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Wacko little amplifier

4/27/2003 5:28 AM
Tim Gagan	Wacko little amplifier A quest for an Alamo Montclair schematic yielded nothing, so I traced the circuit of this one I'm working on. It's got some of the nuttiest stuff in the vibrato circuit--plates tied to grids, and things like that. http://www.gaganbros.com/images/Alamo%20Montclair.bmp The vibrato does sound cool; it might actually be real vibrato more than tremelo. It's weakness is the speed control, which acts more like a secondary intensity control. It's hard to even tell if the speed changes as the pot gets turned, and then the vibrato effect goes away. I tried playing with various components to see if that would change, but nope. It did look like maybe something had been removed from the lug of the speed pot that meets those two 470K resistors. Anybody ever run across something like this, or have any suggestions about making the speed control act like a speed control? Thanks, Tim

4/27/2003 5:28 AM

Tim Gagan

Wacko little amplifier

A quest for an Alamo Montclair schematic yielded nothing, so I traced the circuit of this one I'm working on. It's got some of the nuttiest stuff in the vibrato circuit--plates tied to grids, and things like that. http://www.gaganbros.com/images/Alamo%20Montclair.bmp
The vibrato does sound cool; it might actually be real vibrato more than tremelo. It's weakness is the speed control, which acts more like a secondary intensity control. It's hard to even tell if the speed changes as the pot gets turned, and then the vibrato effect goes away. I tried playing with various components to see if that would change, but nope. It did look like maybe something had been removed from the lug of the speed pot that meets those two 470K resistors. Anybody ever run across something like this, or have any suggestions about making the speed control act like a speed control?
Thanks,
Tim

4/27/2003 1:48 PM
Casey4s	Tim, Nice job on the reverse engineering to get the schematic. I don't understand the 12AX7 with the grid tied to the plate either. It seems to me that it is now a DIODE, but that still leaves me at a loss for it's use. Casey4s

4/27/2003 1:58 PM
Casey4s	Ahhh! Tim, After a second look, is that 12AX7 diode the source for the -197v used in the tremolo/vibrato circuit? Casey4s

4/27/2003 4:42 PM
Tim Gagan	Casey, Thanks for taking a look. If you're talking about the negative dc voltage on pins 1,2 & 8 of V2, that's .197 (point one nine seven), hardly any voltage at all. The funny thing is, there's not really any way for negative or positive dc to enter or exit that V2 circuit; all roads are guarded with capacitors. I can't even tell if that tube is part of the vibrato circuit or not. The funny thing is that the whole signal from the vibrato channel goes through that tube. I don't understand how it even works. Tim

4/27/2003 4:42 PM

Tim Gagan

Casey,
Thanks for taking a look. If you're talking about the negative dc voltage on pins 1,2 & 8 of V2, that's .197 (point one nine seven), hardly any voltage at all. The funny thing is, there's not really any way for negative or positive dc to enter or exit that V2 circuit; all roads are guarded with capacitors. I can't even tell if that tube is part of the vibrato circuit or not. The funny thing is that the whole signal from the vibrato channel goes through that tube. I don't understand how it even works.
Tim

4/27/2003 9:09 PM
Don Symes	For some reason, V1B looks to me a bit like a concertina PI that modulates the voltage from V2Bp to V2Ak. Since both halves of V2 are diode-connected, I'm thinking they set up a diode clamp connected to a variable upper clamp voltage and referenced, not to GND, but to a copy of the signal. I can sort of see it, but not explain it. Odd, odd circuit. Doubt that was any help.

4/29/2003 3:33 AM
Enzo	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.

4/29/2003 3:33 AM

Enzo

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.

4/29/2003 5:01 AM
Tim Gagan	Hi Enzo, Thanks for the thorough analysis! I'll play with your suggestions, and maybe try moving some things around to see what happens. I might not get to it for a couple days, but I'll report back with results. Tim

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