going to work or fail. And they fail short or open mostly.

In standby, you measure very low DC on the hot side of the switch. is there a filter cap on that side? If not, your reading would be normal.

Try this, leave the standby switch OFF so it breaks the B+ path. Connect your meter across the switch set up to read DC current. APply power to the amp leaving standby switch OFF. Now your ammeter is serving as the standby switch. How much current does the B+ draw. Without tubes it ought not be very much. That will tell you if it is loading down or otherwise.

WAIT.

Oh, I just checked your schemo. See there are no filters on the hot side of the switch. So all there is ther is pulsing DC, which will read about what your AC reads. Leave it OFF and clip a cap from that side of the switch to ground and see if the DC doesn;t come up to 400-450 or whatever. 290VAC becomes 410VDC after rectification and filtration.

You know, looking at your schematic, I expect 410VDC on the B+. Not all that far from your 380. Where did you get the idea there should be 500v here? The filter caps are only 400v caps on the schematic. I don't think they were expecting 500v there. The power tubes are shown as 6V6s, which don;t very often run at 500v, 400v being over spec as it is.

[Steve Conner]

Oh it's 6V6s? 50w is a bit optimistic for four of them. But 15 is definitely underperforming. I'd expect something like 30W clean.

Are you sure you connected the right impedance of dummy load, all four tubes are good, and none of the tube failure fuses or screen resistors have popped?

[drewl]

Epic Fail on my diagnoses.
You're right, that first filter cap is only 400v, so it must be a low output problem not a low voltage problem. The output board says voltages in excess of 500vdc, maybe the board is used for other models too.

Okay, all 4 tubes conducting, screens are good, I put a signal into the fx return bypassing preamp and still couldn't get any more out of the power amp.
I'll start with jumping in a new output tranny, the leads are on push on clips so that will be easy.

[tubeswell]

Okay so the plates voltages are ballpark but the vol is low? (Could this be a leaky coupling cap somewhere dragging down the bias of one of the stages?) - Just a stab in the dark

[drewl]

A correct stab, in the phase inverter, which is why running a signal into the fx loop return made no difference.
It's now putting out 50w into 8ohms.
Boy was I way off.

[tubeswell]

Hey whaddaya know? Sometimes I'm wrong and sometimes I'm just a lucky tin-arse (but I had a hunch that is what it might be after having had the benefit of reading all the other posters and eliminating other possible causes )