It was the lightbulb resistor. I'm reading the neccesary 40-45mA adjusting the bias pot. I just risked it, uninstalled the  lightbulb limiter, put a 4 Amp Fast Blow HT Fuse instead a 2Amp Fast Blow HT Fuse (because I haven't found Slow Blow fuses around here yet) and it just works fine. Well, fine is not the word, IT ROCKS!

Thanks for your help, really! I'll submit a video to youtube soon.

It works! It sounds! 

Though cathode current is like 10 times lower it should, right now (maximum 4.6mA!). Is it possible to be because I connected a 4 ohm speaker to the 8 ohm output?

I'll try tomorrow with the 8 ohm load. I connected the 4 ohm speaker because that would drop the energy efficiency, so it doesn't sound too loud for 11PM. And also because I didn't wanna risk my scumback speakers if something was wrong.

If tomorrow, with a matched load, the cathode current is still that low, should I use a bigger bias pot? (I'm using a 25K bias pot).

The small current cathode must be happening too because I'm using a lightbulb  current limiter, that drops B+ Voltage from 400 to about 300 right now. I'm still using the lightbulb limiter because, though everything seems OK, I've realized I haven't got slow blow fuses but fast ones.

No, they are two Valve Art KT66s (and JJ 12AX7 and JJ GZ34), all with a plastic base. I'm waiting for the new Valve Art KT66 to arrive, I will test  them next weekend.

But that doesn't happen. What happens on that situation is that you see 0 VAC on the heaters. In fact you see 0 VAC and 0VDC everywhere in the amp, the amber lap is off, the lightbulb current limiter is shining a lot. And if it wasn't for the current limiter, the fuses would be blown!

I only see those 6VAC if no power tube is installed.

Yes.


I've never closed the SB switch with the speakers and powe tubes installed. This problem arises when the SB switch is open. If I have a short with the SB switch open, closing it could only get things more complicated, I think.


Uhm, I guess that, again,  terminology is confusing. With the amp powered off, it's logical to have continuity between pins 2 & 7 of the socket because it is connected to the transformer. It may also be OK to measure a small resistance between pins 2 and 7 of the tube (without connecting it to the socket I mean). But when you power the amp (with and without the KT66s), there must be no continuity between pins 2 and 7, there should be a difference of 6.3VAC right?

The thing is that, with the SB switch open, almost all the pins of KT66s socket are at or very close to 0VDC and 0VAC, except the heaters. The resistance between pins 2 and 7 of the tube can be very small, but with the SB switch open, I think the only chance the KT66s have to create a short is inside the tube, between pins 2 and 7. It's also consistent with the fact that only if I disconnect those wires or I put a resistor there the short goes away. The voltage seen between both ends of the resistor is going to be zero in any other case than pins 2 and 7, there's just no current there, because the voltage between those pins and ground is zero and the tube is just not working with the SB switch open, this problem can only come from the grid where we connect the heaters.

But, I really wish to be wrong,  I wish the KT66s to be just fine and to only have something miswired! What do you think?

Uhm, yes you are right, I just didn't count well, but everything was connected well.

And now I think I know what is happening: the short must be happening inside the KT66s... I did this thing of putting a 50ohm resistor between the pins of the socket and the ones of the KT66 valve. Only if I put the resistor on pins 2 or 7 the short goes away. And, in that case, I measure the same voltage between pins 2 and 7 of the valve, and 6.5V AC between pins 2 and 7 of the socket. So that's it, the two KT66s must be damaged, because the valves are connecting pins 2 and 7. It's strange because I didn't use the valves until yesterday, and I've never closed the SB switch with the KT66s and speaker in place. Anyway I bought those valves from metroamp, not from ceriatone  , and I'm gonna suppose it's been me who damaged them (anyway I bought them more than 30 days ago so no returns).

But this time, before I buy them, I'd like to know what do you think, because a pair of KT66s cost more than a GZ34...

Sorry about that


Yes, with the difference that I only use one KT66 at a time (because I don't have enough wires with pliers as ends for 2 KT66).



 I've tested it and it doesn't matter which tube I use or which socket I use, the short is still there if I only one KT66. Though, perhaps, using one KT66 can damage the tube? (I think maybe it could, but as I'm using a current limiter I'm a little safer). I really don't think it's a bad tube. Also, if I use the two KT66 but disconnect one of the heaters' wire from only one of the KT66s, the short is still there. What I'm gonna do is to use one resistor or pot  between the valve's pins and the socket's connections to limit the current. That way I'll also be able to measure the AC/DC current through the resistor.

PD: just for reassuring, the KT66 has 8 pins and one gap (a space for another pin), the gap is between pins 6 and 7, right?

Yes, this is a new problem. I've double checked everything but didn't find any mistake (though it's obvious I have at least one). Then I tried to delimit the problem: I only put one of the KT66s and  confirmed that, still, there was some kind of a short. Then I tried connecting  the tube through pliers and wire and disconnected one of those wires to see if still there was a short, and then I connected it again and disconnected another etc...

So I "found" that if I disconnect either pin 2 or 7, the short goes away... I don't know much about how pentodes work but I suppose that heaters are somehow what switches on the tubes, so  this doesn't actually give a lot of information on where is the short. Tomorrow I'l try to measure where is flowing all the current of this new short.

Maybe this helps: with the kt66s in place and the speaker connected, the short happens even with the SB switch off. It must be a very obvious mistake!

PD: yes, I see a good amount of resistance on the primary of the OT.

Also, I see about 1.2 ohms between corresponding pins 2 and 7 of each kt66 valve without connecting them to the socket. I assume it's normal.

Apparently, I used a bad SB, the metal chassis of the switch was connected to its terminals somehow.

Now I've got another problem. I've put in place the 12AX7 valves and, according to metroamp's manual, the voltages of every pin of every valve are OK. But when I install the KT66s and a speaker, the lightbulb current limiter shines a lot and the 120V amber lamp is doesn't shine at all. I'm rechecking everything out again.

The nearest thing to a short I've found is that I measure about 1ohm between the 16 ohm's wire and the secondary's ground of the OT. Is that normal?

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.

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.

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.

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!

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?