Hi All,
This is my first post here, so be gentle
It's been a while since I've done any electronic restorations. A friend of mine picked up a 41-290, and I offered to help with the electronics. I've re-capped it, and replaced all of the rubber wire in it, including the wiring to the loudspeaker/field coil (which was frayed and shorting where it goes through the chassis). I currently have audio going all the way to the input of both 41 tubes, but nothing coming out of either.
I am not sure where to look. The schematics I have at my disposal are incomplete at best. If anyone has any ideas where to look next, they'd be greatly appreciated! Also, does anyone know what the voltages at the 41's are supposed to be?
Ok, I finally got around to checking the voltages on the 41's. The grid voltage (pin 3) is 175v on both tubes. Plate voltage is not there. (-1 or 2 v). I know there's audio at the input to the tubes, confirmed with a test speaker. What should the voltage there be? I did replace the crumbly speaker wires, and I'm beginning to wonder if I got something backwards... Thanks in advance for any help!!!
Could it be you mixed up the wires while rewiring?
Also, check the primary of your output xfmr, could be open. Voltage at pins #3 and no plate voltage hints to that.
(05-27-2016, 07:54 AM)morzh Wrote: [ -> ]Could it be you mixed up the wires while rewiring? 175V at the grids while no plate voltage kinda hints to that.
That's what I'm wondering, but the only thing I rewired was the speaker...I keep checking it, and it looks to be right, at least according to the schematic. Power supply is working great, so I know the field coil is connected correctly. Does it matter which polarity the field coil is in?
Hi Michael and welcome!
Measure the resistance from plate to plate across the 41 tubes, methinks your output transformer is open.
Terry N3GTE
It does not matter what you rewired if your xfmr primary's open which is what I asked you to check later in the post. If this is the case you will need a new one.
Do as Terry told you, measure from plate to plate. And if it's open, make sure the xfmr is connected properly, then if it still is, you can declare it bad.
Plate load for the 41's is 12,000 ohms end to end or 6,000 ohms each side of center tap. This what you need to know to find a proper replacement. Load resistance and dc resistance are two different things. DC resistance is going to be a few hundred ohms.
GLOM
Terry
I measured the output transformer...nothing. Why didn't I think of that! I guess I was just too distracted by checking my wiring to think about it...First time I've ever experienced a bad one. Probably on account of the crumbly speaker wires shorted to the chassis. Thanks so much for everyone's help! I'll see if I can find a replacement...Any suggestions for sources? I have an old 40-180 chassis laying around somewhere...I'll check that one too. I know it has a good output xfmr. Thanks again!!!
You need a pushpull with impedance conversion Terry just listed from your voice coil impedance (I do not know what it is but if you, after removing the transformer (important!!!) measure across the voice coil of your speaker this will give you a fairly acceptable idea of it). A Hammond 125 pushpull is a good candidate.
http://www.hammondmfg.com/125a.htm
Check your impedance and go by the chart, does not have to be exact, just reasonably close.
Thanks. The 40-180 transformer won't work...It's a single ended output...no center tap. I'll go with the 125B...Seems to be most closely suited to the purpose. I'll report back once I change it out!
Edit: Maybe it will. I was looking at the wrong schematic...It is a dual 41 output. I'll check the part number just to be sure.
So Mike (Morzh) can you cheat with a single ended opt by connecting two resistor across the primary and using the center junction as the center tap?? Have done it power transformers.
Terry
No not really. Even if it works it will be bad.
If you cheat on power xfmr output it is possible as you simply divide the voltage to the needed value, only works with light loads. In this case though you substituting the load which is the transformer with resistors, and you want the current to go through the transformer, not through resistors.
Honestly do not see a reason to do that. Especially after he said he has another xfmr for 41 output. Might just fit. Unless also blown
