5F1 Help - 2 - GuitarTalk Community

5F1 Help - 2

modulator

OK, I am at the start-up stage, I checked the power transformer outputs to the heaters and to the rectifier tube. I measure 240V AC going in, and see 750V over the rectifier High Tension input pins (pin 4 and pin 6).

I plugged in the rectifier tube, and switched on, the tube glows up nicely, and I measure a whopping 550 V DC at the first input pin on the turret board, at the first capacitor(22 microfarad, 450 V). The other high tension DC points at the next capacitors also measure much more than they should.

But, while all this is going on, I hear creaking noises, such as something warming up. Tube should not, so? A light draft of smoke starts to curl up from the very hot 22 microfarad 450V capacitor. Switch off.

So, apart from the issue of too much DC going into the system, why does the capacitor overheat? Polarity is correct. Rechecked wiring.

Ideas, please!

modulator

I know you will ask, so:
New battery in multimeter, yes, did check AC / DC setting. And even if my multimeter is lying, the hot capacitor did not lie.

With the power transformer installed, wired up and no load (no tubes installed) I measure

Mains input voltage: 237 - 240 V AC
Rectifier filament voltage: 5.7 V AC
Amp tubes filament voltage: 6.9 V AC
High tension output wires to center tap (ground): 381 V AC
Rectifier tube is a 5AR4

Clearly the power transformer in not in spec, any way I can reduce the output to the required 325 V, or even less? I read on the cleverweb that one should be losing a bunch of volts over the rectifier, but I did not see this in the no-load test. I also do not see plugging in the other valves, to add a load, reducing my B+ to less than 450 V.

Surely, buying a kit means receiving components almost guaranteed to work if assembled correctly?

Ideas please!

klaasvakie

The circuit seems to be fairly sensitive to input voltage --- the official fender schematic even has different windings for 230V vs 240V
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I think you should load your transformer. 381 vs 330 is a regulation of 15% (Regulation = (Vopen - Vloaded)/Vloaded ) 15% is high, but not impossible. I've seen cheap transformers go to 25%, here is an example with a regulation of 24%:

http://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/3FD-3XX.pdf

Not sure how you are getting to 550VDC across the cap, your 381VAC should produce peaks of 538V (381*sqrt(2)), and as you said you should still lose some over the rectifier. Doesn't make sense.

Good luck man, looks like this is turning out to be quite the hairy little project.

modulator

klaasvakie
Thank you. The high voltage at B+ is without a load, so the diode will not drop much anyway, the cleverweb reckons maybe 10V for this tube. Not much. I shall look into adding dropping resistors, if I cannot do a reducing transformer. Just, dissipating heat to reduce B+, a waste of energy, in an already wasteful amp design. I guess an extra 2 watt heat loss will not matter much. I would rather have lower than higher B+, to ease the tubes' life. An overly bright 8 watt 5 watt amp is not what I am after, right now. The dropping resistors should be OK, as the power transformer is a much bigger unit than this design requires. Should handle an extra watt or so without a problem.

I also need to make up a loading resistor to connect when starting up the unloaded amp. This build is "informative", to say the least. In the end, I still will have no clue about how valve amps actually work. Except that the next build will go smoother, if I do not forget about it all by the time I start a next build.