#1
I have just bought an 80's jcm800 100w combo, and i have used it a couple of times, but wanted to ask a few questions.

Firstly after i used it for a little while, the 1 amp fuse in the back blew, any idea what would cause this?

Also the reverb doesnt work, is this the unit which sits in the bottom of the cabinet?

And im not sure if the presence works. Does the presence make a large difference atall? cause it sounds like it changes the sound slightly.

Sorry for being basic but this being my first tube amp, im trying to do things correctly.

Dan
#2
If the fuse blew, maybe it was a power tube that gave up?
Or the fuse was just bad.
GEAR

Dean V79
Randall RG50-TC
Roland MicroCube
#3
For the fuse, check your power tubes.

Make sure the reverb pan is plugged in properly. If it still doesn't work, replace the preamp tube that controls the reverb. Do a search, to find out what preamp tube does what. This might be the list of functions for each tube. Double check to be sure.
V1a- FIRST GAIN STAGE FOR CLEAN AND BOOST
V1b- SECOND GAIN STAGE FOR BOOST BETWEEN GAIN POT. AND DIOD CLIPPING
V2 RECOVERY STAGE FOR PASIVE TON KONTROL FOR CLEAN CH.
V3a THIRD GAIN STAGE AND DRIVER FOR EQ BOOST CH.
V3b REVERB SEND DRIVER
V4a SEND RETURN RECOVERY/ BUFFER
V4b REVERB RETURN BUFFER
V5 PHASE INVERTER FOR OUTPUT STAGE

The presence knob controls the boost of the upper frequencies. Should add a little crispiness. It doesn't do anything extreme, but you should notice it. Try messing with it again and listen to what it does with your treble.
#4
thanks for your help guys,

silly question, but whats the best way to test if a pre/power valve is working? is this a physical check ie heat etc, or can i do a check with a multimeter?

Thanks again guys for your quick responses, appreciate it.

Dan
#5
Ok, the presence works, and so does the reverb. (the box wasnt plugged in! Thanks The Real Ming!) But the gain doesnt appear to be that much.

After heating the amp up then switching it on, i can see that out of the 4 valves, only 2 are glowing. From looking at the back of the amp, the first two on the left aren't glowing, but the second two are.

Ive switched it off as am i right in saying these have gone?

Thanks

Dan
#6
Wait, if a powertube has given up, you wouldn't be able to play at all, so if you can play with your amp, then tubes aren't the problem.
GEAR

Dean V79
Randall RG50-TC
Roland MicroCube
#7
^No, many amps with 4 power tubes can run on two. 1 of those two that aren't glowing have gone bad, in which case you need to get two more since they have to be matched pairs. If you got this thing recently you may as well get a matched quad since you can get better tubes and it will be better for the amp.
I don't give a shit if you listen to me or not
#8
Quote by fila_93
Wait, if a powertube has given up, you wouldn't be able to play at all, so if you can play with your amp, then tubes aren't the problem.

They might be almost dead, and thus he could play (but with very little volume), I think. Also, the tubes should all be glowing.

Edit: I thought about the "remove 2 power tubes"-thing, but would that work with blown tubes in the amp? I guess it would, then.
Quote by Lunchbox362
This thread if fail in almost every way imaniganable.
Last edited by Fama at Jun 14, 2008,
#9
yes the amp does play, and yes both of the first two valves are stone cold.

I will change the complete set i think seeings that i dont think they have been changed since 1994!

Also there are 5 smaller valves which i believe are all 12AX7's,

What do you guys recommend as a replacement for the power tubes?

will these work ...

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5006429

also do i need to change the pre tubes as everything else is working on the amp.

Dan