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Old 10th May 2010, 11:13 AM   #1
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Default Testing for a Damaged Output Trafo

Got a Trace Elliot V4 Bass amp (4 x KT88, B+ = 680V). It claims 220W output, which for bass, I take to be down to 40Hz.

a KT88 lost vacuum and blew the mains(!) fuse. This may be to do with the HT fuse being T2A. with new valves set up at 30mA, the amp works unless you play hard, when it breaks up. A sine wave test into 8 Ohm resistive dummy load shows the waveform just disappears to zero.

Suspecting the output trafo, I tried connecting 28V 50Hz from a high-power toroid to the 8 ohm tap. A 47 Ohm resistor to limit current. Primary totally disconnected from anything. The OT pulls the output down to 21V the other side of the 47 Ohm, which sounds like 140mA rms @ 50Hz to my calculator. The exact same setup measured only 40mA in the OT of a 100W PA amp.

Seeing how 225W is ~42V rms into 8 Ohms, I'm assuming the OT is fried.

Am I missing anything? A new 220W OT is expensive, & I wanted to be sure.

THANKS!
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Old 10th May 2010, 01:06 PM   #2
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Rod --

It sure sounds bad to me. You didn't provide any info as to the power supply's behavior when you were conducting you sine wave tests. I assume all the voltages were holding up well, and it wasn't the power supply collapsing under load? I wouldn't think that would be the problem, but as you say, a replacement for this sucker will be expensive.

One last acid test I would perform would be to monitor the current draw of the output stage under a power output test. Assuming there are no parasitic oscillations going on, lots of current draw with the application of signal, but little power output being developed pretty well puts the last nail in the coffin!

Dave
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Old 10th May 2010, 02:11 PM   #3
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also check the balance of the primary sections. the CT should be exactly CT some OT's go bad and one side of the primary will be 2-3 volts or more lower and this can cause problems.
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Old 10th May 2010, 07:25 PM   #4
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Sounds like a classic shorted turn.
Last resort: check there's nothing jammed between the former and core that could provide a s/c. Older transformers often wern't designed with creepage nor former cheeks and the windings with time get physically closer to the ferrous.

richy
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Old 10th May 2010, 08:05 PM   #5
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Thanks for the help on this!
Dave, power supply seems steady.

here's the 28V applied to the 8 Ohm tap via 47R, on my 7603 scope. See what the OT does to the 50Hz waveform - this is with open circuit primary...
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Old 10th May 2010, 08:33 PM   #6
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Think you made the right call from the jump on this one Rod. With the drop across the 47 ohm resistor you posted earlier, that means the transformer is giving up at a little over 55 watts. A zero impedance drive may have gotten a few more watts, but there is no way that waveform should be clipping at that level and frequency for the power it's supposed to handle. I think you can safely pony up for a new one, although Rich made some great points. Any chance of a visible short that can be repaired? It's probably wishful thinking, but certainly worth one last look before the order is placed!

Dave
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Old 10th May 2010, 09:36 PM   #7
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Shorted section in the o/p tranny forcing flat top saturation. Try backing off the volts and see if decent sine wave returns is a sure sign.
Otherwise it's a duffer. To reconnect to amp with a good tube would be suicide.
A fast 500mA fuse in the center tap is a good insurance policy !
Guitar bass = 40Hz cutoff.

richy
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Old 11th May 2010, 12:11 PM   #8
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Caught it. Used a 10W practice amp connect via 47R to a little mains trafo. Primary of this trafo to suspect OT secondary 8 ohm tap.

Sine signal from MD player to amp, turn it up - up to 20V, no distortion, above that the crushed waveform as before.

Thanks for everyone's help and suggestions! Just got to find a replacement OT that can take the crazy high B+ in this amp (yes, it really does 700V unloaded, with octal socketed tubes)! Any ideas welcome for a robust trafo.
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Old 11th May 2010, 12:54 PM   #9
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The B+ is high, but not beyond what any well built transformer should be able to handle. Is the original one still available by chance? If you have to go the after market route, I would think the bigger issue will be finding one of an appropriate power and impedance level suitable for a bass amp. Any idea of what the primary impedance was supposed to be? Sure would be nice to know the original value, although a good set of curves would work, too.

Dave
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Old 11th May 2010, 02:14 PM   #10
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You'll be looking around 2K A-A; Do you really need all that power ? I find modern day KT88's can't take the stick esp at the voltage you quote. If the tranny heater ratings are up to it, I am using EH KT90 in lieu and these are
more power efficient and robust. Entirely your call.

I've enclosed a graph of what 4x KT90 can do compared to 6550C for the same anode load; (the 6550C is virtually power compatible with KT88's, but NOT B+ voltage/screen compatible !)

Note:- Pout for KT90's with a lower B+ have the upper edge: perhaps food for thought for those fitting out a new project.
As such I am fitting EH KT90's in all my amps now.

richy
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File Type: jpg EH KT90.JPG (64.6 KB, 80 views)
File Type: jpg THD comparison EH KT90_-JSC 6550C.JPG (35.3 KB, 80 views)
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