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Thread: Sessionette 75 problem.

  1. #1
    Junior Member
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    May 2010
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    2

    Sessionette 75 problem.

    Hi folks, don't know if anyone can help me but my problem is this:

    I've got a Sessionette 75 guitar amp that is about 25 years old. It was working fine until the other day when it started to develop a mains hum from the speaker. Not dead loud but far too loud to ignore. Then it started to get distortion and lose power. I took a guess that it might be a dried up electrolytic in the power supply so changed the two big ones. However, it made no difference.

    I had a look at the circuit board closely and noticed that the 1 watt resistor leading to the external speaker socket was burned -and so I suspect it might be one of the MOS-FET power transistors that has burned out. I can get a whole now power amp board from the guy that still services them for about 30 quid.

    However, (and this is the bit I don't understand). I read somewhere that if you plug your guitar into the faulty amp and then take a lead out of the 'send' socket (the output from the pre-amp) and send it to a different good amp rather than back in the 'return' socket it will give you an indication of whether the preamp is OK as it will sound OK through the good amp, if you see what I mean.

    I did this and although it seemed to lose most of the distortion - the mains hum was still there as loud (or louder) than ever.

    So I switched the faulty amp off at its on-board on/off switch - but the mains hum was still coming out of the good amp. Yet when I unplugged the faulty amp altogether the hum stopped.

    What's going on? Why does it still hum when the faulty amp is plugged into the good amp but switched off at the switch - yet doesn't when I unplug it from the socket? I don't want to order a new module if I've made the wrong diagnosis. Can anyone shed any light on the matter? Cheers.

    Oh and I forgot to mention I only got the problem after I connected it to a couple of 2x12 speakers. Don't know if that is relevant or not.

  2. #2
    Old Timer
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    Oct 2007
    Location
    Buenos Aires, Argentina
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    5,548
    That second hum that you get on the good amp is very probably a ground loop hum, I'm sure the internal 230V power connections in both amps are wired differently, nothing unusual.
    Eventually you should correct one of them if you want to use both amps together on stage , but that's not the main point today.
    Please post a couple clean close up pictures of that burnt resistor.
    I guess it might be part of a "Zobel network" that often is soldered right across the output jacks, instead of in the PCB proper.
    It may burn if your amp oscillates, which after that becomes unstable. That oscillation is inaudible, but it causes the audio signal to become fuzzy, dirty, and the amp overheats, eventually burning out.
    Definitely hooking extra speakers without respecting the suggested impedance causes trouble.
    Good luck.

    If

  3. #3
    Junior Member
    Join Date
    May 2010
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    2
    OK thanks for the reply, will post a piccie as soon as I get the chance.

    I wired the 2x12 in series so I would have thought that would have kept the impedance up. Oddly the onboard speaker never switched off when I plugged in the external speakers. BTW I only use it for home use and was actually using it as a bass amp to practice on.
    I've read that this amp is a little bit prone to oscillation so it sounds like you are talking good sense. Cheers for the help.

  4. #4
    Junior Member
    Join Date
    Mar 2011
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    1

    Problem with sessionette 75

    Quote Originally Posted by joe-90 View Post
    Hi folks, don't know if anyone can help me but my problem is this:

    I've got a Sessionette 75 guitar amp that is about 25 years old. It was working fine until the other day when it started to develop a mains hum from the speaker. Not dead loud but far too loud to ignore. Then it started to get distortion and lose power. I took a guess that it might be a dried up electrolytic in the power supply so changed the two big ones. However, it made no difference.

    I had a look at the circuit board closely and noticed that the 1 watt resistor leading to the external speaker socket was burned -and so I suspect it might be one of the MOS-FET power transistors that has burned out. I can get a whole now power amp board from the guy that still services them for about 30 quid.

    However, (and this is the bit I don't understand). I read somewhere that if you plug your guitar into the faulty amp and then take a lead out of the 'send' socket (the output from the pre-amp) and send it to a different good amp rather than back in the 'return' socket it will give you an indication of whether the preamp is OK as it will sound OK through the good amp, if you see what I mean.

    I did this and although it seemed to lose most of the distortion - the mains hum was still there as loud (or louder) than ever.

    So I switched the faulty amp off at its on-board on/off switch - but the mains hum was still coming out of the good amp. Yet when I unplugged the faulty amp altogether the hum stopped.

    What's going on? Why does it still hum when the faulty amp is plugged into the good amp but switched off at the switch - yet doesn't when I unplug it from the socket? I don't want to order a new module if I've made the wrong diagnosis. Can anyone shed any light on the matter? Cheers.

    Oh and I forgot to mention I only got the problem after I connected it to a couple of 2x12 speakers. Don't know if that is relevant or not.
    Dude i just had a right mare with my sessionette! A week ago it started humming Loud and the signal from the input stopped working, just an annoying hum. The trebles/bass/channel swithes etc made no difference.. same annoying sound.. I phoned my mate Brett 'the vision' Robertson who got me to plug my guitar into the effects return at the back and the guitar came through. this showed the output was still ok, which is good because apparently the outputs for these bad boys are quite hard to come by. Next i took it to Blues 'The Spark' Mike who said the 'TL 071 CP Op Amp' had probably blown. online from RS components i ordered a 1 x TL 071 CP and 3 x TL 072 CP (actually i ordered double that amount so i had a spare set), replaced them with the ones ones the board and BAM, Hows yer Father!! Up and Running this bad boy is sweet and once again rippin it up nicely!
    Not sure if that helps but helped me.
    Oh one thing to be aware of, if you get the Op Amps (TL071&72) i was told not to touch the pins with you hands as they can screw them up, something to do with static?!?!!
    Good Luck!
    Chops

  5. #5
    Junior Member
    Join Date
    Dec 2011
    Location
    Ipswich
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    Hi
    If you ever need the amp to be serviced, I have had a few thru my workshop. Great little amps and fantastic sound.
    I can also supply and/or install a new replacement power amp for the Sessionette 75 and other session models and matches the exact input and output specs of the old albeit now more reliable.

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