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The adjustment method varies from amp maker to amp maker. if you're unfamiliar with the procedure, you're better off to take it to the shop and get it done right. If you really want to do it your self, VHT has a meter they sell for about $100 online so you can see where you're at and then you adjust the bias trim pot inside your amp to balance them out to where you want them.
But again, you're best off leaving it to a pro.
Microphonic tubes can be detected by tapping them lightly with a pencil. I cannot find if this uses a spring reverb, but if it does, vibrating the cabinet will get a clanky type sound from the reverb tank. This MAY use a digital reverb however which is immune to that. Next thing is to make sure your guitar can't "hear" the output sound of the amp because the strings can pick up the sound and resonate feeding back into the amp. You can detect if this is happening by quickly dampening the strings and see if ringing gooes away. You probably need 6 to 10 feet of clearance between the guitar and the amp OR put your body between the amp and the guitar. ALWAYS set sound level at safe levels. Get a sound meter and go no higher than 90 Db. I know a lot of near deaf musicians whose careers are near over because of hearing loss using too high level.
When replacing the two output tubes you should check the voltage at TP26 or either cathode pin of the power output tubes to ground. Set the bias by adjusting pot R82 to 60mv which is 60 ma across the 1 ohm metering resistor. This coresponds to 30 ma per tube. Changing other tubes requires NO adjustment.
Scroll down to the link "Get Manual" and download the schematic. Note that it has several pages of parts list preceeding the schematic.
Next, put a 150 Watt incandescent bulb in series with the power cord to act as a current limiting fuse. Now replace the fuse and proceed with testing. You may find that one or more filter caps have shorted. Also pay particular attention the the BIAS power supply. You should see about -35 to -40 volts. Note that the series lamp will come on if excess current is being drawn and voltages will be low if the lamp lights. If the BIAS supply has failed, it is possible your power tubes have been damaged.
When replacing, make sure your MATCHED pairs go into V1 and V4 and the other into V2 and V3 respectively. Many don't realize the proper placement of matched pairs.
Open unit and if you find the tubes with the plates glowing red OR purple haze down between the elements, the tubes are gonzo.
Check the bias at connector CON1, Between pin 1 and 2 is a 1 ohm resistor in the cathode of the V1 and V2 side. Adjust the Bias1 for the number of milivolts matching the required bias current for two of the output tubes. Do the same for pins 2 and 3 of CON1 for the second side using the BIAS2 control. Fir your amp, you will want each of these set to 70millivolts.
It is possible (hopefully) that one of the smaller tubes burned out. These are much less expensive. EACH of these will have TWO points of light visible when they are operating. If you only see one, then 1/2 may be burned out. When turbes burn out, they cool down and sound will fade out.
There are other components that could cause your problem but those are not DIY unless you are adept at electronics.
The scratching is just a bit noisy volume potentiometer, but it pointss out that the power amp is working...
Are you sure you know how to operate the organ? You have to select one or more instrument tab switches to get anything out.
There could be almost anything wrong, but likelly a vacuum tube has failed. Open the back and see what tubes are lit. PLEASE NOTE: There is high voltage in the unit...
PLEASE ALSO NOTE that some tubes have two sections and BOTH must be lit for operation.
I had checked the fuse for the heater circuit and found that while it was testing on a low ohm check meter - it was not sufficiently handling the full demand of the circuit - I put a bypass on the fuse and found the problem. The fuse appears to have been previously replaced with a type not original - the original was a ceramic case fuse 10A.
Check out the rectifier tube, there is only one DC voltage derived from the mains supply and it goes through that Rectifier tube, it is a 5AR4. The other two taps on the secondary of the input transformer are 6.3 Volts AC and 5 Volts AC. BE CAREFUL POKING AROUND ON THESE THEY CAN KILL YOU. Measure the DC output voltage if you have a meter or in most cases just look at the rectifier tube filament, 9 times out of 10 if it doesn't light up it is dead. Good luck.
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