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Old 12th May 2003, 03:36 AM   #1
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Default Buzz, then blown fuses

Hi all,

This week I finished building my first guitar amp, and now I'm in the trouble-shooting phase.
It's a modified JCM800 100W head circuit design.
I added a gain stage and an effects loop, and modified the e.q.
The problem is:
When I first turn the amp on, the tubes heat normally and I get noiseless guitar tone. But as the tubes keep heating, a buzz appears. After about a minute of being on, the 750mA fuses on the secondaries to the rectifier blow.
I should mention, the rectifier is solid-state with two .1uF caps for filtering.
Another thing I noticed is that the 300-0-300V tranny I bought from Hammond actually produces 330-0-330V. Could this cause the fuses to pop?

Any thoughts?
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Old 12th May 2003, 03:56 AM   #2
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Default RE:Buzz, then blown fuses

Hi,

Quote:
Another thing I noticed is that the 300-0-300V tranny I bought from Hammond actually produces 330-0-330V. Could this cause the fuses to pop?
It surely could.
You could try some series resistance before the rectifier to limit inrush current and lower the HT supply a bit.

Cheers,
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Old 12th May 2003, 04:13 AM   #3
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Thanks,
I'll give it a try.
What do you think about the buzz, and the fact that it takes almost a minute before the fuses pop?
Tim
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Old 12th May 2003, 06:43 AM   #4
Duo is offline Duo  Canada
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Your tube might be going into runaway and causing great current to flow from the power supply. This can pull the capacitor down enough that you'll hear hum in the output. It can take a while for the fuse to blow. Are you sure the bias is set correctly? It seems like the tubes are warming up until they hit a sort of threshold point and turning on hard.

Put a current meter in the cathode circuit of the output stage and see what happens as the amp warms up. See if the current flow increases sharply when the buzing starts. If it does, this means the tubes are turning on somehow and drawing too much current... Check out your wiring.

I can see the 330-0-330 causing changing numbers and that but not really throwing the tubes into runaway unless the bias is maxed out or something.
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Old 12th May 2003, 09:01 AM   #5
dhaen is offline dhaen  Europe
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Hi,

750mA Are big fuses. Presumably they are that big to cope with a huge inrush current to the smoothing circuit. All the same, for the valves to take this much current (1.5 amps) means a bias problem, as Duo has intimated. Alternatively it could be rectifier breakdown. A link to the schematic would help the cause.

Cheers,
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Old 12th May 2003, 08:56 PM   #6
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Sure,
I just posted it at

http://www.geocities.com/tduryee2001

I've managed to figure out why the fuses were popping.
The two 10uF caps on the way to the bias pots were in backwards (I had assumed that negative would go to ground).
I'm guessing this explains the runaway current you were talking about.
However, the buzz is still very audible.

Any ideas?
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Old 12th May 2003, 11:54 PM   #7
dhaen is offline dhaen  Europe
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Hi,

There are things on the schamatic that make no sense.
Are you sure it has the 1K's from the grids? I think it is a drawing mistake

Check that there is equal current through each output valve. You can do this by measuring the voltage across the 1 ohm resistors.

Cheers,
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Old 13th May 2003, 04:28 AM   #8
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Whoa,
Good call. Wrong tubes even (my editor doesn't have EL34s).
The 1k resistors are right but the tubes were wrong.
Check it again (it's updated).

http://www.geocities.com/tduryee2001/

Thanks,
Tim
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Old 13th May 2003, 05:25 AM   #9
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I forgot to mention:
Yes, I've measured the voltage drop across the 1 ohm resistors and balance them
~Tim
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Old 13th May 2003, 10:03 AM   #10
dhaen is offline dhaen  Europe
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Tim,

That makes sense now
OK, so they're balanced.
Is the buzz affected by the volume control?
Can you measure the ripple on the following points:
B Output stage supply
C Screen grid (g2) supply
You can use a scope to measure, or a digital voltmeter set to the highest AC range.

Cheers,
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