Ampeg B12-NF Power Trans Question

[jeffreybruton]

Hey All:

I've got a stock B12-NF that worked fine for years and now will pop the fuse the instant its turned on.

I suspect the Power Transformer since the primary side (input) only reads 2.5 ohms. Am I on the right path here?

Thanks very much for any feedback and this great forum!!
Jeffrey Bruton

[JimmyM]

no idea about the primary side impedance, but did you try running it without tubes yet? i'd suspect a power or rec tube before i'd suspect the pt. pull the tubes, replace the fuse, then try it. if the fuse blows, then have it checked out. if not, put the rec tube in and try it. then add the power tubes, and then the pre tubes.

[billfitzmaurice]

Originally Posted by jeffreybruton

Am I on the right path here?

You're not. It won't read a lot of DCR, as most of its impedance is inductive. Disconnect the secondaries and cap the wires with wire nuts, then try it. If it blows a fuse with no load then you know it's the transformer.

[jeffreybruton]

Jimmy and Bill:

Thanks VERY much for the quick replies and suggestions! Just tired Jimmy's and it still blows the fuse. I'll try Bill's next and report back.

Thanks again
Jeffrey Bruton

[beans-on-toast]

Just to let you know, 2.5 ohms is the correct input impedance for your power transformer. Each half of the high voltage secondary winding should be about 55 ohms.

Remove the 0.05uF/600V capacitor that is on the primary side of the power transformer connected to the ground switch. You don't want it in your amp.

Pull all the tubes. With the amp on, the lamps should be on. You should be able to read 6.3 VAC across pins-7 and 8 of the 6SL7 sockets, and pins-7 and 2 on the 6L6 sockets. Pins-4-6 on the 5AR4 socket should have a high voltage AC secondary of about 745 VAC, you can also test each winding to ground and see about half the voltage, while pins-2-8 should have the 5 VAC heater voltage. This assumes that your wall voltage is 117 VAC, if it is higher, your measured voltages will be higher.

[BassmanPaul]

Always be careful working on an amp that is powered up. The full secondary voltage is at a lethal level. If you are using a digital meter, measure the secondary by checking between ground and pins 4 and 6 with the standby switch enabled. Some meters are not so good at high voltage AC. I blowed up my favourite auto ranging meter.

[jeffreybruton]

Beans and Paul:

First off as before, thanks VERY much for the quick replies and the info!

Beans:
I did try pulling all the tubes as Jimmy and you have suggested and with a fresh fuse it still popped immediately. :-( That leads me to try Bill's suggestion. I'll report back as soon as this test has been performed.

Paul:
Roger that!! :-) I'm using my much beloved Tektronix DMM916 which is one of the few DMMs that can read both Hz and dB!

Report back soon.

Thanks again
Jeffrey Bruton

[beans-on-toast]

That capacitor that I suggested that you remove could be shorting and causing your fuse to blow.

[jeffreybruton]

Beans:
Thanks for the suggestion on the Cap.
Do you mind elaborating on why you suggest to remove it altogether? What was the original design thought for including it from the factory?
Thanks again
Jeffrey Bruton

[BassmanPaul]

When two wire mains power was the standard the plug could be inserted either way around. The polarity switch connected the chassis to one side or the other via this cap, typically 100n at 600V. The idea was to pass any AC noise to the neutral (grounded) side of the mains. If the cap became leaky, as caps often do, and the switch was in the wrong position your strings and metalwork would in essence be connected to the hot mains voltage. Dangerous!. When the chassis being grounded by the third wire the cap was no longer needed. However if the ground became faulty, the same shock hazard still exists. Remove the cap and you are safer.

[beans-on-toast]

Originally Posted by jeffreybruton

Do you mind elaborating on why you suggest to remove it altogether? What was the original design thought for including it from the factory?

It is part of a ground polarity reversal circuit. The amps built back then had a two conductor power cord and some, like your B-12, had a polarity switch. This sometimes allowed you reduce hum by reversing the polarity switch.

The problem with this cap is as it ages, it can start to leak. Depending on the polarity switch position, this can result in a voltage on the chassis. If you touch another grounded device, such as another amp or a mic, while touching your strings which are connected to your chassis, you can get a shock. If the cap is completely shorted, you will get the full line voltage through you. For this reason, some people call this the death cap. If you've ever touched your lip to a mic while playing and got a nasty shock you'll know what I mean. The potential is there to get a serious shock. So, why take the chance? Remove the cap and isolate yourself from the primary of the power transformer.

I believe that all amps should have a three conductor power cable installed for safety reasons. It is safer because the chassis ground is tied to a circuit breaker. If the hot AC mains comes in contact with the chassis, current will flow through the green safety ground wire and the breaker will trip. With the three conductor power cord, the cap is not necessary and can be removed. Some people leave it in when the cord is upgraded. It is important to ensure that a fully functional cap is in place. The power cord upgrade is fully reversible should you be concerned about vintage value.

[beans-on-toast]

I see that Paul beat me to it while I was typing. Listen to what he is saying!

[BassmanPaul]

Originally Posted by beans-on-toast

I see that Paul beat me to it while I was typing. Listen to what he is saying!

Y'see, I've got these fat lil' fingers that type real fast!

Personally I'd be less concerned about vintage value and more about stayin' alive! (Gawd I sound like the Gee Gees now!)

[jeffreybruton]

Paul and Beans:

Thanks VERY much for your continued sharing of knowledge! Great explanation!
Be assured my B-12 will have a 3-prong power cord as part of this process. Now I just have to remember which blade of modern grounded AC connector is hot and which one is neutral! :-) I know they are different sizes for polarity.

Also it would be REALLY great if Beans suggestion re: the cap being faulty is the reason for the fuse popping! A new grounded cord and a bit of soldering time is much easier on the wallet than a new power transformer! :-)

Hey, I just had a thought, with installing the grounded power cord and removing the cap, does this means the Ground switch will then have no real effect?

Last comment is...in light of this great discussion, my '71 Bassman head should also probably get a grounded power cord, eh? :-)

I'll report back soon!

Many Thanks again!
Jeffrey Bruton

[billfitzmaurice]

Originally Posted by jeffreybruton

Hey, I just had a thought, with installing the grounded power cord and removing the cap, does this means the Ground switch will then have no real effect?

It will have no effect, and you'll have a very noisy amp. There must be a bypass cap, or even better, a Twin T filter, like those here, which will give you the same level of noise filtering as the average $100-150 'power conditioner'.
http://www.excesssolutions.com/cgi-bin/category/11700

[aborgman]

Originally Posted by billfitzmaurice

It will have no effect, and you'll have a very noisy amp.

Ummm, no.

We're talking about the polarity (aka "death cap") cap - not one of the capacitors forming the PI filter for the power supply.

There is no reason at all that removing the polarity cap from the incoming AC line would cause your post PT B+ to be noisy.

[beans-on-toast]

Originally Posted by jeffreybruton

Now I just have to remember which blade of modern grounded AC connector is hot and which one is neutral! :-) I know they are different sizes for polarity.

Green (ground) for the chassis ground, white (neutral) goes to the longer spade, and black (hot) goes to the shorter spade. Sometimes the colors are different. The standard requires that the green wire is longer than the black and white. It should be the last one to break if the cord is pulled out of the chassis.

For these amps, I use Carol (General Cable) SJ0 18/3 gauge cable, 0.3" (7.6mm) diameter, rubber clad cable that I get from Home Depot. The markings on the cable come off with a little actone. I use a black Leviton 5-15 15A 125V plug. The diameter matches well with the original so the same size chassis hole and the Heyco SR 6W-1 strain relief bushing can be reused.

Here is a procedure for the conversion for a Fender amp (http://www.unclespot.com/2to3prongconversion.html). Follow the instructions carefully. The hot goes through the fuse then the power switch. Note that they use the ground switch as a tie point for the wires. This is ok if you are not using the switch for anything else but I prefer to disconnect the switch so it can be used for some other mod if desired. This involves using a wire nut to tie two of the wires. I then cover it with heat shrink tubing for added safety.

Many musical instrument amps are built this way today without the additional line filtering that Bill was referring to. If your power line is clean the amps will be quiet. The manufacturers are more interested in keeping the amps as vintage as possible while still following the current regulations that require the three conductor cord. For examples, look at the online schematics for the reissues that Fender make.

The better modern amps tend to have a line filter module and they do a good job of eliminating noise. As was pointed out, a lot more cost effective than buying an expensive power conditioner and just as effective.

Anyway, something to think about. As you said, if this cap is the problem, it will be a lot cheaper than buying a replacement transformer.

[BassmanPaul]

Originally Posted by jeffreybruton

Now I just have to remember which blade of modern grounded AC connector is hot and which one is neutral! :-)

The larger blade is the neutral. It is also very often a different colour. If you are using a cable with an existing moulded plug the cable probably will be green for ground, white for neutral and black for hot.

If you get your cable from a box store like home Depot buy the "hospital" style plugs - they are usually yellow. Very much better than the rubber style.

Way to go B.O.T. ya beat me! Now we're quits.

[billfitzmaurice]

Originally Posted by aborgman

Ummm, no.

We're talking about the polarity (aka "death cap") cap - not one of the capacitors forming the PI filter for the power supply.

I'm aware of that. AC line noise must still be filtered with a cap across the incoming AC hot and neutral.

[aborgman]

Originally Posted by billfitzmaurice

I'm aware of that. AC line noise must still be filtered with a cap across the incoming AC hot and neutral.

We'll just have to disagree on the assertion of "must" and "very noisy"