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Tennessee Walker blows ac fuse instantly on key down

Truck

Member
Dec 18, 2012
68
3
18
KY
I have this Tennessee Walker amplifier with 2 4cx250b tubes. It was accidentally driven too hard and got one of thettubes. I replaced them with brand new tubes. Now, on key down it instantly blows the ac fuse. It has to have a short somewhere. Any ideas on this would be great. I know little about AC amplifiers


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wavrider

W9WDX Amateur Radio Club Member
Jun 2, 2009
3,393
1,247
173
If you know only a little about ac amplifiers, take it to a tech. there are deadly voltages in that amplifier.

With that said it is most likely the high voltage power supply that is causing the problems. Just a guess.
 

psycho

Running a special on our rooms!
Aug 25, 2006
3,435
1,295
173
Floriduh
If you know only a little about ac amplifiers, take it to a tech. there are deadly voltages in that amplifier.

With that said it is most likely the high voltage power supply that is causing the problems. Just a guess.


Many a DIYer been electrocuted or lost finger(s) playing with HV tube amps.
 

jammeejoe

Member
Aug 8, 2005
50
0
16
East Texas
98 % sure it is a diode or diodes in your high voltage power supply. In your first picture it is the board with the 20 diodes on it. I would replace all of them. They look like 3 amp @ 1000 volt. Cheap---replace them and I think it will solve your problem.
 

Truck

Member
Dec 18, 2012
68
3
18
KY
98 % sure it is a diode or diodes in your high voltage power supply. In your first picture it is the board with the 20 diodes on it. I would replace all of them. They look like 3 amp @ 1000 volt. Cheap---replace them and I think it will solve your problem.

Those failing would cause it to blow a fuse when Keyed only? As I said in my first post, I know little aboutaAC amplifiers but I would think that if that diode bank was defective it would blow the fuse even in idle
 

137

Member
Jun 6, 2011
54
1
18
Middle TN
Is it radio drive only? If so, John's screen supplies are switched on when keyed, Likely a diode in the screen rectifier, which is around the big gray capacitor in the pic showing the underside of the whole unit, or at least it looks that way (I have a 4 tube 400 of his and its in about the same location). You should measure 325-350vdc On the outbound side of the screen circuit if it's working correctly. I'd do most of these tests with the H.V off and possibly disabled if possible, and tubes removed, you don't generally want screen volts on the tubes without plate voltage. Good luck and be safe.
 

Shockwave

Sr. Member
Sep 19, 2009
3,754
3,215
273
If it were the HV supply, the fuse would blow on power up. Same thing for the screen supply. While the screen voltage is switched on during TX, the supply is already energized. Just the output is being switched. You more than likely have lost the bias supply. Common problem when overdriving these amps.

See that 50 watt stud mounted zener diode that is not mounted? Check it to be sure it's not shorted. That is the diode for the bias regulation. If this diode shorts, it will remove bias voltage and cause the tubes to draw excessive current on key up. One of the few cases where a loss of voltage can cause an increase in current.

This is an area where tubes are much different than transistors. If you remove bias from a transistor amp, it will draw a little less current. If you remove bias from a tube amp, it will draw a lot more current. If you remove bias from a tetrode tube amp, it will draw extreme current to the point of saturation. Bias for tubes is a negative voltage that holds the tubes back from drawing too much current.
 
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Shockwave

Sr. Member
Sep 19, 2009
3,754
3,215
273
There is some interesting background information on these amps. Johnny Frank, Dolomite, Cocaine Charlie, Tennessee Walker and about five other guys all copied the same flawed circuit starting in the late 1980's and now there are hundreds of them out there. They all had this bias issue if you drive them with too much power. Many also had plate transformers that were way to high in voltage and would arc the tubes.

It was almost funny to watch each builder attempt to deal with the bias zener diode shorting as they always overlooked the root of the problem. There is very little current being drawn by the bias feeding the control grid. It's a high impedance circuit. Even a small 5 watt zener can handle this current. So why do we see the biggest zener diode you can buy still failing sometimes?

It's not failing from DC current, it shorts out from RF getting across the zener diode. Way back in the beginning, someone forgot to install a proper choke in the DC bias line off the control grid so that the blocking cap has no place to drop the RF. Just about everyone after that copied the same mistake. Now if you feed a little too much RF into the amp, it shorts the bias zener and blows the fuse. Sometimes it can ruin the tubes too.

This lack of a proper choke in the DC bias line causes another problem. It's loading the RF drive feeding the control grid of the tetrodes. This reduces gain and wastes drive power as the RF heats the zener diode. I pointed this out to one of these builders many years ago and he told me "That's so they can't overdrive the amp and blow the tubes up". My response was "So if we get the tubes drawing lots of current by driving them hard, it's OK to let the bias fail under this condition and approximately double that current?" The look on his face was better than his response.
 
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Shockwave

Sr. Member
Sep 19, 2009
3,754
3,215
273
Replace the zener diode shown inside the red circle. Add a 10 uh RF choke in series with the wire that has the red arrow pointing towards it where it connects between the two resistors. Then the RF drive will just be feeding the tubes instead of the bias circuit at the same time. This assumes you've checked the zener and found it to be shorted.
 

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dxChat
Help Users
  • No one is chatting at the moment.
  • @ ButtFuzz:
    on the connector with the green wire, if you short that to a nearby black wire, the power supply will turn on. (for use as a bench supply or whatever)
  • @ ButtFuzz:
    And IIRC, the purple wire is -12VDC, but that is from memory and it does not serve.
  • @ ButtFuzz:
    So if you need, say, 7v, connect + to the Yellow wire and - to the Red wire.
  • @ ButtFuzz:
    or or you need 9v connect + to the yellow wire and - to the orange wire. note: not all PSU's have the orange wire nowadays, as many manufactures had the voltage regulation on the motherboard, and all the power supply gives is 12 vdc
  • @ BJ radionut:
    Thanks ROB will check it out!~!! got to clean the work bench a little make room for that big case :)