cooked '66 deluxe reverb power transformer....wondering why??

[acorkos]

looks like smoke came from the green filament windings and then took the fuse out....correct 2A fuse. it smells horrible. what should i look for when i test it? did a power tube short? everything else inside the chassis and under the doghouse look ok and everything i metered checks out ok.

[Raybob]

looks like smoke came from the green filament windings and then took the fuse out....correct 2A fuse. it smells horrible. what should i look for when i test it? did a power tube short? everything else inside the chassis and under the doghouse look ok and everything i metered checks out ok.

Probably a short in the filiment wiring, probably at the pilot light.

[Inventor]

Howdy Y'All, New Meat,

Sounds like you may have had the original filter caps in your amp?
Those are what cooked most likely. Were your cap[s original?
Always a good idea to replace them.

Also, if the amp calls for a 2 amp fuse, try using a 1 amp fuse and
if that seems to blow easy, try a 1.5 amp fuse, the smaller the better.

And two 100 ohm-1/2 watt CC resistors soldered on your heater wires at the pilot light location
to the chassis ground is a good thing to do to help you save the trannys in case of a failure.

If you have a GZ34 or other rectifier tube amp, the wire that runs from the GZ34 to the Standby switch
you can install a 500 mA in-line fuse there and it will save the trannys in case of a rectifier tube failure.
Those new stock J&J and others are not reliable. The old Mullards are well worth the investment as they are rock solid.

I use a 1 amp fuse in one of my Tweed Bassman amps along with the other mods I mentioned above.
It has never blown on my yet.

[pdf64]

To check that the PT is ok, first isolate all PT secondary windings, then apply power to the primary via a light bulb limiter or similar. If ok confirm how much current it's drawing from the line in that condition; expect <0.25A for a good PT.
Second, if you're still in doubt, try the neon 'shorted winding' test as on geofex.
http://www.geofex.com/FX_images/xform_test.gif
Pete

[acorkos]

transformer was definitely cooked and shorted...i disconnected all secondaries, and my bulb limiter torched brightly. and it smelled horribly...i pulled of the bell covers and it was a nasty smelly goo in there.

filter caps were all recent replacements.

after installing the new transformer (no filament CT on the replacement...and the amp hummed until i added the artificial CT 100 Ohm resistor pair on the filaments), i fired it up and watched as one of the power tubes started sparking inside, with hum, and slight smoke from one of the power tube screen resistors.

i replaced the power tube screen and grid resistors, installed a new pair of power tubes and rebiased, and all seems well.

could this be the cause of the transformer blowing?....the amp had a grounded power cord previously installed elsewhere, with the hot black wire going to the mains switch, and the white neutral wire going to the fuse holder. when i install a grounded cord, i route the black wire through both fuse and switch, with the white wire going directly to one of the transformer primary leads. just wondering if the white wire going to the fuse prevented the fuse from blowing when it needed to, to save the tranny.

[jmaf]

Howdy Y'All, New Meat,

Ahoy! And welcome to the forum, Inventor!

[jmaf]

after installing the new transformer (no filament CT on the replacement...and the amp hummed until i added the artificial CT 100 Ohm resistor pair on the filaments), i fired it up and watched as one of the power tubes started sparking inside, with hum, and slight smoke from one of the power tube screen resistors.

could this be the cause of the transformer blowing?....the amp had a grounded power cord previously installed elsewhere, with the hot black wire going to the mains switch, and the white neutral wire going to the fuse holder. when i install a grounded cord, i route the black wire through both fuse and switch, with the white wire going directly to one of the transformer primary leads. just wondering if the white wire going to the fuse prevented the fuse from blowing when it needed to, to save the tranny.

Arcing in the power tube was definitely your culprit. Those filaments are referenced to the chassis, when 500V start arcing into the 6.3 V loop you get what you got.

If I understood correctly, you just changed wiring colors, but you did not bypass the fuse, so as far as where the fuse is located it should not make a difference, unless your mains network had a difference between neutral and hot(impedance-wise, bad neutral, etc...). The HT arcing into the filament supply will fry things.

I can feel your pain about that original trafo going down.

[kenda47]

Hi. guys kenda47 here recent new member. I am glad you have got your amp working again. The reason your tranny went south is if this is a 66 Deluxe with reverb it must be a blackface amp and therefore either has the AA or AB763 chasis and I was pretty certain that Fender changed from 2A used in pre blackface deluxes 6G3 chasis to a 1A slow blow. I checked my book of Fender schematics and it should be 1A. The fuse in yours is double what it should be and unfortunately your tranny burnt out before the fuse blew. The 1A would have blown before the tranny got damaged. It would have got hot but it would have lived. Inventor is right put a 1A slow-blow in ASAP and if that blows we need to help you identify the real problem.

[Steve Conner]

Arcing in the power tube was definitely your culprit. Those filaments are referenced to the chassis, when 500V start arcing into the 6.3 V loop you get what you got.

If this had been happening, then the 100 ohm "virtual center tap" resistors he added would have smoked instantly. But they didn't. Instead, the smoking screen resistor suggests the screen is shorting to the beam plates or control grid.

Also, the heater winding is made of thicker wire than the HT one: if the B+ were shorting to the heaters, the HT winding would have burnt out first.

So I think the problem was something else. Shorted pilot lamp holder, etc.

Kenda47: Is that 1 amp fuse for the 120V version or the 240V export version?

[kenda47]

Hi Steve. Iterestingly the schematic is drawn as per USA and although it shows the export model tranny alternative it only shows 1A fuse. I have an old UK Blues Deluxe non re-issue I am restoring at the moment and the schematic also drawn per USA and again illustrating export tranny alternative for that shows 3A fast blow for 100/120v and 1.6A 230/240 which is what I would expect. So like you if this is non export I would have expected 2A and if export 1A to 1.6A. You are probably as puzzled as I am. Do we know whether the amp in question is USA or export version?

[acorkos]

If this had been happening, then the 100 ohm "virtual center tap" resistors he added would have smoked instantly. But they didn't. Instead, the smoking screen resistor suggests the screen is shorting to the beam plates or control grid.

Also, the heater winding is made of thicker wire than the HT one: if the B+ were shorting to the heaters, the HT winding would have burnt out first.

So I think the problem was something else. Shorted pilot lamp holder, etc.

Kenda47: Is that 1 amp fuse for the 120V version or the 240V export version?

the original setup had no virtual CT resistors...I added them after installing the new tranny - which needed them to reduce the hum

[jmaf]

If this had been happening, then the 100 ohm "virtual center tap" resistors he added would have smoked instantly. But they didn't. Instead, the smoking screen resistor suggests the screen is shorting to the beam plates or control grid.

Also, the heater winding is made of thicker wire than the HT one: if the B+ were shorting to the heaters, the HT winding would have burnt out first.

So I think the problem was something else. Shorted pilot lamp holder, etc.

Kenda47: Is that 1 amp fuse for the 120V version or the 240V export version?

The Deluxe Reverb doesn't use 100 ohm virtual center taps, it uses the transformer itself. Also part of my reasoning is that it wouldn't arc from plate to screen, they're at similar potential....so I figured the heaters was more probable. If it was cathode, it'd have smoked the OT, not the PT.... Anyway, just guessing and you're far more knowledgeable than I am. All I could do is regret that that original transformer is gone, I'd be in pain right now.

Edit: I replied to you before I read the other replies, as I have the forum in linear mode...my bad, acorkos already mentioned the heater CT. I'm in a big hurry and I guess that's what you get not paying attention, I just wanted to acknowledge your post before I had to head out again. My mistake.