'77 Ampeg V4B Transformer Repair

[ajunea3]

I took my '77 V4B to an amp repair guy because of a bad hum problem, and he said he isolated the source of the hum to somewhere in the output transformer section. He didn't want to go any further with the repair because he couldn't do it "cost-effectively". I translated this as "it might cost you an arm and a leg to replace those old output transformers".

Anybody know of an Ampeg tube amp repair specialist somehwere in the lower 48 states that might be able to help me out? I wouldn't mind shipping it somewhere. I do dearly love this amp!

[will33]

As far as I know, you can't really "repair" a transformer, you replace it and yes, they're the most expensive part of the amp by far but they also rarely go bad. When they do, it's usually user error like playing it without speakers connected, or cranking it up with the bias way off or just continuing to play the amp after there's already signs of trouble. Either that or rusting due to improper care/storage.

I don't know what model is in the v4 but there are a million different kinds of those things. If you can't find the exact replacement, a tech should be able to figure out an alternative that will work properly.

Can see if you can get part numbers off of it and try mouser .com or fliptops.

I'd also be 100% sure that's the problem before dropping that kind of money. Perhaps one of our real techs will be along here to help further.

[beans-on-toast]

Mercury magnetics is another option.

They can test your transformer to verify that it is the problem. I believe that this is don't free of charge. You pay the shipping. If there is a problem, they can unwind your old transformer and rebuild it keeping the original iron but replacing the windings. This is usually expensive but will give you what you had.

They also sell replacements.

[BassmanPaul]

Originally Posted by beans-on-toast

Mercury magnetics is another option.

They can test your transformer to verify that it is the problem. I believe that this is don't free of charge. You pay the shipping. If there is a problem, they can unwind your old transformer and rebuild it keeping the original iron but replacing the windings. This is usually expensive but will give you what you had.

They also sell replacements.

Not as expensive as you might think. I was quoted a price around the cost of a good replacement. that price included un-potting the old unit and re-potting as per the original.

[ajunea3]

Originally Posted by will33

As far as I know, you can't really "repair" a transformer, you replace it and yes, they're the most expensive part of the amp by far but they also rarely go bad. When they do, it's usually user error like playing it without speakers connected, or cranking it up with the bias way off or just continuing to play the amp after there's already signs of trouble. Either that or rusting due to improper care/storage.

I don't know what model is in the v4 but there are a million different kinds of those things. If you can't find the exact replacement, a tech should be able to figure out an alternative that will work properly.

Can see if you can get part numbers off of it and try mouser .com or fliptops.

I'd also be 100% sure that's the problem before dropping that kind of money. Perhaps one of our real techs will be along here to help further.

I NEVER play the amp without a load connected. I've been told that's a good way to destroy the transformers, as you say. I also use two different cabs, and always make sure that i have to the ohm load set to the proper resistance.

The wierd thing is the hum developed almost instantly. Using the hum balance, I used to be able to make the amp dead quiet. Then one day I powered her up and BAM!, loads of hum.

I just looked up transformer prices on the internet... yes, they are VERY expensive. I hope I don't need a new one.

[will33]

Beans on toast and BassmanPaul are real actual amp guys, I would follow their suggestions for best results.

[beans-on-toast]

Originally Posted by BassmanPaul

Not as expensive as you might think. I was quoted a price around the cost of a good replacement. that price included un-potting the old unit and re-potting as per the original.

Paul, I like their products and have ordered a lot of their stuff. The price gets even better as you order more as they accumulate the piece count over time to determine the discount.

When I mentioned that it can get expensive, I wasn't talking about buying one of their off the shelf transformers or having a new one repotted in the original can as you had looked into, but rather if they blueprint the one that you own and make a new one based on those specific specs. There is more labor involved in doing this so it costs more. They meticulously winding it, measuring and recording every detail, then build a replica. It is nice to have this option available.

[beans-on-toast]

These hums can be really tricky to track down and they can fool you into thinking that it is the transformer when it may not be the case.

[BassmanPaul]

They do seem to be a good company with a very reasonable code of business.

I have two complete sets of iron from MM, chassis made and painted, components in house the whole ball o' wax. I've just got to get down to construction. Worst part is that when I had the blank chassis made, had I thought to add in the needed holes I could have had them all laser cut at the same unit price! Next time!

[ajunea3]

Originally Posted by beans-on-toast

These hums can be really tricky to track down and they can fool you into thinking that it is the transformer when it may not be the case.

I'm not too sure how one goes about diagnosing hum in a tube amp, but the tech I brought it to did say that the hum was 60 cycle hum (he used an oscilliscope), and that he checked the grounding everywhere in the preamp stage.

I will probably bring the amp to a reputable guy who used to be in my area but recently moved about a half hour up the road. If he also agrees that the hum is originating in the output/power transformer, I will look into having the tranny re-built as you guys have mentioned.

Thanks for the input guys!

[will33]

Originally Posted by ajunea3

I NEVER play the amp without a load connected. I've been told that's a good way to destroy the transformers, as you say. I also use two different cabs, and always make sure that i have to the ohm load set to the proper resistance.

The wierd thing is the hum developed almost instantly. Using the hum balance, I used to be able to make the amp dead quiet. Then one day I powered her up and BAM!, loads of hum.

I just looked up transformer prices on the internet... yes, they are VERY expensive. I hope I don't need a new one.

Sounds like you're treating it right. I didn't mean to imply "you broke it", just that that's most often how it happens. You could do everything correctly and there's still a chance some $3 part could fail and take a $300 part down with it.

You say the hum balance made it quiet, then suddenly got bad hum again. It's possible there's something wrong related to that circuit/adjustment and your transformer is OK. I don't know enough about it to say but I wouldn't convince myself to go dumping money in transformers unless I knew for sure that's what the problem is.

[johnk_10]

personally, i think that your amp has been mis-diagnosed since a bad output transformer doesn't usually cause hum. it's symtoms are usually a weak distorted output or it can possibly blow fuses.

if you do end up needing one, fliptops sells a V4B replacement for $150. Fliptops
i've used one of their's for a V4B repair in the past and it sounded exactly like the original. or you can send it out and have it rewound (usually for about the same price).

[ajunea3]

Originally Posted by will33

Sounds like you're treating it right. I didn't mean to imply "you broke it", just that that's most often how it happens. You could do everything correctly and there's still a chance some $3 part could fail and take a $300 part down with it.

You say the hum balance made it quiet, then suddenly got bad hum again. It's possible there's something wrong related to that circuit/adjustment and your transformer is OK. I don't know enough about it to say but I wouldn't convince myself to go dumping money in transformers unless I knew for sure that's what the problem is.

Right. I want to make sure I know exactly what the problem is before I start throwing money at it.

[ajunea3]

Originally Posted by johnk_10

personally, i think that your amp has been mis-diagnosed since a bad output transformer doesn't usually cause hum. it's symtoms are usually a weak distorted output or it can possibly blow fuses.

if you do end up needing one, fliptops sells a V4B replacement for $150. Fliptops
i've used one of their's for a V4B repair in the past and it sounded exactly like the original. or you can send it out and have it rewound (usually for about the same price).

Thanks John. When I went to Mercury Magnets' website and looked at the prices on Ampeg V4B transformers my face did this: . It's good to know that there is a cheaper alternative (if the tranny is in fact the problem).

[beans-on-toast]

Originally Posted by ajunea3

I'm not too sure how one goes about diagnosing hum in a tube amp, but the tech I brought it to did say that the hum was 60 cycle hum (he used an oscilliscope), and that he checked the grounding everywhere in the preamp stage.

If it was confirmed 60 cycle hum the first place I would look is the heater circuit as it operates at that frequency. The hum balance pot might have blown.

As you said, another tech might have better luck at isolating the problem.

In case you don't have one, here is a schematic for a (1971, close to your 77) V4B. Click on the image to expand it.

[Jerrold Tiers]

I also am suspicious of hum due to output transformer..... there are too many other possible causes that are more probable (and less expensive).

Quoting a part problem with a high price part is one way to make a customer go away if you really can't fix the amp....!

it IS true that a certain amount of hum cancellation occurs in the transformer..... and anything that affects the balance of the two parts of the winding will affect hum. But mostly that is a shorted turn or more likely a number of turns shorted... And THAT has lots of effects on sound, power, and heating that go way beyond hum.

Severely mismatched tubes, capacitor failures, a heater-cathode short or severe leakage in a tube, all can cause hum. That last one is not unlikely in any amp that uses an off-ground cathode, such as many phase-splitters. Tubes from the usual suspects may not have good insulation on the heater (usually a ceramic type coating).

Do not give up..... get someone who is really tube-savvy to look at it.... you might try some tube-swapping first....

[christw]

I know several quality tube techs in the SW Ohio area but I'd hope you can find a local guy to diagnose. I'm with the rest of them, don't worry about finding replacement parts until you have a second diagnosis at least.

[B-string]

I'm also having trouble with the "60 cycle hum" being the fault of the output trans. The B+ feeding the OT is full wave rectified (120Hz or cycles), heater to cathode leakage or faulty hum balance on the heater circuit is much more likely. If the 60Hz hum is being feed from the output tubes to the OT of course you will see it at the OT (that is not the transformers fault).

[BassmanPaul]

I'm SOOO glad that others also had difficulty with hum and the OPT. It was grating on my mind from the start. As others have said there are so many causes that you have to get the amp to TUBE tech for a proper diagnosis. Could very well be as simple as a bad output tube. See if any are glowing red on the plate. Keep us informed of your progress.

[T-Bird]

Hi.

Originally Posted by ajunea3

The wierd thing is the hum developed almost instantly. Using the hum balance, I used to be able to make the amp dead quiet. Then one day I powered her up and BAM!, loads of hum.

Either You blew a cap or the hum blance pot just broke down (IMLE pretty common on CBS Fenders) as suggested earlier. Neither is difficult to diagnose nor expensive to repair.

Unless the tech is regarded to be a pro, I'd get a second opinion.

As a sidenote, non-welded construction EI transformers are pretty easy to repair, even DIY style, it's just a certain number of turns of certain wire, wound in a certain way on a bobbin. The quality (or more precisely the lack of it in MI amps) of the iron pretty much makes the transformer coupled amps behaviour what it is. For better or worse. I for one say for the better .

Most of the "direct replacement" OT suppliers know that, and therefore offer competitively priced repairing as an option as well. They won't have to source the correct "vintage" core material, so they're happy, and the customer gets a near-exact copy of their original transformer, so they're happy as well. Win-win situation if You ask me.

Regards
Sam