Putting the Simple SE back together - blown PT - Flashing Rectifier
I got my new Allied PT today - 6K7VG and hooked everything back up. When the old PT blew, one of the FRED's melted and fell off the board so I took the other one out too.
When I plugged everything back in, the rectifier after about a few seconds gave a heck of a lightshow and R1 started glowing. I quickly unplugged the amp.
The new FREDs arrive tomorrow but at this point it would seem that I need a new rectifier tube.
The driver and output tubes filaments glowed as normal for the brief period that it was plugged in.
I have a choke now but did not hook it up for the test. Once I get the new FREDs installed I will remove R1 and hook up the choke.
Any other obvious things I should check for the next time I fire it up?
Re: Putting the Simple SE back together - blown PT - Flashing Rectifier
Massively overdrawing the power supply will also the cause the arcing you observed in the rectifier. It sounds like you have a dead short somewhere along the B+ rail.
I'm surprised the fuse didn't blow.
Indeed a glowing ceramic resistor was surprizing to see but it after a few seconds it light up.
I am remounting everything on a new sheet of metal so I did not actually have the fuse in line yet - I have everything laid out on my "test" bench.
Here is a question, what is the "proper" way to run SS rectification for the Simple SE?
Put a fuse on it. There's no sense burning up a transformer.
Check your wiring carefully. Since you've moved everything to a different chassis, it's likely you made a simple wiring mistake somewhere. I'll bet you can probably find it with an ohmmeter. Keep the amp turned off, obviously. Maybe you somehow put a jumper in the wrong spot, and shorted the B+ terminal of the T1-PRI or T2-PRI straight to ground? I can't recall whether you have this amp wired up for ultralinear or cathode feedback.
It's also possible that a failed C2 cap could short to ground, but I can't imagine what would have killed C2. It worked fine the last time you had the amp power up.
Basically, you have the correct idea. Just put a jumper across the SW1 terminals. You may leave the 5AR4 installed, or not. It doesn't really matter. Removing the 5AR4 will take a two amp load off the power transformer's 5 volt winding. That will let the PT run a little cooler. Leaving the 5AR4 in won't really affect much at all.
Keep in mind the solid state rectifier diodes have very little voltage drop, and no warmup time. If you have SW1 jumpered when you turn on the main power, the B+ will almost instantly rise to 1.4 times the power transformer's secondary winding voltage. In your case, I guess that's 375 volts (750 VCT is 375-0-375) times 1.4, which equals 525 volts. It'll probably be even higher than that, since I expect the 6K7VG rating is for a loaded secondary winding. When you first turn on the amp you don't have a 100+ mA load from the power tubes, since they aren't warmed up yet. My Hammond 374BX is also rated 750VCT, but in reality the secondary is about 395 volts unloaded. I measured peak B+ voltages in excess of 530 volts when I was using a 5U4 rectifier tube.
What does all this mean? Your power supply caps (C1 and C2) will see an initial voltage surge that exceeds their working voltage. I recall the Panasonic caps which I bought from DigiKey are rated 500 working volts DC, and 550 volts surge. They didn't specify how long the surge can last. It also means your output tubes are subjected to >500 volts on the plate with no electron cloud surrounding the cathode. If you believe in cathode stripping, maybe this concerns you. I'm still on the fence about the issue of cathode stripping, so I can't offer much guidance here.
I might suggest keeping the 5AR4 in place and using it to soft start the amplifier. After everything is warmed up (30~60 seconds) close a switch installed at SW1 to enable the solid state rectifier diodes. However, I suspect you are wanting to use the SS diodes in lieu of a working 5AR4. In that case, I would recommend installing a CL90 (inrush current limiter) in series with the power transformer's primary winding. You can get them from Mouser:
Good luck with your rebuild. Post a photo of your new chassis layout and wiring. Maybe someone will spot a wiring mistake.
Tonight I installed the new FREDs and put the jumper wire back in. I inspected the board and checked for ground faults to the best of my ability.
Everything looked good so I plugged it in without tubes or OPTs hooked up to get some voltages. Everything seems to be in order EXCEPT that C2 does not read any voltage.
This does not seem right correct? C2 and C1 should be reading the same voltage right? Ty mentioned maybe blowing the C2 and it looks like when the old PT blew it took the FREDs and C2 with it.
I did doublecheck the bell housing on the new PT and ran some shrink tubing around the wires where they exit - for some added precaution.
Carl! Those Panasonic caps ARE NOT rated for 579 volts!
Exploding electrolytic capacitors are messy business. Ask George...
Either way, if the resistor is getting more than warm something is wrong, really wrong. As stated before continued operation in this state WILL fry your new transformer. Look at it this way, the wire inside the transformer is thinner than the wire inside the resistor. It is getting hot too.
I just noticed your new post as I was typing this. Your voltage readings show that R1 has had enough of this and is now dead. You probably still have a serious short down stream. A short like this is best found with an ohmeter.
Disconnect the power source. Let the amp sit for at least 5 minutes. Connect the black lead of the ohmeter to ground. Touch the other lead to the L1 terminal that connects to C1. The resistance reading should be fairly high, over 50 K ohms. Then probe the other L1 terminal. The resistance should also be in the same range. If the reading is low (under 10K ohms) there is a short. Disconnect both OPT's and take the reading again. If the reading is still below 10K C2 is probably shorted. If the reading increases, there is a possible wiring error in the OPT's or one OPT is shorted.
OK, something about the R1 resistance reading did not seem right. I doublechecked the wiring schematic on George's site and asked myself how would C2 not be getting any power?
R1 was indeed dead...
With all the other readings similar to before the incident and no other ground faults apparent I plugged in the choke, the tubes, and hooked up the OPT's. Been playing off the ipod for the better part of 30 minutes with great results utilizing SS rectification.
I suspect the old 5ar4 tube to be shot - new one is on order. Thanks for the suggestions guys. I think we are back in business.
With load now
B+ at the OPT=493
Wall voltage is 122
Watch your tubes carefully for signs of red glow on their plates. 39 volts at the cathode over 560 ohms is just under 70 mA. 493 volts B+, minus the 39 volt bias, leaves about 450 volts across the tube and transformer. There will be some voltage drop over the transformer, but not more than a few dozen volts or so. Your tube is dissipating somewhere near 30 watts, which is on the high side for 6L6GC.
I'm also worried about exceeding the rated working voltage of C1.
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