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Stumped again?  Ampeg J12T cathode bias.
Stumped again?  Ampeg J12T cathode bias.
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Old 4th May 2018, 03:47 PM   #1
CapnDenny1 is offline CapnDenny1  United States
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Default Stumped again? Ampeg J12T cathode bias.

I have an Ampeg J12T that has an issue. It has a cathode biased EL84 class AB output stage. It distorts at really low volume. Signal is good up to the PI stage. After that at around 1W or so output it starts to distort really badly.

I measured the DC voltage at the 120 ohm cathode bias resistor at 9.9V, as the schematic says. However when I read the current level using an in circuit probe inserted below the tube in an extender socket, I read 13mA in each tube.

.026A x 120 ohm = 3.12V not 9.9V ?

If I use the ohmmeter on the resistor it measures 120 ohms.

How can ohms law be violated? I mean it's the law!

This amp had one side not working for a while, and only 1 tube was conducting. So the OT had an unbalance DC current through it for a while and could have damaged the OT by being way out on B-H curve and going into saturation.

But even that doesn't explain the failure to obey ohms law?

Has anyone seen a 5W power resistor that ohmed out good, but when you put 20mA through it, the resistance increased?

I have new resistors on the way, but I suspect that won't be the issue.

I am going to remove the tubes and see if there is still a voltage on the cathode resistor? There must be some other path for current besides the tube.

Click the image to open in full size.j12t by Dennis Kelley, on Flickr

Last edited by CapnDenny1; 4th May 2018 at 03:58 PM.
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Old 4th May 2018, 04:05 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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9.9v across 120R = (V/R) 82.5mA, including the screen grid current.
Only half conducting will not usually affect the transformer.
Check the anode load resistors on the phase inverter.
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Old 4th May 2018, 04:19 PM   #3
CapnDenny1 is offline CapnDenny1  United States
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But I only measure 13mA per tube with a current meter in series with the cathodes.

Testpoints 2 and 3 (The anodes of the PI tube) look correct, according to their chart.
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Old 4th May 2018, 04:37 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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They may look correct, check the load resistors.
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Old 4th May 2018, 05:39 PM   #5
CapnDenny1 is offline CapnDenny1  United States
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You mean the 220k past the coupling caps?

I donít see how tgat can affect the cathode current or voltage of thd output tubes?
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Old 4th May 2018, 06:49 PM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Anode load resistors, R15 and R16.
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Old 4th May 2018, 08:37 PM   #7
CapnDenny1 is offline CapnDenny1  United States
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OK. I re-read my post about this from 2 years ago. The ohms law issue is the fact that the ammeter adds resistance to the reading. So with the ammeter removed I can reply on the voltage reading across the bias resistor. It doesn't tell me how much is on one side vs the other.

I don't think that is the problem.

So i measured the resistance of the primary windings of the OT. One side is 105 ohms, but the other side is 144 ohms. that sounds like a BIG difference. I suspect I have some shorted windings on the 105 ohm side.

There is no short the the ground or from primary to secondary. But that seems like too much imbalance. That was measured between the windings with the rest of the circuit disconnected.

So i guess I am ordering a OT for my J12T.
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Old 4th May 2018, 10:48 PM   #8
PRR is offline PRR  United States
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Quote:
Originally Posted by CapnDenny1 View Post
...OT. One side is 105 ohms, but the other side is 144 ohms. that sounds like a BIG difference. ...
No, it's not. Even 40% diff is normal. The 40 Ohms diff is "nothing" in a 8K load.

They are different because they wind one half primary, secondary, other half primary. For the *same* number of turns each half, the outside winding MUST be longer; and thus higher resistance. But ideally resistance is "small" compared to load, and resistance unbalance is lower.

You could have shorted turns. But I've seen far too many cases where someone spent for new iron and shipping, and it wasn't the OT.

I'd strip the 120r and install two 220r one per cathode. 1/2 Watt resistors won't smoke in an hour, and you just want a quick cross-check on resistor(s) and each tube individually.
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Old 5th May 2018, 07:52 AM   #9
CapnDenny1 is offline CapnDenny1  United States
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I would also have to use two cathode bypass caps. I could just put a 1 ohm resistor in series with the cathode. It may not be wired in a way to allow that to be done.

I can measure the voltage drop across the primary windings. If I know the resistance I can calculate the current.

If it is not the ot then I don’t what else to look at?
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Old 5th May 2018, 11:03 AM   #10
Parafeed813 is offline Parafeed813  Netherlands
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Is there still 9,9V across the 120 ohm resistor when you connect the probe adapter between socket and EL84?
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