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One tube getting to much current: Audio Innovation  25 W Class A Series 500
One tube getting to much current: Audio Innovation  25 W Class A Series 500
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Old 14th June 2018, 10:34 AM   #1
leiflende is offline leiflende
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Default One tube getting to much current: Audio Innovation 25 W Class A Series 500

I have an old Audio Innovation
25 W Class A Series 500
One of the main tubes (the large left one) is getting to much current and is burning hot. It appears also with new tube. Can something be adjusted/swapped or what is the problem?
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Old 14th June 2018, 11:13 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Location: The Jurassic Coast, England. GB
Check the voltages on the valve. 400v on the anodes and 395v on the screen grids plus 0v on the control grids. If they differ, replace the pair with a matched pair and if the control grid voltage is other than 0volts, with no signal, replace the coupling capacitors, 470nF 400v.
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Old 14th June 2018, 02:19 PM   #3
Alan4411 is offline Alan4411  Wales
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I have repaired 2 of these, I still have one and it is a great amp, they had both cooked their 15 ohm cathode bypass limiter, or worse, because of the green ERO 0.47uF coupling caps leaking.

As John says, start by checking voltages BUT replace the 0.47uF coupling caps if they are as in the picture. The 47k ohms resistors tend to drift high too so do them at the same time. Then check all is still well with the bypass cap and resistors.

I have enclosed a schematic, I think it is in the public domain so is OK to show. Mods, if it is not allowed, please remove it. Alan
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