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?

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.

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.

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?

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...

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?

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.

By any chance, are your tubes made by TungSol and do they have a metal base?