Maintaining the 12AX7 phase inverter

I recently bought a 212 HD-130 in near immaculate condition. Eventhough I paid collectors price, and didn't get a fair shake on my trade ins, I don't feel an ounce of buyers remorse, because I've fallen hard in love with this amp!
I've always heard that Music Man amps are super reliable, and that's half of how I rationalized buying this 75 pound bohemoth. The other half was that it had the coveted 12AX7 phase inverter. Now I come to find out that under the right/wrong circumstances that tube has the potential to wipe out the transformer, and then all you got is a nice open back speaker cabinet with nice alnicos, pretty much.
My question is this: Can anyone elaborate on what can cause the 12AX7 to short out and kill your amp?
I've read here that it's mostly due to "mechanical abuse", and I'm wondering what that could mean?
Also, how can I avoid these circumstances, outside of just using a realy hardy NOS tube? The previous owner left a spare GE 5751 in the back,I hear these are the most reliable 12AX7's you can get.
Anyway I got a bunch more questions regarding this
awesome amp, and alot of bragging to do too. But this is something that's realy been bugging me, any help is much appreciated. Thanks.

The only advice I can give is to bias the amp a little cooler, like around 23mA PER TUBE. Even a mA or two cooler won't hurt. This should give you a little breathing room. I keep mine around that mark and haven't seen any signs of failure yet. Mike.

What are some signs of failure?
Has anybody ever had thier amp crash like this, and had to get a new transformer?
I've always heard that once you change out the transformer it'll never sound the same.

Well, I don't know about that. Usually, if you change a transformer and you think it sounds different, it's either in your head or you had something else go when you took out the transformer and you didn't catch it. My Twin sounds better NOW that I replace the OT than it did before. Yes, I've had a Music Man "melt" down. I was there quick to turn it off and never DID eat a trans. If you are paying and all off a sudden it distorts and then goes quiet, the tubes will be glowing. TURN IT OFF! It may survive if you're quick. Mine did.

you know, chris, I've wondered about that same question: what chain of electronic events cause the 12ax7 meltdown that takes the OPT along with the power tubes. Several guys have alluded to that "event" and have said that it was this that caused Musicman to retrofit any 12ax7 PI amps that came in on warantee calls and hastily convert their line production amps to ss phase inversion....so quickly, that many early solid state PI amps still have the sealed-off hole where the 9-pin tube socket would have gone. Does anyone know the details of that electronic event. Terry, perhaps?
ed

I am not 100% sure, but I think the failure mode that is mentioned in whispers but never discussed has to do with a certain set of elements shorting internally in the 12AX7 phase inverter tube.
You will notice on the schematics that the feedback signal from the hot side of the output transformer's secondary connects to the Cathode of the first half of the P.I. tube. When connected like this, the feedback is negative feedback and this keeps the output stage stable and under control.
If this feedback signal was connected instead to the grid of this stage (Cathode-to-Grid short for example) or the Cathode of the second half of the tube (Cathode-to-Cathode short), the feedback becomes POSITIVE feedback with the opposite effect... the amp goes critical, unstable and out of control! In a very short period of time it can generate full power high frequency sine-wave oscillations that can wipe out a set of output tubes and the output transformer.
What is insidious about this possible failure is that the frequencies that output stage might be oscillating at are probably beyond your range of hearing and the frequency response of the speakers so everything is either silent or it is modulating your guitar signal so it may sound funny.
High Frequency energy outside of the audio frequency band will cause the output stage to heat up faster and more dramatically than the lower frequency signals it usually amplifies. If you don't get it shut down quickly it could melt-down before the fuse will blow!
Since this could be considered a fire and electrical safety issue I think Music Man moved quickly (and silently) to minimize any product liability fall-out from such failures.
I have never seen one of these failures personally, but talk of them among Music Man afficionados seems to take on a mythical quality because of the lack of hard information.
Since the tube P.I. has BOTH elements of the inverter circuit inside the same glass envelope, you can have numerous (and interesting) connection short possibilities due to mechanical failure of the tube! This is physically impossible with the solid-state P.I. and an electrical failure of one S.S. element can't cause this mechanical "rewiring" of the output stage!
The best insurance is to have an N.O.S. American-made JAN (military) or Industrial version of the 12AX7 in this socket and baby your amp when moving and transporting it. Alternatively, you could install a "Socket Saver" (if you can find one) to prevent wear and tear on your chassis socket and simply remove the tube when transporting the amp. Simply bring the tube with you in a foam padded security case and install it just before you need to turn it on!
Steve

I understand that the post I'm about to make isn't really concerning the question that was asked, but it's a point that I think should be made; --just to be technically correct for those (like myself) that don't always know any better. Chris said in his question, that he understood that the 5751 "is the most reliable 12AX7 you can get." Actually, a 5751 isn't really a 12AX7 since it has slightly lower gain--even though the 2 tubes are pretty much interchangeable. I just didn't want anyone to take away from the conversation that they should buy a 5751 thinking that it's a higher quality tube. And no, I'm not a 1/3 the tech that alot of the contributors here are, I just wanted to help.

Steve, thank's loads for that explanation. the question of what exacly caused the meltdown in the output section has bugged me since I first heard it mentioned here on the site. I don't know how you became privy to information that was a mere rumor in 1976, but I love it! The idea of a cathode to cathode short caused by metal fatigue due to HF oscillations just makes such perfect sense.
music man management reaction to this potential liabitity issue would explain the company's actions after discovery of the inherent product flaw: hasty retrofit of every chassis with PI tube within reach and hastily coming up with a new production design to keep the merchandise flowing out the factory doors. holding this knowledge about the failure as secret proprietary information to avoid liability litigation is also all too plausible. Great post, steve.
ed

I didn't become privy to the info, it is just an educated guess based on a posting I saw in a newsgroup some years ago that postulated the cascade failure of the output stage could probably only be caused by the 12AX7, since most shorts of consequence in the output tubes would probably blow the fuse before anything else could get started! That is all the clue I needed to create the scenario above!
It is interesting and important to note that this isn't really an electronic design flaw in the amplifier circuit (one that could be easily determined by simply looking at a schematic), but a rare physical occurrence that WAS a real possibility. The crux of the possibility was the design of the 12AX7 tube itself! You either note the data, see how common this scenario is and live with it if it doesn't happen often OR you take steps to see that it isn't possible anymore (which is the most responsible move to take when you are making large quantities).
Having worked in a variety of electronics-related industries in my life, I am all too aware of the increasing climate of product safety lawsuits and corporation's efforts to minimize any leaks of info that might feed an effort to take advantage of a potential problem. It seems only natural for Music Man to move "swiftly and silently" to try and mitigate a problem once it has become obvious (and I have seen such efforts first hand in other industries). It is just sound business practice!
Doug is right... the 5751 is basically an industrial cousin to the 12AX7 and its overall gain sits between the 12AX7 and 12AU7 I think. I would argue that it IS a higher quality tube from a MECHANICAL standpoint (the important quality in this story) since it would be an American-made NOS industrial-quality tube which would almost be guaranteed to be more physically stout than most of the 12AX7 tubes in current production.
Steve

steve,
your modesty does you credit. anyway one slices it, your explanation above was a flawless piece of logical deduction. I do have one question, however, this postulated design flaw in the 12ax7 used as phase inverter; has it shown the same tendency to 'melt down' in other amps that use this tube for PI? Personally, I've never heard of it happening in any other amp manufatured here or abroad.
be that as it may, I'm running a NOS Amperex 12ax7 in my '75 210-sixty five (perhaps you read of my recent good fortune); would it behoove me to purchase a good NOS 5751? If so, what brand(s) should I consider? While I don't run my amp in high power mode any more (unless I've get a wild hare up my butt and want to "crank it" out in the garage for a couple of minutes to hear the beautiful op-tube distortion), I'd like to take every reasonable precaution available to protect my amp.
Rebuilding my 'baby twin' sounds like a wonderful project in theory. but I know how long it would sit on my bench awaiting my attention--I find myself busier since early retirement; rather than relaxing I seem to have more demands from my old employers (at least now I can pick and choose between the jobs 'requiring' my attention these days).
on another subject, steve, thank you for making this site a little more exclusive. it is a small price to pay to avoid having to see those annoying spammers' posts cluttering up our board.
ed

Thanks guys! This is realy fascinating stuff. You've been a great help. I had no idea this subject was so mysterious.

Yes, very fascinating and educational. I never really knew why Music Man switched out the 12AX7 tube until now. And I've been using a Music Man amp (with the 12AX7 tube) for many years, Jeez..
Great Post!!
Thank you,
Dave.

I have not only been the winess to such a failure, it was mine. I used to go through a set of 6CA7's at least once a month. And remember, that was back in the days when we HAD good(or at least better) tubes. I still go back to my original premise that these tubes, when biased INDIVIDUALLY, don't have that trouble. What I don't like is taking a voltage reading for BOTH tubes and setting the amp that way. I set my 210 up by measuring the current at each tube and setting modestly for the HOTTEST tube. Back in the so called day, they set them in pairs and got an average for both tubes and called it done. This was also when they weren't checking for balanced tubes(matched) and could have one tube conducting at one rate of current flow with a certain control grid voltage and the other concucting a much larger amount. Same total but one's got a much higher current flow and at 700v, there isn't much room for error. I've got some sylvania's that I got red SEVERAL times back then that I kept that run just fine now that I grouped them by their current flow and biased accordingly. I know Steve's explanation is 100% right and the people at MM basically told me about the same thing back then.They also told me they couldn't get GOOD tubes anymore. I had one of the tube inverter amps converted BY THEM to the SS driver and it was never the same. THe frequencies generated by the tubes can cause them to melt down, so don't push them into resonance. THe MM folks also told me back then that Sylvania's sucked-go figure. I have a quad of Amperex EL's that I've held on to that all measure within 1 mA of each other and the boys at MM back then loved them but just couldn't get them. I honestly think they designed those amps around the Amperex tubes but I'm probably all wet.If yu want to hear something with balls, get a quad or a pair if that's what you need, and put them in the old MM and bias them about 23mA tube-you'd be amazed at the difference. I didn't like them back then, that's why I took them out when it was new. Kept them all these years 'til I grew up! By the way Steve, great explanation.

It sounds plausible anyway... After I wrote that I too had a question about what might be different with a Fender or Marshall circuit since they also use a tube phase splitter.
I imagine there actually might be a design difference that is not susceptible to the same problem so maybe I misspoke regarding the Music Man design not being at fault... it works but it may have this problem due to the design topology that other amps designs don't have!
I know that Fender has had at least 4 or 5 different phase inverter designs throughout the years. Maybe one of theirs had this problem too?
Steve

you've given me an idea for a great research topic, Steve. I'm gonna investigate Fender PI circuit designs (historically and electronically) and see what I can glean from the data. I think it is still on topic because I consider a musicman amp as just another in a long list of Fender designs.
I think I have most of the relevant schematics already filed on my computer, so it should just be a matter of straight investigation. I think I can develop a partial chronological list of changing PI designs from a careful re-read of Weber's ...Hip Vintage Guitar Amps. I'll let you folks know what I've discovered and any conclusions I glean from the data. Again, thanks for the shove Steve.

I was thinking that perhaps the failure mode could also be related to a runaway bias condition (although I would expect that to blow a fuse before things would self-destruct).
Any more educating guesses and scenarios will require some study as Ed is doing. Gerald Weber's books and the Groove Tube (Aspen Pittman) book should provide enough classic schematics to choke a horse!
Steve

Steve, Mike, Ed, and others; Thank you once more from another inexperienced, but hungry for knowledge, MM lover. Steve/Mike/et all: How about you guys writing a "Tube Amp Primer"??
Anyway, my serious question regarding this thread, is how does the Groove Tubes "Mullard" 12AX7 measure up in terms of being a good, reliable PI tube to use, as far as likelihood of "crashing"? I think they now have even a "hand-picked", "balanced" version of this tube, which would be optimised for use in a Phase Inverter application. What experiences have folks had with this tube?
Thanks again for the generous sharing of your time and knowledge, and Ed, I for one, will be looking forward to your findings, when you are ready to share them.

Groove tubes are just a graded sovtek tube that they "supposedly" have matched and picked out the "GOOD" ones. I'd better keep my opinion of sovvy's to myself this time. I'd wait until they have a little more track record before I'D use them.

Reed,
one of the department heads at groove tubes has a site called "amplifier blueprinting" oR something along those lines. this guy, his entire job with the company is grading and matching 12ax7 and ec83 tubes for use as phase inverters.... claims "matched" triode halves within the tube make all the difference in the world in your sound. seems like a hard sell case to me--we had good sounding amps long before some fool got the idea of matching dual triode halves. excuse me, I stand corrected: the man is not a fool, he is practicing "marketing."
now, Reed, I may be all wet on the subject. Maybe balanced triodes in the dual triode make a big difference...my only problem with that is that we had good sounding amps before GT began this practice. (I happen to think my 210 sixty-five is an ass-kicker, heart breaker and name-taker; I never used a balanced PI tube...never heard of one until some tone guru decided it was another service "required by the music industry" for its continued smooth functioning--these are the same folks selling beach front properties in arizona).
but beware, all of the foregoing discussion of the phenomenon of tone gurus' marketing products we didn't know we needed until we were told is just my opinion on the subject. as I said, I could be all wet (but I doubt it). ed

If the PI tube is causing the problem with the output tranny, why not add a 500ma.-1amp(?)fuse in line, like the Marshall amp did, this would save the tranny.
And keep a good 12ax7 around to swap when the opertunity comes up. And always keep an eye on the biasing on the tubes perhaps add a 1 ohm resistor to ground on pin 8 of all 6CA7/EL34's.

Normally I have seen the fuse inline in series with the RED B+ wire going to the OT. Also the Concertina splitter was used on the old 5E7 Bandmaster if I remember. There was a mod putting a 2.2meg resistor in front of the second 1/2 of the 12ax7 for stability. I will try to find the schematic and see if I can post it here.

The "Paul C" mod which DC-biases the phase inverter. The result is a slightly smoother breakup at higher volumes and slight increase in headroom (though not much).
The .047 uf cap off the wiper of the master volume, from that .047uf theres a 2.2meg resistor connected to the B+ where the 33k plate resistors connect section "F" of the MM schematic,(not the 12ax7 plate). Then the .047uf on the same end that the 2.2meg is connected goes to the grid of V1B (12ax7)the 1meg resistor that was connected to the grid of V1b, with the other end between a 33k resistor and the 1.5k cathode resitor, get disconnected from the 1.5k/33k junction and now goes to ground(1meg). So you have a 1meg resistor from the (V1B) grid to ground, leaving the 1.5k and 33k in tact. So there's a 2.2meg off the grid of V1B to the B+. thats the only additional resistor. From the Bandmaster schematic, there are 2-56k resistors where the MM's 2-33k resistors are. The MM's 33k's would add more voltage to the tube, maybe stressing it somewhat, where maybe the 56k might drop the voltage a little. The 56k might add a subtle bit of headroom over the 33k's. This is a concertina splitter by the way if anyone did not know used in older Fender Tweed amps like the 5E7 Bandmaster. Heres the link, go 1/2 way down the page to January 2003 update. The Paul C mod.www.kilback.net/homebrewtweaks/amps/amps .htm
or ask me for a copy of the schematic comparing the mod to stock.

Steve, is there a way to pst this schematic here?
thanks
Ed
PS, the inline fuse I posted above, I have seen in Marshall amps, usually. A 500ma I believe on the B+ output transformer wire.

I just checked my work again on the concertina splitter. on the second stage of the 12ax7 there is a 33k plate resistor and a 1.5k and 33k at the cathode. On the mod I was trying to explain, I see that the 1.5k is omitted! just a 56k on the 5E7 modded bandmaster, so Im thinkin if theres a 33k there on the cathode to ground, a 1meg on the grid to ground and a 2.2meg to the B+. Maybe I will try changing the 33k's to 56k, that should take some stress off the 12ax7 a bit. Also I have hear that the powertube overvoltage protection diode D-7 and D-8 can short out and that takes a transformer like that! I remember a Mesa Boogie amp that someone had added the diodes to power tubes plates and the tech at Mesa said that is not a good idea, since its been known those diodes go bad and it takes out the transformer! Look at the later HD-150 with 6L6's the schematic I have, has NO diodes on the 6L6 plates, at least on the latest revisions. I wonder if there were any issues with the solid state drivers using those diodes... Just a thought. I have not seen them on the RD amps either. Well I will post my findings for you when I get some. I hope this is useful info and I am not rambling too much!

I just done this mod and so far it sounds really good, I need to run it loud tomarrow to hear what that sounds like at gigging levels, low volume it sounds smooth and more alive. I decided to use 2-47k 2 watt Metal oxide resistors on the 2nd stage 12ax7 plate and cathode instead of 33k. 33k sounds a bit compressed compared to these 47k's. It seems to have slightly, more headroom and presence. I feel that the power tube plate protection diodes are a disaster waiting to happen. If a diode was to short out, it would be a direct short and that WILL take out the transformer. I undersatand that it was a design to protect those power tubes, It is supposed to absorb the tranient surge when the power in applied to the power tubes but if you let the tubes warm up properly in standby mode for about a minute and start out in low power for a few minutes, I think they will do fine without those possible shorting diodes. Does anyone have any thoughts on this?

nice work, Ed. let us know how it sounds at performance volumes, I'm really interested in this mod...I have a 5751 NOS GE coming in the mail any day now to hopefully guard against the PI failures we've discussed on the thread. [btw., since I NEVER use my 210 sixty-five full balls-to-the-wall open anymore this is a little bit of overkill; but.... "an ounce of prevention..." old ben franklin knew his shit, too!]....anyway, let us know how the volume tests go.
one question, ed? is a "concertina phase splitter" the same as the "split-load phase splitter" described by Crowhurst? my study of the Fender-genesis of the Music Man phase splitter has been sidetracked by an examination of the whole concept and technique of inverting a signal to the push pull pair....man, is there ever a lot to digest on this subject!
again, nice job
ed

Hi Ed, your too nice, and thank you, anything to help keep these amps happy and the people who own them anyway... Well I tried it up loud today, and something is not quite right. It sounds a little fuzzy and I need to see what may be the problem. So I put it back to stock to make sure that was the problem, now its a matter of backtracking. The circuit was as above from Kilback's site (www.kilback.net go down half way and look for January update on the Bandmaster Paul C. Mod), and its for a Bandmaster 5E7 circuit. I really dont know why the mod started making the constipated sounds, since I also tried it in low power to emulate the Fender voltages, but maybe later I will try something else and find where the problem is. I did disconnect one end of each diode on the 6CA7 Plates (2) that I mentioned above, its a great idea originally to supress over volatages to the power tubes, as long as a diode does not short out, since that would be a direct short and take out the output transformer. As long as you start the amp as you already know, in standby for a good minute, and start with low power, it should go easier on the tubes, switching to high voltage after the amps has warmed up a bit. I have heard a few stories about those diodes causing problems with the OT. If anyone knows anything about this, please feel free to say something. I will take another shot at it when I get a chance to see where the problem is and let you know. The 2.2 meg resistor that goes to the second 12ax7 grid and the other end to "F" B+ section, might be the problem for some reason, its supposed to smooth out output distortion and add headroom slightly. I will dig more into it. The Paul C. phase inverter mod is supposed to be good, maybe I overlooked something...

Ed, I thought I better look to make sure. I looked in my Dave Funk book and looked up the split load inverter, which is what is in the Music Man or the Cathodyne inverter. This came out in 1953 in the 5E3 Deluxe and almost all Orange and Hiwatt amps have it as well. As the signal goes through the a tube stage from grid to plate, the signals phase is reversed. When the signal goes from the grid to the cathode, the phase is unreversed. This means we can get a push/pull signal off of one tube. The outputs are opposite, but are they in balance? By dividing the plate resistor in half, and putting half of it on the cathode, and half on the plate, we create a cross between a plate loaded triode, and a grounded plate triode (Cathode fallower), at the same time. The same current flows through the cathode resistor, also flows through the tube's plate resistance, and the plate resistor. So basically its a cathode falower. It has a low gain, less than 1, basically a loss of gain, so the stage before, drives the second stage that used to be very much like the concertina PI. So actuall the Split load or Cathodyne is really what is in the Music man. Which is strange maybe since after this design way back then, Fender came out with the long tailed pair that is used in most amps like the 59 Bassman, most Fender Blackface amps and Marshalls etc. Boogies... On the Fender design Split Load PI, the plate and cathode resistors are 56k. and theres a 25uf/25 vdc cathode bypass cap on the first triodes cathode. I tried that oneday just for kicks, it does beef up the signal, but does nothing for the tone, It sounds somewhat harsh to me. It does not need it maybe cause of the solid state preamp. In the Paul C. mod, the 1 meg resistor stays on the second stage grid and gets disconnected from between the (33k on Music man)-56k/1.5k cathode connection, and that end is then grounded (of the 1meg). Also there is a 2.2meg resistor added to the second stage grid and the other end connected section "F" of the power supply in a Music Man 12ax7 model. Maybe people reading this should not just try this, thinking its a fix for the 12ax7 issue, but I am curious if it helps promote stability of the circuit. I think the 2.2meg resistor is some form of (current?) feedback to improve distortion charactoristics, since the report says it smoothes the output distortion and adds headroom slightly. I understand that some Dumble amps have a negative feedback in one of the preamp stages, and it does change tone, it seems to cut the bass a little, roll the highs slightly and fatten it up a tad... More of a smoothing effect, probably takes away some harshness too. I hope this answers the question and helps others to understand more about how amps work.

I had a chance to rework the Paul C. mod on the HD-65 PI. I found that keeping the 33k plate and cathode resistors there, and instead of using a 2.2meg resistor, 4.7meg does the trick. The overall effect is that the harshness goes away if thats what you want, it smoothes out the distortion, slightly fattens it up, and does give a sight bit more headroom. The notes are Dumble* fat! If thats to your liking its not too difficult to juggle, its a little different sound, but if you like the Dumble-like sound, the fat smoothness, its there. If you prefer the Fendery bite, (harshness?) keep it stock. With this mod, the gain all out, you can get close to Robben Fords smooth tone, but you better have the fingers.

Wow, you guys have really gone deep into PI territory!
Ed G: If you can create a PDF file (the best for schematics) you can send it to me via e-Mail and I can put it on the server and post a link to it here.
Otherwise, you would have to post it as an image using the simple formatting commands shown in the Help/Instructions area you can click at the bottom of the page.
I havr wondered if the negative implications of those plate protection diodes had ever occurred to anyone else! For every problem solved two more are potentially created! Fuses would certainly prevent damaging that sweet piece of iron!
Steve

I agree, steve. this thread is becoming really informative--my understanding of the electrical mechanics of phase inversion is steadily progressing. ed g has added some great insights. as part of my "heater circuit rehabilitation plan" those diodes are HISTORY.
since I've always warmed up on standby before firing up my sonic rocket, the potential hazards of retaining D7 and D8 outweigh any circuit protection they once may have had. and I don't let some bonehead just plug in while I'm on break, so I should be safe (the liberties some people take with another's property has always amazed me).
ed s
*keep up the great work, ed!

Thank you fellow Music Man amp heads, lol I will try to send you the schematic I have, but I kept the 33k plate and cathode resistors there and the4.7 meg in place of the 2.2meg, sounds much better for that mod. I have not had a lot of run time with it yet, but it really fatens up the tone!

hi ed,
in an earlier post on this thread you mentioned removing the connection of the D7/D8 diodes from the power tubes. which pin did you disconnect? wouldn't it be better to just remove both of the diodes and be done with it?
On another front, wanting to keep my amp as stock as possible I decided against installing a filament xfmr. I'm still waiting on my new power xfmr from mojo musical supply so I'm trying to stay busy with improvements until my major project part arrives.
ed s

Hi Ed S. I would just be done with it myself, I took the diodes completely out so the loose end would not get jarred upon a dangerous connection and blow the amp amp that way. I would put the diodes in a safe place, so that you will have them incase you want to put them back. I have just heard and even seen where the diodes in other amps with plate protection go bad from one too many high voltage spikes when powering up. Maybe its the first turning on in the HI position with all the reserve capacitance in the power supply being fresh and so eager to power up the tubes the first time its powered on each time that weakens the diodes... Thats why I would just be safe and do the warm up, LOW power first till the amp has been on a little bit before full (HI) plate voltage. I think that might have something to do with the diodes going bad. It seems that the later MM amps did away with them anyway. Can't wait for you to get the replacement tranny, I hope it goes well for you. Do you have a way to check the tubes to make sure they are not shorted?(The filaments)How are the filter caps, have they been changed? If not, it would be worth the investment to get them replaced as well before turning the amp on with a new tranny. Sometimes old caps can start to short out and that would kill the tranny too. But maybe thats not the case with you amp. I have run across that in a Traynor amp, but the tranny's are so overbuilt in them, fortunately the fuse blew before the tranny, Th filter caps were a direct short, I put a capacitance meter acreoss the cap and it was a dead short! Just be careful, you know :^)

hi gents,
I hope you all can appreciate the irony of my situation. I've been involved here in this thread immersed in discussions about the theoretical failure of a 12ax7 due to some form of mechanical grounding of one or more electrodes. Some saturdays back, the 19th august to be exact, I blew my amp up at the start of the gig... the immediately ascertainable cause of my amp not working was No Heaters. ominous, no?
I got the amp back up on my bench sunday afternoon and began looking around with a meter in hand; I first encountered a dead short in the heater circuit. after removing the tubes I followed the short through the heater circuit with the same result. next I disconnected the circuit from the transformer feeders and discovered the short had shorted my 6.3 vac circuit in the xfmer secondaries.
this morning, in response to your suggestion ed, I went back and started checking the tubes for the source of the original short. I didn't have to go too far to find it: the 12AX7 had shorted the 6.3 vac to ground which had in turn boiled the xfmr terminations....6.3 volts don't sound like much until one factors in the amount of current present in that circuit...it'll definitely burn metal and probably arc like a bastard!
so, I am trying to be patient about the arrival of the new xfmr (I did order ground shipment, after all) because I want my amp back really bad. for one thing, I have my brown dogs and wish to become involved in the op amp tasting...
oh, ed, I did a full recap last year along with refurbishment of the tube sockets with new caps and resistors along with new caps for the driver board... so I was pretty confident there was no problem originating in the filter circuit. but as you so wisely pointed out, one must be careful with our old amps and investigate every possibility until we are certain of the cause of trouble and its solution. take care, MM-folk
ed s

obviously not patient enough (and I've been an electrician for 23-years!)....of course there is a short in the heater circuit of the 12AX7 (pins 5 and 9)! that's how we turn the filament red hot through the application of all that current to whatever filament material is being used (tungston with a boride coating on the cathode?).
the short is in the trannie. has anyone a hypothesis as to why it went dead short on the windings? old age, inability to withstand the rigors of transport and set-up at 30+ years old? now I'm at a loss again for an explanation--I'd sure like to figure it out before the new transformer arrives--I don't want to install the new one with that question unanswered. [damn, and I was feeling so smug ten minutes ago until I thought about what I had just posted...idiota]
ed s

Hey Ed, and Ed; This is more exciting than "Desperate Housewives" !!??#!!$%
Thanks for the interesting and stimulating updates. (I'm not kidding) I hope one day to be able to follow things through as good as you guys, so just remember; "One man's 'idiota' is another man's 'eureka' ", so to speak.
Reed

thanx, reed
I'm glad someone is deriving something worthwhile from my elementary mistakes in troubleshooting logic. personally, the problem is driving me crackers...the only problem I can ascertain is the shorted 6.3 vac xfmr windings.
I suppose I'm going to have to chalk this one up to old age in an amp--it isn't as if my 210 sixty-five has led a sheltered life. she's been rode hard and put away wet more than once in the last 30+ years. her early years were spent in hard, steady gigging; both local and on tour--and she never let me down until the 19th, so I really have very little to complain about. I definitely got my money's worth a long time ago.... it's just that I love this little powerhouse of an amp.
there's been a new development in my transformer story but it rightly belongs on the transformer thread....
ed

Woosh! Right over my head! I'm glad people are trying to get to the bottom of this troubling issue thow. I've decided to just stick with the 5751. and never play my amp in the "Hi" setting, not even live. In a perfect world I'd never have to push my amp down a cracked sidewalk, but that's not realistic (this thing weighs 80 pounds!) I just have to accept that one day my bad karma might catch up with me, and try not to wince every time my amp gets knocked around.
Just so you know how serious I took this issue, I did purchase some "socket savers". They have to be installed from the inside of the chasis right? Is it hard? And does anyone think that that would be a bit extreme and overprotective?