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Old 9th July 2011, 05:31 PM   #1
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Question 5E3 Build Problems Cont'd

Well after much more head scratching and rebuilding I am still having major problems with my 5E3 project.
Allthough audio is clearly going through the amp it has a terrible hum with volume and tone controls all the way down.
If you turn up the volume the hum gets much louder and you get a loud screech. Same if you try turning up the tone control.
Grounding pin 2 of the 12AX7 silences the amp.
Grounding the input does not.

The DC voltages are:
6V6's : P 378 vdc K 20.4 vdc S 325vdc G .3 (both tubes)
12AX7 : P1 142vdc G2 0 K3 1.3vdc P6 147vdc G7 88.2 vdc K 89vdc
12AY7 : P1 95vdc G2 0 K3 1.1vdc

Should I be seeing a positive voltage on the grid and cathode of the PI ?
I have tried (everything) substituting 12AX7 and it has no effect.
These are all new or suspected good components and the circuit seems correct.
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Old 9th July 2011, 05:43 PM   #2
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There is positive feedback somewhere in the amplifier.
One of the circuits after the volume/tone control circuits is feeding back into one of the circuits before the volume/tone control circuits.
Perhaps the problem is the layout of the amplifier.
It's important to keep the low-level input signals away from the high-level output signals.
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Old 9th July 2011, 07:20 PM   #3
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Frank.
The components are much closer together than standard as I built using an
existing chassis.I could try moving the 12AY7 away from the rest.
What about the positive voltage issue?
Is this (88vdc)normal bias for the PI side of the 12AX7?
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Old 10th July 2011, 01:09 AM   #4
jjman is offline jjman  United States
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Yes there should be DC on the grid of the PI. I don't know the expected figure but it sounds high. I'm not finding the expected voltage for a 5e3. My cathodine PI 12ax7 in my non-Fender non-6V6 amps shows 260 for the plate, 17 for the grid and 21.5 for the cathode. I'm sure someone can advise the expected figures for a 5e3.

The noise problem sounds like it is before the PI since grounding the grid of the previous stage stops it. The PI voltages may or may not be related to the noise problem, and may not be "wrong" at all.
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Old 10th July 2011, 04:44 AM   #5
Mr. dB is offline Mr. dB  United States
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Maybe you need to reverse the primary leads on the output transformer?
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Old 10th July 2011, 05:09 PM   #6
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Thanks JJman
Hopefully someone will post with correct voltages at that stage.
I am suspicious that they would be exactly the same and that high.

Note to Mr DB. Reversing the leads only has an effect with circuits with neg feedback. The 5e3 does not. thanks.
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Old 10th July 2011, 05:23 PM   #7
jjman is offline jjman  United States
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The dc on the grid is formed from the ratio and sum of the 2 resistors on the cathode to ground. Check those resistors. The difference you show between the cathode and the grid is 0.8V which is not outlandish on it's own.

Actually you need to measure that grid's voltage by measuring the other side of the 1meg on it. Measuring the grid directly provides a path to ground thru the meter. This results in current thru the 1meg that's not their otherwise. This is significant to the reading, in this particular case. Same with measuring grids on a LTPI.
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