I mean, with only the GZ3 tube installed, the power switch ON and the standby switch OFF (standby mode), the 500mA fuse blows after about 10 seconds. I don't know if it helps, but I measure 42 ohms DC between the 350V (high voltage)  ends and their common (0V) end.  Is that OK? Or it's that what's blowing all the fuses I have?    I mean, while the fuse is not blown, I actually measure 350V between he terminals and the common, and 42 ohms DC is gonna be  much more in AC.

Perhaps it's something I've connected incorrectly, but I've been unable to locate any errors... Other option would be a bad GZ34.

Well, I hope it's the rectifier wiring! I checked and it looks like the standby switch is OK. Talking about the rectifier's diodes... I used the schematic that uses no diodes. I guessed that would be fine, as it's not used neither in this schematic nor in metroamp's manual.

But now you mentioned it, should I use those diodes? I thought the valve rectifier already worked as a diode rectifier. Oh, I also wanted to ask if it's normal to measure about 0.4 ohms between pins 2 and 8 of the GZ34 valve rectifier.

Anyway the amp is at my parent's house so I won't be able to check this until this weekend. And this time I'll be using a lightbulb current limiter.

Here (ceriatone.com) you can see that the rectifier tube has 2 diodes (and also the 0.01uf cap):



But, for example, on the layout found in this thread (ceriatoneforum.com) or the layout found at metroamp.com, you can see there are no diodes there:




I also have other question: On the jtm45 step-by-step metroamp instructions , inside the testing part, it says that if I only connect the GZ34, with the standby switch OFF and the power switch ON, I should measure about 325V DC on one side of the standby switch... that must be the side connected to the rectifier (pin 8 ), right? Therefore, if I don't see those 325V DC I can assume it's not the big caps what's failing, right?

Well, it's confusing, but what I meant is that (according to metroamp's manual), if the standby switch is open, I should read about 325V DC between one leg of the standby switch and ground and what I remember is reading 0V DC... and after about 20 secs the amp blows the 500mA fuse. I'll make more measures this weekend (because the amp is at my parent's house).

Oh, and I didn't say it before but: Thank you all for your help!! Really!

I believe so (230V for Europe). L goes to the 2A fuse, then to the power switch and finally to the 230V transformer's wire.


There's 5V AC between pins 8 and 2. And 0V DC between pin 8 and ground.


Yes.


Or the 5 or 0 from the transformer are shorted to ground, but it's not the case. Having checked everything thrice, I kind of figured I had a bad GZ34, so I bought a new one. Tomorrow I'll go to my parent's to make some measures and recheck everything, and I'll try the new GZ34 whenever it arrives.

In the transformer, E is ground, which is connected to the E of the mains plug, and to the "ground" called 0 of HV(350V) and to the "ground" of the heaters(3.15V). But the 5V wire and it's "ground" 0V are not connected to E, the question is: is it normal to measure about 0.4 ohms (and the tester is not realiable at those scales, but it certainly is less than 1 ohm) between any of those 2 wires (the 5V and it's "ground" 0 of the PT) and E? If it's not, then the problem is there, a short in the PT.

I've done some measures (this time with the lightbulb current limiter) and, just like metroamp's manual says, with only the rectifier valve, I measure about -45V DC between pin 5 of V4 and V5 and ground. I measure 0V DC between pin 8 of the rectifier and ground (when I should measure about 325V DC). I also measure 220V AC (it should be about 350V AC but that's because of the lightbulb current limiter) and about 80V DC between the ends of the 220K resistor connected to the diode. 80V DC and 220V AC through a 220k resistor amounts to only about 1mA, so I guess it's not there where the amp is wasting all the power that makes the 500mA fuse blow without a current limiter.

I put a 100ohm resistor between pin 8 of the rectifier valve and the 5V wire of the PT. The result was that the lightbulb limiter wasn't lighting strong anymore and that there was 5V AC between then ends of the resistor, but I measured 0V DC between both  ends of the resistor  and ground and also  between pin 2 and ground. I think it's logical to measure 0V DC between pins 2 and 8 with the 100ohm resistor at pin 8 because there's also 0V AC between pins 2 and 8, the 5V AC produced by the PT are disipated through the resistor. As it appears, the resistor is limiting the current that passes through pin 8, which is much bigger than it should when I don't put the resistor. But what is generating that much current? Is it created by a short in the PT or the PT is fine and it's caused by a bad rectifier tube?

We'll see about that when I receive the new tube, or if you tell me whether it's OK or notto measure less than 1ohm between the 5V wires and E. Until that, you are welcome share your thoughts.

Thank you. THANK YOU. Thank you. It was that. I desoldered the tranformer's wire from pin 8 and pin 8 was still grounded! And the transformer's wire wasn't grounded anymore! And now pin 8 was only connected to... the standby  switch. I unmounted the standby switch from the chassis and finally pin 8 ceased to be grounded!

Such a simple thing has made me wait about a month, but now I'm gonna have my amp finally running. If I don't find more grounding problems, the jtm45 should be running tomorrow.