jcm 900 no sound blown OPV fuses

[augie1]

I would love any help I can get... I just got a used jcm 900 model 4100 dual reverb 100 watt head. It was missing one of the el34 groove tubes, has three, and they all turn on and glow. the preamp tubes turn on, and glow as well..but no sound at all!! I have 3 diff marshall cabs, all good, and several known good speaker cords, tried all. no sound. I read that these amps can run on just 2 tubes, so i tried 1,4.. then 2,3, etc... no sound. I then proceeded to install my new el34 groove tubes out of my jcm tsl100 watt head, and as soon as i turned it on..poof.. blew both T500ma OPV fuses. the tubes were working perfect in my tsl-100. why would having all 4 tubes installed into the jcm 900. cause immediate fuse falure in th OPV. One note, the main fuse is still good. did NOT blow. [I] am so upset, and want so dearly to get the jcm 900 working..I will order a new output transformer if that is what any of you pros think may be the culprit. I am going to go get some extra fuses, and see if it blows with all 4 tubes out of socket. but as I said earlier, no fuses were being blown with just 2 tubes installed. even moving them around to any/all socket combinations. someone please help me, I will greatly reward you..promise..augie in california...cell#.707-413-8127

[Amp Kat]

No reward needed but we will help you fix you're amp. Seems to be a power supply problem and it could be from several sources. You're going to need a meter and some basic skills to troubleshoot the amp. Hopefully you just have maybe an open resistor or fryed screen resistor because those symptoms are what blown output transformers do when they go. First off do you have a multimeter where you can take some measurements. ?

[Gibsonman63]

To run just two tubes you need to pull either the two outer tubes or the two inner tubes. Also, removing two of your power tubes will make your output inpedence double what is indicated on your output inpedence selector, but it is generally not a big deal, just less efficient power transfer.

You may want to build a current limiter as well so you can troubleshoot the amp under voltage without frying your output tubes.

Try this one:

Do it Yourself - Light Bulb Current Limiter

[bluesfreak]

Having both OPV fuses could also being caused by incorrect bias setting, did you check it before you popped the new valves in?
Before shelling out the bucks for a new OT I would first check the screen and gridstopper resistors in your amp (R24-27, R33-36) for any failure, remove the additional 2k2 if they are in your amp. To quickly check the OT measure from red to brown and from white to brown if you have resistance and it is is approx the same on both side.
if all turns out to be good, pop new fuses in, adjust bias to around -38V and test voltages on the empty power tube sockets. Post values here. If within a range which seems acceptable test with a set of 2 tubes (either 1-4 or 2-3) if it works.
If no sound is heard I would jumper the FX loop to exclude a bad contact there.
Still no sound then the best is to take a signal generator and a scope to trace the signal through the amp...

cu
bluesfreak

[nickcom]

hi i need help to fix my amp i am running 3 2 ohm dual kicker comps which would be at 1.33 ohm at amp, my amp i just found out after having it hooked up for about 1 week that its not 1 ohm stable briged at amp which is how i had it hooked up. my amp now is turning on but no sound, the power light blinks a couple times and goes off but the amp is still on and the power light sometimes is on but still on sound? the amp is a cerwin vega exl 1000d 2 channel, can some one please help with diagnosing the problem on my amp so it will have sound though it . thank you much, nicholas

[bluesfreak]

First welcome to the forum and enjoy the time here.
I needed to google the amp to understand what your problem is as this forum here is for guitar and bass amplifiers and not for car stereo. However, your amp is probably toasted as it requires a minimum impedance of 2 Ohm on each channel. You had less connected which apparently has killed the MosFET Powerchips. Unfortunately these Amps are nearly impossible to repair so you will have to shell out another 100 Bucks for a new one....

cu
bluesfreak

[Enzo]

Ni nock.

Yeah, car stereo is a whole different game from guitar amps. While electronics is electronics, there are many application assumptions that we wouldn;t have here.

And unless I don;t understand your terminology, three 2 ohm speakers in parallel makes a .67 ohm total, not 1.33 ohm.