Safety/how do i stay alive during amp tube r/r

[KC0MJP]

I have a Al811 3 tube that is getting three fresh tubes. am i correct in reading a jumper wire should be connected between the cathode? or top of the tube
to the chassis?? there are three coils(chokes in the
wire)leading to the top of each tube.Or should i just short from each filter cap screw to chassis?? The amp has Not been On(powered up in weeks) Thanks Mark

[N9XTF]

Mark,

You are in dangerous waters here, be very careful.  
First off, look at the power cord three times and make sure it is not plugged into the wall outlet.  (This is not meant as sarcasm! It is very easy to forget!) You need to be sure that the filter caps in the HV power supply are bleed down.  

The top of the tubes are the anode.  There are descriptions for making a 'shorting stick' to dissipate any power by clipping one end to ground and touching the other end to each cap and tube anode.

There are others here that are much more knowledgeable and can give more precise descriptions.  Please wait for their advice before you proceed.

73 - Doug N9XTF

[N9FME]

Mark,
Listen to Dougs advice or better still, it actually might be best to find a local ham who is familiar with amplifier repair or if nothing else, package the amp up and send to Ameritron if you are not sure. They can go through the amp and completly check it for a reasonable fee.

This is not meant to insult your intelligence but it only takes one mistake.

That's my two cents..Good Luck!

Greg
N9FME

[KC0MJP]

thanks to those who have posted, i am standing by for
further instruction, as i have been lit up as a youngster retubing a NC 270 RX it had only 350v stored
 but this one has 1700v which needs a safe path. but not thru Me..

[KE4DRN]

hi,

Never ever work on any amplifier when you are alone,
always have somebody in the area to check on you should you get into trouble.

Having another ham that has experience with this model
amp is a great idea, keeps the knowledge  flowing to the next generation of amateurs.

is this amp from factory or used ?
no matter, always check any safety switches and interlocks.

73 james

[K7GRR]

What everyone else said. plus:
I used to work on megawatt radar systems with 117KV drive.

1)  A shorting stick is your friend.  After touching all the HV locations, leave it connected to one of them while you work.

2) Never work alone

3) Never trust bleeder resistors.  We usually found *something* that went POP when we shorted it.

4) Really - find someone who's done it before to help you out.

[KC0MJP]

thank you guys for your info, the amp was used when i got it. it worked fine till till i powered up w/o
changing bandswitch, which promptly took out a tube
or two... so it's getting new tubes since being tested
otherwise as ok.. btw it's getting 572b's and a nice Large checklist to see to it properly being cared for
you know senior moments,and momentary lapses of reason.
thanks once again kc0mjp Mark

[KE4DRN]

hi


why do you want to use 572b ?

not sure if you have read this,

http://www.eham.net/forums/Amplifiers/7927

73 james

[K7GRR]

Agreed.  572's are, I think, a waste of money.  If keying up on the wrong band blew a tube or two, I think something else is wrong....  too much drive, high SWR, exceeding grid current, tuning technique, something....  if you blew a tube or two, you probably took out a grid resistor or two as well.

How I tune my 811H (four tube version of the 811) (this will generate a lot of traffic, I'm sure hihi)

5 watts in.
plate and load to spec per the manual
Mode: FM
Antenna: dummy load
Key the mike and adjust 'plate' and 'load' for max output.  With practice, this takes five seconds or less.

20 watts in.
Repeat the above, but turn the 'LOAD' a bit clockwise from max output. Watch for high current on the grid.

Key the mike and increase the drive until you hit the max factory specs on either plate or grid.
Tune for max power out with PLATE and LOAD.  They interact a bit, so twiddle between the two for max smoke, then turn LOAD a bit more clockwise, which will reduce the power out by a fraction, but increase your linearity (good) and allow for occasional higher power peaks (SSB).

Now turn the drive down about 10 - 20 percent.  The other end won't tell the difference, and your tubes will last a loooooong time.

Note:  Each tuning sequence should be short - like under ten seconds.  Wait a minute between tuning attempts, let the tubes cool off a bit.

Note also that if you tune into a dummy load and it's SWR is a lot better than your antenna, you may need to retune to the antenna a bit, at least that's been my experience.

If you get 572's, you'll get zero additional power, and move the critical failure point from the $65 set of tubes to the power supply and associated circuits.  I'd rather replace the tubes once in a while than replace the HV circuitry..... plus, you'll be tempted to overdrive them, since they are rated higher.

All of these interact, so it takes practice.  I can tune in about 20 seconds max (worst case) now.  

Really, though, if you blew up tubes, there's a reason:
1) They're really old
2) Tuning process stressed them out
3) They're over-driven

AL-811 should work just fine for years and years if properly handled.

-TimLa
K7GRR
Former USAF GCI Radar tech (radar spelled backwards = radar, we've got you coming and going!)

[HA6SST]

What you need to remember is SIDE.

S witch off the amp.
I solate the amp by removing the power cord and putting it in your pocket.
D ump the high voltage to chassis using a grounding rod through a 100K 5W resistor
E arth the anode caps to ground using clip-on leads.

Now, some other tips.................

1) If you ever watch a TV engineer he will probably work with his left hand in his pocket. It stops you grabbing on with both hands and getting an electric shock across the heart.

2) Never work alone.

3) Some safety gear such as protective goggles or glasses might be a good idea.

4) If you have a variable transformer handy then use this to bring the linear back up the first time.

HA6SST

[KE3WD]

Seriously reconsider the "helpful advice" about using 572b's in there.  

Doing so will not solve your original problem, which is what lunched the 811s in the first place.  

There could be a situation where, if you do the same thing again with the stouter 572b's in there that much more expensive damage will occur to other components in the amplifier instead of just losing the cost of 3 8lls.  

Your amp, your call.  

Many will likely come out of the woodwork and say that this is nonsense.  Many others are likely lurking here just awaiting the possibility, with firsthand accounts of having switched to the 572 without problem, too.  But over the years I've even seen folks tell others that they have run for years without grounded antennas, understand?  


KE3WD

[N2VIN]

Now I have a question since I did the same thing blowing two 811 tubes and having to replace them. I was told to take a long screwdriver with a plastic handle and touch the braided wire connected to the anode cap to the amp chassis. Of course I removed all power from the amp first.

I did as I was advised and I could see the sparks as the voltage discharged. To double check I ran my voltage detector all over the inside of the amp and it did not pick up any signs of voltage. I then removed the tubes and then removed the anode caps. Put the anode caps on the new tubes and seated them back in. While in there I did the 10 meter mod.

I then used my multimeter to check some diodes I was told may have blown but they were OK. Replaced the case and all was well.

Having spent many summers as an electrican's helper I always use tools with rubber or plastic handles.

Am I being unsafe the way I did it?

[KE3WD]

A shorting stick with an appropriate resistance in it can keep you from becoming the guy who welded the screwdriver to the chassis or punched a spotweld into the tube cap.  


And trusting the screwdriver handle only can indeed be a recipe for life threatening disaster if you hand should slip...

[WB1AEX]

When I was a kid I grabbed one of my dad's nice Craftsman screwdrivers while I was stupidly "probing" some microphonic connections in an old AM table radio I was attempting to "repair". Imagine my surprise when I hit a solder joint on the high voltage tap of the power transformer, probably around 175 to 250 VAC. It whacked me quite nicely right through the plastic handle and impressed me enough to never do it again. Live and learn, or not...

I'd definitely stay away from the screwdriver method and use a proper shorting stick with a ground strap.

[K7KBN]

Once things appear to be discharged, it's always a good practice to connect a short piece of wire with insulated alligator clips on each end between the connector on top of the tube and chassis ground.  THEN remove the connector(s) from the tube(s) and proceed with what you were doing.  Don't remove this little piece of wire until the connectors are back on the tubes securely - just before you put the cover back on the amplifier.

Don't forget to remove it, though... ;>)