Jtm 45 fuse keeps blowing

[thetragichero]

1w is plenty overkill. 2w is okay too. one of the nice things about higher wattage resistors is their leads tend to be thicker so much better for soldering to sockets/lugs/etc

 

[william vogel]

I use 1ohm 1/2 watt 1% tolerance for cathode current resistors. Even at 500ma it’s only 1/4 watt dissipation.

 

[thetragichero]

the 1% tolerance is the important part

 

[pleximaster]

Jtm 45 fuse keeps blowing

Hi,

I just started my jtm45 build using the valvestorm parts and instructions.
Everything went fine until I got to the testing phase.

With the preamp and rectifier tubes installed everything seems fine but as soon as I install the output tubes my fuse blows.
I have already swapped a tube because one of them had a vacuum leak and I replaced a tube socket just to be sure that it doesn't cause the problem.

Here are my voltage measurements without the output tubes and only preamp and rectifier installed.

V1
Pin1 239V
Pin3 1,9V
Pin4/5 3,3V AC
Pin6 234V
Pin8 1,9V
Pin9 3,3V AC

V2
Pin1 205V
Pin3 1,2V
Pin4/5 3,3V AC
PIN6 335V
Pin7 205V
Pin8 206V
Pin9 3,3V AC

V3
Pin1 275V
Pin3 48V
Pin4/5 3,3V AC
Pin6 277V
Pin8 48V
Pin9 3,3V AC

V4
Pin2 3,3V AC
Pin3 491V
Pin4 490V
Pin5 - 54V
Pin6 490V
Pin7 3,3V AC

V5 is identical to V4.


Can someone help me figuring
out what's wrong with my amp?

When nothing else work...

 

[Marcelcar]

That looks like a great idea! I can try and desolder the fuse holder completely. If there's no fuse then it can't blow. Easy as that

 

[william vogel]

Only problem is that the power transformer isn’t rated/ designed or capable of passing 500mA but for just long enough to pop the fuse. The fuses are usually speced to go two times the rating of the transformer.

 

[thetragichero]

cigarette foil ya newbs

 

[neikeel]

cigarette foil ya newbs

seen that so often!
Don’t forget the cut down bolt too

 

[thetragichero]

^^^^^ don't actually so any of these. i don't want ken to come in and yell at us again

 

[neikeel]

That looks like a great idea! I can try and desolder the fuse holder completely. If there's no fuse then it can't blow. Easy as that

Don't joke! The original JTM 45s that Ken and his team built did not have an HT fuse at all just 2A mains fuse on the back panel. The 64/65s had HT fuse holders mounted north of the mains transformer by the power input socket and late 65 between the mains transformer and the output transformer. In 66 they both went on the back panel.

 

[Gunner64]

You pussies. Use a .22 lr cartridge like men.

 

[Marcelcar]

Will any 5W 470Ohm resistor do it for the screen grid or does it have to be the special Ohmite one? My ohmite ones have been desoldered so many times, i'd like to put in some new ones..

And I just noticed that my speaker jack has continuity to ground on the tip and on the "sleeve".
Is there supposed to be continuity to ground on the tip at the speaker output?

Red is A1, Brown is B+ (which I hooked up to the filter cap), Black is A2.
Blue is 16 Ohms, green is 8 Ohms, orange is 4 Ohms and yellow is 0 Ohms (common?)

Is my output section wired correctly?

 

[neikeel]

Yes you will get continuity - the meter is reading through the OT secondary.
The screen resistors can be any type but at least 3w and flame proof. Welwyns are good as are the 5w cement block type.

As per my post above you cannot tell if the primaries are necessariy the right way round so leave the wires long until confirmed.

 

[Chris-in-LA]

Will any 5W 470Ohm resistor do it for the screen grid or does it have to be the special Ohmite one? My ohmite ones have been desoldered so many times, i'd like to put in some new ones..

And I just noticed that my speaker jack has continuity to ground on the tip and on the "sleeve".
Is there supposed to be continuity to ground on the tip at the speaker output?

Red is A1, Brown is B+ (which I hooked up to the filter cap), Black is A2.
Blue is 16 Ohms, green is 8 Ohms, orange is 4 Ohms and yellow is 0 Ohms (common?)

Is my output section wired correctly?

What OT are you using? Your wire colors are definitely different than what’s shown in the Valvestorm directions.

 

[Marcelcar]

I'm using the TAD jtm45 OT. I explained the color coding some posts earlier

 

[Marcelcar]

I firered the amp up with all tubes in and began biasing. Everything seems fine.

But after getting to about 43mA the amp began making a motorboat like noise through the speaker.

Would motorboating occur from the beginning or is something like this normal and I just have to turn around the A1 and A2 leads?

 

[neikeel]

Primary:
A1 Red
B+ Brown
A2 Black

Secondary:
100V White
16 Ohm Blue
8 Ohm Green
4 Ohm Orange
0 Ohm Yellow

Yes, just flip the primary wires

 

[Marcelcar]

Nope, after flipping the leads I'm getting a short.

Could the motorboating be because of a too high bias voltage? And I'm not getting continuity on the primary with this OT either..

I also noticed something like this around my screen grid resistor:

Edit :

I tested again and got a beep every couple of seconds (caused by the resistance of the primary?)
I got the same result on the old OT. When connecting the test leads I'm getting a short beep after some seconds.

 

[Marcelcar]

I decided to take the amp to an experienced technician. Don't want to make any more damage to the amp..

Thanks for your extensive help, will report back when the amp is running with some sound clips

 

[Chris-in-LA]

I'm using the TAD jtm45 OT. I explained the color coding some posts earlier

I looked for it, couldn’t find it, I hope your tech gets it all sorted out.