Jtm30 output tubes not lighting up?

[the great waldo]

Hi All

I just popped over from the pup makers forum. I have a question . I lent my Jtm 30 combo to a friend and he just told me that after a month the amp stopped working. The main tube were not lighting up. I haven't had the amp back yet to check it out but was wondering if the output transformer might have blown. I realise that with such limited info it's hard to make a diagnosis, but I thought I would enquire before I get the amp back to check out.

Cheers

Andrew

[Tom Phillips]

Tube heaters being off (If that even turns out to be correct) is not a symptom of a blown output transformer. You just need to wait until you have the amp so you can do first hand troubleshooting.

[g1]

The JTM30 has an internal fuse for the heaters (F101).

[oc disorder]

Waldo .. there is a T6.3amp fuse internally for the heaters, (the filaments) which is what you see glowing.
Usually it (the fuse) blows for a reason eg the output tubes/valves are kaput !
In this situation I wouldn't even consider fitting a new fuse until the two output tubes are replaced.
Regarding the output transformer ... Last time I contacted the Marshall agent here in Aust. I was told they were no longer available
and that was at least 5 years ago. As Tom has pointed out no glow in the tubes doesn't mean the output transformer has blown.
BUT if there is a serious problem with the o/p tubes and you refit the fuse you may find within the first few seconds you have just burnt out your transformer.
The fuse is one of the smaller ones M205 (5mm x 20mm) and a slow blow or timelag as it has a "T" before the amperage-T6.5 amps.
A non timelag would probably go open circuit with an initial cold switch on with the high current surging through the cold filaments.
Incidentally the pre-amp tubes also depend on this fuse , and as they operate on a DC supply , a bridge rectifier is connected after the fuse .
It's harder to see the pre-amp tubes glowing due to the design of the tubes and also when the amp is in the cabinet.
Good luck and maybe your friend could chip in towards the replacements.

[g1]

What kind of power tube fault could cause failure of the heater fuse? Heater to cathode short?

[Alan0354]

My JCM 900 blew the HT fuse for no apparent reason too!!! I troubleshoot all the way to the fuse and found no problem. Replaced the fuse and was good to go. Don't ask me why.

[g1]

Sometimes they blow from the turn on surge, especially if the line voltage is high. I recall one of the JTM models was prone to this and sometimes needed a little bit of over-fusing on the heaters. But I believe it was the JTM600 which had EL34's drawing more heater current.

[the great waldo]

Hello All

Well I got in touch with my friend who had borrowed the amp and guess what,? he had flipped the standby switch to quickly before the valves had a chance to warm up DUH!!! . So anyway thanks to all the people that responded.

All the best

Andrew

[Tom Phillips]

...he had flipped the standby switch to quickly before the valves had a chance to warm up DUH!!! ...

That should not cause a problem unless he continued to flip the SB switch multiple times in quick succession.

[Alan0354]

That could not cause the problem.

But I am not surprised it blew for no reason either. Look at the big tubes, make sure it's not glowing on the side( the greyish metal), if it's not glowing in the dark, and let it go for now.

[the great waldo]

That could not cause the problem.

But I am not surprised it blew for no reason either. Look at the big tubes, make sure it's not glowing on the side( the greyish metal), if it's not glowing in the dark, and let it go for now.

Hi Alan

There was actually nothing wrong with the amp. It was just that he wasn't waiting long enough for the amp to warm up !! It reminds me of a customer who bought a Martin acoustic guitar in for a setup, the fingerboard was swimming in lemon oil. I mentioned to him that he shouldn't oil the fingerboard as it was made out of plastic (corian or whatever martin are using these days) but we saw the funny side of it.

Cheers

Andrew

[mort]

It's a different application, but after years and years of hvac work I've learned to accept nuisance over current tripping as just that, a nuisance. If I come across a blown fuse or tripped breaker, I perform simple checks to rule out grounded or resistive shorts, replace or reset the protection, and move on.

[Archie Speed]

FWIW...I worked on a JTM60 recently that blew it's T6.3A fuse at random intervals. Replaced the bridge rectifier, floated it a bit higher off the pcb than stock in an attempt to get better airflow/cooling & no more problems.

These amps have the bridge rect & main filter caps nestled amongst a few cement block 5W resistors, all of which sit directly over the horizontally mounted power tubes. Warm in there. Just a thought.

Archie

[g1]

The JTM60's have EL34 which draw much more heater current than the 5881/6L6 of the JTM30. As a result they are much more prone to blowing the heater fuses.
I seem to recall they were blowing during operation rather than at startup, so it may be heat related as you mention.
Thanks for the tip Archie.

[Chuck H]

Hi Alan

There was actually nothing wrong with the amp. It was just that he wasn't waiting long enough for the amp to warm up !! It reminds me of a customer who bought a Martin acoustic guitar in for a setup, the fingerboard was swimming in lemon oil. I mentioned to him that he shouldn't oil the fingerboard as it was made out of plastic (corian or whatever martin are using these days) but we saw the funny side of it.

Cheers

Andrew

So is that to say that the amp is working correctly? No blown fuses and heaters are warming correctly? How long did the guy wait to see if the tubes were glowing, like two seconds?!? The tubes will show a glow after only a few seconds. Standby has nothing to do with the heater supply. Just sayin'. This got confusing and mislead somehow. And more importantly...

Are you saying that Martin is using Corian instead of wood to make fingerboards?!?