1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
     
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Does this sound like a blown transformer?

Discussion in 'Amps and Cabs [BG]' started by Naggon, Aug 7, 2012.


  1. Naggon

    Naggon

    Nov 8, 2006
    Vancouver, BC
    Hi,
    I've had some problems with my amp, and when I called the local tech shop on the phone they said that it was likely a blown transformer and would cost $300 to fix. The local tech shop is also notorious for ripping people off, so I wanted to check with my trusted talkbass community to see what you guys think before I shell out.

    This actually happened on my guitar amp - but I swear I'm usually a bassist. :bag:

    I was playing a Fender Bandmaster VM (the reissue, not the classic bandmaster) As it stands right now, my amp will only give me a BIT of volume when it's cranked to 10, and nothing noticeable on lower settings. The tubes and LEDs, all light up fine, there were no bad burning smells, and everything looks OK inside the amp. (I took it apart just to check)

    What happened was, I was playing this amp (through a different cab than usual actually), and it started sort of suddenly getting much quieter when I hit a certain volume, like almost 'bouncing' off this volume 'limit', dropping suddenly down to a much lower volume before rising back up again, until it hit that apparent limit and bounced back down again. These cycles took about 5-10 seconds each.

    I went back to try it again this morning, and the amp wouldn't give me any sound, except for a really low volume when it was cranked to 10, as I said above.

    The cab I was playing through is a strange old unlabeled cab, 1x12 and 1x15. I assumed it was 8 ohms, and was told this when I bought it on craigslist for cheap, but I suppose its possible that it could actually be a 4 ohm? The output on my head is 8 ohm.

    I have tried different speaker cables, different patch cables, a different guitar, as well as a different cab (AFTER the 'damage' was done), all to no avail.

    Please let me know if you have any idea what this could be, or if there are any possibilities that you think could be eliminated! I really don't want to let these guys rip me off again.

    Thanks so much!!
     
  2. It sounds like a blown tube to me. You may have blown one or more of the tubes pre-maturely because of a impedance missmatch between the cab and the amp. I can't speak to that specific amp but many tube amps have sacrificial parts to protect the costly transformers in cases like this. I'd try to replace the tubes first before the repair man. If it's not the tubes, then you'll have a spare set. One day you'll need them.
     
  3. Naggon

    Naggon

    Nov 8, 2006
    Vancouver, BC
    Thank you! I am so relieved to hear you suggest that. I'll be getting some new tubes and checking this out. I'll let you know how it goes.

    Other ideas are still welcome! Cheers.
     
  4. spider_legs

    spider_legs

    Jan 11, 2011
    an impedance mismatch wont blow tubes it will blow your power transformer
     
  5. Naggon

    Naggon

    Nov 8, 2006
    Vancouver, BC
    ahhh...I'm afraid of that. But if the power transformer was blown, wouldn't that prevent me from getting any sound at ALL? Cus, as I said, I can still get it to make noise, just not much noise...
     
  6. Naggon

    Naggon

    Nov 8, 2006
    Vancouver, BC
    Ok. I just got back from testing the tubes, and to my dismay, they are all in great shape.

    Is there anything this could be besides the transformer??
     
  7. P Town

    P Town

    Dec 7, 2011
    If the power transformer had a shorted winding, it would not produce any sound.

    If the power transformer had an open winding, it would not produce any sound.

    If the output transformer had an open winding, it would not produce any sound.


    If the output transdformer failed by shorting some percentage of the windings, it may produce reduced sound output.

    Trying to diagnose the problem over the phone is not likely to be successful.

    A good Ohm meter, and knowledge of the resistance of a good output transformer would tell if the transformer was at fault.
     
  8. Naggon

    Naggon

    Nov 8, 2006
    Vancouver, BC
    Thank you P Town. I'll see if I can get an Ohm Meter and look into this...

    Cheers!
     
  9. my gut instinct is it isn't the output transformer, they tend to just kind've fade out and never get power back.

    I'd check preamp valves / power resistors etc. You wont know till you get it on the test gear
     
  10. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    It almost always only manifests itself when the cab is a higher impedance than the output transformer demands, but tube amps are pretty good with a cab with a lower impedance. Jess Oliver would even recommend that folks use the B-15 with a 4 ohm cab because it sounded a little tighter and the transformer's fine as long as it doesn't see infinite impedance (no load at all).

    Really tough one to call, but if it IS a transformer, that's likely a fair price unless he's using absolute junk for the transformer.
     
  11. I'm not sure where you arrived at that conclusion.

    As long as you do not exceed the power rating of the transformer, it does not really care.

    Another tidbit of info...speaker impedances are not constant! Ever looked at a speaker response curve? The impedance really takes off as frequency rises.

    Bottom line, I agree with P town. The OT is very likely NOT the source of the problem. Some unscrupulous repair shops will indeed claim "it's the transformer", charge $300, and instead replace a preamp tube that costs $10.
     
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    And, that's pretty much the point, isn't it?

    If the amp is driven hard into a lower-than-design impedance, it works BOTH the transformers AND the output tubes harder..... More current, more losses, more heating...... I know a guy who did a 6550 conversion on a Fender... really sounded GREAT..... but the power transformer was leaking tar like crazy...... it was as hot as a five dollar pistol.

    In this case, there are all sorts of possible causes, including a funky bias circuit.

    You already know the opinion of the one outfit..... you probably need to find a different one and see if they can actually fix it instead of guessing over the phone.
     
  13. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    So Jess wasn't entirely accurate about it, or was he specifically referring to the B-15? Not trying to be sarcastic, either...just trying to understand why Jess would say it's OK if it's not.
     
  14. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    he was accurate enough..... it's usually not an issue. And it makes a tonality change. I like the idea.

    Point really is that it depends what you do..... Most do not get into a B-15 quite the way they might with an SVT.... they kinda are for different things.... So you can depend on the built-in margin that is always left in a design for "surprises".

    If you BOTH run a lower impedance, AND belt the daylights out of the amp, (yes, it happens) you may get a nasty, smelly, hot, but silent surprise........ When you are using up margin, you don't want to count the same thing 2 or three times.....

    My friend with the 6550 conversion.... for guitar..... the 6550 let him get more power out, AND he really hit it hard on overdrive, plus the 6550 filaments were drawing more than the original tubes did..... So, let's see..... start by drawing more filament current...(attack #1 on power margin).... then have the tube pull more power (attack #2 on power margin), and finally, run the amp into heavy clipping (attack #3 on margin). he tried to use the same margin 3 times over..... wasn't much left :rolleyes:........ he was heavy into negative territory...... Gettin it?
     
  15. I am NOT on board with a Power transformer or output trans either. You DO have an effects loop, plug your guitar into the effects return and if the volume output is better (still shouldn't be real loud) you can eliminate the transformers. If the output is no better it could be a bias circuit as JT mentioned, or something in the PI/Driver tube circuit. If the volume is higher then the preamp or DSP circuits have the fault. Could even be the internal contacts in the effects return jack :)

    After looking at the schematic http://support.fender.com/schematics/guitar_amplifiers/Band-Master_Deluxe_VM_schematic.pdf there is a whole lot of places for the signal to go wrong in between the tube sections as well.
     
  16. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I'm gettin' it! And you're right...those who want to stick a 4 ohm speaker into a B-15 aren't looking for better distorted sounds from it ;) So I see what you mean. As always, thx for the great explanation!
     

Share This Page