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Tennessee Walker blows ac fuse instantly on key down

Truck

Member
Dec 18, 2012
68
3
18
KY
If you don't have extra tubes, I'd consider testing it with no tubes. Make sure the plate connectors for the tops of the tubes are not touching ground. Turn it on and key it with your radio for a short test. Obviously it won't make any power and your radio will see a fairly crappy SWR since there will be little load on the drive power. The goal is to see if all power supplies can fire up without blowing the fuse.

I have the set of tubes in which one is shorted. I also have one nos Eimac. It was sealed in package but data is 1960 so I was scared to put it in and fire it up. Afraid of gasses being built up.
 

Truck

Member
Dec 18, 2012
68
3
18
KY
Well, I put the old tubes back in, even though one is bad. It would load out somewhat before. Now it just blows the fuse just like the new tubes. That about rules out the tubes. I give up...
 

Truck

Member
Dec 18, 2012
68
3
18
KY
Could this cap cause this? I remember the leg breaking off real close to the cap and soldering it back?
U6ymjRD.jpeg
 

Shockwave

Sr. Member
Sep 19, 2009
3,752
3,213
273
That cap looks like a screen bypass cap to me. If it is bad or disconnected it will most likely cause the amp to self oscillate when keyed. There should also be more than one of these screen bypass caps on each tube. Can you take a closer picture of the bottom of the tube sockets so that I can examine this wiring better?
 

137

Member
Jun 6, 2011
54
1
18
Middle TN
When you put the nos tubes in, did you let them idle with the filaments on and no plate volts, or just hit the switch and go?
 

Truck

Member
Dec 18, 2012
68
3
18
KY
When you put the nos tubes in, did you let them idle with the filaments on and no plate volts, or just hit the switch and go?

They were not NOS, these were new production tubes but I did let them burn in anyway
 

Truck

Member
Dec 18, 2012
68
3
18
KY
That cap looks like a screen bypass cap to me. If it is bad or disconnected it will most likely cause the amp to self oscillate when keyed. There should also be more than one of these screen bypass caps on each tube. Can you take a closer picture of the bottom of the tube sockets so that I can examine this wiring better?

The other side also has the same. 001 cap. I just have it removed right now for replacement
 

Truck

Member
Dec 18, 2012
68
3
18
KY
Nomad got my Tennessee Walker back up and running! The culprit was a open 47ohm resistor. I had replaced both of them but one was open. I didn't think to check a new resistor. Thank you Nomad.
Only issue I have now is that on some random unkey the relay chatters for a second. A minor problem.
 

Shockwave

Sr. Member
Sep 19, 2009
3,752
3,213
273
That resistor would certainly stop the bias voltage from getting to one tube. The relay chatter when you unkey is likely related to the fact the amp is not neutralized. There is probably some low level oscillation keeping RF at the sniffer circuit for a second. Most of these non neutralized 4CX250B boxes used a 100K non inductive resistor to swamp the control grid. That would probably help make it unkey clean.
 

Shockwave

Sr. Member
Sep 19, 2009
3,752
3,213
273
Two watt carbon 100K ohm should be fine here. See the red wire coming off the input tuners that goes to the big yellow .01 cap, then feeding the two 47 ohm resistors? You'll want to put the 100K on the red wire side of the .01 cap to ground. You'll need to readjust the two input tuners because this will lower the drive impedance a little.

The pre amp board inside the tube compartment is also not so good. The long green wires from the relay are unshielded. I'm sure the feedthrough SWR on this amp when it's in standby is somewhat high as a result. Unless you really like that preamp I'd hack it right out of there and shorten up that green jumper across the normally closed contacts.
 

MudDuck442

Active Member
Mar 24, 2013
130
33
38
I agree with Shockwave on everything he said. I would only add that sometimes depending on the amp I used to use a little lower impedance resistors..but I think all of his advice is excellent and right on the money.
 

Shockwave

Sr. Member
Sep 19, 2009
3,752
3,213
273
I don't even see any grid load resistor in this amp. The actual value used in these amps is somewhat of a compromise between a value low enough to stabilize the amp and one high enough that it won't scrub off a good deal of the stage gain. The use of a grid load resistor also helps maintain a stable input impedance to the amp over a wider range of drive.
 

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