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No Bias Votage on 12AX7
No Bias Votage on 12AX7
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Old 30th October 2015, 04:30 PM   #1
Electrobumps is offline Electrobumps  United Kingdom
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Default No Bias Votage on 12AX7

Hi

I have a Bassman build and I made some adjustments to the tone stack and the channel has just stopped working.

The only fault that I can find is that I have no bias voltage on the second stage of the 12AX7 V1. The tube is fine and I read correct bias voltages when in other positions and the 1.5K to ground reads okay.

I do not fully understand how this bias voltage works. Any ideas on how to track this fault?

thanks

Last edited by Electrobumps; 30th October 2015 at 10:28 PM.
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Old 30th October 2015, 04:38 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check the anode voltage and load resistor and check the grid leak resistor.
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Old 30th October 2015, 07:10 PM   #3
Electrobumps is offline Electrobumps  United Kingdom
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Thanks for your reply.

Anode is 123v.
Grid Load Resistors measure 100k on DMM
The Grid goes to ground via a 250K pot (volume)
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Old 30th October 2015, 09:34 PM   #4
Enzo is offline Enzo  United States
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Look inside the tube, are BOTH heaters glowing inside that tube?

Did you measure the 1.5k resistance from the cathode pin on the socket to chassis? or did you measure it right at the resistor? In other words did you check the part or the circuit?
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Old 30th October 2015, 10:28 PM   #5
Electrobumps is offline Electrobumps  United Kingdom
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Both heaters are glowing and resistance is good measured to the chassis.

If turning volume pot it is very noisy.
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Old 30th October 2015, 11:02 PM   #6
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Cathode bias works by the current through the cathode resistor (1.5k) raising the cathode voltage a bit above ground (say 2V). Since the grid should be at ground (0V), when there is no signal coming into it, this means that the bias voltage from grid to cathode is (say) -2V.
The current coming through the cathode resistor (in normal circumstances) is equal to the anode current. So you can measure the voltage drop across the anode resistor and use Ohms Law to calculate that current.
You can measure the voltage to ground (at the grid) to confirm that it is very close to 0V.
A very noisy vol. pot suggests that there is significant DC current going through that pot (there should either be zero DC current or an extremely small current through it).
By the way, which Bassman schematic should we be looking at?
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Old 31st October 2015, 01:59 AM   #7
Enzo is offline Enzo  United States
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Maybe I was confused. The bias would be a positive voltage on the cathode, a volt or two. If you were looking for a voltage on the grid, then no. I had assumed you did not have voltage on your cathode.

Bias is the relationship between cathode and grid. The grid could be negative wqith a grounded cathode or the cathode could be positive with a grounded grid. Either way, the grids is more negative than the cathode, and the difference is the bias.
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Old 31st October 2015, 10:17 AM   #8
Nigel Goodwin is offline Nigel Goodwin  England
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Quote:
Originally Posted by Malcolm Irving View Post
A very noisy vol. pot suggests that there is significant DC current going through that pot (there should either be zero DC current or an extremely small current through it).
I agree, which would totally mess up the bias on the triode, perhaps the OP would care to post his exact modified schematic?.
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Old 31st October 2015, 10:28 AM   #9
Electrobumps is offline Electrobumps  United Kingdom
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Thank for your replies and information.

The clear light of day and some sleep has helped. I had miswired the grid and the cathode. Blind to the obvious in this case.

This happened while chasing the initial fault and rewiring V1. The fault I still have is bass pre amp is just much quieter than the guitar. Tone sounds normal and fault is still present when I bypass the tone stack by connecting the 500p directly to the volume pot.

The Schematic it is built from is AA165
The tone stack on the bass ch is AA865

Last edited by Electrobumps; 31st October 2015 at 10:38 AM.
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Old 31st October 2015, 11:51 AM   #10
Malcolm Irving is offline Malcolm Irving  United Kingdom
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The bass channel has an extra triode, which should make it louder. Have you tried a different 12ax7 in that position, and checked the DC voltages around that extra triode?
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