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Ampeg M-12 oscillator

3/30/2006 8:25 PM
David Deen
Ampeg M-12 oscillator
Hello all,  
I while ago I bought and rebuilt an old 1962 Ampeg M-12, the one using 3 6SL7 preamp tubes, 2 6V6 power tubes and a 5Y3 rectifier. My question is regarding the oscillator for the tremolo section. Right now it is very weak but still present when engaged but the speed control is uneffective (same speed at all settings). I was wondering if tired tubes could be a culprit for weak oscillation?  
A few possibly helpful debug notes:  
Before the rebuild the entire control section was scratchy (sounding) but improved dramatically upon rebuilding it. New tubes for the power section, rectifier and one 6SN7 replacement in the preamp section but I'm using the tubes that came with the amp for the oscillator and phase inverter. These two tubes I highly doubt are 1960s tubes. I suspect they're no more than 25 years old tops by their condition but I know that's a guess and 25 years is quite a while. I've checked the wiring for the speed and depth controls and all looks sound to me. One last thing, immediately after the rebuild the oscillator worked fine, then became intermitant and now is a constant weak and speed-invariable one.  
Any suggestions would be greatly appreciated. Thank you.  
3/30/2006 11:54 PM

Like most trems, this one has two stages - the LFO and the driver. You need to isolate the problem. Either the LFO puts out a weak signal and the driver has nothing to work with, or the LFO works and the driver is weak. Or both.  
You have theee 6SL7s in there, well two now anyway. Swap places with them. If the trem works just the same, the tubes are probably not it. If it makes a difference, get a new 6SL7.  
That the speed control does nothing is suspcious. That means the control is open or the resistor in series with it, most likely. Make sure the low end of the pot is actually grounded. Is the footswitch circuit clean and of low on resistance? Use a clip lead across the FS connector to be sure.  
The oscillator relies upon feedback from the plate to the grid. I would just replace the three feedback caps and be done with that. Leaky old caps make for weak LFO. Verify resistance of all seven resistors in that tube's circuits. Make sure the cathode cap is right. The LFO gets into the driver by sharing that cathode and the grid of the LFO.  
How large is the signal at the plate of the LFO? ANd then at the plate of the driver? I expect them to be similar. I expect the output of the LFO to be fairly large signal. SCope it, or lacking a scope, set your meter to AC and read the signal level.  
Get the LFO freq to adjust before worrying about any thing else.  
Once there is a fat signal from the LFO, and assuming we find it at the plate of the driver side, it then has to go through a 0.1uf cap to block the DC and put the trem signal on the intensity pot. I'd probably just change the .1 anyway. Caps are cheaper than spending all day fussing.  
If the drive output is healthy, it has to get to the power tubes. The intensity pot is 1 meg, and there is a 470k in series with it. That means that by voltage division, only 2/3 of the LFO signal gets to the intensity pot. The pot wiper then samples off however much of that signal to send on to the power tube grids.  
If the tube circuits are working well, and the signal is just too weak, you can reduce the value of that 470k. That increases the signal level to the control.  
In my experience, even relatively weak tubes usually still oscillate. I often have to recap the LFO circuit. If any of the resistors to ground are open among the feedback caps the circuit will work poorly if at all.
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