I was thinking it may be the rectifier .. seen it happen before.. in the future please install backup diodes on the recifier socket
I have been using the chinese 5AR4 rectos for years now.. physically they are dimensionally like the mullards of lore and they are heavy as in weight.
they are substantial. If you cannot get a NOS mullard or a Japanese mullard. (matsushita) then definitely go for this one..
https://www.tubesandmore.com/products/vacuum-tube-5ar4-str-tube-amp-doctor
@mresistor
Digging into why the diodes can be saviors I surfed around and found this explanation, makes a lot of sense. I have 3 other tube rectifier amps I've built in the last 3 years, seriously planning to upgrade them all in this fashion. Thanks for the lead.
https://boogieforum.com/threads/simple-mod-for-more-reliable-rectifier-tubes.44168/I see alot of amps and recieve alot of phone calls from people wanting to know why their amps are blowing fuses. I would say the majority of the time it's due to shorted rectifier tubes. These tubes tend to take alot of abuse, more so than your other tubes and I'll explain why as well as show you a very simple and inexpensive way to greatly increase the reliablility and the life of your rectifier tubes. The voltages I'm going to use here are for example only but you'll get the idea. All amps will produce different voltages. Ok.... here goes.
Most of us know the rectifier tubes function is to take the AC current from your wall and convert (rectify) it into DC current. This is considered full wave rectification since both cycles of the AC current is being used. The tube is a twin diode meaning it's 2 tubes in one. Your Rectifier series amps use two of these tubes.
Your power transformer has a lead attached to each end of it's coil and a center tap that goes to ground. Each end of the coil goes to one of the plates in your rectifier tube. The cathode of the rectifier tube is where AC current enters the tube. Since AC current flows in both directions the plates change back and forth from positive to negative. Now this is where the problems come in. When one plate is positive, the other is negative. The problem that arises here is something called PIV (peak inverse voltage).
Lets just say for argument sake that your power transformer has 300 volts from each end of the coil to the center tap. Will also say that when in use the cathode of the rectifer tube may have 500 volts. By using these numbers we can determine that when the tube is conducting with 500 volts positive on the cathode and 300 volts positive on one of the plates there is only a 200 volt difference. Now lets look at the other plate that is not conducting. It will have a negative 300 volts on it leaving the difference in potential between itself and the cathode at 800 volts. This really increases the risk of arcing inside the tube and could reduce the life of the tube.
What we can do to avoid this is simply install a diode in series from each plate to the lead on the transformer. I like to use 1N5399 diodes. They are rated at 1000 PIV at 1.5 amps. To install you simply discontect the transformer leads on the rectifier tube sockest and install these diodes in series with the diode's cathode toward the tube socket and the transformer wire on the diode's anode.
What this will do since diodes are like check valves and only conduct in 1 direction is this: when one plate has 300 volts positive on it, instead of the other plate having 300 volts negative, it will have 0 volts since the diode stops the plate from seeing that negative 300 volts. Now the tube will only see a 500 volt difference in potential instead of 800 volts. This greatly reduces the risk of arcing inside the tube and increases the life of the tube as well.
Another really cool feature about this mod is if your playing on stage in front of hundred of screaming fans and your rectifier decides to short cathode to plate the amp will still work since you have installed a series solid state rectifier. How is that for reliability!!!!!
So you see for about 2 dollars and 15 to 20 minutes of time you can beef up your rectifier circuit for better safety and reliability and these diodes in no way effect the sound or sag of the tubes.
Topic: AB763 single channel PT smoked (Read 3239 times)