Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gtr2 on October 30, 2013, 08:13:12 am
-
Hello all! I introduced myself yesterday in the welcome area :icon_biggrin:
I recently acquired a 65 blackface bandmaster. Its an AB763 with vibrato.
I bought it as not working but took a gamble on it. The seller said it was his friends and that it was given to him. It pretty much sat in a basement for 30 years. It's original minus a mod that someone did to the heaters.
The center tap for the heaters is fried/damaged and has damaged the insulation on the 320V center tap. Someone had the idea of tapping into the 440V control grid pin to create a voltage divider with a 470k/12k which works out to around 10V to power the heaters. Without any tubes in I have continuity between the two sets of heater wires. I'm not sure if a power tube originally shorted from pin 3 to pin 2 and fried the center tap and then they tried to mod it?
One of the 6L6's also had very loose pins on the tube.
I'm assuming I need a new power transformer and to rewire the entire heater system. I also need to recap all the electro's under the doghouse, bias cap, and board. Other than that, I'd like to leave it stock as much as possible. It does need updated with a 3 prong cord and rewired properly (+ remove the death cap).
Is there anything else I should check? I'd hate to fry a new transformer because of something I've overlooked.
Thanks!
Josh
(http://1776effects.com/wp-content/uploads/2013/10/IMG_1380.jpg)
-
If you want it back as original as possible, you will need a new PT. I would simply "go through" the amp and replace the filter caps and any paper caps that you can see a blister on. I would leave the blue ones for now.
If you use a current limiter, you will not fry anything on startup. I cannot find the instructions to build one. If you do not have one you will need to make one.
Once repaired and checked with the light bulb getting dim, the i would give the Bandmaster a test. You may have a few resistors making noise, like the plate resistors. Changing only what is necessary will keep the Bandmaster close to its original tone.
-
Ed's advice is good. Look here for more info on lightbulb limiter. You may be able to save some money and repair the insulation on the HV
leads and use an outboard 6.3V 4 amp transformer to supply the heaters. Make sure your fusing is appropriate for the bandmaster.
-
Someone had the idea of tapping into the 440V control grid pin to create a voltage divider with a 470k/12k which works out to around 10V to power the heaters.
I doubt they were trying to 'power' the heaters. I'm thinking maybe elevate the heater. I don't see anything connected to the junction of that 470K/12K voltage divider. Just remove both resistors.
I can only see the filament string from the PT to the pilot lamp to the two 6L6s and that looks fine. If the string carries on down the line to pins 4/5 and 9 of all the little tubes then the filament string wiring is probably OK. Check to be sure.
The melted center taps on the PT need investigating. I'm guessing that since the filament circuit is such a high current circuit, there was a short from one side of the filament string to chassis. This would create enough heat in the filament winding to melt the insulation as well as the insulation of the HT center tap wire that was twisted together with it.
You need to check the voltages on the PT. I would put the amp in standby mode (open the standby switch) and pull all tubes. Plug it in and turn on the power switch. KILL THE POWER IMMEDIATELY if you see/smell smoke or hear unusual noises and investigate. If OK at this point, does the pilot lamp illuminate? If so, the filament winding is probably OK, but measure the 6.3VAC across pins 2 and 7 of a 6L6 socket . Next measure the AC voltages for the two red leads (≈320VAC) and the red/blue lead (≈60VAC). If any of these voltages are messed up the PT is probably shot.
If all the above voltages are OK, then plug in the tubes ONE AT A TIME and verify that each tube lights up. If all tubes light up then check for ≈ -48v on pin 5 of each 6L6. If this voltage is OK then turn on the standby switch and look/listen/smell for any problems.
Let us know how far you get into this checkout procedure.
-
If the tranny is fried instead of changing it out see about getting it rewound, I think Mercury Magnetics does it but Im sure you can find out for sure here. I read a article here on checking your caps and it was really interesting, you'll have to have a running amp tho to check for leaking caps. The article is in here and I'll look thru what Ive been reading here .. I wish I still had a 3 of my early blackfaces :BangHead:
-
"Someone had the idea of tapping into the 440V control grid pin to create a voltage divider with a 470k/12k which works out to around 10V to power the heaters."
Absolutely flaming not, the heaters need in the area of 3 amps of power and the 320 volt winding is capable of supplying (guess) 125-150 ma, thus the heaters need something like twenty times the current available from the HV winding, so eliminate any whisp of that thought from your thinking.
"Without any tubes in I have continuity between the two sets of heater wires."
Ummm, this amp as schem'ed in Doug's library and as pic'ed in your pix has no tube rectifier so there are not "two sets" of heater windings, there is only the single center-tapped 6.3 volt winding. If you mean you have continuity across the 6.3 winding, including to its very own own center tap, from both sides, then OK. Nominally, the DC resistance of a heater winding is very, very small, it can easily be less than 1 ohm. Which is about the resistance of your test leads.
If this amp was working right now, there would be no problem at all with the insulation being cooked or burned off the heater CT and the HV CT (other than esthetic) as both are common and both are grounded. However, whatever created that condition may well have burned stuff up inside the power transformer. Many times, a PT can survive this. A stickler would probably immediately go for replacement and it could be argued that that is the smart thing to do in any case. I wouldn't, in the first parts-buy.
It's not dead certain that you need to replace the power transformer. It's likely, maybe even a greater then 50/50 chance, but not certain.
I think what I myself would do is as follows:
Get your nose as close as you can to the power transformer and smell it. Burned smell? Not good.
-Build a light bulb limiter.
-Take some stranded wire, strip it 1.5", pull a single fine strand of the wire from the bundle, and solder it across the fuseholder. Remove the fuse.
Look at the fuse, if there is one. Blown? Did someone throw a 20 amp fuse in there? In any case, remove it and place a new batch of fuses of correct size on your parts shopping list.
Ideally, I would like to have a 50 watt, 5K resistor, and connect it to the unfiltered output of the HV rectifier, to see if the PT can handle roughly 100 ma of current. I will assume you don't have such a thing.
-buy & install all the parts needed to replace all the parts under the doghouse (all the filter caps) including all the power resistors, the 2 x 220Ks and a 1K and a 4700 and 4 qty 100K resistors, all of these 3-watt metal films like what Doug sells. IOW, on the assumption that you wish to proceed to fix this versus throw it out, completely rebuild the filter section with all new parts. It wouldn't be a bad idea to replace the six diodes in the rectifier with 1N4007 or the 3-amp ones if you want to go NASA. They are cheap enough.
-plug the amp into the LBL
During the following, you are NOT going to turn on the stdby switch, only the power switch! Be sure you know which is which and if you are in the habit of flipping both switches on at the same time, be sure you are not about to do that. Even better....CUT a wire that connects the rectifier to the filter caps, or cut & tape a wire leading to the stdy switch.
Put one tube, hopefully the worst-looking 12 a_7 into the amp. Or...if the pilot lamp is still good, that could do the deed. Position the chassis open-side down.
Power on. Tube lights up? Add another. another, another, another.
Now you know if the heater winding can power up the tubes. leave the amp on for a while. Hours.
[ Ultimately, I think you are going to want to take the PT apart, meaning, cut the wires, remove it from the chassis, unscrew the four bolts, and examine the interior to see if any burning occurred inside. Unless you want to directly proceed to buying & installing a new one. ]
Next, you are going to find out whether the HV section works.
-
I don't think I clearly stated that the center tap for the heaters/filaments is gone, the center tap for the HV is melted, but I can't see any copper exposed. The only thing left of the heater center tap is a stub end of the wire, connecting to the chassis.
I also forgot to mention that the previous owner also put in to high of a fuse which was blown when he powered it up before he sold it.
Lego - I did look into rewinding. It's not worth the exorbitant cost.
Josh
(http://1776effects.com/wp-content/uploads/2013/10/IMG_1387.jpg)
-
"I also forgot to mention that the previous owner also put in to high of a fuse which was blown when he powered it up before he sold it."
Thanks for clarifying that. Your chances of needing a new power transformer just tripled. If there is a 20 AMP fuse in the fuseholder....that tells a big story and I'll leave that to your imagination. Think "welding". IMO a new tranny is almost certainly in your future and maybe you should just go buy one and be done with it.
-
Thanks for your help eleventeen!
I believe it was only a 3A 250V fuse.
Sorry, I made an assumption on the voltage divider. Any idea why the resistors might have been placed there?
Josh
-
No idea, really. Your task here is to get the amp working in stock format, a known thing that's known to work via many many examples...(unless you plan to hotrod it to 4 x 6L6 or something like that)
I would just rip out anything that is not stock and work towards the stock deal.
It is still possible, by the way, that the PT is OK. It's clearly the most expensive part on your shopping list and it would be nice if you didn't have to buy one. If you don't care that much about the price, you are unquestionably better off popping for a new one. To determine whether the old one is workable, IMO you are going to have to rebuild things (which is in your plan anyway) and proof-test the old PT in some way. Whether you do this via rebuild - power-on - burn in for numbers of hours...or take apart and examine the innards....
I've seen some remarkable transformer rescues....that's about all I can tell you. They are rugged parts, but if you have charring and a bad discoloration on the inside of the tranny, then maybe it's only a matter of time before it fails. It's not a big deal to take it apart, of course you have to cut the leads and remove it from the chassis.
-
You may indeed need to replace the transformer. But, you really should verify the AC voltages as I described above. That filament center tap is not necessary to light the tubes. The damage to the filament center tap probably goes right on inside the PT and may have caused some damage inside too.
-
The melted center taps on the PT need investigating. ...
Indeed.
But I'll tell you one way I boogered an amp with a similar result: I plugged EL34's into an amp made for 6L6's. I must have also botched something with the socket wiring at the same time, and when I flipped the power on the heater center tap insulation melted away almost instantly.
That said, I un-did whatever mod I had made and put the 6L6's back in the amp, and the amp worked fine. However, the center-tap wire was still intact.
As Sluckey says, you don't need the actual CT, cause you can make an artificial one with resistors. But you do want to know what killed the original CT, even if you don't use the existing PT.
-
Another thought: If as Sluckey suggested, the added resistors to the heater ckt were added to raise the heater voltage (in an effort to reduce hum) yet the CT of the heater supply was NOT removed from ground (as it appears it was NOT, then Houston, we would have had a smelly (literally) problem. Under those conditions, one or the other of the filament winding or the HV winding would have become a de facto fuse of some sort! I can't of course name from a distance which one would have fried. But fried is fried. Did the current of the heater winding overload the ampacity of the HV winding? Or, (more likely in my mind) the initial turn-on of the amp the first time this misguided mod was installed, the HV surge thru the rectifier punched through the insulation of the heater winding, shorting it out.
While the thought exercise is kind of interesting...it's sounding more & more like you have a transformer buy coming up. But I repeat...if you can test it and run it for a while (if it works) and convince yourself it'll stay that way...then have at it.
-
Thanks so much everyone! Your comments and suggestions are much appreciated.
I'm definitely going to check out the PT before purchasing one. Whats the best order to do things in.
Recap then check the PT as described above?
Josh
-
I would simply check the transformer first.
What I noticed, which is why I mentioned replacing the PT is there is a lot of rust on the housing like it got so hot the heat induced oxidation. From the photos, it looks like the PT is the only thing that is very rusty which I have never seen in an amp that has not been stored in high humidity. Simply stated, it looks like the Power Transformer is rusted unusually. Does the other side look the same and are the laminates rusted as well?
It is very possible to repair if the windings look good, but I am betting it is rusted due to heat. I have seen these sell for $750 recently with a replaced Output Transformer. If it were my amp I would simply get a replacement as it is worth putting $70 into it for the transformer. You can keep the old one and if the day comes where the value exceeds the cost of rewinding the old one, then it is simply an investment to have the old one rewound.
I collect a few amps here and there and really would be more putoff by the original as opposed to a replacement unless the original were rewound.
-
Ok, so I checked out the PT.
I got 340V on each red and 50V on the red/blue lead.
The filaments have 7V across them.
Plugged in the tubes one by one on standby, all the elements lighted.
Pin 5 on each 6L6 has -49V
What i found when I went to put the 6L6's in is that one of them was missing the center black plug that lines the 6L6 pinout properly in the socket. I think I remember when I took them out that the text on one of them was oriented differently but I didn't think much of it and I didn't see the broken middle tab. When I compared them the text is on the exact same spot on both 6L6's so the tube with the broken "center tab" may have been in wrong! I'm assuming that this caused the heater CT on the PT to melt.
After everything seemed good, I took it out of standby. I heard a pretty loud buzz/hum at first but then it went away. I noticed a nasty smell, I pretty much located it to the caps that would be under the doghouse cover.
I did power it up again, and took it out of standby and it didn't make any strange noises that time.
I'm guessing the PT could be ok after all?
Josh
-
Could be, sounds like it!
"I'm assuming that this caused the heater CT on the PT to melt." Ab so LUTEly possible.
I'd go ahead and assume the PT is good if you get those voltage readings. The bad news is that if you want this to be a reliable gig-amp that will stay working under heat and time-on conditions, you still have the semi-obligation to check out the innards of the power tranny to see if there is charring...it would be a nice thing to place some tape over it, if so. I could also imagine that would not be much of a welcome task if you haven't done it before. OTOH, maybe you got real lucky and the heater winding CT opened up on an external nick in the wire and the inside of the PT is clean.
In any event, you should proceed to rebuild the complete doghouse (and get those 100K @ 3 watts to replace the plate resistors) full speed ahead. Get it working, run it overnight as a torture test, maybe with the bias just a tad cold, 2-3 volts cold, make sure you run it with a proper resistor as a speaker load....ideally with a signal generator input so the output section is working.....and you have a cool amp. (well it won't be cool after such a test, but you need to stress it some) Be on the alert for burnt-electric smell during burn-in and have the right sized fuse installed. Very nice!
-
I agree with what Eleventeen said. You may be lucky & all it needs are new e'lytics.
-
Cool, onward then...
Josh
-
1st off congratulations on joining in, and secondly for a 30 yr ''Leo Van Winkle''!. Great friends are worth their weight in Goldtops. Of course the info you got was great, and may I add,...and I'll differ to those that have already responded ahead of me, is it worth checking heater windings at bulb area; I've seen the cardboard insulator that separates the holder center from touching the chassis compromised on numerous oldies. Can't that also lead to troubles like described?. And yeah, recently picked up '72 Super Lead,...fresh cap job, unmolested mods wise,.....and wait for it.................. 20 amp mains fuse!? WTFudge?. No tellin' what horrors these baby's have been through, but seems win win,...little time, little money,...Lot O Amp. s&p
-
I got my hand slapped recently for suggesting taping.
My hand is still stinging. :sad2:
Bullshit! I simply expressed my opinion.
-
Took out the PT while rewiring for the 3 prong, etc.
One of the primary wires is a little melted but the CT for the secondary looks good where it goes into the windings. The heater CT wires are exposed.
Comments/suggestions? Use it or save it and put in a replacement? I'm not really sure what it should look like. Never took the cover off one.
Josh
(http://farm8.staticflickr.com/7293/10632414844_022d11f9ff_b.jpg)
-
If this was a repair for a paying customer, I'd replace the PT. If it were my amp and I had to depend on it on stage, I'd replace it. If it were my amp and it was only gonna be used in my home (this is my case) I'd use it and put a pair of 100Ω 1/2W resistors between the pilot lamp terminals and chassis to create an artificial center tap for hum reduction. It may last a long time or it may fail tomorrow.
-
If this was a repair for a paying customer, I'd replace the PT. If it were my amp and I had to depend on it on stage, I'd replace it. If it were my amp and it was only gonna be used in my home (this is my case) I'd use it and put a pair of 100Ω 1/2W resistors between the pilot lamp terminals and chassis to create an artificial center tap for hum reduction. It may last a long time or it may fail tomorrow.
Yeah, that's what I was thinking. This won't be a gigging amp. I really just want to preserve it properly and jam with it at home. Should I leave the wiring as is? I don't see any exposed copper where the insulation is melted.
Josh
-
100% agree w/Sluckey including the dual 100 ohm resistors.
Other than the zapped heater CT, it doesn't really look too bad. (It's ~~50 years old, remember!) Maybe take some isopropyl (rubbing) alcohol and get as much burnt stuff as you can off the thing...anything that is carbonized is a potential short or short path. Wherever possible, slip some heat shrink or some of the jacketing from a piece of CAT5 cable over any charred or melted insulation to get it/them outside of the housing. It would be nice to get rid of the loose cardboard/fibre sheet if it's carbonized...not sure what you'd replace it with...maybe a dead credit card with the mag strip OUTwards.
-
100% agree w/Sluckey including the dual 100 ohm resistors.
Wherever possible, slip some heat shrink
Yes and yes, I agree with both Sluckey and 11teen. If it were either mine or I was asked to fix it that's what I'd do too.
Brad :icon_biggrin:
-
Many times on forums people ask for help and then you never hear from them again. Resulting in no closure. :dontknow:
Well, she is fixed! Sounds really nice.
I put in the two 100R resistors to make the center tap for the heaters, recapped it, and put some heat shrink on any of the PT wires with damaged insulation. Also put in a 3 prong cord and eliminated the death cap. Fired her up with the lamp, everything looked and smelled good. Tubed her and played away. Voltages look good. I do need to do some fine tuning on the bias and clean up the cab a bit, but its ready to rock. I didn't do the plate resistors until I heard it roar, no popping or fried bacon in the background. Really quite amp (noise wise)
Thanks again everyone for your help!
Josh
-
Sweet!! Congrats on bringing a great amp back to life! :occasion14:
-
Thanks for the follow up! Always nice to hear of a happy ending! (apologies to Larry David)
-
:guitar1 :wav:
You are going to love that amp.