b15n output transformer question

[z6hatchboy]

I have a b15n that was given to me by my mother (mom, i know right?! Im near 30 now lol) She's owned it since the 80's, and best i can figure out it is early 70's. story is about 10 years ago (shortly after she had begun to teach me on it) it had begin to suffer from the tubes getting really hot and the sound lessening during a session of playing (maybe during 20 minutes of playing). she had taken it to a shop and they replaced the tubes and said that "A" transformer may be going on it, and that if it did, no sound would come from it. well it eventually got the point where no sound would come from it. that same 10 years ago she had bought a BA110 ampeg for me and her to use, then the b15n went into storage and now i have it. im not a master in tube amp or design but i am very confident in automotive electrical design (my profession), so i can perform some test on this. i started testing the output transformer since it does have leads that goes to the plates, and is somewhat responsible for audio output (makes since to me?). from the OT secondary, the black (-) to the yellow (8 ohm) and (-) to green (16 ohm) i am getting 1.5-2.0 ohms continuity. this is with the transformer still soldered to the board, just all TUBES TAKEN OUT. is this a accurate measurement, and are the windings indeed shorted internally (have read where the OT measurements can be taken with just tubes removed)?

 

[beans-on-toast]

Most often, the power transformers need to be replaced. I would test that first.

You’ll have to identify what year it is. Posting an image here would help. Pot codes can give you an estimate of the date of manufacture. Look for a code starting with 137, it is followed by year and week of the year. If there is a schematic on the tray, there may be reference voltages. If not find a similar revision that has these voltages. There are also service manuals with voltages for the power transformer. They also provide the DC resistance values that you are looking for.

The TalkBass Portaflex Wiki has schematics and manuals, as well as loads of helpful information.

Ampeg Portaflex Wiki | TalkBass.com

Also look for schematics and service manuals at Vintage Blue

If you need a new power or output transformer, they sell excellent ones made by Heybour at Fliptops Home of Parts for Ampeg Lovers.

Finally, there’s the Official TB Portaflex Club. Drop by and ask questions.

Official Ampeg Portaflex Club

 

[dune2k]

Hi,
I would highly recommend bringing the amp to a reputable tech.
What you are doing is poking around without exactly knowing what you are doing. Since these amps carry lethal voltages, I can only highly recommend against doing this. You might kill yourself.
Also there is the chance that you break more than you fix, although this would be the lesser problem.

 

[BogeyBass]

Tubes getting really hot sounds more like unstable bias.

And a basic extended run through of the amp which is rather difficult to completly cover in a short post

 

[byacey]

Take it to a qualified technician. Based on the symptoms, it sound more like the bias voltage has been dropping over time, perhaps the bias filter cap going bad.