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Author Topic: Mixing old and new tubes  (Read 3205 times)
W5DQ
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« on: May 17, 2012, 12:51:50 PM »

I have an AL-811 3-tube amp which has 572B's in it. It had been running fine up until a week or so ago when the max power out rolled off to just under 300 watts. It appears to still have a clean signal from looking at the monitor scope and on-the-air reports. I haven't cracked it open yet to see what the problem might be but I suspect possibly one of the tubes has gone to the tube graveyard in the sky Smiley There was no smoke, no burned smell (unlike when one of the 811A died and took out the original VHF parasitic trap resisitor - POP! and stink), no 'snap, crackle or pop' notice before the power roll off. With the cover in place and amp situated at its present location, I can't see the third tube (one farthest inside the amp) to tell if it is lit of not. Other 2 are lit up and look correct as far as colors go. Operationally, the Ip at full out was running around 450mA and now it is just shy of 300mA with the Ig around 125mA or so, down from around 210mA before.

If it turns out to be the case of a dead tube and no other problems, I was wondering if anyone has had any experience in replacing just a single NEW tube in a set of 3 that has had several hours on them. I suspect there is the risk of a NEW tube may cause the old tubes to fail but have no empirical data to back up that suspicion. 

I have no problem in replacing all three 572B's but if not required, I'd like to keep the costs down thus the question here.

Tnx in advance for any info,

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
WB2WIK
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« Reply #1 on: May 17, 2012, 02:30:06 PM »

If it is a bad tube, I'd have no hesitation replacing just that one.

I can't think of any reason one new good tube should harm the other two older ones.
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W8JX
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« Reply #2 on: May 17, 2012, 02:57:43 PM »

If it is a bad tube, I'd have no hesitation replacing just that one.

I can't think of any reason one new good tube should harm the other two older ones.

I see no problem either. I would suggest same type of tube as others though (meaning Chinese or Cetron)
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
W5DQ
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« Reply #3 on: May 17, 2012, 02:59:31 PM »

If it is a bad tube, I'd have no hesitation replacing just that one.

I can't think of any reason one new good tube should harm the other two older ones.

Tnx for the response Steve. I wasn't sure if that was a sound way to go or not. Thought maybe there would be interaction that might be bad on the others but after thinking about it, each tube is going to operate on its own so probably no bad juju there. If that is the problem (plan on opening it up this weekend), I'll probably order a couple tubes from RF Parts and fix it. The 572Bs came from RF Parts about 3 years ago. Not sure why one would go belly up now but I do run the little amp a bit harder than I probably should, even though still with a clean output signal. Maybe time for a bigger amp Smiley

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
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« Reply #4 on: May 17, 2012, 03:31:33 PM »

The lifetime of tubes varies all over the place.  In my SB200 I had installed two new
572B's from RF Parts and after 2 1/2 years the filament in one burned out and the other was
just fine.   The original tubes were Cetrons and had lasted about 25 years.  Alas Cetrons
(NOS) are just about unobtainium these days.  There are tales of Chinese tubes lasting for
several years though I think most have shorter lives  from practically zero to a few years.
You pays yer money and makes yer choice and hope they last.

Allen
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KH6AQ
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« Reply #5 on: May 17, 2012, 04:39:11 PM »

If you want to see if the 811A tubes are sharing current reasonably well key down until the plates show some color. If the plate of the new tube is considerably cooler or hotter it's not a good match.
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W5DQ
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« Reply #6 on: May 18, 2012, 08:57:35 AM »

Went home last night and was going to try and work 6O0CW with my crippled amp. Could not find them anywhere on the band or cluster. Called a retired buddy who is on the radio alot and he told me they were not on. So after a while of being pissed off and upset that I had missed ANOTHER one (besides 7O6T) that I needed, I decided since the bands were dead, I'd just see what happened to the little Ameritron amp.

Took it offline and took the cover off, bypassed the interlock (don't try this at home boys and girls, I am a professional Smiley hooked up the exciter and a dummy load and fired the ol girl off to see if I had fire in all the bottle. NOPE!!! #1 bottle was dark and cold. Ah ha! Just as I suspected .... a bad tube. So I shut it all down, bled the HV (they don't call it a "chicken stick" for nothing) and started to pull the dead bottle when I notice a little fuax pax in the heater wiring. The heaters are wired in parallel and just so happens that the dead socket and the one next to it are wired with stiff wire while the 3rd bottle has actual flex wiring going to it since it is offset a little more and they had to wire around the #2 socket to get to #3. One of the stiff wires had come loose. Looked like a bad (crystalized) solder joint. Upon further inspection, I found the stiff wires had been put thru the solder lug hole on the #1 (dead) tube socket but just tack soldered on top of the #2 solder lug for the heater connection. One of the tack solder points had come loose and the wire was tilted up like it had been under a small amount of tension. That was the culprit. When I put the new wiring in, I made sure both ends were sucure and there was NO tension on the wiring.

Long story shortened, after about 2.5 hours of surgery the patient awoke and was capable of spewing 600W RF peak (with 75W drive) just like before. Checked all the vitals and all was well with Ip @ 450mA, Ig was 180 mA and HV holding steady at 1600V under load. Held the key down for 45 seconds to allow all the new solder joints to get toasty warm and no problem. Let her sit for a minute or so and did another 45 sec to a minute of keydown. Did this about 5 times and all appeared well. Buttoned the old girl up, gave her a peck on the cheek and reinstalled her back in service.

Now if I can't work'em I'll have to find something else to blame it on Smiley

Thanks for all the inputs. Glad I didn't need to buy new tubes right now.

Gene W5DQ

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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
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« Reply #7 on: May 18, 2012, 10:33:34 AM »

Aha!  You got an Ameritron "semi kit."

I've worked with those before.  To make it more fun, they do not provide any pointers about exactly which parts have been left unsoldered and stuff.  I appreciate that, as too much information can make us lazy. Wink
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W5DQ
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« Reply #8 on: May 18, 2012, 11:52:28 AM »

I just glad I have oodles of kit buidling experience and many, many hours of bench tech work to help solve these little puzzles. I worked my way thru college as a bench tech for the Heathkit store in Dallas, TX back in the 70's. Seen the insides of way too much Heathkit ham gear so I've seen the good, the bad and definitely the UGLY in solder jobs. I especially loved the 'drip' method of soldering, where the builder would let the solder actually melt and drip onto the connection. Those were way too much fun to fix.

I could see someone having this sort of problem and either not knowing how to troubleshoot/fix it or worse getting in there and getting hurt as I had to nearly gut the tube section to get under the riser mounted tube sockets to repair the wiring. Had to remove the plate VHF suppressor circuit board, HV choke, some of the chassis like RF shield between the tube compartment and the RF input tuned circuit back section. Not hard to do but definitely not for the faint of heart towards equipment repair.

No matter how much I ensure the HV is down and out, I still get uneasy sticking my hands into the HV section of amps, TV and such. Just too much respect for Mother Nature I guess Smiley I once pulled the 25Kv 2nd anode socketed style power connector from a POWERED UP X-Y vector monitor I was working on. I looked at the power cable and got it mixed with another that was unplugged and also checked the power switch which was down in the OFF position or would have been except it was UP in the ON position cause the unit was sitting up side down. I stood there with a nice 2-3" blue lightening arc drawing across the gap, me frozen like a statue but not getting shocked or fried. My work mate got a wooden stick and killed the power to the unit. I went to clean my shorts  Shocked

73

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
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« Reply #9 on: May 18, 2012, 02:58:51 PM »

That's what's called MFJ Quality Control.  They've had lots of problems with that issue and it was getting worse rather than better.  I heard via the grapevine that MFJ hired a well-known ham to work with them for a few months to see what can be done with improving the QC of their products.

BTW, with regard to replacing just one tube, although I realize it's a moot point now.  I've never had a problem doing that with 3-500Zs, 572Bs, 3CX400s, and 3CX800s.  In fact each time I replaced a single tube the others were several years older.
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