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  1. Replies
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    Good plan, I'd also check C308 for leakage. With...

    Good plan, I'd also check C308 for leakage. With the base voltages you have on the LTP Q301 should be off and Q302 should be on. Is the collector of Q301 at 0V and the collector of Q302 at 8V? That...
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    Yeah, but the output impedance of the usual LTP...

    Yeah, but the output impedance of the usual LTP PI is only around 40K so the PI 'wins' :)
    A 40k resistor to ground from each output of the PI would divide the signal by two as would an 80k between...
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    Are the base voltages the same on the...

    Are the base voltages the same on the differential pair? The output voltage should be roughly 4 x the voltage on Q301 base. Can R304 be adjusted to set 11V on C304 + (and Q301 base)? If it won't go...
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    Here's the schematic I have. It should work...

    Here's the schematic I have. It should work without the missing parts. As the output is sitting between the rails I'd suspect the LTP transistors. Will R304 adjust the output voltage?

    53993
  5. You could have saved time by first checking with...

    You could have saved time by first checking with a DMM to see if the mystery wire was grounded ;)

    For it to have negative feedback there would need to be two extra wires, one to ground one side of...
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    Even with a tube rectifier there's still the turn...

    Even with a tube rectifier there's still the turn on surge of the transformer and heaters.
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    Looks great from 4400 miles away :)

    Looks great from 4400 miles away :)
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    53870

    53870
  9. Sticky: You should really swap the OT leads but for a...

    You should really swap the OT leads but for a quick test try disconnecting the negative feedback wire at the board. It's the right hand end of the red wire on Chuck's marked up layout.
  10. 53858

    53858
  11. I don't think a shorted C7 could burn out the...

    I don't think a shorted C7 could burn out the resistors because there would still be R111 100k in series to B++ Perhaps there was a short from grid to plate or screen in either V5 or V7 then the...
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    What is the input connected to? If it's DC...

    What is the input connected to? If it's DC coupled to a low impedance source the op-amp will have a gain of 48 to its own input offset voltage which will turn a 4mV input offset voltage to 200mV at...
  13. Thread: soldering irons

    by Dave H
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    I bet it's not as old as mine. I've been using it...

    I bet it's not as old as mine. I've been using it for decades.

    53721

    No wait that was my dad's.
    This is like mine but mine has a black base unit.
    It's temperature controlled by tip selection...
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    The EL84 data sheet doesn't have plots for a Va...

    The EL84 data sheet doesn't have plots for a Va and Vs of 190V so some interpolation is necessary.

    I'm guessing the peak voltage available will only be about 160V. If the OT is 5k then peak (bias)...
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    What is the impedance of your output transformer?

    What is the impedance of your output transformer?
  16. I don't think 9V rms is enough to generate 12.6V...

    I don't think 9V rms is enough to generate 12.6V DC. I'd run it through PSUD2 to check.
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    I'd pull the PI tube before turning the amp on.

    I'd pull the PI tube before turning the amp on.
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    You could think of impedance as resistance to...

    You could think of impedance as resistance to Alternating Current (AC)

    Edit: I see Helmholtz has already said that :)
  19. If this is the correct schematic and PCB layout...

    If this is the correct schematic and PCB layout it looks like a really messy mod to do. All the pin 8s are connected together and pin 3 is connected through pin 1

    53699

    Edit:
    I see we have...
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    You could always pull the PI tube. I use the...

    You could always pull the PI tube.
    I use the shunt method myself. I've not had any oscillations with the OT primary shorted and there's no calculations to do do.
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    The centre tap of the output transformer isn't...

    The centre tap of the output transformer isn't ground, it's connected to B+ which is several hundred volts so take care!

    With the amp off measure the resistance between OT red and blue (call it...
  22. EL84 and 6L6 are too dissimilar to run in...

    EL84 and 6L6 are too dissimilar to run in parallel. It would be better to make two amplifiers, one with the EL84s and a second with the 6L6s.
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    Or electric, I once opened the door and a...

    Or electric, I once opened the door and a neighbour was standing there holding a toaster. I was told you work with electronics. Can you fix my toaster? It won't pop up she said. I looked down the...
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    I think the choke has to come first (no...

    I think the choke has to come first (no capacitor) for it to be considered a choke input filter. If a capacitor comes first (even if that node isn't used) it's a capacitor input filter.
  25. "GC" means Glass Container ;)

    "GC" means Glass Container ;)
  26. Then we have a measure for the plate current of...

    Then we have a measure for the plate current of 11/215 = 51mA
    Plate dissipation is (413-27.3)*0.051 = 19.7W!
  27. There's a safe way to find out. Connect a DMM set...

    There's a safe way to find out. Connect a DMM set to 'mA' where the missing ground wire should be.
  28. I prefer to think of it as +300V on the guitar...

    I prefer to think of it as +300V on the guitar input compared to the chassis (which is mains earthed). It's probably normal if you have removed the ground reference from the PCB 0V rail making its...
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