Kalamazoo Amp Field Guide:

Adding a Standby Switch to an Amp

DISCLAIMER:

Tube amplifiers contain potentially lethal, high voltages even after they are unplugged, that may cause personal injury or death. Do not attempt to repair, modify, or work on any amplifier unless you are absolutely certain you know what you are doing.

NO GUARANTEE

These mods are all things I have tried, someone I know has tried, or are recommended by people who work on amplifiers for a living. Nevertheless, if you try any of these, you assume all responsibility for anything that happens, whether the amp explodes, you get zapped, or the amp suddenly increases in value because everyone falls in love with it. The glory, the pain, whatever, they're all yours. If you can't live with that, don't mess with the amp!

A standby switch allows you to warm up the tubes before applying the higher voltages used for amplification. Proper use is to make sure the switch is in standby (open), turn the power switch on, wait a minute, then flip the Standby switch to play (closed). At this point, the amp is ready to play.

While this is optional with the standard Model One and Model Two (or any amp with a controlled warmup rectifier), it may make more sense to add one to a Reverb 12 (or any amp with a solid state rectifier, or rectifier tube without a separate cathode). OTOH, there is great debate within the tube amp community whether this is a problem at the relatively low voltages prevalent in guitar amps (compared to the thousands of volts typically found in RF transmitters)

The following is verbatim from email by Justin Belshe.

``For this, I drilled a hole in the chassis and put in a big switch. Cut the red & yellow wire that runs from your power transformer to the rectifier socket and insert the switch.''

This is a common way to handle standby. But as someone at ax84.com pointed out, this can be a safety issue as you are interrupting a ground connection. It's somewhat safer to add the standby in the B+ line, either immediately after the rectifier or after the first cap or two, assuming that's still before the power tube[s] B+ line. If you put it after any caps, make sure those caps have a voltage rating high enough to handle 50V to 100V over the normal B+; without a load the voltages will be higher. Check the voltage after the rectifier, with everything else disconnected, to be sure you won't run the caps at too high a voltage.

Example:If the normal B+ with tubes in place is 250V, the B+ with the tubes disconnected by a standby switch might easily be 300V to 330V. While a 350V cap might seem safe here, a very minor spike on the power line would result in an over-voltage. I'd want to use a 450V cap in that case.

[Wiring diagram is here]

Combined Power / Standby Switch

If you move the power switch off the tone (treble on Reverb 12) control (moving power wiring away from signal wiring is a good idea; it reduces hum and in extreme cases improves safety) you can use a DPDT On/Off/On switch for power and standby; wire one side of the switch for power, and the other for standby. Wire it so that when it's in Standby, you have AC but no DC (B+); in the On (Play) position you should have both. These switches are usually off in the middle, but that's OK; just mark things appropriately. Switching through Off to go from Standby to Play seems scary to some folks (it did to me initially) but the duration is brief enough that no harm is done. Lots of Ampegs did this, and Ampegs are known as tough amps. Use a switch with a 250VAC, 1A or higher rating.

This can be a toggle or slide switch (or rotary or whatever you like). A standard toggle fits nicely in the fuse holder hole (you can move the fuse holder to a new hole by the power cord hole). On the Reverb 12, a standard slide switch can replace the auxiliary outlet, which you probably want to dispose of after the three wire power cord conversion.

Thanks!

Special thanks to Justin Belshe <JBelshe@AOL.com> (who would like to thank Tim Swartz <TimTube@AOL.com>) for some of the ideas on rebuilding this amp.


Last updated: 28 October 2007

Copyright Y2K, 2004, 2005, 2007 Miles O'Neal, Austin, TX. All rights reserved.

Miles O'Neal <meo@XYZZY.rru.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514