Classic 30 blows phase inverter tube

vbjamin

Member
Messages
1,116
My Classic 30 blows the phase inverter tube and takex out the 1 amp fuse with it. This has happened twice now. I am not a tech at all. I this problem at all common or does it ring a bell to anyone as being symptomatic of a common problem. I replace the tube and fuse, crank it up, it sounds great, go to a jam and crank it up and bang, gone!
Frustrating :(
 

Blue Strat

Member
Messages
30,194
It's an unusual scenario. I can't think of any particular reason why this should happen repeatedly.

Sounds like you need to get it to a tech for a full checkout.
 

vbjamin

Member
Messages
1,116
Yes I have taken it in and am waiting for the verdict. Hopefully it isn,t a major issue.
Thanks for the reply
 

fullerplast

Senior Member
Messages
6,781
Originally posted by vbjamin
My Classic 30 blows the phase inverter tube and takex out the 1 amp fuse with it. This has happened twice now. I am not a tech at all. I this problem at all common or does it ring a bell to anyone as being symptomatic of a common problem. I replace the tube and fuse, crank it up, it sounds great, go to a jam and crank it up and bang, gone!
Frustrating :(
Are you using a 12AX7 or 12AT7 in that slot? A 12AT7 will usually be a bit more robust as a PI. It is an odd type of failure, so it may be something else entirely. Report back if you get it figured out.
 

vbjamin

Member
Messages
1,116
Would speaker impedance mismatch be the problem?
I say this as the only thing I have done recently that was common to both failures was, unplug stock speaker and plug into a Mesa cab with a 16 ohm Greenback in it.
Although I did use the same cabinet at home with no problemo.
 

John Phillips

Member
Messages
13,038
Originally posted by vbjamin
Good point, I have a Jan Philips at7, that I will try in this socket.
Don't!

Don't waste a good tube. Don't fit anything decent until you know what the problem is. If it keeps blowing the same tube, that's the symptom not the cause.

My best guess is that it's probably some kind of filament connection fault, since even a direct short through the plate circuit couldn't draw enough current to blow the fuse. If I remember rightly, the preamp tube filaments are in series (not the usual parallel) on that amp too, which means that quite high voltages are present if there's a problem.


No, a speaker impedance mismatch couldn't blow the PI tube. In fact, is the C30 not a 16-ohm amp anyway?
 

vbjamin

Member
Messages
1,116
Yikes!
Good thing the amp is at the shop or I would prolly be down another tube!
I will let you guys know what the tech says,
Thanks again
 

fullerplast

Senior Member
Messages
6,781
Originally posted by vbjamin
Yikes!
Good thing the amp is at the shop or I would prolly be down another tube!
I will let you guys know what the tech says,
Thanks again
vb, my question was regarding whether or not you were using a 12AX7 when the design called for a 12AT7. I wasn't suggesting you try it as a test. Check the tube chart or a schematic.

What other clues do you have? How long did you play it at home with no trouble? What was different at the jam? Louder, being on a longer time, using high output pedals, etc?

What tubes are you using? There have been reports of GT12AX7M's with infant mortality. I don't really see this blowing a fuse either, but I'm sure I've not seen all failure modes.
 

WailinGuy

Member
Messages
1,290
If it keeps blowing the same tube, that's the symptom not the cause.
+1

Clearly, there's a serious problem with the amp's circuitry. There's nothing wrong with the tube.

I remember once a customer complained that his Princeton Reverb kept blowing rectifier tubes. He must have ruined two or three 5AR4 rectifiers before finally bringing the amp to me. As soon as I opened it up, I found the cause. The tip of a 1/4" phone plug had broken off inside the amp. Of course it eventually had wound up in the lowest part of the chassis which was....

... that's right, the shallow round well created by the bottom of the multi-section filter can. The tip nestled between the edge of the chassis hole and one of the cap terminals, creating a direct short on the main B+ rail.
 

fullerplast

Senior Member
Messages
6,781
Originally posted by Jim Salman
+1

Clearly, there's a serious problem with the amp's circuitry. There's nothing wrong with the tube.
Actually, I don't know that we have enough information to draw that conclusion. This happened two times. We don't know if these incidents happened over the course of two years or two hours. We don't know what tubes he used or what the line voltage is both at his home and the jam. I agree it is likely a circuit problem, but not clearly a circuit problem. We just don't have much to go on. People on this board have had two 12AX7 failures in a row with new tubes.

I just looked at the schematic. It calls for a 12AX7. B++ to the PI circuit is 275V. The 1A fuse is in front of the diode rectifier supplying B++. The filaments are DC in series, rather than the usual AC. You need to have all the tubes in place to get filament voltage down the line to all the tubes.

Since we don't have the amp in front of us it's hard to guess what components or traces around the PI may have failed. A visual inspection may make it obvious. My best guess at this point would be a PI coupling cap failure allowing DC to pass between the power tube bias circuit and the PI.

Vb, please get back to us when your tech discovers what the actual problem is.
 

vbjamin

Member
Messages
1,116
I will let you guys know 4 sure.
I played the amp at the same jam in consecutive weekends. Played there many times before with other gear and no issues. Other amps ran fine. I do remember a few months ago a guy blew up a DR a Mesa F30 in one night. I was playing a DC-5 next to him with no issues. I think it was just his bad day.
 

fullerplast

Senior Member
Messages
6,781
Originally posted by vbjamin
I will let you guys know 4 sure.
I played the amp at the same jam in consecutive weekends. Played there many times before with other gear and no issues. Other amps ran fine. I do remember a few months ago a guy blew up a DR a Mesa F30 in one night. I was playing a DC-5 next to him with no issues. I think it was just his bad day.
Have you had the amp a long time and had you been playing it consistently with no troubles prior to this happening or is this amp a new acquisition and you are just finding this problem? Did you play the amp at home between jams after changing the tube and fuse? When you play at the jam, what are your volume settings relative to your home settings? How long did it take before the amp failed? What tubes failed (manufacturer)?

Sorry for all the questions, but long distance debug can be a difficult thing, especially for a "non-classic" type problem like yours...
 

vbjamin

Member
Messages
1,116
Have you had the amp a long time and had you been playing it consistently with no troubles prior to this happening or is this amp a new acquisition and you are just finding this problem? Did you play the amp at home between jams after changing the tube and fuse? When you play at the jam, what are your volume settings relative to your home settings? How long did it take before the amp failed? What tubes failed (manufacturer)?

Sorry for all the questions, but long distance debug can be a difficult thing, especially for a "non-classic" type problem like yours...

Had the amp one month only
Cranked it at home for hours on end , no issues, put the Greenback in myself.
First tune at jam, dead ,
Replaced tube and fuse, cranked again and used OD-3 AND other pedals , no issues
First tune at jam in a differant ac outlet than the first time. dead

Volume settings not really differant at the jam as I had an empty house the same day at home and really let her rip.
 

WailinGuy

Member
Messages
1,290
I agree it is likely a circuit problem, but not clearly a circuit problem. We just don't have much to go on. People on this board have had two 12AX7 failures in a row with new tubes.
The fact that the fuse blows along with the tube makes it, in my mind, extremely unlikely that the problem is being caused by the tube alone. It's not easy for a 12AX7 in the phase inverter or in the preamp to take out the mains fuse.

Now it might be helpful if we could find out exactly how the tube "blows". Does it stop lighting up? Do the plates glow red? Does it lose vacuum (getter flash turns white)? This info might give us a clue.
 

vbjamin

Member
Messages
1,116
Th first tube I did not notice until I checked/ replaced the fuse. It has a cloudy white residue near the top and does not light up at all. The second one does light up at the bottom of the tube only/ faintly, has no residue and no sound.
I did not bother putting in another tube and or fuse , just took it to a tech I know.
 

fullerplast

Senior Member
Messages
6,781
Originally posted by vbjamin
Th first tube I did not notice until I checked/ replaced the fuse. It has a cloudy white residue near the top and does not light up at all. The second one does light up at the bottom of the tube only/ faintly, has no residue and no sound.
I did not bother putting in another tube and or fuse , just took it to a tech I know.
Vbjamin, you've added quite a bit of new information in your last two posts. The odd part is that you were able to play it at volume for a long time at home with no problem. It also sounds like you may not have lost a second PI tube, just the fuse. I strongly suspect the problem will be obvious with visual inspection of the boards. The Classic 30's are good sounding amps, but are not built especially well from a durability and maintenance point of view. Again, let us know what he finds and good luck.
 

fullerplast

Senior Member
Messages
6,781
Originally posted by Jim Salman
The fact that the fuse blows along with the tube makes it, in my mind, extremely unlikely that the problem is being caused by the tube alone.
Jim, I have to agree with you there. Even a straight short plate to cathode would draw much less than an amp, given the plate and cathode resistor values in the circuit. What I don't get is the fact that he played it hard after replacing the PI and all was well.....I wouldn't be surprised if it was something rolling around in the chassis, much like your princeton problem.

Edit to add: as I mentioned earlier, a shorted coupling cap between the PI and the power tubes would draw an amp or more, based on the values in the schematic.....
 

Blue Strat

Member
Messages
30,194
Originally posted by fullerplast
Are you using a 12AX7 or 12AT7 in that slot? A 12AT7 will usually be a bit more robust as a PI. It is an odd type of failure, so it may be something else entirely. Report back if you get it figured out.
Using the incorrect tube type is not generally recommended unless you're going to change the circuit (the cathode bias resistor) to operate the different tube correctly.

Not changing the resistor in this case poses no danger however.
 




Trending Topics

Top