June 25th, 2012, 10:02 AM
Tomorrow night I get to meet a holy grail amp, a black face Vibrolux, '66 I think. It belongs to a buddy's neighbor. The neighbor doesn't play, the amp belonged to his Dad, who recently passed on.
Any way, my buddy does play and is a serious gear ho, so he's helping his neighbor get the amp up and running. It's already been to a tech two or three times. First for the typical "recap and new tubes". After that, the amp made weird sounds, crackling and such. I played it briefly at this stage, but don't remember the specifics. It went back to the tech, who eventually concluded that the OT was going bad.
Now my buddy is asking me for a second opinion. Here's my question: how does one diagnose an OT that is "sort of" working? It's passing audio, so it obviously doesn't have an open winding. Are there any other tests that can be performed with just a good DVM? Or, is the next step to swap in another OT and see if it solves the problem?
June 25th, 2012, 11:14 AM
Is the tech any good ? Changing a tranny in this amp would be the last thing you'd want to do. In terms of value anyway. If it needs it thats fine but I think a lot of transformers get changed that don't need it.
A lot of things could be doing what you describing here.
June 25th, 2012, 12:08 PM
I don't know about the tech, nor what diagnostic work he did leading up to his conclusion.
We're taking the value of the amp, as well as the future intended usage, into account. Chances are, if we determine the OT is bad (or dying), we'll swap in another one for troubleshooting/temporary purposes, then have the original rewound and put it back in. That's assuming the owner is going to hang onto it as a keepsake.
June 25th, 2012, 06:40 PM
There are a lot of problems that could cause those symptoms. Imho, that amp needs to see a good tech. The tech who dignosed the problem as the OT may be a good tech, but he has not given specifics..or someone didin't take notes....as to how he came to that conclusion. THat tech needs to convince the owner that the amp needs and OT. IME, if an OT is bad, a tech would tell the owner not to run the amp. Why? The amp doens't work properloy, adn it can only fail while sounding badly, right? NOthing positive comes from turning the amp on.
After all other possibilities are exhausted, a tech will assess the output of that OT. A 'scope will tell them what they need to know. OR....subbing in a known good OT temporarily will either confirm or eliminate the OT as the problem.
Example of one problem that will cause those crackling sounds.....bad connection of a reverb cable ground. The late Buggs HEnderson's VK was making useless soudns like that....techs in theMetroplex area were unable to find it. I luckily found theproblem in a couple of minutes. Moral of the story: Sometimes simple folks find simple problems easily!
But...sometimes it is not that easy. TEch time right now for that amp. IT is too valuable to be in the hands of someone who is wondering what to do for it, imho.
June 25th, 2012, 08:38 PM
My problem here is that I don't know the tech. I'm a DIYer, and visit my buddy occasionally when I'm in town on business. I *assume* the tech did sufficient troubleshooting to rule out bad cables, dirty pots, etc.
Anyway, I'm going over there tomorrow after work, and my buddy has obtained a Classic Tone replacement OT. I'll swap it in for him, and see what happens. Then we'll sit down with the owner to figure out the path forward.
With any luck, he'll just decide vintage tube amps are nothing but trouble, and give it to me. LOL.
June 25th, 2012, 10:20 PM
Just my experience and opinoin....but I think it would be a pity to
'interrupt' original solder joints on an OT just to find out that it is a good OT....that is, IF--big if-- the amp is otherwise pristine and correct. IF it isn't all of that Excellent condition, all-original BFVR, then it makes 'no nevermind'.
IF it is a pristine original amp, I would exhaust all other possibilities before I undid those solder joints on the OT. Yeah.....got it bad, don't I? LOL
June 26th, 2012, 06:32 PM
Ya know with the symptoms you describe, the OT is the last thing I would suspect. Pops and static and various weird tones like that are not usually the OT.; They are things like cvold solder joints, failing caps, bad tubes, bad tube connectors, bad solder joints, or cold solder joints and failing components. In my experience there's only been two symptoms associated with bad OT's depending on which OT winding went bad. In most cases, when a winding goes bad you are lucky to get any sound at all, or not get a cascade of other problems. Example: Lets say the 6V tube heaters go out... you'll know it's that wininding and the tubes wont light up on Standby. Let's say its the high voltage winding....guess what....no sound....no power. None of the OT windings can go bad without bad repercussion. usually you'll get a very low volume amp if it works at all with bad OT windings. If it's the speaker output winding, they might gradually go bad resulting in scratchy sound, or low volume, but thats about it.
I'd want to know which winding was bad, what it tested at that was off spec, then you can get an intelligent reason an OT needs replacing. Usually when they fail, they are just done. Usually, it's a shorted winding that that will show burn marks on the bobbin.
I don't want to say the tech is wrong because he may not be, but I'd ask which OT winding was bad, then ask us here on the TDPRI what that would mean.
June 27th, 2012, 12:01 AM
I have to agree with all the skepticism regarding an OT failure.
An OT is nothing more than two coils on a core. There is not a lot that can go south there, and usually when it does failure is rapid, catastrophic, and not remotely intermittent.
I'd also agree that anything is possible, but would want to positively rule out many other potential sources prior to swapping the OT.
June 27th, 2012, 03:38 AM
A 46-year old Fender amp. Brittle solder joints? Conductive tagboard? Corrosion between the brass grounding plate and the chassis?
I wouldn't tip the OT either. Plenty of other things are more likely to go wrong with an old Fender.
June 27th, 2012, 04:14 AM
It might be that the tube sockets need cleaning, and if you got all new tubes, [or any, as far as that goes], it could be a bad tube. Or maybe a tube is a little loose in it's socket. Or you could just try using the old tubes and see what happens. Or you could wait for your buddy to look it over, since you know that, he knows what he is talking about..
June 27th, 2012, 09:08 AM
Lots of good suggestions here, many of which, sadly, I received after I left work for Mike's house last night.
I expressed my concerns about the OT diagnosis, and asked Mike if the tech had given him any info about what led up to it. He said the guy mentioned he'd looked at the OT with a 'scope, and concluded it was going bad. ??? I was still suspicious, but that's all I had to go on.
So, I opened it up, looking for other possible causes. I was relieved to see that the amp had had some work over the years, so I no longer had to worry about violating a pristine, all-original amp. There were some minor changes and repairs that obviously pre-dated the recent cap job/3-prong power cord/death cap removal mods.
For one, the main speaker output jack didn't look original, and the soldering on it was kinda wonky. The nuts was loose, too. Hooray! That could cause the problems we were hearing. When I went to pull it out and re-solder it, the green secondary transformer lead came off in my hand. It had either been hanging on by one strand, or maybe had been sort of just sitting there loosely in the middle of a cold solder "donut". Either way, I resoldered the whole jack, confident that the problem would be gone. No such luck. The sound cut out a lot, with the symptoms coming and going at random, but correlated with louder, or lower-register playing.
Then Mike noticed one of the white speaker lead was broken off its terminal. Upon closer examination, I discovered the other white speaker lead was loose, hanging in the middle of another cold solder donut. I could slide the wire back and forth throught the hole in the terminal. (Cut and paste from previous paragraph.) I resoldered the speaker leads, confident that the problem would be gone. No such luck. Same symptoms.
At this point, I rationalized that we should just swap out the OT and see what happened. My reasoning was: 1) the tech (who may know what he's talking about) says it's bad and may actually be right, and 2) the messed-up speaker connections we found would make intermitent open circuits on the OT secondary, which would be rough on the OT, and could lead to its demise. So, since we had the new Classic Tone sitting there, we swapped it out. We hung the chassis loosely in the top of the enclosure, just sitting on top of the battens, without being bolted in, and played it.
It sounded GODLY. I played first, with a Strat, amp volume on 4, tones and about halfway up. I hit a couple percussive, low-E chords, and it was instant "See No Evil" (from Television's "Marquee Moon"). I played some Dire Straits rhythm parts, too. Mike took over with a PRS, cranked up the volume and got some great dirty sounds. We were both in awe, how well this old garden-variety amp stacked up against many of his fancy, modern boutique amps. I dug my DOD compressor and Ibanez chorus out of my toolbox, and played some Police stuff... wow. Like I said, we were in awe.
To be continued...
June 27th, 2012, 09:30 AM
Mike went off the dick around with a PRS through one of his other amps, while I let the BFVR cool down, and then buttoned it backed up. I checked the wiring one last time, and then bolted the chassis back into the enclosure. Time for one final check, AND THE PROBLEM WAS BACK. GAAAAAAAAHHH!!!!!
So, the problem stayed away for an hour with the chassis sitting loose, but comes back nearly all the time when it's bolted in solidly. That indicates loose connections, and I started poking around while Mike beat it up with yet another PRS. I zeroed in on the middle of the chassis, and then localized the problem to the PI tube. When the amp started cutting out, I could push the PI firmly into its socket with a towel, and the tone would return its full, normal glory.
Some tapping around with a chopstick indicated that perhaps the 1st vibrato channel preamp tube was a bit wonky, too, but the primary problem we were hearing effected both channels, so I'm going with a new diagnosis: the PI tube socket needs to be cleaned and re-tensioned (and do all the others while you're in there). Sadly, it was 12:30 am by that time, and we both had to get up for work this morning, so we called it a night. The plan is to tighten up the sockets, and put the original OT back in.
However, Mike's going away on vacation, so I guess the amp's going to sit for a while. :sad:
Anybody want to buy a very lightly used Classic Tone replacement OT for a Vibrolux (or Super) Reverb? :roll:
All in all, a frustrating but educational evening.
June 27th, 2012, 10:46 AM
IMO the issues you dealt with last night are exactly the kind of things the tech should have assessed the first time he had it on the bench.
Working with a newer amp it might be possible not to do a general once over, but with an old classic - one that probably hasn't seen recent use - it is imperative that everything be checked out and given a thorough tune up (re-tensioning all sockets, cleaning and checking all pots, checking all caps for leakage, thorough chopsticking etc.)
That he 'diagnosed' a bad OT while missing a whole mess of other problems (some that could have been found simply by giving every lead a gentle tug) says he wasn't particularly thorough.