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Sure enough, those voltage drops and differences in resistance accounts for about 10mA difference in plate current. Does this actually account for the higher current? This fuse measures a tiny bit smaller resistance from end to end. This is the other fuse, for the V2/V3 pair plate circuit. The fuse for the V1/V4 pair of tubes measures over half an ohm (meter zeroed for test lead resistance.) Does that tiny voltage drop make any difference? Remember seeing separate fuses for plate current on the back of the amplifier? Checking voltage drops in the entire plate circuit, we see that this fuse drops about 0.2 volts across it more than the other fuse.
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We paid big money for matched tubes (which, when swapped around, make no difference…) More work! The temperature difference confirms the validity of the different idling currents… but why are they different? They share one transformer winding. The V1 and V4 tubes are about 114 degrees C. Plate current causes heat to be dissipated in each tube. Both meters are in good agreement with the values measured, but I’ll stay with my good Fluke to investigate the situation. Here, I’m using my good Fluke bench meter to confirm that one pair of tubes is idling at 50 milliamps, while the other pair is idling at about 41 milliamps or so. So with the tubes installed and operating into an 8 ohm resistive load, we set the idle current for one pair of tubes. We will also wipe off the pins on the bottom of each tube. The pipe cleaner works well to clean and recondition each individual octal tube socket contact. Here, we have sprayed a little D100 into the cap, and then soaked a pipe cleaner in the solution. The Unbrokenstring Crew are big fans of DeoxIt products. The current splits thru R28 and R29 to manage a pair of tubes each, part of the 50W/100W power control circuit. This blue control sets the idling current (bias) for all four tubes. The power transformer and power switches are mounted directly to the chassis. These are in excellent condition and will not be replaced today. The large blue items are the filter capacitors.
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Tube sockets are in the middle.Īt the lower right side of the output circuit board is the power supply power resistors, rectifiers, and fuses More views of the preamp board on the left and the output jacks on the right.
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The tube sockets are discretely wired, and on the right is another circuit board handling the effects loop jacks. On the left is the preamp circuit board containing the input jack, tone controls, and signal switching. The red and black leads to to the reverb tank. You can read Internet posts regarding the battles between Marshall in England and American importers the latter changed the tubes on new amps to 6L6GCs because they believed the 5881s would not last through the warranty period.Ī walk through the bottom of the unit shows us the output transformer. Later 6L6GCs dissipate more power and take higher voltages. Interestingly, Marshall delivered these heads with 5881 tubes, a military 6L6. These great tubes have delivered a long service life and are now just about worn out. Do you see the frowning face in the upper insulator? The brown scorch mark is his beard. These power tubes have pushed billions and billions of electrons around, and some of those electrons have interacted with the inert gas inside the glass envelope. IEC mains power socket and a line fuse rounds out the rear panel. Two fuses are used in the high voltage plate supply for this amp, which is a nice touch and will add something to the story later. This amp can be switched to 50 or 100 watt output power. This unit is recording-friendly, with outputs for ‘wet’ and ‘dry’ signals. The effects loop is accessible from the back. The amount of reverb, as well as the gain and volume, are independently adjustable. In simple terms, this head has two channels that share a common tone stack, effects loop, and reverb tank. Could the Unbrokenstring Crew refresh this head and resolve the tiny issues that had arisen over the years? It was due for a set of tubes and a million-mile checkup. This wonderful old Marshall JCM900 lives in a recording studio.