jcm 800 v2 trouble

[dewg]

My band was practicing a few nights ago and my jcm 800 blew its v2 preamp tube. The amp is a combo channel switcher. I popped the metal guards off of the pre amp tubes, planning on rolling them out. The v2 not being lit up made it easy to diagnose and fix. I had some jan ge 12ax7wa's in each position. It was surprising to me that they blew so fast. They were NOS tubes that had seen about a year of use. Is a v2 in these amps are more likely to blow rather than any other positions? I have heard of certain circuits being more demanding on different preamp tube positions. I want to replace with something that sounds better than the jj ecc83 in it now, but I dont want to throw something in that cant handle the position. These are the functions of each tube: V1a - first gain stage for both normal and boost channel V1b - second gain stage for boost channel V2a - second gain stage for normal channel V2b - third gain stage for normal channel V3a - third gain stage for boost channel V3b - reverb drive V4a - reverb recovery V4b - final gain stage for entire pre amp V5 - PI

 

[ampmadscientist]

If V2 is blowing, there may be some part of the circuit which causes it to blow.
Normally, V2 should not blow over and over.
ANY good 12AX7 should work without a problem....for a long time.

Rule: don't blame the tube. Find out what causes the tube to go bad. Don't assume anything.

JAN = Joint Army Navy (that's WWII, folks)
If you are using really old tubes, don't expect them to be reliable. Use brand new tubes, and try it again.

 

[johan.b]

If it's not lit up, then the filament is bad...can happen, but usually they last for decades.
the filament doesnt care what position the tube sits in or what the circuit looks like. As long as the heater voltage stay 6.3 volt + - 10%, you'll be fine

Pop a new one in and off you go...
J

 

[Guitar-Sam]

Jan tubes aren't nessasarily that old I've see jan sylvania 12ax7s dated as late as 1987. Ges from mid 80s.
Personally I'd grab another ge and go with it,of the tone was doin it for ya.

 

[Gordon Thunderfuck]

I would have to agree with trying a new tube... So, toss in a new tube in v2 on off you go. If that one burns up, well...now we have something to look for. I don't think I have ever blown out a preamp tube. swapped them lots of times tone surfing, but never needed to.

Good luck!

 

[RussBert]

ampmadscientist is the only guy making any sense here. Find out WHY the tube failed

 

[jgab]

I had two JAN GE 12AX7 tubes a long while back and both of them failed on me. I have only had 3 preamp tubes ever physically fail (stop working) on me and two were JAN GE. Maybe we had tubes from the same batch? Mine had the blue print.

I would try another tube in your V2, but not the JAN GE's. Try something less valuable in there in case it is a component causing the tube to blow. I would try a cheap tube before you start poking around. If the tube in V2 fails again, then I would measure your voltages to see what is going on. Could be getting too much voltage through a bad resistor or something. If it is the filament circuit then all your tubes will be feeling the effects. Measure the A/C voltage on your filament wires (pins 4,5,or 9 of your preamp tubes). Should be around 3.13VAC per side.

 

[RussBert]

I would try another tube in your V2, but not the JAN GE's. Try something less valuable in there in case it is a component causing the tube to blow. I would try a cheap tube before you start poking around. If the tube in V2 fails again, then I would measure your voltages to see what is going on. Could be getting too much voltage through a bad resistor or something. If it is the filament circuit then all your tubes will be feeling the effects. Measure the A/C voltage on your filament wires (pins 4,5,or 9 of your preamp tubes). Should be around 3.13VAC per side.

Why wouldn't you measure and check first?

 

[jgab]

Why wouldn't you measure and check first?

Because not everyone is able to use a meter, understand what they are even measuring, or knows how to do this safely. Its easy to throw in a worthless tube to determine if it was the tube, regardless of your technical skill level. Like I said, in my experience, those JAN GE tubes that I had were trash, so that is why I suspect the tube. Different experiences dictate different strategies, so I just offered mine.

Tubes can go bad on their own without any circuit related components being bad. Tubes more often go bad compared to any other non-tube components. That is why they are plug in components and everything else is wired in. If the tube is not working, then why not suspect it first? Seems pretty logical. Regarding the three preamp tubes I blew in the past 25 years, I replaced them and never had another problem in those slots again.

But in saying all of that, if you are tech savvy then by all means measure the pins inside the chassis when the amp is fired up, and then go from there. You should use a known good tube in this slot to perform your tests anyway.

 

[dewg]

Decided to measure the heater pins for some peace of mind and they all measured between 2.90VAC and 2.98VAC.

If it's not lit up, then the filament is bad...can happen, but usually they last for decades.
the filament doesnt care what position the tube sits in or what the circuit looks like. As long as the heater voltage stay 6.3 volt + - 10%, you'll be fine

Pop a new one in and off you go...
J

How does the voltage measurements I took relate to the heater voltage being at 6.3V +/- 10%?

 

[Jazz P Bass]

It appears that you took your voltage measurement with respect to chassis ground.

It's best to go directly across the heater supply.

 

[Ken]

ampmadscientist is the only guy making any sense here. Find out WHY the tube failed

I disagree. Tubes go bad, so before you tear the thing apart, just replace the tube. If the new one blows too, yes, there's an underlying cause. But in the real world, my money is on a bad tube, especially in the V2 position.

Ken

 

[mickeydg5]

What hardly anyone noticed is that he had placed a JJ in V2 which apparently is still there and working,

No one asked which JCM800. There are what, more than a dozen amplifier designs in the JCM800 series.

 

[Ken]

What hardly anyone noticed is that he had placed a JJ in V2 which apparently is still there and working,

No one asked which JCM800. There are what, more than a dozen amplifier designs in the JCM800 series.

Ah, I missed the JJ as well.

Does the model matter? V2 isn't a high stress position in any amp, unlike the PI. But that being said, if the amp isn't smoking, it's still pretty likely it's the tube itself.

Ken

 

[Blacque Jacque]

What hardly anyone noticed is that he had placed a JJ in V2 which apparently is still there and working,

No one asked which JCM800. There are what, more than a dozen amplifier designs in the JCM800 series.

The OP did say it was a channel switcher combo

 

[mickeydg5]

Ah, I missed the JJ as well.

Does the model matter? V2 isn't a high stress position in any amp, unlike the PI. But that being said, if the amp isn't smoking, it's still pretty likely it's the tube itself.

Ken

Does V2 matter? Why yes it does. V2 does not have the same job in every JCM800.
Refer to the next post.

 

[mickeydg5]

The OP did say it was a channel switcher combo

Right. So that does narrow it down.

I had to reread the first post, myself.
It must be a later 2205/2210.

If it were an earlier one V2 could have been part of the cathode follower.

 

[Blacque Jacque]

I tend to agree with Ken's first assertion that a bottle swap is the simplest test. If it fails, then obviously more in depth diagnosis is required, but other than some preliminary voltage measurements, a new V2 would most likely be my move if it was on the bench in front of me.

 

[mickeydg5]

And as stated he already has a JJ working in V2.

I still wonder why he is averaging 5.88VAC on the heaters. That does work though.

 

[Blacque Jacque]

Depends how accurate the meter is, not everyone has access to a calibrated, quality meter.