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Topic: Ampeg B-15-N ... no workie |
Bill Terry
From:
Bastrop, TX
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Posted 3 Feb 2007 1:17 pm |
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I've got a friends Ampeg B-15 on my bench (gee Al, it's only been a couple of years, relax....  ) that is not working, absolutely no sound whatsoever, not even a hiss, buzz or thump coming off standby . I checked all the power supply voltages, all seems good, although a bit lower than the schematic shows. I swapped in known good tubes, no help, read all the voltages at all the tubes, all good. BTW, the voltages throughout the amp are about 20% low, but I've seen that on working Fenders, so don't think that has anything to do with the problem?
The fact that there is not any sound whatsoever, led me to look at the OT, and I read about 260 ohms across the primary side, and an ohm or two across the secondary. I've never worked on an Ampeg before, and I did some googling but couldn't find a spec anywhere for the OT. Anybody know if that's in the ballpark?
Thanks for any thoughts.. |
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Ken Fox
From:
Nashville GA USA
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Posted 3 Feb 2007 6:25 pm
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The Ampeg is different from the Fender type circuit in that the amp is put in standby by removing the center-tap of the secondary of the power transformer, thus eliminating the B+ to the amp, Check for an open center tap or a bad standby switch.
Did you for sure check plate voltages on all tubes? If the voltage are all there, the the above would not apply.
Is the center-tap on the output transformer primary reading good to both sides of the primary windings? If not you would not have plate voltages on the power tubes. Also is there screen grid voltage on the power tubes (pin 4)?
Just some thoughts! |
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Larry Jamieson
From:
Walton, NY USA
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Posted 3 Feb 2007 7:36 pm
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| I am assuming that you have checked the speaker to see if it is OK... I had a B-15-N and the cable that you plug in to connect the speaker went bad at the connecting end. I replaced it with a 1/4" phone plug and put a 1/4" female in the amp, problem fixed. Larry J. |
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J Fletcher
From:
London,Ont,Canada
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Posted 4 Feb 2007 5:35 am
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| If I remember right, the XLR type speaker plug and jack has a safety type of circuit in it so you can't operate the amp without the speaker connected. Plugging into the 1/4" jack only, won't give you any output....Jerry |
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Bill Terry
From:
Bastrop, TX
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Posted 4 Feb 2007 10:48 am Thanks..
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All good points... thx
Ken, yes, B+ is there, I suspected the standby switch early on, but it seems ok. I did not check Grid voltage on the Power tubes, but I will.
Larry, and J Fletchher.. I've tried the connected speaker wire and plug (it's a 1/4" plug for some reason, not an XLR, looks stock maybe not?) and the external speaker jack to a known good speaker, neither works, although I can read the OT secondary DC resistance off the attached plug, so it seems ok.
I'll keep poking.. thanks again for the suggestions. |
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Bill Terry
From:
Bastrop, TX
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Posted 9 Mar 2007 7:40 am
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Got back to this amp this week, and checked grid voltages. Everything looked great, so I put a test tone in the front of the amp and traced signal all the way through the preamp and to the output tubes. Gain goes up as expected, and I've got a big signal at the primary side of the output transformer, however, zero on the output side.
Is that a definitive test for a bad output transformer? Seems there isn't anything else that could be wrong, but I read that there are special ways to test the OT with a Variac.
I did notice if the input test tone got big enough, i.e. if there was enough gain, I could hear it very faintly at the speaker.
Any thoughts? OTs are about $200 bucks for this amp, so I want to be sure. |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 9 Mar 2007 7:55 am
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Have you checked for continuity on both sides of the OT? Sounds like an open winding.
_________________
Best regards,
Mike |
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Bill Terry
From:
Bastrop, TX
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Posted 9 Mar 2007 8:14 am
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Yep, 250 ohms DC resistance across the primary, about 2 ohms across the secondary. Can't find a spec for the x-former, so I don't know what I should read.
Since it's stepping a big voltage down, (and from what little I remember of x-former theory ) I sorta expect the DC resistance to be a lot higher (more windings) on the primary side. |
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Bill Hatcher
From:
Atlanta Ga. USA
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Posted 9 Mar 2007 8:35 am
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| Be careful with the the OT and PT on these. If you have ever seen one of these transformers loose the "black oily contaminated with maybe PCB mess" that is used as some sort of insulator inside the tformer you have not experienced one of the absolute nastiest things you could ever imagine running out onto your amp or your workbench or YOU!! BE warned. |
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Mike Wheeler
From:
Delaware, Ohio, USA
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Posted 9 Mar 2007 8:51 am
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You also need to check for shorts to ground at each transformer lead. By the way, all these measurements should be done with all the xformer leads disconnected. Any shorts=bad xformer. If you don't find any, you could try the Variac test.
Since ohmmeter readings are done at very low voltage, they don't always reveal a short...the arcing kind is still possible. Here's a one link to the Variac test you mentioned, Bill.
http://www.geofex.com/ampdbug/outtrans.htm
Be very careful doing this test. You can seriously hurt, or kill, yourself. The above Variac test should never be done by anyone who doesn't have thorough training in electronics repair practices.
edit: When you tested the continuity of the windings, did you test all the combinations? Like center tap to each end?
_________________
Best regards,
Mike |
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Bill Terry
From:
Bastrop, TX
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Posted 9 Mar 2007 12:08 pm
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Thanks for the tips Mike. I was also thinking that the low voltage volt-meter test was inconclusive. There's about 425V on the primary side, so that could easily be breaking down inside.
I don't have a Variac, and I'm not real interested in buying one, but I've talked to a friend who works the bench at a local repair shop and he told me could load it and let me know, gratis. Sounds like the best plan.... |
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Bill Hatcher
From:
Atlanta Ga. USA
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Posted 9 Mar 2007 2:23 pm
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| Find out when you local ham radio club has a hamfest. I bought my variac at one for about $5. |
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Bill Terry
From:
Bastrop, TX
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Posted 9 Mar 2007 2:27 pm
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| 5 bucks??? wow!... I would pay that for one. thx.. |
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Robert Leaman
From:
Murphy, North Carolina, USA
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Posted 9 Mar 2007 5:13 pm Variac Xfmrs
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Remember that a Variac (a trade name) is NOT an isolation transformer. It is an autotransformer and its secondary has a galvanic (metallic) connection to the primary. Basically, this type of transformer is really an high impedance inductor (choke coil) with a variable tap. Carelss use will ferry a user into the next world (whatever and wherever that is).
Also, there is a reliable, elementary method to measure impedance, either inductive or capactive, without the use of an impedance (RLC) bridge. Anyone interested, email me. You will need a signal generator that can produce audio frequencies.
Output transformers that are used in vacuum tube (antiquated) amplifiers are not used as step down transformers, they are used for impedance transformation. Vacuum tubes are high impedance voltage driven devices and speakers are low impedance current driven devices. Although a few transformerless vacuum tube amplifiers were manufactured, they were never popular and they were very expensive. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 10 Mar 2007 9:25 am
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Variacs are commonly found for $5-10 at electronics swap meets...but only should be used if you know what you are doing (the infamous Eddie Van Halen "brown sound" reportedly using a Variac to put out around 90-100 volts...and hundreds of blown-up Marshalls as a result...comes to mind).
Honestly, I think you've found the problem with your transformer testing, and more testing is just a waste of time. The OT is toast. I'd go to the Mercury Magnetics site and just pay the piper.
FWIW the 1/4 jack is not original - and mine and every one I've seen had an amphenol plug of some sort, not an XLR.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Mike Hoover
From:
Franklin, TN, USA
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