Jtm 45 fuse keeps blowing

[neikeel]

Unsolder the choke wires, measure the DCR (resistance between the two wires) and between the wires and ground. If there is no measurement to ground and the resistance is 100ohms for a Drake 3H type or 690ohms for an RS 20H type then you are good.
Next measure the resistance between the desoldered ground lug and the isolated lugs on your filter can (make sure the caps are drained first!). You should get around 1M on the terminals and nothing to ground. Same goes for your PI can on the chassis wall.
Double check your under board connections - particularly that you have not nipped a wire up between the board and the stand offs.
Next thing I would do is isolate the power tubes from the board by isolating the yellow screens feed so all you are powering up, with your lamp limiter in circuit is the HT, rectifier and choke but not the rest of the board (take a peek at the schem and you will see where I mean).

 

[Marcelcar]

Thanks that's really helpful!

So to isolate the output tubes I just have to disconnect the screen grid wires?

 

[Marcelcar]

The resistance of my choke is somewhere around 115Ohms, I can't get any resistance readings on my filter cans though. I am connecting one lead of the multimeter to ground and the other one to the poles of the filter can

 

[neikeel]

That is good re the choke (assuming no continuity to ground - chassis). You should normally get a resistance reading across the filter cans between positive and ground. What meter have you got? sometimes they charge and the meter struggles! It seems that you do not have an internal short though.
Normally it is the yellow wire between the pin 6 of V4 and the board that carries the B+ to the board (because of course you want the tubes to have bias, plate supply and screen supply) that you unhook from the socket).
There are other things that I would test as routine if it were in front of me but do these things first.

 

[Marcelcar]

I isolated the power tubes from the rest of the circuit and got the same result. 10 seconds from startup (standby and power turned on simultaneously) the lightbulb starts to light up brightly and the indicator light in the amp is flickering. I think the problem is really limited to the output sockets. Can I operate this amp with only one power tube to see if the socket is at fault or if one of the tubes is bad?

I also noticed that after turning the amp off, the filter caps are almost completely drained. Without the output tubes they are storing around 200V

 

[thetragichero]

so there's a short through the tube, likely with the socket wiring

 

[Marcelcar]

But I can't find out what's wrong with the wiring...

 

[william vogel]

Run the amp with the preamp tubes and v5 only. It will not hurt anything with only one power tube.

 

[Marcelcar]

I figured it out guys!! I've got a bad tube..

I installed only one tube in the V5 slot and my limiter flashed again as I reported. After changing the tube I got no problems in V5 and the amp runs stable.

 

[Trouble Free]

All that for a tube? Welcome to amp building, lol

you should have a great sounding amp now enjoy.

 

[Marcelcar]

Yea, it's really a strange world . Thanks for your help guys, I really appreciate it !

 

[neikeel]

Phew!
There is an Oscar Wilde quote about losing one parent may be regarded as a misfortune but to lose both looks like carelessness!
Which tubes were you using? Were they sold as matched and fully tested?

 

[Marcelcar]

I initially bought a pair of matched Tad premium kt66s. One of the had a vacuum leak so I asked the support if they could send me another matching tube. Funny enough my bad tube is actually the one that they sent as a replacement.

 

[J Saw]

I initially bought a pair of matched Tad premium kt66s. One of the had a vacuum leak so I asked the support if they could send me another matching tube. Funny enough my bad tube is actually the one that they sent as a replacement.

Where did you get the tubes?

 

[Marcelcar]

Directly from the TAD store.

 

[J Saw]

 

[Marcelcar]

What do you guys think about Ruby tubes? How is their quality compared to other manufacturers?

 

[J Saw]

Glad you got it going. Been following this thread. So many helpful folks 'round these parts.

 

[neikeel]

We should bill TAD for our time and grief!!

My preferred current production KT66s are Gold Lions (Russian). Fortunately I still have a pair of GECs left.

 

[J Saw]

We should bill TAD for our time and grief!!

X 2! I didn't help but agree entirely.