Fender Pro Reverb 69 - phase inverter problem

[charliepop]

Hi. I'm having a problem blowing the phase inverter tube in my Pro Reverb. The amp stopped working and I noticed the PI 12AT7 tube had blown. I replaced it, it worked I played for a couple of hours, then the next time I tried it didn't work. The PI tube had blown again.

I'm a pretty inexperienced at electronics but can use a soldering iron and follow a schematic and indeed did a cap job on this amp recently and generally tidied it up a bit as I don't think it had been touched since assembly and someone had stored it in a swamp for nigh on 40 years. I've noticed a dodgy wire going to pin 7 on the tube which I'll replace when I get chance - which will be at the weekend. I just wondered if there was a commonish cause for this that you guys know about to get me started if that doesn't sort it. You know the sort of thing a ham-fisted amateur might have done to his amp with his blind stumbling attempts to work on it... I can only afford so many tubes! And I've just bought a tele 52 reissue that I NEED to play through this amp...

Cheers
Charlie

[5DGuitars]

When you did the caps, did you change any other parts?
Are you sure the caps are all oriented to the correct polarity?
Did you use the amp before you changed caps or did the problem start after your maintainence? And lastly is it an AT7 in the phase inverter?
That's where I would start. Leave an update and also let us know which circuit you have.
P.S. I just blackfaced a SF Pro and you will love yours with that Tele when you're done!

[charliepop]

Hi 5D - thanks for the reply.

I replaced a lot of the caps then blackfaced it following Gerald Weber's guide - so replaced some resistors and did a little rewiring. So it was the AA1069 circuit which I've converted to a AA165. I was pretty methodical when working on it but when I get chance this weekend I'll go back and double check everything as well as replacing the dodgy wire (think I damaged it myself with the soldering iron but didn't notice before.

I bought it from a guy a few years back who had inherited it from his uncle and didn't know what it was (don't hate me - I paid £15 for it!). Didn't play it for ages as it crackled a lot and generally didn't sound very good. Decided to take it to an amp tech to get it fixed up, then decided I'd have a go myself. So it was working before I got my hands dirty with it, then I played it a little afterwards (30 - 40 hours? - and it really sounded great!) when the tube blew, then for a couple of hours before the new tube went. Not when I was playing it though - just didn't fire up the next time.

And yes it is a 12AT7 in there. I'll report back when I've done the above...

[charliepop]

Just noticed that the AA1069 circuit has a 5U4GB recifier tube and the AA165 has a GZ34. I've got a 5U4GB in, but being as I've changed the circuitry from the 1069 to the 165 could this be causing me problems? Or would it just be some sound / tone difference..?

[Enzo]

I have to admit I have no idea what you mean by "blown" as regards the 12AT7. Filament open, no heater glow? Cathode to heater short? Some other interelement short?

Your rectifier tube couldn;t matter. It provides B+ voltage or not to the whole system. That one tube has no idea where its 300v (or whatever) came from. All a different rectifier tube would do is raise or lower the overall B+ a few volts. And besides, isn't the reverb tube also a 12AT7 in that amp? and doesn;t it run on an even higher voltage node than the PI?

Or perhaps it is the case that the amp stops making sound, and when you put a new 12AT7 in that socket is works again? In that case, try this: pull the bad 12AT7 out and then put it back in the socket - same tube replaced by itself. Does it start working again? it is possible the problem is the socket rather than the tube. A spread female pin in the socket could cause the tube to stop functioning. In that case, just wiggling the tube should also restart things.

[charliepop]

Hi Enzo.

By blown I mean the glass cracking and the silver at the top going white. Don't think there's much point in putting it back in... And yes the reverb input driver is also a 12AT7.

[MWJB]

Check for anything that could be physically damaging the tube, read voltages at PI tube pins with a working 12AT installed.

[charliepop]

Hi MWJB.

Can't see anything physical - ie. something that I could poke my finger at that is actually in contact with the tube that could be a problem. As I said I don't have much knowledge but can carry out instructions (I should probably be tried for war crimes to tube amps...) - what should I be testing as regards voltage and what readings should I be looking for..?

[Amp Kat]

You need to get a DMM and put the black lead on chassis and the red lead on pin 1 and measure DC Volts and then pin 6. Be extremely careful what you touch and where your hands go especially if your touching chassis with a part of your body. There are voltages in there that can possibly hurt you are even kill you. Anything over 400 volts on that tube will most likely take it out and that is most likely what's causing it.

[charliepop]

Hi Kat.

I'll be as careful as Careful Jock McCareful, winner of last years Mr Careful competition. So 400+ is bad and I should then try and find the reason for that if that's the case - I'll post the readings. I only checked the output tube voltages when I was biasing the amp and not the other tubes... What values would you expect for pins 1 and 6 on the phase inverter tube?

[Enzo]

I can't think of anything the amp could do that would crack that tube. Something might hit the tube, or if something cold hit the hot tube like someone's drink or maybe dog slobber.

But electrical stuff just doesn;t have many ways to physically blow up a small tube.

And you amp could have taken a drop or hard jolt. If your spare tube was in it, then it would be subject to the same stress.

[pdf64]

You've either been unlucky, careless, or there's an evil elf armed with a toffee hammer tapping on your tubes. Put a good tested tube in there and you should be fine. Peter.

[charliepop]

Damn that elf and damn his teeth - he's cost me a house 2 jobs and 3 wives already...

I replaced the visually dodgy wiring, spent a while checking voltages - not finding anything untoward, and replaced the tube. Played for a few hours (sounds great) and will keep my fingers crossed and all toffee hammers out of sight.

So it sounds like it's unusual to physically damage this tube - I've left the chassis out of the cab for the time being (not gigging at moment) and I'll keep playing and see what happens. Thanks everyone for your thoughts - I'll let you know if it happens again and indeed if it doesn't...

[Amp Kat]

I didn't think he meant physically blew but more like blew it out like I've seen in SVT's from over voltage. He should be fine now.

[drewl]

If one of the grids or cathodes was shorting to ground would that cause something similiar?
Too much current draw?
I've never seen it, just hypothesizing......it's at least as valid a theory as the elf.....or would it be an evil troll with hammer?