Fender 5e3p from Wurlitzer organ amp

[Phil_S]

I've never actually bent a chassis. It strikes me that any first attempt at anything involves a learning curve and one should not expect a 1st rate result because of that. On the particular task, having no access to a brake is a real problem

If you have the skills and equipment to drill AND bend, it is best to drill the whole thing, to the extent you can plan, then cut and bend. As you probably know, it is easier to drill the metal when it is flat. Things to watch out for:
-sockets not too close to the edge.
-transformers not too close to the edge
-adequate room between components...particularly if you will be using coke bottle shaped tubes or chicken head knobs
-on front and back panel, remember to make sure placement (of switches, for example) allows adequate clearance for soldering wires, etc.
-bend a pair of lips for mounting to a cabinet -- simplest mount is about .75" on each (short) side bent outwards, but probably not the simplest bend -- simplest bend is probably two inward lips on the long sides.
-no oversize holes You can always make them bigger later.

Look around for drilling plans that already exist. There will be good hints for things.

Give it a try, particularly if you think you can do it. You can always move to plan B if that doesn't pan out!

[Zombie_stomp]

I've got to know a lot more before I proceed with a chassis, but we've got a whole set of hole saws, a pan brake, and an 'iron worker' type of machine that can die-punch many sizes of holes needed up to probably only switch and pot size. It can punch a lot larger but not up to tube socket size since it is designed mainly for thicker steel.

If I plan it right, I could probably work the offset outside bend for screwing it into a cabinet, I think. I may mess with some scrap to test the theory out. It doesn't have to be exactly like another chassis, I could piece together certain features. I'm pretty sure I could at least bend myself a box the right size with tabs and holes where I could make it one-piece and plug weld it at the corners in lieu of a spot-welding rig. I need to find out what gauge of sheet metal most chassis are and see if the local air conditioning engineers have a scrap I could have. They have a major metal shop that makes stuff you wouldn't believe. They made us some super-thick press braked stair step pieces we picked up today for an ultra-heavy duty staircase we're building. They know where the galvanized metal is!

[Zombie_stomp]

I got the 5u4GB in the mail and not much seems to have changed in the voltage chart.

Maybe some part of the amp that interfaces with the organ needs to be removed to let it fully turn on to where these readings become available.

[Phil_S]

Let's have a good picture of the wiring on the tube socket. Something's not right.

[Zombie_stomp]

If this had something to do with 6l6s being in sockets labed 7027, you would have said something by now. Right now I'm thinking it's more to do with some sort of controller device from the rest of the organ not letting us have the amp all the way on right now, or something of a factory design, because although it was really quiet, it did make sound and is all original under here. I may fire up my photo hosting site account again to get you as many high quality, large size photos as possible. Stay tuned.

[Structo]

Not sure if this has been mentioned but I would drill/ punch all the holes in the chassis before bending.

Flimsy sheet metal can be hard to drill when the chassis is formed.

[Zombie_stomp]

The 'control' socket on the amp doesn't seem to turn anything else on. It's got a pin connected to ground, and a couple more that branch through a pot labeled 'bias' to two different capacitors. So I think that's not the problem. I've made a mini layout diagram to try to help show the wiring of the main 3 tube sockets we've been measuring. If it's hard to read, I can expand it onto a larger sheet of paper. And then I've got the pictures of the bottoms of those tube sockets.

[Phil_S]

On the 6L6's, your drawing shows a wire between pin 4 of the two sockets and nothing else. This wire needs to be there, it is missing the B+ supply. Pin 4 is the screen. In an earlier picture I see another orange wire heading somewhere else, and I assume that's to the screen supply.

I am thinking maybe this amp just isn't working. We know you've got a good PT because we have 5V filament, and high voltage secondaries. I didn't ask about 6.3V filament supply, so verify that.

Assuming the amp is wired correctly and caps aren't totally spent, I'd look next at the OT to see if it is shorted or good. There are two tests, both relatively easy to perform. First, test for turns ratio. That's described here: http://www.radioremembered.org/outimp.htm

You will need a low AC voltage supply. You can use the 5V or 6.3V secondary from the PT with some homemade jumper wires. It is probably easiest to run jumpers from the rectifier socket (2 and 8 ) to the OT. I like to input voltage to the OT secondary. 5V in will produce something in the range of 100V on the primary. If you input 5V to the primary, you will see very little voltage on the secondary, making it hard to know anything.

Pull all the tubes, and disconnect the OT secondary from the speaker when you do this test. Assuming a 4K primary impedance and 8 ohm secondary, the turns ratio should be about 22.3:1; 4 ohm secondary 31.6:1; 16 ohm secondary 15.8:1. Of course this will vary depending on the actual primary impedance, which may well be different. This just gives a ball park example.

The other test is for weak internal insulation. We can look at that next, but first do the turns ratio test. If your OT fails that, no need to go further.

[Zombie_stomp]

The amp was working quietly when I began to dismantle it. I have not had a speaker plugged in while testing voltages, and it just clicked in my head that people will sometimes put a 'speaker load' on the amp. Should I hook the speaker up and test again?

[Blackburn]

Don't ever test an amp without a load connected. That's one of the first rules in the amp world.

[Zombie_stomp]

Well shiii.... Did I risk damage leaving it plugged in a really long time without load, or just whilst poking and prodding?

[Blackburn]

If you're moving wires around you don't want the amp to oscillate and try and amplify it. People do get away with not using a load, but why risk it? Those are generally lower gain amps and don't oscillate easily. I've read about guys on TGP that say they set the bias without one connected. It's a pretty dumb thing to do if you ask me. If things still light up and there doesn't seem to be a problem, there probably isn't. Plug in a load and if all your voltages look good, you're probably alright.

[Phil_S]

I moved to the OT test prematurely. Take a few seconds with your Ohm meter to check the OT for continuity (open load indicator, or infinite load from melting the insulation and making it one big hunk.) If there is no continuity on either winding, there is no reason for doing the test. Now, learning you operated the amp without a load, you may have toasted the transformer. I'm sorry to hear you did that. Do what Blackburn says and maybe you got lucky. With no signal, sometimes you get away with it.

[Firestorm]

You might want trace out the connection of the PT's center tap to ground. It was pretty common to make that ground through the speaker socket so if you accidentally unplug the speaker, it shuts off the HT to protect the output transformer.

[Phil_S]

I wondered why there was no report of smoke or burning. I think Firestorm has hit the nail on the head! When you ground the CT, make sure you have a speaker load and you'll get your voltage readings.