During a recent start up, the Zoom G9.2tt started to boot up and just died. I have confirmed the power supply is working, and the two 12AX7 tubes are glowing when the unit is switched on. This unit is approximately two years old and was recently updated with the latest Zoom operating system.
However, the display does not light up, none of the console lights come on, and I'm getting no output from the G9.
While on the surface it would appear to be a power supply problem, it seems to me more like a CPU type issue.
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Check your power supply output voltage. It may be putting out (under) the correct voltage. Also I believe there is a small internal transformer in that pedal that may supply the tube voltage? ... that might be your culprit.
The preamp tube fillaments are run from DC with all three in series. The 36 volts for the heaters come through a 15 ohm power resistor. Check voltage across C46. You should find about 36 volts. if that voltage is there, one of the 12AX7 tubes has burned out one of the heaters and like a series Christmas light set, the string is all dead. Here is the problem: Various brands of tubes will vary in heater current and as such replacing one of a different brand may cause it to not take its share of the heater voltage, either more or less. So sorry to say, you should replace ALL three of the 12AX7's with the same brand tube and preferably from the same production lot at the same time. If you happen to find the voltage across C46 is way below the 36 volts, cease to use the amp until the cause is repaired since this voltage is also used in biasing the power tubes. The arrangement they have used in this amp is stupid as far as the heater circuit and having these in series... I would have fired the designer.
The problem is very likely a blown tube. You may be able to see which tube is dead by looking in the back of the amp with the room lights turned low and the amp on. If you can that it is one of the four 12AX7 tubes that isn't lighting up, that one should be replaced. If you can see that it is one of the KT66 tubes that isn't lighting up, buy a new matched pair and replace both at the same time.
* WARNING: UNPLUG YOUR AMP BEFORE CHANGING TUBES *
If all of the tubes are lighting up, then one or more may be bad, but not yet completely dead. Determining the problem would require more testing. If you have one spare, good 12AX7 tube around, try replacing them one at a time and see if that fixes the problem. TURN OFF AND UNPLUG YOUR AMP IN BETWEEN TUBE SWAPS. If a new preamp tube doesn't fix the problem, order a new set of KT66 power tubes and you should be back rocking out in no time.
* ALWAYS UNPLUG YOUR AMPLIFIER BEFORE REMOVING TUBES *
If you can see that a tube is definitely not lighting up then it may be blown. If this amp has seen regular use then it may be time for a new set of tubes. Try replacing the 12AX7s first before committing to a new set of 6L6s. Though, if you have the cash available, a new set of power tubes isn't a bad idea either. I would advise you to do the preamp tubes first and play through the amp for a few days first before changing the power tubes.
Keep in mind... if a tube (or tubes) has blown, there could be another issue in the circuit. Swapping in a new tubes is a quick way to determine that. Then it would be time to take it to your local repair shop.
First thing is that if you bought MATCHED pairs of the EL34's a COMMON mistake is to install the pairs wrong. Correctely install a pair as the OUTSIDE tubes of the line and then the remaining pair as the middle two tubes of the line of tubes before biasing.
Funny thing, I just ran across this problem you have with a Bedrock brand amp that uses the same configuration as your V3 tube does. This is really a stupid configuration engineering wise as they tie a plate at high voltage to the grid of the second stage in the tube as a cathode follower, This means that the second section has the cathode at a reasonably high voltage relative to the heater with wide audio swings on it as well... Tubes are NOT designed to work that way and result in noisy behavior, One would have to select tubes that have very low heater to cathode leakage in the second section of the tube... What you are seeing is the result of very poor engineering practice. Marshall is probably testing tubes for the leakage and branding them as such...
I think that designers of this circuit have NOT had experience with tubes and have applied transistor type designing to this use... it is plain stupid and myself I would re-engineer the circuit if it was mine or build a tube tester capable of measuring the heater cathode properties to select a suitable tube to use.
Just because the voves are lit doesn't mean there is the high voltage power to the valves.
ALSO the smaller valves have TWO heaters in them and if one half has burned out that would cause the symptom. Check if the smaller 12AX7/12AT7 types all have two heaters burning. Verify that the standby switch has not gotten bumped off.
Bad tube? maybe Try removing each tube 1 at a time starting at the first preamp (12ax7) If the noise stops after a particular tube is pulled replace that tube with a known good tube. Shut down the amp between tube pulls, Power tubes don't usually cause this problem but I'd pull them anyway,just remember they get very hot. Make sure all tube shields are on and replaced after tube pull test The most common cause of oscillation is a open or leaky filter cap,this would require a cap job and unless you have a good knowledge of tube electronics and the voltage dangers present,I wouldn't suggest doing it yourself. Also if the output transformer was replaced or rewired or the negative feedback circuit was touched oscillation could occur also. Make sure your guitar cables are in good shape, a cable with a bad shield can cause problems too. Excessive noise can occur when one of the 1/4" input jacks shorting bars doesn't make contact to ground. A duumy plug would confirm this but you could also plug two guitars with good cables, into each input and see if the oscillation disappears. Hope this helps