A15Mk1 working but hums

    Hello I built this marvel in 2007 and used it for about a year, then switched to bass guitar and didn't used the A15mk1 until today.
    Today I switched it on and it works but produces a sensible hum, not very loud, on both channels, also with volumes at 0.
    The hum diminishes and disappears when I turn the volume between 40% and 60%, then it reappears when I continue to rise the volume.
    I really can't remember if the hum was there 10 years ago...
    Any suggestion on how can I investigate the problem?

    thank you
    Mauro

    The volume knobs on both channels have the same (strange...) effect?

    yes. I just checked voltages inside.
    all voltages are ok in the range of +- 5% except the pin number 3 of V3 that reads 83v instead of 117.

    ?

    Did you try to spot if it's the power stage or the preamp? When you plug your guitar in the "return" input you can test just the power stage. If you leave a plug in the "return" and connect the "send" to another amp you can test the preamp w/o the power stage.
    //Martin

    Hi Martin, I just did the tests as you indicated.

    In both cases (preamp and power stage) there is the hum.

    I tried also the amp in other places, just to exclude home electrical circuit problems...

    I measured the values of the resistors around the pin 3 of V3 (R11, R12, R13) desoldering them, but they're ok.

    what else to try?

    thanks

    What do you mean with "the hums is here"? What is the effect of removing C9? I the wiring is correct, putting a plug in the return socket should do the same as removing C9 => breaking the audio path between pre amp and power stage.

    So you know that the hum comes from the phase inverter or the power amp. No check what happens if you pull the phase inverter tube. And find out if the hum is 50Hz or 100Hz (test tones on youtube...).
    Have you checked the filter caps?
    ...and did you get my private message?

    Hi Roseblood I left the amp at my friend house so I can't try to pull the phase inverter tube.
    the hum is 100hz
    Tomorrow I'll check the filter caps (and do a search for faulty component with an oscilloscope)
    and no, I didn't receive your private message

    thank you for the support
    Mauro

    well, yesterday I brought the amp to a friend that has oscilloscopes to check for any faulty component and so on... at the end, we removed R2, used a shielded wire to connect input signal to pin 7 of V1, and soldered R2 in parallel with R19.
    Great part of hum is gone, almost silent, great amp!
    BTW I will experiment with ground connections to check for ground loops....

    thank you all for support

    Mauro

    Unfortunately, we don't know the schematic...
    I only have the version 2, where R2 is the gridstopper for V1a and R19 is the gridstopper for V2a. If the A15mk1 uses the same numbering, your operation doesn't make any sense.

    But that's the point to stop the free assistance - as long as Martin doesn't publish the schematics, it would just be guessing

    Hi Mauro,
    1) so you're removed the grid stopper (R2) of V1a. Please be aware that this could result in high frequency oscillation and radio frequency getting into your amp.
    2) The second action (reducing the grid stopper of V2A from 470k to 31k by adding 33k to 470k in parallel) might result in so called "blocking distortion" when using high gain, so I do not recommend this change. What was the reason to reduct the grid stopper that much?

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