Jtm 45 fuse keeps blowing

[Marcelcar]

 

[neikeel]

A few things to check (given the results it's going to be something weird) is to check for shorts to ground on each tube pin (of course 1 and 8 will) and that both screen resistors are in spec.
I think that any other tests would merit using a current limiter - make up a light bulb to clip in circuit in lieu of the HT fuse.
Things I would want to test would be shorted filter cap, correct choke measurements.
Can you take a photograph to show the keyway in your sockets.
Oh and what is the value of your NFB resistor? Looks like a 1R in the pictures (supposed to be 27k and hooked up to the 16ohm tap on the output selector.

 

[Marcelcar]

I'm measuring about 26.5 kohms on the nfb resistor

 

[Marcelcar]

The output tubes are shorted to ground on pin 1 and 8 and the heater pins

Could my rectifier tube be bad?

 

[coolidge56]

I'm thinking a head fake. The bad tube took out the slo blow fuses. Sounds like the issue is fixed but you don't have any slo blow fuses left so the regular fuses are blowing as expected?

 

[william vogel]

I messaged you on TGP. Build a light bulb limiter. Once you have it you’ll be able to test live and find the excessive current draw.

 

[Marcelcar]

I have slow blow fuses, they blow as well. I'll build a limiter today

 

[Marcelcar]

I've finally built a limiter.

I noticed that after the last test, with the output tubes in place, the mains fuse blew aswell.

Without the power tubes installed the amp works fine and the light bulb doesn't light up, no fuse blows or any sort of arching.

What should I test with the power tubes installed? What should I look out for?

 

[coolidge56]

It looks like the two leads on the presence pot cap are touching the grounded rear pot cover?

 

[Marcelcar]

The leads are not touching the pot, it just looks like that on the picture

 

[coolidge56]

It looks like you have your mains L crossing the indicator lamp before reaching the power transformer?

 

[Marcelcar]

It's hooked up according to the instructions.
The blue wire is my common wire (orange in the picture) and the other lead of the lamp is hooked up to the 120v tap of the PT because its a 120v lamp.

 

[coolidge56]

It's hooked up according to the instructions.
The blue wire is my common wire (orange in the picture) and the other lead of the lamp is hooked up to the 120v tap of the PT because its a 120v lamp.

Yeah this wiring scheme is making no sense to me. I'm guessing the mains brown wire is 240vac. So you have mains 240vac going directly to the on/off power switch and then to the power transformer. Meanwhile your mains neutral passes through a fuse, then over to the power indicator 120vac lamp, through that to the 120vac tap on your power transformer.

 

[william vogel]

Yeah this wiring scheme is making no sense to me. I'm guessing the mains brown wire is 240vac. So you have mains 240vac going directly to the on/off power switch and then to the power transformer. Meanwhile your mains neutral passes through a fuse, then over to the power indicator 120vac lamp, through that to the 120vac tap on your power transformer.

It’s perfectly fine to run the indicator lamp from the 120 volt tap. The fuse is in the neutral on the original amps, it’s not the safest way to wire an amp but it still protects the “building” from the amp if fault occurs within the amp.
What wattage bulb is in the limiter? It has to be incandescent.

 

[coolidge56]

It’s perfectly fine to run the indicator lamp from the 120 volt tap. The fuse is in the neutral on the original amps, it’s not the safest way to wire an amp but it still protects the “building” from the amp if fault occurs within the amp.
What wattage bulb is in the limiter? It has to be incandescent.

Forget the lamp for a second, he has mains L going to the 240vac tap and mains N going to the 120vac tap. That said I'm wondering what happens if that lamp burns out, a break in N?

 

[Trouble Free]

Looks like he has 2 wires to the fuse, instead of splicing it at the transformer. using the 120v tap for the light. Not going through the light.

 

[Chris-in-LA]

The leads are not touching the pot, it just looks like that on the picture

The right lug in your first pic is supposed to have a grounding lead attached to it. Not saying that’s causing your fuses to blow, but, who knows?

 

[Marcelcar]

I have a resistor to use my lamp at the 240v tap. Should I change the wiring of my lamp?

Technically it shouldn't matter whether I split the common wire near the transformer or if the common and the lamp connect at the fuse

The right lug of the presence pot is grounded. One of the cap's leads is soldered to ground.

 

[neikeel]

Some of the things you need to test now are:
- shorted mains filter cap
- shorted screens filter cap
- integrity of choke
The problem does seem isolated to the power tube sockets, based on the photos so far and the voltage readings you gave earlier. Other things are how the OT is wired up but I have no clue what the colour codes are on the particular OT you have used but if you have 450v or so on pin 3 of each socket then you have not got the centre tap/primaries mixed up. I would check that the impedance selector works and that the selector is on right setting for your load (speaker cab?).
The lamp should not be an issue (you are just treating the primary as a voltage divider - 240 to the main coils and tapping off the 120v to the lamp. Personally I would wire the live mains to the fuse and, ideally use a DPST switch for mains and switch both neutral and live as that is safer and mandatory if you have a fixed power lead on modern appliances. As you are now should not be causing the current issue.

 

[Marcelcar]

How do I check for a short? Using the contunituy setting on my multimeter?
How do I check the integrity of my choke?

Sorry for the dumb questions but it's my first build.