So I snipped off the offending diodes, and replaced the dropping resistor and voila! we are no longer blowing fuses and voltages are back in spec. We don’t know what damage may have been caused by those damn things… I stuck in a real 6X4 rectifier and I still get the loud hum.

To add to the fun, a buddy of mine, the guy who came up with ‘Paleo-Electronics’, found the Weber Copper Cap rectifiers. These are the RIGHT way to do solid state rectification. They are simply tubes with solid state components, but done right, they protect your other tubes in a way that diodes can’t.

In a tube rectifier, high voltage is not passed until the tube warms up. As the tube gets closer to temperature (various widely, but well under a minute) it produces more voltage. It’s not until the tube reaches operating temp that it puts out full power. This is a good thing because the other tubes are also warming up. If they get full voltage before they are warm, they can damage the tube immediately or at least have their lifetimes seriously curtailed.

The Copper Cap rectifiers mimic this warm up with what I assume is a capacitor network inside the bottle. They even mimic ‘sag’ (the failure to produce full power when driven hard and quickly) although that is a conversation for another time.

You can get them here

I tried swapping the Weber out vs. the Original 6X4 I had in my other GA-5 and while I could hear a slight difference in the reaction to hard fast strumming, that was it.

So I took off the back to have a first look. And sure enough.. there was something I had missed. There was a piece of silver tape over one of the tube sockets. WTF? The missing tube is the 6X4, which is a rectifier (converts AC into DC, sort of). There are two diodes soldered onto the tube socket. So I go googlin’ for a reason and find out that some people replace the tube rectifier with a diode network.

I email the seller, explain what I found and ask if the amp’s been modded. He says he doesn’t know anything about amps, but it was blowing fuses and he took it into the shop and now it sounds great, honest!

Modern rectification has come a long way from the old tube style rectification. With a little thought, you can replace the old tube with a solid state circuit. There are good ways to do it, but this wasn’t one of them. Lets look at the circuit :

V5 is the rectifier tube. When you power up the amp, two things happen : 1. About 260 VAC starts hitting the plates of the rectifier (on the left and right sides) and 2. 5 VAC starts hitting the heaters of the 6X4 tube (the ‘V’ with x’s above at the top of the tube).

Once the tube has warmed up, current will start flowing up the cathode (shown leaving straight up from the tube). However, until it warms up little if any current will flow. This gives the other tubes in the circuit that are being powered a chance to warm up before they get hit with 240 V on their plates.

Diodes, of course, don’t warm up. For reasons that I don’t want to go into, you really want the tubes to warm up before you hit them with power. Mind you, I’m not talking about playing the guitar, just powering up.

So now all my tubes are suspect. Yay!