Valco Model 1200 help - diyAudio
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Old 11th September 2011, 03:51 PM   #1
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Join Date: Sep 2011
Default Valco Model 1200 help


I'm a newby to this site and am searching for some help finding the source of a problem. The amp I'm referring to has a similar schematic to the following link: with the exception of the 10MF and 20MF caps in the supply section (right most caps). My amp has a multi-cap 15MF & 30MF value respectively.

The amp does not produce any sound. I've checked the simple things like fuse (power light is lit), transformer primary, secondary and heater voltages and all seem to be to spec. I've replaced the selenium rectifier with a Si 1N5204/100 ohm, 10W series combo and replaced the 40MF field coil shunted cap plus the 10MF shunted caps in the supply (left most in the schematic).

I disconnected the choke (30hy) and measured both series (@1.8K ohm) and to ground (Open), but am not sure if the 1.8k reading is ok, I assume so. I further measured the input signal to the 6SL7 grids and found them to be ok. Where I'm stuck is the output of the plates. What would I expect to see, voltage wise at this point? I've read some other threads where they've stated some "typical" plate to cathode voltages of 200V, so I'm planning to check that next.

From my research and understanding, this amp is configured as a dual input preamp (via 6SJ7 & 6SL7) with a differential amp (6SN7) driving a push/pull 6L6 output.

Having cut my teeth (20 some years ago) on solid state and shelving my analog brain (15 some years ago), I'm re-educating myself on amp theory, using this nice vintage amp as my training tool.

At this point I could use some help on the next level of troubleshooting to breath some life back into the amp and my technical self confidence......... : )
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Old 11th September 2011, 07:44 PM   #2
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Join Date: Sep 2011
Update on my measurements. I show an active signal from the guitar input to pin 4 (control grid) of the preamp (6SL7), however the plate voltage is a steady -.495vdc.
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