As noted previously, the amp had a loud hum with no inputs connected and the volume and tremolo turned all the way down. Nothing I could do for it, I was going to have to open the case.
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!
Hello ! Where would you cut to install the Stand By switch then? I love the sound of my skylarks with the 6X4, but just in case I wanted to try the solid state rectifier in one of them, can you recommend me the best place you think to insert the stand by switch?
Thank you !
Good for people to know.
Ignacio,
Well.. I used a Weber Copper Cap rectifier (22 bucks) as a replacement and it actually delays for you. It’s built to be a drop in replacement and goes right into the tube socket.
If you want to do standby, You need to make sure that the heaters are getting juice and plates etc. are not.
Referring to the drawing above, I would put the switch on one of the lines out of the high voltage secondary from the power transformer.
Right at the point where the ‘V5′ is.
(Sorry this took so long)
Perhaps it would be better to put the switch on the cathode line leading out of the rectifier (the line going straight up, next to the x’s )
That way the Transformer is not floating with power on the primary. Not really sure that it matters.