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Old 30th October 2012, 06:27 AM   #1
scrimpu is offline scrimpu  United States
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Default homebrew 18 watt problem

I have an old hammond amp that I am converting for guitar, the amp worked before but now I only get sound for maybe a second after it warms up then the sound fades out to nothing. If I turn it off for a second then turn it back on I get another second or so of sound then nothing, any ideas?
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Old 30th October 2012, 07:02 AM   #2
Phildog is offline Phildog  United States
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I'm assuming that when you say it "worked before", that you were getting guitar sounds and the amp worked properly. If that is the case, check your preamp tubes one by one. Try swapping them until the problem goes away. It sounds like one could have a bad heater.
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Old 30th October 2012, 10:10 AM   #3
DF96 is offline DF96  England
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Look for missing or open-circuit grid leak resistors. 'It works for a few seconds' is a classic symptom of this problem. Every valve grid must get a DC supply from somewhere.
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Old 30th October 2012, 04:52 PM   #4
kevinkr is offline kevinkr  United States
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Moved to Instruments & Amps where all guitar amplifier related threads belong. Note sub headers in Tubes / Valves and Instruments And Amps.
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Old 30th October 2012, 05:09 PM   #5
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Quote:
Originally Posted by DF96 View Post
Every valve grid must get a DC supply from somewhere.
Not always it is true. In a work called "Some augmented cathode follower" the author explain that if you leave the grid without DC return, the positive and negative grid currents cancels out, and then the tube gets his own bias.
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Old 30th October 2012, 06:08 PM   #6
DF96 is offline DF96  England
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No, you still need a grid resistor even if it is quite a high value (e.g. 10M). The reason is that if for some reason the grid happens to acquire a high potential then secondary emission could push it even higher. A grid resistor helps counteract this.
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Old 30th October 2012, 06:29 PM   #7
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Again no! If there exists grid emission, then it will be sometime it will have a so large positive charge in which future emission will be impossible.
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Old 30th October 2012, 06:38 PM   #8
DF96 is offline DF96  England
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I said secondary emission. The higher the positive charge on the grid the greater will be secondary emission as electrons will strike with greater momentum so more likely to cause secondary emission. A grid can be coated to suppress secondary emission, of course, but keeping its voltage low will always help.
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Old 31st October 2012, 03:49 AM   #9
scrimpu is offline scrimpu  United States
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Turns out there was a problem in the PI, not sure what it was but after changing it back from LTP to the original cathodyne all is well.
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