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Old 12th February 2011, 03:35 PM   #1
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Default SE 2.5k secondary 2w guitar amp

Guy I work with has an amp using 12AU6, 50C5 nd 35W4 tubes and it needs new tubes and should have the caps changed, isolation transformer. Almost at the point of replacing the whole amp so I thought why not go to a different output tube. Not sure what tube to use, want to keep the 2.5k output transformer and still run in between the 100-200v range. Would like to stay with a 7 pin miniature tube and am looking for some suggestions for relatively common tubes.

As an alternative I was thinking of trying a 6AQ5, which normally likes 250-300v and 5k load, and running it at a reduced voltage into the 2.5k transformer. Possibly use a 6AU6 as a preamp tube. Thoughts on that would also be appreciated.
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Old 12th February 2011, 03:40 PM   #2
taj is offline taj
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6GK5 a popular 7-pin preamp tube.

..Todd
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Old 12th February 2011, 04:00 PM   #3
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Not too concerned about the preamp, either going to the 6AU6 (I have 50) or a 12AX7, more looking to find a tube to match the output transformer.
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Old 12th February 2011, 04:27 PM   #4
taj is offline taj
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A quick scan of my GE tube manual shows the following (besides the 6AQ5) for 6 and 12v heaters in 7-pin.

Listed as Va-max, class A operation: Zload for Wout as show in the manual tables.

6AS5-150V/4500Z/2.2W
6BF5-250V/2500Z/1.9W
6CA5-130V/4500Z/1.5W
6CU5-135V/2500Z/2.3W

12A5-180V/3300Z/3.4W
12C5-135V/2500Z/2.3W
12CA5-130V/4500Z/1.5W
12CU5-135V/2500Z/2.3W
12DM5-135V/2500Z/1.9W
12ED5-150V/4500Z/1.5W

Check their datasheets for better info.

..Todd

Last edited by taj; 12th February 2011 at 04:37 PM.
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Old 12th February 2011, 06:13 PM   #5
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That 2.5 KOhm primary O/P trafo would work nicely fed by a 2A3.

The B + needed could come from voltage doubling a Triad N-68X isolation trafo. If sagging B+ under load is wanted, use a 6BY5 dual damper as the doubler diodes. The "taller" B+ rail would be good for a pentode (6AU6) driver, too.
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Old 12th February 2011, 06:26 PM   #6
jjman is offline jjman  United States
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Expected heater resistance/current is also relevant since it is in line with the other tubes and all tubes will see the same level of heater current.

Sometimes there is a resistor in the heater circuit to drop the "remaining" volts. Perhaps that value could be changed if needed.
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Old 12th February 2011, 10:01 PM   #7
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Quote:
Originally Posted by jjman View Post
Expected heater resistance/current is also relevant since it is in line with the other tubes and all tubes will see the same level of heater current.

Sometimes there is a resistor in the heater circuit to drop the "remaining" volts. Perhaps that value could be changed if needed.
Actually I wanted to get away from the series heater thing. Stick to 6v or 12v and just enough B+ to get the job done. This is just a cheap home practice amp, he wants to replace the tubes and caps because they are old and worn and I just do not want him to electrocute himself.

Thanks taj for the list. I'll look them up.

Thanks also Eli for the transformer info. Not too sure about the sag as this is a class A amp though.
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Old 12th February 2011, 10:45 PM   #8
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Looks like the 12CU5 might be the winner. Thanks Todd.
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