Voltages too high

[Mark]

So, I’m getting 427, 381, and 249 volts at each of the filter cap terminals (C1, C2, C3, respectively), which is too high according to the book. I tried a different rectifier tube (brand new 5AR4) and got higher values (e.g. 440 on C1). I’m seeing 165 and 145 volts at pin 1 and 6, respectively on the 12AX7, which is in normal range IIRC. How do I bring the voltage down at the filter caps? Could I drop it with a resistor before C1? If so, what spec would you recommend?

The primary on the PT is right where it should be, 350vac.

[Andy]

Im assuming the first tube is the 5Y3? Is it new production?

Also, what Capacitor values did you go with for C1, C2 and C3?

[Andy]

Can you give us the reading of the input voltage from the wall and then the Red, Yellow and Green wires from the secondary of the PT?

[Mark]

Yes, 5Y3, new production Sovtek from Hoffman Amps.

C1= 20uf/500v
C2= 16uf/475v
C3= 8uf/450v

Input voltage = 124vac
Red = 352vac each
Green = 3.4vac each
Yellow = 0vac, ?? it reads 435vdc each, same as at C1

The spec for this PT (from AES) is 120vac primary, 325vac secondary, 6.3vac green, 5v yellow (AC or DC?). Is the 352v secondary reading that I get higher because the input voltage is higher than spec? I totally don’t understand what’s up with the yellow pair readings… Help!

[Andy]

So your red secondary is a bit high, it may be due to the higher input voltage.

The green is a tad high, so that points to the same as the red. That is what powers the power lamp and the filament/heaters for the 12ax7s. Just double-checking, did you use the artificial center tap?

The yellow is the most concerning. I would do some digging there. That is what powers the rectifier tube. It should be 5 VAC going into pins 2 and 8 on the rectifier. The DC voltage is the rectified voltage post 5Y3. So it seems you have both a 427 VDC and 435 VDC readings there at different times?

[Mark]

The different rectified DC voltages are from different tubes, 427VDC with a 6L6 plugged in, 435 VDC with a 6V6 plugged in. Sorry, forgot to mention that. Also, yes, two 100ohm resistors are attached to ground for an artificial center tap.

Ok, so I used a different meter and got 5.5 VAC on pin 2 and 3.2 VAC on pin 8. Why on earth my other meter (a new Klein Tool MM400 auto ranging) reads 0 VAC on the rectifier filaments but reads exactly the same on the other filaments as my other meter (3.4 VAC, BK Toolkit 2704B manual ranging) is baffling to me. Anyway, those readings on pin 2 and 8 are too high, they should be 2.5 VAC each, correct? Could that be due to the higher primary too?

Back to the meters. Why would the Klein meter detect VAC at one point, but not at another point where a different is able to detect VAC? The voltages aren’t even supposed to be that different. The meters are shown below.

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[Andy]

As far as the meters go, I dont have any idea about that. I’ve not seen that problem!

I would disconnect the yellow wires from the tube and measure them open. The only reason I can think of for the difference is that pin 8 has a load where pin 2 does not. Even at 5.5, that is a bit high.

It seems that your voltages are just high overall. I don’t know if thats due to a higher input voltage or if the PT you got just has some higher output. Really, around 330VAC on the red is what you are aiming for. 350VAC is high.

[Brent]

Mark, I think you need to know how much current you have through the 6L6. Measure the actual value of the cathode resistor with the amp off and carefully measure voltage drop across the resistor with the amp on. Check the cathode to plate voltage (pin 3 to 8).

[Author]

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