Why install a vacuum tube rectifier?

With a vacuum tube rectifier your amplifier will sound different because of the sag of the power supply voltage. Once the tube socket and 5V filament transformer are installed, you may install your favorite rectifier tube. A SS rectifier plug containing diodes may be installed in place of the tube. A switch may be installed to select SS rectifier or tube rectifier.

Some amplifiers have power supply voltages at or above the ratings of the output tubes. While the original output tubes may have been selected to "take it," you may have problems with replacement tubes. A tube rectifier helps by producing a lower voltage (50 Volts less for a 5U4 for example).

See Figure 2 for a generic power supply schematic with hyrid bridge rectifier.

Can I install different output tubes?

Possibly but each amp is different and the electrical ratings of the tubes and of your amp must be taken into consideration and adjustments made as required. For example, it may be necessary to change the screen resistors to a larger wattage and add an additional filament transformer if installing EL34 in the place of 6L6.

Can my tube amp be modified for more power?

Well, yes but this can get expensive and your amp may not sound much louder even if the power is doubled. Doubling the power would require changing the power and output transformers and changing or adding more output tubes. Maybe all you need is more speakers or more efficient speakers. Or place a mic in front of your speaker and play through the house PA system.

How should the power cord wiring be connected?

Your amp should have a chassis-mount 3AG fuse holder so the fuse may be replaced without taking the amp apart. The power cord black wire should go to the rear connection of the fuse holder. Reason is, as you withdraw the fuse it will not be electrically "hot" as you remove it. Then run a well-insulated (600 Volts) wire from the side terminal of the fuse holder to the power switch. If possible, run this wire through the air via the shortest path and do not tuck it next to the chassis. Connect the other terminal of the power switch to the transformer primary. Connect the other transformer primary wire to the white wire of the power cord using a blind-end crimp splice. Do not use a terminal strip as was done on some old equipment. The green wire needs it's own dedicated screw that is not being used to hold something else, and on painted chassis the paint must be removed to assure a good ground connection. Remember you are relying on that ground connection to pass enough current to trip a circuit breaker somewhere if there is a fault and current may be greater than 20 amps. There are lots of amps out there that are not wired this way and they have not passed UL either. Figure 2 shows proper power cord wiring.

What about a ground lift switch?

The green ground wire must be connected solidly to the chassis. Do not use solder which may melt if there is a fault. Do not install a ground lift switch.

What about those old amps with a capacitor from power line to ground, or a switch to select the cap to either side of the power line?

Any old equipment with a capacitor from power line to chassis is a safety hazard. Any power line to chassis caps should be removed and a 3-wire power cord installed as described previously.

What about those cheap amps with tubes like 50L6, 50C5 and 35W4?

Those were and are dangerous because the AC power is connected directly to the chassis and therefore to your guitar. When using those amps you had to avoid stepping on a floor furnace or touching the mike stand or other players. But those amps can be fixed by adding an isolation transformer and 3-wire power cord. Most can be further improved with different tubes such as 6AQ5 and 6AU6 and by adding a full-wave rectifier and a quality speaker. Below is a schematic for a power supply for operating these amps from a 12V wall-wart transformer:

There is usually room on the chassis for the 12.6V to 120V transformer.

Old projector amplifiers contain quality parts and may be modified into guitar amps.
The amplifier pictured below uses push-pull 6V6's.

Disclaimer: The information presented on this web site is for experimental or informational purposes only. There is no guarantee or warranty associated or implied with the use of this information. Some of the circuits have not been built or tested and may contain inaccuracies. The described voltage levels are lethal and vacuum tubes become hot and can cause severe burns. Use this information at your own risk. We are not responsible for damage, monetary loss, injury or other issues resulting from your use of this information.