Tube amp power question

[Electron Tornado]

I'm working on a Harley Benton GA5, which is a copy of an Epiphone Valve Jr with a tone control. Here's a schematic  http://www.sewatt.com/files/sewatt/HB-GA5-Stock_0.pdf

When it's powered on there is no glow in the tubes. I checked the power transformer voltage that goes to the filaments. According to the schematic it should be 6.3 VAC, but only measures 3.5 VAC. Resistance on that section of the transformer secondary reads 0 ohms.

Voltage at point C in the schematic is 175 VDC.


Am I correct thinking the power transformer is bad, or should I check something else?

[amptramp]

It could be a bad transformer or it could be a shorted load on the filament output.  the filaments use rectified and filtered DC and an open or shorted diode in the diode bridge or a shorted or leaky C12 filter cap could cause the problem.  Disconnect the filament winding and measure the AC filament voltage without a load.  If it is back up to normal, check the wiring for shorts or pieces of insulation flaked off the filament wires in the chassis.  Note that the filament voltage will be two diode drops below the peak voltage with no load and below the average with a load.

My advice would be to run the filaments off AC (delete the rectifier bridge and C12) and see if it works.  If it does, I would leave it that way with one side of the filament line connected to the cathode of the output tube.  Having the average filament voltage above zero reduces hum by reducing direct emission of electrons from the heater (as opposed to the cathode).  Some tubes need this more than others.

[Electron Tornado]

Thanks for the reply amptramp. Here is how I measured the transformer voltage: I disconnected the transformer leads from the lugs on the PCB and measured directly at the ouput of the transformer with no load and got 3.5 VAC. If I reconnect those leads and measure at the tubes themselves I get 2.5 VDC.

[merlinb]

Are you in a country with 230V mains, but the transformer is set for 120V?

[Electron Tornado]

Are you in a country with 230V mains, but the transformer is set for 120V?

No. I'm in the U.S.

[JRB]

You sure your not measuring between a centre tap and one side of the whole transformer? 3,5V sounds like roughly half 6,3V unloaded.

Edit: just noticed on your schematic that the 230VAC primary is drawn as being connected. So the next question to ask is are you sure the primary is connected properly?

[R.G.]

The schematic, if accurate, implies some oddities about the filament supplies.

It shows "6.3Vac" full wave bridge rectified, filtered, and fed to the filaments. The problem is, 6.3Vac rectified and filtered doesn't produce 6.3Vdc.

If you FWR 6.3Vac you get 8.9V peak, minus the two diode drops in the bridge, or about 7.5Vdc. It is possible that they set up the transformer internal resistance and the ripple on the filter cap to let this sag down to something like 6.3Vdc, but that's a very odd way to do it.

3.5Vac doesn't rectify/filter to 6.3V either - it's too low, about 3.6Vdc.

But be that as it may.

The right way to test this is to (1) disconnect the PT from the board, as you may already have done; (2) study the schematic very carefully, and apply a known AC voltage to the primary section of the PT - that is, 115Vac to the 115Vac taps. Then measure the primary taps and the secondary windings for what voltage they have. For instance, the 0-240 connection on the primary should be about 240 +/- a bit. The HV secondary should be 260V +/-; the 6.3Vac winding should be 6.3Vac, probably a bit higher because of the high current loading. The 12V windings should be about correct too.

As usual, BE VERY, VERY, VERY CAREFUL MESSING ABOUT WITH AC POWER LINE VOLTAGES - YOU COULD BE KILLED. I'm assuming you have already obtained the skills to do this safely or you would not be doing this.

If the voltages come out about right, the PT is probably OK. If the voltages are off, take Merlin's suggestion in the more general sense, that being that the taps on the primary winding may be set up wrong. Double check your connections and which wire is which before assuming the PT is dead.

It might be - but I don't see anything in your description that makes me think it is.

[anchovie]

No. I'm in the U.S.

What AC voltage are you getting on the HT secondary? Schematic says 260V. If you get roughly half that (like the filament voltage) then that'll confirm that you're running 120V mains into a primary set up for 230V.

Your 175VDC at point C seems suspiciously low.

[PRR]

I think Merlin's fingers typed backward.

If you have a 230V amp in a 115V land, you expect 3V and 175V where should be 6V and 350V.

Check your 115V/230V jumpering.

(The 350V at point "C" will sag-down when V1 sucks current, but at 3V heat the suckage is almost zero.)

The 6.3V AC into FWB and 4,700uFd to a heater load "is" 6V DC, near enuff.

Duncan gives 6.21V RMS for assumed PT loss and 1N5404 rectifiers (1n400x will not last with EL84 and an AX7). It also gives 1.4V p-p ripple, which pretty much ruins the point of running DC on the heaters.... nevertheless this is what many modern cheap amps do.

[merlinb]

I think Merlin's fingers typed backward.

Yep, I wrote my post back to front!

[StarGeezers]

  If there is a fuse on the PT (fils) , check that the ends of the fuse holder are clean and bright ... many problems reported about that...