Need Bias help with mesa mark iv

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stephenphilp

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Went to bias my Mesa mark IV and experienced an issue.

The multimeter had no reading on the two inside tubes (JJ 6l6's). I threw in some Mesa, and other JJ 6l6's, but still no readings. It has to be a socket problem, right? Where Do I start?
 
Check how the Poweramp Switches on the back of the amp are set....if the switches are on Pentode and Simul Class there should be readings on all sockets
 
Thanks for the help. I was on class A, so I switched it, and it did fix one of the tubes which now can be read. But the left inner tube still has no reading. Any Ideas?
 
If you have no current on one tube, it may be due to a loose pin in the socket.

Looking for blown resistors is more than just looking for damaged parts by observation. Resistance check as well as solder joint test may be required (determine if there is an open circuit between the resistor and the solder tab of the socket). The mark IV (at least the one I have) uses ceramic coated flame proof 2W (1K) metal oxide resistors for the screen connections on the outer tubes, 470 ohm 2S metal oxide resistors on the inner tubes (directly connects to 448V supply rail. When the power section is configured as Pentode, the 1K screen resistors connect directly to the Screen (pin 4) terminals of the inner tubes. In Triode configuration, the outer tube screen resistors are switched to connect to the OT. The cathodes on the inner tubes should be connected together with a bus wire (pin 8). When Simul Class is enabled, that bus will be pulled to ground to enable the inner tube current flow. The grid (pin 5) is connected to the output of the PI tube network though 2.2k resistors on each. Note: you will still have bias current present in the tube if the screen resistor is open, however, if the grid resistor is open, there will be no bias voltage present to promote idle current. A resistor may look good by observation, however if you do not measure the resistance you cannot confirm it is any good. Your issue may be a loose pin in the socket, cracked solder joint on the solder tab, open resistor on the grid pin. If you decide to measure the resistance of the screen and grid resistors, keep in mind they are tied to the high voltage supply, verify the supply caps have been properly discharged before doing the measurements.

Before opening up the amp and trying to discover any issues with the components, check your probe for issues (if it is the plug in socket type that fits between the tubes and socket). May have an open circuit on the probe. I would at least check that first. In order to measure current, the probe will have small value resistors on one of the pins for a voltage drop measurement to determine current. If that current sensing resistor is open circuit, you will not measure current in the tube. If the same probe works in another position, and not in the suspect positing, rule out the tube (swap the inner tubes with each other to determine if the issue follows the tube, keep the probes in the same sockets.) Hopefully this will help you before any other investigation is required.
 
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