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  1. Replies
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    If you really gotta know, 4 wire measurements...

    If you really gotta know,

    4 wire measurements to the black common wire (no averaging):

    4 Ohm tap, 0.36390
    8 Ohm tap, 0.48331
    16 Ohm tap, 0.71211 (1047 Ohm feedback network in parallel)
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    I just took resistance readings on a Dynaco 40W...

    I just took resistance readings on a Dynaco 40W amp that sits near my computer as a chrome monument. Plate to plate, 185 Ohms. One side to center tap, plate 102.4 Ohms, screen 34.2 Ohms. Other side,...
  3. In the Dallas area we are blessed with one of the...

    In the Dallas area we are blessed with one of the best "surplus" electronics stores anywhere. It started out in the late 70's almost exclusively surplus electronics from local companies going out of...
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    It's not an issue yet, but when one of those...

    It's not an issue yet, but when one of those little computers in your car fails, you might not be able to get a new or rebuilt one from the manufacturer. Even if you could find one at a wrecking...
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    What type of impedance switch does this amp have...

    What type of impedance switch does this amp have ? Is it the same kind of PCB mount slide switch that Marshall uses on the back of speaker cabinets that allows the 4x12 to be split into stereo ?
    ...
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    This amp has no short circuit or over current...

    This amp has no short circuit or over current protection other than the fuse on the output. Amps like this typically don't have an extension speaker jack. It's designed to run the internal speaker...
  7. I always thought the 5AR4/GZ34 had controlled...

    I always thought the 5AR4/GZ34 had controlled warm up but looking at the RCA data sheet I'm not seeing any reference to that.
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    It's all relative. A really long time constant...

    It's all relative. A really long time constant might seem like forever, but it's not. Any decent modeling program will show you that the capacitor is still blocking DC or holding the DC up. Like the...
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    Could the pins of Q8 be scrambled ? Could the...

    Could the pins of Q8 be scrambled ? Could the Base and Emitter be reversed ? What is that note in the lower left corner of the schematic ? Shouldn't the pinout be B C E if you are looking at the top...
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    Back in the day, Tektronix made a special scope...

    Back in the day, Tektronix made a special scope called the 570. Using it you could display the plate current curves of many tubes. Connections to the tube socket were made by short jumper wires. When...
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    What do they use to probe circuits in China ? A...

    What do they use to probe circuits in China ? A spork ?
  12. Another guess, it could be a fractured solder...

    Another guess, it could be a fractured solder joint around the input jacks or the power amp in jack. These also have something to do with the Mute circuit.
  13. Look for the Volume pot in this example of the...

    Look for the Volume pot in this example of the Marshall 2203.

    https://el34world.com/charts/Schematics/files/Marshall/Marshall_2203_jmp_master_volume_lead_100watt.gif
  14. This is just a WAG (Wild Ass Guess). The FM212...

    This is just a WAG (Wild Ass Guess). The FM212 has a Mute circuit. It appears that it only acts on the power amp. It also appears that it is only (supposed to be) activated when the amp is turned on....
  15. I would use 1K or 820 on the V1a cathode. As long...

    I would use 1K or 820 on the V1a cathode. As long as B+ is 300 or so, you won't run out of headroom on a first stage. You can make a Voltage divider by sticking a resistor above the Volume pot and a...
  16. The blackface tone stack is pretty bass heavy to...

    The blackface tone stack is pretty bass heavy to begin with. Many people run with the bass control on 2. To get away from this you need some HF lift in a couple of places. Try a little bass cut by...
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    Nobody would use the 50's style wiring from the...

    Nobody would use the 50's style wiring from the original schematic. Rockybottom16 stated that the CT of the heater supply was connected to the output tubes cathodes.



    Rockybottom16 states that...
  18. Model T's are awesome for Bass, but require a...

    Model T's are awesome for Bass, but require a strong set of 6550's. Current prices in the 2K-3K USD range because they are hard to find. OT has a 2 Ohm tap and an impedance switch that won't melt....
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    The purpose of the diode is to limit the Voltage...

    The purpose of the diode is to limit the Voltage at the back panel test point (where the user can touch it) in the case that the resistor fails open for any reason.
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    Does the transformer have wires on both sides ?...

    Does the transformer have wires on both sides ? How are you going to deal with that ?
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    I'd knock those 47uF caps down to 22uF.

    I'd knock those 47uF caps down to 22uF.
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    It's very hard to protect the tweeter with a...

    It's very hard to protect the tweeter with a passive crossover system. You usually need to resistively attenuate the tweeter and add some type of limiter. In fact the first thing I would do is verify...
  23. The tremolo is just the opto type. If the opto...

    The tremolo is just the opto type. If the opto goes bad, you'll have to experiment to replace it. Some low value electrolytics in the LFO that go bad. Reverb is tube drive, solid state recovery.
    ...
  24. Worth about half that with non-original 2x10...

    Worth about half that with non-original 2x10 cabinet. It should have a 4x12 cabinet. Chances are it won't sell. Keep your eyes open for it to be re-listed in a month or two.
  25. Sunn transformers are not very corrosion...

    Sunn transformers are not very corrosion resistant. Fender transformers from the same era (mid to late 60's) are better. Going back to the Tweed Fenders, you see more corrosion. The Sunn chassis and...
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    A couple of things bothered me. 1) The diode...

    A couple of things bothered me.

    1) The diode drop at 1 Amp is closer to 1V than it is to 0.7V.
    2) The diodes distort the waveform with a dead zone around zero Volts.
    3) The peak of 6.3VAC is...
  27. What are MBB power relays ? I tried running...

    What are MBB power relays ?

    I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a...
  28. My reasons for doing something like this are...

    My reasons for doing something like this are slightly different. I want to set up an experiment to see if people can tell the difference between two amps, one tube and one solid state. The problem I...
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