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Old 25th February 2013, 10:35 PM   #1
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Default Low output from two 6L6's for no apparent reason?

I recently bought a new Bell Carillon PA to restore. It has two good RCA black plate 6L6's, three 12AX7's, and one 12AV6. I knew it would need a filter cap job, so I went ahead and turned it on to see whether or not it had any other problems, and sure enough, cranked, it's giving me a pathetic amount of volume. No other problems; the master volume still affects the gain, the mic knobs do the same, and it sounds sorta like distortion from unsaturated power tubes. Should I be worried about it or should a cap job fix this? What could potentially be the problem otherwise?
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Old 26th February 2013, 04:59 AM   #2
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recap it and expect to change the 12ax7's.
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Old 26th February 2013, 05:50 AM   #3
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Is that the Bell Carrillon 6060 which came out in 1959?? You powered it up first without re-capping it? OOps! Low volume would be one of your least worries. Re-cap it now......then try it, not the other way around.


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Old 26th February 2013, 11:57 AM   #4
DF96 is offline DF96  England
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Replace or reform electrolytics. Replace leaky paper capacitors (which may be all of them). Check DC voltages. Until it is fully repaired, don't let it run for more than a few minutes at a time and never unattended.
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Old 26th February 2013, 12:43 PM   #5
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When recaping, try to do in a reduced group of say, 2 or 3 units and power the set between them, so if anything goes wrong, you can go back. Also use a incandescent lamp in series with any of the power line cord, so if a short circuit or a reversed cap, the lamp will bright powerful meaning an error has happened. The lamp, in this case acts as a current source protecting the equipment. Keep attention to extraneous sound on the speaker and or the light in the tubes.

The lamp or group of them will be about 5 times the power drained by the DUT.
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Last edited by Osvaldo de Banfield; 26th February 2013 at 12:47 PM.
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Old 26th February 2013, 12:58 PM   #6
chd_diy is offline chd_diy  United States
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I had an amp recently that lost most of it's output, and sounded really distorted.
I changed the coupling caps, but it still sounded the same.
After more testing, I found that the plate resistor for the input tube had gone bad.
Changing that resistor totally cured the problem.

I would recap the entire amp, and if you still have the problem, look for a bad resistor somewhere.

Good luck.
Chris
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Old 26th February 2013, 12:58 PM   #7
DF96 is offline DF96  England
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Yes, good advice (post 5).

Last edited by DF96; 26th February 2013 at 12:58 PM. Reason: clarify
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Old 27th February 2013, 12:33 PM   #8
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Quote:
Originally Posted by DF96 View Post
Yes, good advice (post 5).
Thanks, DF!. Actually I'm 43, but from the secondary school (Here known as ENET's, Escuela Nacional de Educación Técnica, National School for Technical Education, some years ago disappeared), when in 4th year (To complete was 6 years), a professor say to use always in a doubt, a series lamp.

This actually includes a three phase test board here in my job, in where there is a 100W lamp in each phase, shunted by a switch which permits to normal voltage and current flow into the DUT, but in case of doubt, always first is triyed in series.

This avoids or almost reduces risk of personal and equipment injuries.
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Old 27th February 2013, 12:54 PM   #9
Tesla88 is online now Tesla88  Italy
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I always use the series lamp trick expecially for PA amps , the trick save me from replacing all the ouput devices anytime a fault occour during repair
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Old 27th February 2013, 10:53 PM   #10
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Thanks you guys! I have the new filter caps in place and will solder them in after I get back from New York. The coupling caps are all good, I tested all of them for voltage and resistance and they're all good. A few resistors are out of spec so I pulled the tubes and I'm replacing a few of them as well. I'll tell you all how it goes!
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