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#1
Posted to rec.audio.tubes
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EL34 output tube balancing?
Hi,
I have just dusted of a Heathkit AA-121 that I remember sounded pretty good when I last used it. I used a variac over several hours to get it warmed up and used the rear bas-bal to balance socket to balance the output tubes. One channel was rock solid and stayed at zero volts, the other channel would slowly rise to .1 volt for a second ot two and then slowly return to zero and repeat. I removed the VOM, hoping that it would steady itself over time, no such luck. There was a muted "pop" and when I inserted the VOM the balance was way off zero volts and could not be adjusted back to zero, also, AC voltage was present across the bas-bal socket which was not measured on the good channel. I did an output tube change from one channel to the other and the results was the same. Any hint at what could be the problem would be appreciated. Thanks, Combover |
#2
Posted to rec.audio.tubes
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EL34 output tube balancing?
Hi RATs!
Prolly a leaking coupling cap. Buy a pair and replace both at the same time. Happy Ears! Al |
#3
Posted to rec.audio.tubes
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EL34 output tube balancing?
combover wrote
I have just dusted of a Heathkit AA-121 that I remember sounded pretty good when I last used it. I used a variac over several hours to get it warmed up and used the rear bas-bal to balance socket to balance the output tubes. One channel was rock solid and stayed at zero volts, the other channel would slowly rise to .1 volt for a second ot two and then slowly return to zero and repeat. I removed the VOM, hoping that it would steady itself over time, no such luck. There was a muted "pop" and when I inserted the VOM the balance was way off zero volts and could not be adjusted back to zero, also, AC voltage was present across the bas-bal socket which was not measured on the good channel. I did an output tube change from one channel to the other and the results was the same. Any hint at what could be the problem would be appreciated. Blown selenium rectifier, perhaps. Replace with 1N4007. Check associated electrolytic caps too. I looked briefly for a circuit diagram but couldn't find one. cheers, Ian |
#4
Posted to rec.audio.tubes
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EL34 output tube balancing?
Thanks Guys,
I fired up the amp after replacing 2 x .047mf coupling capacitors, I must have changed them before and had 2 spares. The gentle pop I heard the first time, happened again and it was was a flash within one of the output tubes which blew the fuse immediately. I found another fuse, 3 amp rather than 4amp SB and it blew again I'm starting to remember that I was having trouble with the amp years ago, the bias voltage was difficult to adjust, the VM would do nothing then just go off the range and one tube would glow like Rudolph's nose. Thanks, Combover wrote: Hi RATs! Prolly a leaking coupling cap. Buy a pair and replace both at the same time. Happy Ears! Al |
#5
Posted to rec.audio.tubes
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EL34 output tube balancing?
I would rather suspect that a shorted tube element is causing this. I
had the same thing happen with my Dynaco ST70. New tubes cured it. |
#6
Posted to rec.audio.tubes
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EL34 output tube balancing?
Oops, forgot to mention that those old Heathkit printed-circuit board
amps were infamous for burning the board traces and components around the output tubes, so check those carefully, too. An AA-121 is a mighty nice sounding amp when it is working right. |
#7
Posted to rec.audio.tubes
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EL34 output tube balancing?
Sometimes in threads like this, when the OP goes AWOL, I wonder if he
has electrocuted himself. Or become demented from selenium poisoning... "*******s...you killed my son" Combover, if your bias supply uses a selenium rectifier, please change it for a 1N4007. It is wise to do this whether it has failed or not. If it fails to a short, it may make a soft pop as it releases a puff of poisonous fumes. Then, if you escape dementia, you may measure AC on the bias and watch the following electrolytic cap explode with a rather louder pop, as your valves glow and your output transformer bursts into flames, etc. As you brought the amp up to voltage, you really should have monitored the bias at the grids, or the current through the valves. It helps a lot if you use a metered variac, or an ammeter in series with an unmetered one. Preferably analogue. cheers, Ian |
#8
Posted to rec.audio.tubes
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EL34 output tube balancing?
"Ian Iveson" said:
Combover, if your bias supply uses a selenium rectifier, please change it for a 1N4007. It is wise to do this whether it has failed or not. In addition to this good advice, observe the polarity of this diode very careful, the resulting voltage has to be *negative* with respect to ground! Also, use a 47 ohm/1 watt resistor in series with the 1N4007 to compensate for the losses in the former selenium rectifier. -- "Due knot trussed yore spell chequer two fined awl miss steaks." |
#9
Posted to rec.audio.tubes
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EL34 output tube balancing?
Ian, thanks.
I blame my dementia on the coating they put on the inside of beer cans! The Selenium diode looks to have been replaced and it looks nothing like the one in the construction book. The replacement is a pea sized, round ceramic looking thing. I purchased a pair of Yorks about 6 years ago and have only listened to them through a SS Intergrated amp even though I been meaning to try them out with some tube stuff. I also have tucked away 2 Heathkit W-5Ms and a Quad II outfit so perhaps I should start looking at these? Combover Ian Iveson wrote: Sometimes in threads like this, when the OP goes AWOL, I wonder if he has electrocuted himself. Or become demented from selenium poisoning... "*******s...you killed my son" Combover, if your bias supply uses a selenium rectifier, please change it for a 1N4007. It is wise to do this whether it has failed or not. If it fails to a short, it may make a soft pop as it releases a puff of poisonous fumes. Then, if you escape dementia, you may measure AC on the bias and watch the following electrolytic cap explode with a rather louder pop, as your valves glow and your output transformer bursts into flames, etc. As you brought the amp up to voltage, you really should have monitored the bias at the grids, or the current through the valves. It helps a lot if you use a metered variac, or an ammeter in series with an unmetered one. Preferably analogue. cheers, Ian |
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