Fuse blowing in Wurlitzer 112 (I think) tube-driven E-Piano

[Pianokeys]

I accidentally posted this in "general discussion" first... Mods, you can delete that one if you want.

I am the proud owner of a wurlitzer electric piano that I got for the right price (FREE!!!) some time ago. I had to put on new dampers, one tine, and adjust a few settings, but it wasn't anything I've not had to do on a "real" acoustic piano before so it was pretty straightforward.  It had a HUGE fat tone because of its tube drive, I ran it through two amps, a bass amp and a guitar amp for stereo sound and it had the greatest tone of any instrument I own.

Unfortunately, a few days ago the fuse blew. I thought "well, I hadn't replaced the fuse before, it's probably just time for a new fuse." I bought a new one. I put it in. It blew again, instantly.  Now, I am wondering, could it be a short in the mix, or perhaps just a blown tube? Which tubes are more likely than others to die, and if it's a short, how the hell do I get the amp out to fix it? (this thing is built like a wooden tank, but I can't get it apart!!!) Any help would be nice.

By the way, If you need help to positively identify it: It's got a wood case with a flat metal top (curves down for the front), wooden action, a full tube amp in the back with a 6x9 speaker, and the thing looks freaking old, though I know that doesn't mean much. I am still somewhat new to the world of ACTUAL electric pianos (which I've learned you just can't really synthesize properly) and can't wait to fix this thing, but I also can't afford to have someone else do it.

[jeffwuollet]

Your problem might not be tubes, but caps.

If you happen to find huge electrolytic capacitors on the amp/power section, I can guarantee what the problem is. I'll give you my grand piano if it isn't. They dried out, and now you'll get hiss, hum, and power inconsistencies. My diagnosis? Replace ANY electrolytic caps every 15 years and call me in the morning.

[Pianokeys]

I found one of the tubes to be cracked down the side. I've ordered and received the new tube, but I'm out of town right now so I won't get back to checking it until probably next week. If it turns out to be the tube and not the cap do I still get that piano?

[axg20202]

...not if the dried out caps caused the broken tube  

[Pianokeys]

I'm wondering...what WOULD cause a broken tube? All I can think is heat because no one goes near the thing except for me.

[Bim]

Dried out caps. Dead caps. Causing a bad tube? No freaking way can  bad caps do that. I've been working on old amps for ages and never have seen a cap take out a tube. It's usually a bad grid or screen RESISTOR, an incorrectly set bias adjustment  or a bad solder joint that will take out tubes. Bad caps give you hum.

Next question.

[axg20202]

Yeah, well, you';re probably right. My reply was just a joke. "Next question" ? - Arrogant or what!

[jeffwuollet]

I check caps all the time with any kind of electrical problems. I've seen bleeders fry and lead to all sorts of trouble. ESPECIALLY on older electrolytics. I've never seen the actual power section of a wurli, but I know old gear(well enough, anyway). Work backwards with a multitester and find where the voltage and amperage inconsistencies lie.

You can have my piano if you come and get it. Maybe I'll do a trade for some wurli schematics.

[Pianokeys]

Ha, the only problem is that I'm in the process of trying to sell the grand I already have... but that's only because I don't like it much, not because I don't like grands in general.

So, I found the problem: apparently the rectifier tube (big 5 pin dealie near the transformer) fried out. I found this out when I got in and replaced the one that had broken, then turned it on, I was treated to quite a nice light show with lots of blue plasma...then the whole thing shorted out and blew the fuse. I just so happened to have a box of spare tubes with me, so I checked the ratings between a rectifier from an old Conn organ against the one that had just blown, replaced the fuse and switched out the tube, and now it plays fine... At least it did until yesterday, when I accidentally severed one of the speaker wires trying to get something out of the back. I need to replace that wiring anyway, though... Does it decrease the value if you replace the old rotting speaker wire with modern, low loss cable?

[jeffwuollet]

My grand isn't that great either. Need to voice it.

Anyway, I'd replace the cable based more on the fear of shorting something. I love cloth cable for aesthetic reasons, but the grade of cable in this case would probably be determined by what kind of abuse it'll 7

[Pianokeys]

Is it possible to replace the driver, too?

[jeffwuollet]

If it's messed up, sure. If not, why?

If the driver ain't broke, don't fix it.

Although if you do like modding the hell out of your pianos, start with tube and component upgrades. You can drill a 3/4" hole somewhere on the wurli and install a Male XLR jack for a DI out. A 75 dollar jensen transformer and some raw cable is all you need to make a great direct sound.

As for tubes, read up on the different kinds of triodes available and their gain structure. A lower output tube can offer much different sonic characteristics than your basic 12AX7(ECC83 for you Brits).

The resistors in the unit are probably a mix of carbon film and ceramic with tolerances between 5-10%. Newer metal film resistors offer tolerances down to 1%. You might want to look into replacing some of your non-audio circuit paths(HeaterV, GridV, etc) with these as they'll make any tube upgrades more noticeable.

Caps are another upgrade path. Electrolytics are now the standard in power sections because of their long life and resistance to leakage, but if you want a more musical sound at the cost of having to replace them in five or so years, paper-in-oil capacitors are what I consider to be the best. They are perfectly suited for audio whereas electrolytics are less specialized.