No bias reading

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LP Freak
Posts: 3
Joined: Mon Jul 09, 2012 10:00 pm

No bias reading

Post by LP Freak »

Hey guys, first post here.

I've got an issue with a Mojotone 2204 kit I built and could use some help.

I get a reading on all the pins on all the tubes except V4 pin 1. I've gone over the entire amp countless times but just can't seem to figure it out. I've even reflowed all the connections with no luck. They wire their kits slightly different than a Marshall I've noticed too. I do get sound out of the amp also.

Here's a few pics. If anyone has any advice I love to hear it. :lol:

[IMG:1024:768]http://i647.photobucket.com/albums/uu19 ... ild033.jpg[/img]

[IMG:1024:768]http://i647.photobucket.com/albums/uu19 ... ild031.jpg[/img]

[IMG:1024:768]http://i647.photobucket.com/albums/uu19 ... ild026.jpg[/img]
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xtian
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Re: No bias reading

Post by xtian »

V4 is one of your two power tubes? Pins 1 and 8 are grounded, so you should get 0 volts DC.

OTOH, if you're talking about one of the 12ax7s, it's a plate, and you should be reading around 180vdc. Which tube is it?
LP Freak
Posts: 3
Joined: Mon Jul 09, 2012 10:00 pm

Re: No bias reading

Post by LP Freak »

I forgot to mention, I've got a 1ohm resistor going from pin 8 to 1 and to ground on the power tubes so I can get a bias readings off of pin 1.
gingertube
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Re: No bias reading

Post by gingertube »

I assume you mean that you have 1 Ohm from pins 1 and 8 connected together to 0V. If you are not getting any voltage reading across that 1 Ohm then you are not getting any current through the tube.

Checks:
Measure the resistance form pin 1 and 8 to )V rto make sure the 1 Ohm resistor is intact and properly connected.

Swap tubes and see if problem follows the tube, if so then the tube is probably bad.

If the problem stays on the same socket then:
check that the heater circuit is working to that socket, no heater means no current. You can also look at the tube to see if the heater is lighting up.

Check the screen supply voltage to that socket, no screen supply will also result in no current. I've been caught by this one several times.

Cheers,
Ian
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Structo
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Re: No bias reading

Post by Structo »

Is that your bias pot clear over by V1?
Tom

Don't let that smoke out!
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Colossal
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Re: No bias reading

Post by Colossal »

Structo wrote:Is that your bias pot clear over by V1?
Looks that way. It should be on the other side of the board, next to the bias circuit (this is shown correctly on the drawing).
LP Freak
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Joined: Mon Jul 09, 2012 10:00 pm

Re: No bias reading

Post by LP Freak »

Structo wrote:Is that your bias pot clear over by V1?
Yea, they have it drawn under the PT. I moved it to the other side of the board.
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