Chris Scott
02-13-2007, 11:49 AM
Hey Folks,
I have a Twin Reverb that’s giving me problems, and my somewhat limited experience is really showing on this one.
Amp is a MV silver-face 100watt Twin Reverb. I have back-dated it to an AB763-style circuit, eliminating the MV circuit, changing back to true bias adjust, as well as a few tonestack tweaks, all of which has greatly improved the sonics on this unit- the other guitarist in my band claims it’s better sounding than it has any right to be, and it does indeed sound good. What is giving me fits is the reverb circuit. It has always had a very weak and “boingy” sounding reverb. I’ve replaced the tubes, pan, and transformer, checked all components in the circuit, all ending with the same symptom. And now, after recently removing the M/V circuit, I have been having oscillation problems when I turn up the reverb control, as well as when I turn the speed control for the vibrato- the vibrato control gets a classic “motorboat” sound that speeds up when you dial it up (duh!), while the reverb, in addition to the oscillation, also adds a sound vaguely reminiscent of a very slight envelope follower kind of effect.
The voltage check throughout the entire amp is within spec, save for the vibrato tube. Pin 1 plate will not register a voltage unless the .02 cap in the vibrato chain is removed. All other pins on this tube show within-spec voltages, and the 220K resistor between the oscillator and pin 1, like damn near every component in this amp, has been checked, and then replaced, to no avail.
One question I have at this point is with the power supply, as it is the section just before this area that is not functioning. The latest schematic I have for a Twin Reverb is the AA270 circuit, which shows the same value resistors tied into the three 20uF filter caps as what is in this amp- 10K 1watt between “D” and “C”, and a 2.2K 1watt between “C” and “B”. The AB763 shows these to be 4K7 and 1K respectively- could this be my problem? I would simply sub these components, but I was told these are only to control the voltage of the PT, and have nothing to do with the problem I’m having. The other question is with the phase inverter section- should I be showing the – voltage figure off the bias pot when the unit is on standby as well as in play mode? (remember, I’ve changed this amp to true bias adjust specs) This amp does, and since I’m such a comparative doofuss with this stuff, I don’t know if it’s in order or not.
Thank you in advance for any help you can send in my direction- I’ve tried to avoid asking for help with this as I’m aware of how the only way to learn is to just muddle through it, but I’ve already burned up so much time with this one, I’m ready to throw in the towel. Thanks, Chris
I have a Twin Reverb that’s giving me problems, and my somewhat limited experience is really showing on this one.
Amp is a MV silver-face 100watt Twin Reverb. I have back-dated it to an AB763-style circuit, eliminating the MV circuit, changing back to true bias adjust, as well as a few tonestack tweaks, all of which has greatly improved the sonics on this unit- the other guitarist in my band claims it’s better sounding than it has any right to be, and it does indeed sound good. What is giving me fits is the reverb circuit. It has always had a very weak and “boingy” sounding reverb. I’ve replaced the tubes, pan, and transformer, checked all components in the circuit, all ending with the same symptom. And now, after recently removing the M/V circuit, I have been having oscillation problems when I turn up the reverb control, as well as when I turn the speed control for the vibrato- the vibrato control gets a classic “motorboat” sound that speeds up when you dial it up (duh!), while the reverb, in addition to the oscillation, also adds a sound vaguely reminiscent of a very slight envelope follower kind of effect.
The voltage check throughout the entire amp is within spec, save for the vibrato tube. Pin 1 plate will not register a voltage unless the .02 cap in the vibrato chain is removed. All other pins on this tube show within-spec voltages, and the 220K resistor between the oscillator and pin 1, like damn near every component in this amp, has been checked, and then replaced, to no avail.
One question I have at this point is with the power supply, as it is the section just before this area that is not functioning. The latest schematic I have for a Twin Reverb is the AA270 circuit, which shows the same value resistors tied into the three 20uF filter caps as what is in this amp- 10K 1watt between “D” and “C”, and a 2.2K 1watt between “C” and “B”. The AB763 shows these to be 4K7 and 1K respectively- could this be my problem? I would simply sub these components, but I was told these are only to control the voltage of the PT, and have nothing to do with the problem I’m having. The other question is with the phase inverter section- should I be showing the – voltage figure off the bias pot when the unit is on standby as well as in play mode? (remember, I’ve changed this amp to true bias adjust specs) This amp does, and since I’m such a comparative doofuss with this stuff, I don’t know if it’s in order or not.
Thank you in advance for any help you can send in my direction- I’ve tried to avoid asking for help with this as I’m aware of how the only way to learn is to just muddle through it, but I’ve already burned up so much time with this one, I’m ready to throw in the towel. Thanks, Chris