... Pin 7 of V3 is 8.3Vdc n Pin 7 of V4 is 310, what I would expect on both tubes. ...
Did you mean
Pin 3 of V3 & V4 are 8.3vdc, while pin 7 of both tubes is 310vdc?
Assuming your chart is DCV correct, along with what you wrote above, the you have an open half-primary in the output transformer. No need to go further.
... I didn’t unsolder the OT primary yet but when I ohm ct to V4 I get 175ohms, V3 to ct I get like cap charging megohms ...
In-circuit testing will often give confusing results unless you already know what to expect. If you're uncertain of the measurement (as above), then isolate the part from the circuit. If you pull the tube out of the socket and/or unsolder the output transformer center-tap, you should be able to get a resistance reading.
If you get the "slow charging" effect you had before, there is a break in the OT primary on that side, with a kind of capacitive coupling manifesting across the break.
When I ran a 1khz signal through I get 100v V3 Pin7 n 178v V4 pin 7. I didn’t unsolder the OT primary yet but when I ohm ct to V4 I get 175ohms, V3 to ct I get like cap charging megohms, did resolder V3 pin 7, no change, swapped tubes, no change.
Tube Swap = No Change on the a.c. volts difference? (BTW, please specify vdc or vac, as you're mixing both in the same post; makes it easier to figure out what's happening).
The loudness is in the range of my 5E champ build, way lower than my parallel SE el84. Thanks for the help.
That might be abnormal, or it might be typical for that version of the GA-5T Skylark. I had one that used 6V6's, had weak output and sounded horribly bright.
The signal test was probably unnecessary, given you showed serious issues with d.c. voltages (if you truly got 8v d.c. on an output tube plate, you know it's a problem to be fixed).
But you also need to have a sense of reasonable signal voltages to apply. Your DCV chart shows the EL84 cathode voltage as 9vdc. The amp will run into the limit of input signal it can handle when the grid is driven with an a.c. signal
whose peak equals the bias voltage (9v). Converting from peak-volts to RMS sine voltage, 9v peak/1.414 = 6.36v RMS.
If your scope is indicating RMS a.c. volts, you should have no more than 6.3v RMS at the EL84 control grid; if it registers peak volts, no more than 9v peak; if it registers peak-to-peak voltage, you should have no more than 18v peak-to-peak.
So the 25vac shown on the diagram is absurdly high by any standard. If it weren't for the fact you have a inverter/driver transformer, the voltage into the EL84 grids would have almost certainly collapsed because the output tubes were drawing grid current.
But that's an aside... unsolder the OT and measure primary resistance. You probably need a replacement OT.
Topic: skylark GA5T question (Read 1950 times)