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Hi all,
First off, sorry for the double posting. I also put this in the tube/DIY asylum, but a bunch of you guys hang out here....so wanted to make sure I covered all bases.
I have a George Wright AG1000 preamp on my bench from a customer. The power supply electrolytics have been replaced with good F and T axials of the exact same values as original design. I did this about a year ago. No other mods made. It ran for a few months and then blew a fuse. I put exactly the same component values as the original caps. The 6X4 is rated to have a 40 uF first filter cap and this one has a 22uF. I have determined there is a dead short in the 6.3V filament winding. This drives the 6X4 rectifier and a 6BM8 that is being used as a regulator. Of course there is no schematic and no specs on anything and unfortunately George Wright passed away. A web search is not turning up anything. There is no sign of any damage to any power supply component. The 120 ohm resistor off the 6X4 ahead of the first 22 uF filter cap reads exactly 120 ohms. Caps are all fine. It appears the transformer just decided to die. There is no sign of anything that could have killed it. All caps are less than a year old.
So the 6.3 V spec on the transformer is easy in that the combined 6X4 and 6BM8 filaments are 1.38 amps. So a 1.5 amp trafo is fine or a 2 A with a resistor or two to get the voltage back down to 6.3 VAC. The preamp has 3 x 6GU7 tubes and their filament supply is DC which is rectified off the only high voltage output of the power supply. The 6X4 is rated at 70 mA and has a 325 VAC max input to the plates. So I am trying to figure out what voltage secondary to use in a replacement. Does anyone have a schematic or transformer spec? Since the preamp is dead I have no idea what the B+ was.... Anyone have any leads?
Oh, and the resistance of the high voltage secondary is 2.6 ohms, but maybe it was damaged when the filament secondary died...hard to know for certain. Those are the clues....
Anyone with info on this unit please chime in..
thanks in advance,
Don
Follow Ups:
I moved the thread over to DIY tubes. I pulled a filter cap and got a good look - it is tight in there! Anyway, further description of circuit posted in DIY thread for anyone interested. Am awaiting an email reply from Steve, who was George's tech. Hopefully he knows the B+ secondary voltage spec of the original transformer...
Don
Hi Don,
I sent you an email about a person who would likely be able to provide any information to do with Wright Sound gear you may require.
Regards Dan
Don,
FWIW, the triodes in a 6GU7 are "the same" as those in a 12BH7. Pin out is the same as a 6FQ7 and, more often than not, 6FQ7 circuitry operates just fine with 6GU7s in the sockets. Perhaps thinking in terms of the 6FQ7 will allow for proper selection of the replacement power trafo. Also, the 70 ma. B+ current limit of the 6X4 is an important clue.
Can you post a quick sketch of the PSU? I'm curious as to how the late Mr. Wright obtained 2 rails from a single secondary, especially since 1.8 A. of heater current are drawn by the 3X signal tubes, but a 600 mA./"18" V. series heater string is possible.
Except for the signal tube heaters, a Hammond 261G6 might be suitable. Since the object of the exercise is restored functionality, perhaps an external regulated heater supply is the path of least resistance.
Eli D.
Hi Eli,
I will have to trace out the PS schematic. There is only the 6.3 VAC secondary that drives the 6X4 and the 6BM8. They are a 1.38 amp load. The only other trafo output being used drives the B+. The filament supply for the three signal tubes is DC that George derived from the B+ supply and then filtered with a 22,000 uF above chassis cap that I left in there because it seemed fine. Am going out now, but will trace the schematic this afternoon and post by tonight. I was hoping someone knew the spec from having worked on one:) There is no sign that anything is damaged so I guess the winding just died. The lytics are all really nice F and T axials that I got from Jim when I put them in 6 months to a year ago.......
later...
don
Don,
I saw your photo over on Tube DIY. The failed power trafo looks pretty good sized. Maybe you have enough room for 2 off the shelf items to replace that failed part. Mouser stock # 511-L7818CV-DG is a low cost, 1.5 A./18 V., 3 terminal regulator. Antek's AN-0220 toroid, when bridge rectified and filtered, should be adequate to provide for the signal tube heaters. IF the Hammond part I previously mentioned will yield a "tall" enough B+ rail, you might have the makings of a nice repair.
Eli D.
Hi Eli,
I can think of a number of ways to skin that cat too... However, I think the filament winding was probably just barely adequate in the rating of the original trafo and it died. A 2A trafo would be fine for the 1.38 amp load. I have an Antek toroid sitting here that is 240 VAC secondary with a 2A 6 vac winding. There are several Hammonds that have 2A 6.3VAC windings with 220 - 320 VAC secondaries and all are small enough to replace the original. It has mulitple windings that were not used. I am sure George got a deal on a bunch of trafos and used them in various products. This one only needed the two windings. I emailed a guy who was his tech, who Dan (mossback) was kind enough to provide the email address of. Hopefully he knows what the trafo specs were and I can just get an over spec'd replacement that will never fail again. Otherwise, I can improvise anything I want as you suggest, but I need to know that B+ voltage I am shooting for. I know the rectifier voltage drop and current rating. I have many puzzle pieces....but if I get the spec from George's tech then it is easy....
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