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Odd Problem with Vibroverb Clone

6/12/2005 12:34 PM
Dominik Odd Problem with Vibroverb Clone
Hi there,  
I have a VERY odd problem with my newest project, a vibroverb clone.  
I bought the Export Power Transformer from Hoffman Amps. TBPT for Tweed Bassman.  
So I wired up the heaters, Bias board and connected the HV secondaries to the rectifier socket.  
The Export primaries are to be connected as follows for 220V operation:  
connect brown-white/black, heat-shrink brown, orange and black-white are supposed to be the line connections.  
I flipped the Mains switch and the Fuse blew. Short flash of the indicator light while doing that. So I rechecked the wiring. Found nothing. Measured the secondaries, no apparent shorts.  
I found a small problem in my heater wiring, where I mixed up the two leads. So I redid that, still the amp blows itīs mains fuse.  
Btw. this was with no tubes in the sockets, just the indicator light. Checked the heater wiring again, desoldered it from the PT and used a 9V battery, indicator light shines nicely.  
Removed the red-yellow CT and green-yellow heater wiring from chassis ground, still blowing the fuse.  
Connected my trusted VOM across the red an red secondaries, blows the fuse and takes out the VOM for good, now the display on the VOM has artifacts. GRRRRRRRR :-( Btw, the CT was back connected to the chassis then.  
Could a short in the heater wiring (my only obvious mistake) have done permanent damage to the PT? The first time I fired up the amp, the fuse blew ever so quickly, no smoke! The PT was stone cold after the fuse blew.  
What can I do?  
Since I have seen quite some heater shorts in other amps before and never ever witnessed a permanent PT failure, what could cause this?  
Seems like the PT is drawing quite large current.  
Even with all secondaries disconnected.  
6/13/2005 4:42 AM
I'm not sure if this is a feasible cause, and I don't know that PT, but could having your two 110V primary windings hooked up in reverse phase cause such a problem? :-?
6/13/2005 8:06 AM

I agree that the most likely problem is that your two primaries are not connected properly. Check the polarity of the two primaries - it Does make a difference! Also check all of the windings and make sure that they are not shorted to each other or to the transformer case. The fuse should have prevented damage if it blew immediately.  
Hope this helps!  
6/13/2005 8:44 AM
Hi Loverocker and Charlie,  
thanks for your answers!  
This seems to be indeed the only explanation, but I am a bit irritated, since I hooked up the Transformer as per the wiring diagram on the Hofmann Page.  
I will try to reverse the polarity of the two primaries.  
Wouldnīt the PT hum badly mechanically with the primaries being in wrong polarity?  
6/13/2005 11:55 AM

Yes the PT would hum loudly but the fuse may be blowing too quickly to let that happen. After checking that none of the windings are shorted to each other, I would check the primary polarity.  
I would also recommend that you limit the voltage / current when you continue debugging. I use a 60 watt light bulb wired in series with the power cord. With tubes out, the lamp should not glow brightly unless there is a short somewhere. The lamp will limit the current and allow you to measure things without blowing the fuse. I keep the lamp in series until I have debugged the power supply with the rectifier tube installed. This also soft starts the power supply caps which is good for forming them.  
6/16/2005 4:19 AM
Dominik Thank you and Follow up!
First of all, thanks again for the replies.  
I swapped around the primary leads, thus hoping to get the polarity right and still the Fuse blows.  
I checked for shorts between the secondaries and ground and found nothing on the B+ side, Rec. Filament (5V), and bias line. With the CT disconnected of course.  
If I attach the Filament (6.3) CT to ground I get a very low impedance reading, somewhere around 0,04 Ohms. Still, even with the Filament supply CT disconnected from ground, the Fuse Blows.  
What is going on here?  
I checked for shorts between the primaries and ground, but seem to remember that the resistance was infinte.  
Still scratching my head on this.  
The bad thing is, that I ordered the PT from Hoffmann amps and sending it back would cost too much money. I am a bit reluctant to buy another one.  
I guess I will have to talk to Mr Hoffman, maybe we can work something out.  
Still thankful for any input on this,  
6/16/2005 1:16 PM

Did you check the resistance between the two primary windings when they are disconnected from each other? They should be isolated. For 240V operation, they should be in series with same polarity. For 120V operation, they would be in parallel - same polarity. For both to be possible, the two windings cannot have a common connection.  
There has to be a short someplace or a miswired primary if your fuse is blowing with no load on any of the secondary windings.  

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