Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sigkarl on August 16, 2010, 07:45:20 am
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I have an old JTM Bluesbraker wit a faulty tremolo circuit. Regarding to the original owner the tremolo has never functioned. There has been done many attempts to fix the amp and some modifications done and components have been replaced.
I have undone all the modifications but nerveless the tremolo doesn’t work. For other parts the amp sounds perfectly. I have done all the regular things to troubleshoot the amp: replace tubes, try another footswitch, replacing FET1 and other things but without any luck. There seems like there is no oscillation in the tremolo tube V6.
Regarding to Gerald’s Weber’s book ”Tube Guitar Amplifier Essentials” on page122: “….the oscillator tube for the tremolo needs 350 volts”.
Measuring the voltages on V6, plate voltage point “B” 214V and on the other end of R39, 209V.
Could this be causing the problem?
Now is my goal to solve this problem.
Is there anyone familiar with this problem?
Regards
Sig
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Tube is the 1st thing to try replacing. Trem oscillator tubes wear out to the point where they eventually become ineffective. The oscillator stage does the 1st 180 degrees of inversion, and the combined RC network between the oscillator triode's plate and its grid does the other 180 degrees, so replacing those resistors and caps in older amps will usually get it working again if the tube replacement doesn't work. The other thing I guess it could be is the J174 FET has had a meltdown (esp if the previous modder had removed the resistor/diode at the gate). JM2CW - by no means the last word