• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

funky power supply

I picked up an EL34 PP amp from a garage sale that I am now going to restore. The power supply looks pretty interesting and I was wondering if I could safely remove the 75u cap from the circuit?

I mapped out the circuit and the power supply indeed looks like that...


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Hey HollowState, I'm pretty sure that's how its connected. There are four red leads from the power transformer. One is grounded. The other supplies 7.5V The third one is connected to the 10R resistor and it measures 140Vpk to ground. The fourth red lead is connected to the 75u cap which also measured 140Vpk. These two points are finally connected to a dual section planet cap. The other leads on the power transformer is the heater grn-grn/yel-grn leads so I assume that the two red leads are the HT. Also the diode going to ground has a 1F snummber cap.

I'm pretty confused myself about how this works; the amp plays music and the only problem is that it is quite noisy (mostly 60Hz hum). I double checked all the wiring again to be sure as I'm definitely no expert I yanked out the transformers today to mark out the new chassis bit I'll post a pic of the chassis soon. Thanks.
So heres a pic of the PSU section.

1. One red lead from the power transformer was connected to A, a 70u cap. A was connected to C via a dual section 50u+50u cap. I measured A at 140V.
2. The other red lead was connected to B, to 10W power resistors measuring 10 ohms. At B was also at 140V.
3. The power resistors are connected to C through a diode. C measured 280V.
4. From C we have 2 130R resistors connected to reservoir caps that connect to the EL34 plates.
5. Also note the diode that goes from the power resistors to ground (chassis!).


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The 4th red lead that measured 7V was connected to a couple of tube socket heaters that formed the radio receiver. The second heater pin on all these sockets were connected to ground at the chassis. Also, shouldn't this red lead also have a 0V lead? Or is the CT lead the reference for this 7V lead as well? The main amp circuit (EL84's and 12AU7) had heater supplied via two green leads which also had a CT connected at one of the EL34 cathodes.

There are so many chassis grounds its amazing. Whats worse, the amp power plug is only two pin :att'n:

Thank you for being patient, HollowState.
The 130 ohm resistors are completely rediculous....
It was a way of trying to reduce capacitor in-ruch currents from poping the diodes or such...but they add to the caps ESR and put a nasty ZERO in the response....
The ZERO is obviously above the 120Hz ripple, but have an adverse affect on the higher frequency audio signal currents that these caps are suppose to bypass to ground and back to the cathodes of the output stage...
I would remove those 130 ohm resisitors and make sure to use diodes that can handle the start-up surge current...

Yes, I will definitely use some sturdier hexfred diodes. Also all grounds are on chassis including the psu grounds and the CT too. The socket lugs and the extra pins on the input selector were used as high voltage nodes to distribute plate voltages throughout the chassis. One such hv node also happens to be on TOP of the chassis connected to one of the radio tuner knobs. This point was exposed probably because the knob cover was destroyed over time. I hope this amp dint fry any of its previous owners :eek: