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previous: Tim Gagan A quest for an Alamo Montclair sche... -- 4/27/2003 5:28 AM View Thread

Re: Wacko little amplifier

4/29/2003 3:33 AM
Re: Wacko little amplifier
In looking at your schemo, my first reaction is that it is wrong. The V3 input stages look right, and also V4 and the rest of the output stage looks good.  
The oscillator V1A looks right.  
But V2 looks like you drew half the circuit, turned your head, and came back and were off a couple pins or something. I don't mean that snotty.  
Having said that, no doubt it is completely correct and just downright odd.  
Taken at face value, you have two diodes in parallel pointing opposite ways. Out of the input stage, your signal goes through these to get to the volume control for the rest of its path through the amp - V4 etc. I can see that phase splitter sort of circuit at V1B as sending out of phase waves from the oscillator and banging them on the signal at each diode. These diode circuits are symetrical it appears. Geez I am starting to believe in this thing.  
So the invading wave will affect the signal level coming through, thus giving the trem effect.  
Where it does not work for me is V1B. Those voltages are not right. With B+ at 246 volts, this thing has its load split into two 100K resistors. I would be expecting 100VDC on the cathode, not 3VDC. And a lot lower at pin 6 too. I think your problem is in the intensity control. It is just wired right to the grid of V1B - DC and all. I think there needs to be a cap inserted between the wiper of the intensity control and the grid of V1B. The way it is now, I bet if you remeasure the cathode voltage at V1B it will vary as you turn the intensity control. The control is loading down V1B. Remove the wire to the control from the grid and see if your voltages on V1B snap back into place. What size cap? I dunno, the oscillator produces low freqs, so a big cap is suggested - is .05 enough, is .2 too much? EXperiment.  
Not only that, but you mentioned interaction in the controls - undesired interaction. The oscillator, I am going to call it LFO now for ease of typing, the LFO is a classic phase shift type where the signal at the plate is sent through several stages of phase shift through the caps. Both controls act at the same node in the train of caps. The DC from V1B is swamping the LFO. The amount depends on the settings of the controls.  
So all of this adds up to a weak trem that wants to stop the LFO at certain settings.  
Least that's what it looks like from here.

Tim Gagan Hi Enzo,Thanks for the thorough... -- 4/29/2003 5:01 AM
Tim Gagan Playing with some of Enzo's suggest... -- 5/4/2003 8:44 AM