JCM 900

Hammeron

My JCM 900 sounds very unwell on the 100w setting but perfectly good on the 50w setting. Any thoughts?

VellaJ

I'm not sure how these different settings work exactly, but to me it sounds like there's something wrong with one of the main amp tubes... I assume the 50W setting only uses some of the main amp tubes? (Amp gurus please ? ) In which case, one of the tubes that's used when you're on the 100W setting is ill, probably dead...

MikeM

Sounds like some powertubes saying goodbye.. Got any spares to test?

Jacquesg4j

Does sound like tube issues! One of the power tubes probably. I've not used it on the 100w setting... Are you doing a stadium gig?

MikeM

Of course she is ? You forget she's a rockstar already ?

Nicholas-L

best start would be to start with a Tube check... If I'm not mistaken the Wattage switch still uses all the tubes, but disconnects grid 3 off of it's supply and connects it to plates increasing the resistance to the OPT, thus reducing the wattage for those who were wondering.

Hammeron

Jacquesg4j wrote: Does sound like tube issues! One of the power tubes probably. I've not used it on the 100w setting... Are you doing a stadium gig?

Was in a bigger practice space and decided to try out the 100w setting, was fine for a day then failed.

Nicholas L wrote: best start would be to start with a Tube check... If I'm not mistaken the Wattage switch still uses all the tubes, but disconnects grid 3 off of it's supply and connects it to plates increasing the resistance to the OPT, thus reducing the wattage for those who were wondering.

So altho the 100w side has failed, the 50w side should be fine, so to speak?

Nicholas-L

So to speak it should "sound" fine yes, but have it checked out.

Hammeron

Nicholas L wrote: So to speak it should "sound" fine yes, but have it checked out.

Absolutely, problem is I live 400 k's from the nearest tech! But we make a plan. 8)

Nicholas-L

??? eish.... What you need is dial a tech. Someone who drives around with a van fully stocked up with equipment and gadgets and repair stuff.... ?

Hammeron

Ok. It has been getting quite warm in the Practice space lately, so I put a fan on the back of the amp when I played yesterday and the failing never occured after several hours of playing, whereas without the fan the problem occured, on both 50 and 100 watt settings after about half an hour. So the failure seems to be related to overheating, any ideas ?

Nicholas-L

It still sounds like the tubes are getting old... How long have you had those tubes in there. The only other thought that comes to mind is the Output transformer, but that's not likely.... What color do the tubes glow? Orange, red, scorching blood red?

Hammeron

Nicholas L wrote: It still sounds like the tubes are getting old... How long have you had those tubes in there. The only other thought that comes to mind is the Output transformer, but that's not likely.... What color do the tubes glow? Orange, red, scorching blood red?

Nic yea. I am not sure exactly how old the tubes are.
I watched them and I would say they glow orange.
The fact remains that they are probably getting old you are right.
Now, how difficult is it to replace them?
I mean, would it be possible to do it ourselves ? { In light of the fact that we are far from a tech }

Nicholas-L

yeah it seems they are getting a bit old. As for changing the tubes for new ones not a big problem you carefully wiggle the old ones out and wiggle the new ones in. The problem however comes in with the biasing of the amp, there you'll need someone who knows something about electricity and using a multimeter.

Hammeron

Nicholas L wrote: yeah it seems they are getting a bit old. As for changing the tubes for new ones not a big problem you carefully wiggle the old ones out and wiggle the new ones in. The problem however comes in with the biasing of the amp, there you'll need someone who knows something about electricity and using a multimeter.

Yea.
Would new valves glow red then?

Nicholas-L

That all depends on the bias setting of the amp.