5F6H
566 posts
Mar 21, 2011
11:34 AM
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There have been a few posts lately touching on the subject of reducing volume for an Epi Valve Jr, for home practice.
To cut volume by 75% (6dB, or from 5W to 1.25W output) the speaker output can be attenuated by using 2 resistors in a simple voltage divider. The "load" resistor is installed in parallel with the speaker, e.g. from the "+" to the "-" on the speaker terminals. This resistor will be of an equal value as the speaker, in ohms. For a 4 ohm speaker, use a 4ohm resistor, 8ohm speaker requires an 8 ohm resistor, etc.
The second resistor is the "dropping" resistor. This goes in series between the output transformer "+" wire (not the "common" wire) and the speaker "+" terminal. The value of this resistor is half the value of the speaker ohms/load resistor. So for a 4ohm output the dropping resistor is 2ohms, for an 8ohm output the dropping resistor is 4ohms, etc.
These resistors should be wirewound, 10W (or preferably better, like 12W to 18W) rating. They will be quite heavy, so if mounting to the speaker basket/terminals don't let the "dropping" resistor swing in the breeze. You could use an INSULATED (not grounded) terminal post to secure the end that connects to the OT "+" wire, to the speaker frame? Alternatively, the resistor network could be mounted in its own enclosure, with 1/4" jack plugs between the OT & speaker. Remember, NEVER turn on a tube amp without the appropriate speaker load attached...you will kill something! So always make sure however you do it, that everything connects securely.
This is called a resistive attenuator, there are lots of articles on "how to" on the web, with a bit of thought you can have a switchable box, going from "attenuated" to "stock".
For 1/8th power (9db drop, or from 5W 0.6W output) the load resistor will be half the speaker value in ohms, the dropping resistor will be 66% of the speaker value in ohms. So a 4 ohm output will have a 2ohm resistor in parallel with the speaker and a 2.7ohm resistor dropping the signal from the OT "+" plus wire.
This same principle can be used for any amp of <5W.
Last Edited by on Mar 21, 2011 11:39 AM
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