bradley_beauchamp

okay i have had this amp since i was 16, and there has always been this problem with losing output, first we (my dad and i) though it was the impedance select switch, we fixed that, still the same problem after it worked for about 3 months then i lost the output again, then we though it was the tubes, changed them, it worked for a week and now i hit a dead end we don't know why it has such a low output, if the volume was up at 12 o'clock, then you can hear it, except it should be blowing my ear drums out at the level not just a hum any suggestions!? I NEED TO FIGURE THIS OUT!!!

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thrawn86

When you replaced your valves, did you just do the output (power) valves? And did you properly re bias?
Did you also replace the preamp valves? Let us know what you've done concerning these things. 90% of the problems we see are linked to valves......even ones that have been put in brand new can be bunk.
Do all the valves light up? Are the power valves glowing a bright red color on the plates? (NOT the filament wires inside; those should glow. The plates around the wires.)

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bradley_beauchamp

we didn't rebias, and yes they all light up everything looks like it works fine just really low output we have see the schematic for it doesn't really help my dad much (he's an electrician by trade so he kind of knows his way around the amp but, not enough haha) so if it biased you think that could be it?

bradley_beauchamp

and yes we replaced all the tubes sorry didn't mention o.o;

thrawn86

It's entirely possible. If the bias was set too high it can cause a power valve to fail, which if played for a while could damage the transformers. If it's too cold, that could detract from the sound and make it sound weak. Rebiasing after a power valve change is an essential. They should also be a matching pair (or quad if your amp uses four).

From your response I'd guess that the preamp valves haven't been replaced....how old is the amp? Preamp valves last much longer than power valves, but even they have a lifespan. A new set might help to narrow down the problem, and it certainly won't hurt at all to do so. A plus is that you don't have to rebias after replacing Preamp valves.

Hang tight....some of the more technical members will be along to add to this discussion.

EDIT: Ok, I see you replaced the preamp valves. Still think the biasing could be to blame.

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See me 'n the boys mean business, bustin' out dead or alive.
2002 DSL 401-NOS Mullards & Martimus Maximus High Gain Pack....WOW!
2001 Gibson LP Studio Pewter 2003 PRS Santana SE 2002 1960B

bradley_beauchamp

we replace both power tubes, and all three pre amp tubes with a hotter set of grove tubes those are the newest and oh also, my previous set of grove tubes one power tube blew out, and it also blew the output fuse, so as an educated guess i'm going to say that the bias is set to high? and my dad and i are going to get a biasing kit to see if that helps, and you woudln't happen to know the voltages off of the transformers? (my dad is curious) that way we can see if there is something else wrong if that doesn't work. i hope that's all it is...

bradley_beauchamp

the amp is from the early 90's then the amp was discontinued for the newer JCM 900 models

thrawn86

The power tubes can go regardless of the bias. That's the way things are in the moder era of Current Production (CP) tubes. Good call on the biasing kit. No, sorry....I don't know the voltage numbers.

I'm sort of the 'First Responder' around these parts; kind of a receptionist, if you will. I attempt to steer people in the right direction. If you do a search for 'bias a JCM 900' here on the forum you should come up with something. Keep checking back, as someone will chime in with more advice for you. Sunday is usually slow, and sometimes it takes a day or two for all the regulars to get a look at the topic and comment.

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See me 'n the boys mean business, bustin' out dead or alive.
2002 DSL 401-NOS Mullards & Martimus Maximus High Gain Pack....WOW!
2001 Gibson LP Studio Pewter 2003 PRS Santana SE 2002 1960B

bradley_beauchamp

okay we found out we just need a flat blade to bias it... we we did that... no change just a low output... so biasing didn't do anything it was way high though it's suposted to be 40 - 42 v but it was at 46 v so maybe the transformer is blown? and thank you for the help =] i keep an eye out for other people who post anything else =]

bradley_beauchamp

okay, there was a lose connection between one transformer and the power tube, but biasing the amp defiantly made this amp sound 100% better (which is a revolation to me because i never heard it sound like that but i bet it mad you guys chuckle haha anyway thanks again all)

thrawn86

Glad you got it sorted out man.

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See me 'n the boys mean business, bustin' out dead or alive.
2002 DSL 401-NOS Mullards & Martimus Maximus High Gain Pack....WOW!
2001 Gibson LP Studio Pewter 2003 PRS Santana SE 2002 1960B

bradley_beauchamp

well so i thought, my dad found out that something it wrong with one of the transformers. now i'm out 100$ lol but yeah other then that yeah it's solved my dad come to the conclusion that it was a defect after it was made. thanks for all of the help. i need to get rid of the other post

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