Seeburg AVC switch

The four position AVC switch goes from 'off' to 'max'. The manual states there is a 20 db compression at max. When playing a record with 'average' volume and switching the selector switch from off to max I get an marked increase in volume. I would have thought that a 20 db compression at max would result in a decrease in volume not an increase. Is this the correct function of the switch settings?

Hi Steve,
What model amp are you speaking of ?? Ron Rich

Steve, the process of analog compression here, and in other circuits of this nature is attenuative.

An excess of gain is designed into the circuit and the reactive element, in this case a tube acting as variable shunt "wastes" it at a rate corresponding to the control voltage applied to it.

In the case here the gain was based on the output from a "loud" record and the redhead cart with mono spike styli. Under these conditions the 6SK7's Grid one would be near the same potential as the cathode thus the tube would be conducting heavily and reducing gain somewhere near 20db.

If the AGC feature is to be made optional via a switch and is turned off something has to be done about all that excessive gain or distortion will result from over driving the following stages --in this case the tone amp and cathode follower. So when the AGC switch is turned off resistors are introduced to reduce gain and prevent overload. If the pickup and styli and-or preamp are not generating as much level as originally intended there is often little-no gain reduction actually going on and when the switch is moved toward "max" or "on" the resistors are removed and level increases.

On an MRA-5 you can check the gain reduction at the junction of R38-R40 and C20-C31 (note that on later designs a single 1mfd was used instead of the two 0.5's here. Because of the high impedance here even a typical 10 megohn meter will load the circuit to some extent and cause the volume to rise about 2 db. If your meter is able to read quickly you'll be able to get a better idea of the voltage reading before the two caps are bled down by it. You can also quickly short this junction to chassis ground and the level will rise according to how much reduction was actually taking place. --Use caution and make sure you have the right test point here--.

I routinely check this voltage while playing a "reference" record (one whose level was noted when the amp was fresh from rebuild ) to track any deterioration as components age in the preamp or if new, lower output styli are used.

So, to sum up (finally -whew!) what you are seeing is normal if the output from the pickup is low.

Rob

Very informative, great explanation. In addition to knowing what something does it's always nice to know how it does it.
Thanks.