Building by section

[gui_tarzan]

After a few failed attempts I figured I'd build this amp by section and see where I'm going wrong. My goal is a Princeton 6G2 base model with a few tweaks after I get the darned thing running.

Using parts I have on hand, I paired the two 16/475 caps in series (pre-standby) which I know only gives me 8uf there, but this is a test so the capacitance value shouldn't be all that important right now. However, when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too.

Is 505v too much for that type of resistor? And why am I getting 505v (DC) instead of mid-high 400v with a 5Y3? I realize this is unloaded but my pre-rectifier voltage is only 373v per leg with the CT going to ground so it seems a bit high.

[Tillydog]

...when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too.

Sounds to me like you have a short downstream of that 470 ohm resistor. (Is that middle 22uF cap OK?)

[ToneMerc]

After a few failed attempts I figured I'd build this amp by section and see where I'm going wrong. My goal is a Princeton 6G2 base model with a few tweaks after I get the darned thing running.

Using parts I have on hand, I paired the two 16/475 caps in series (pre-standby) which I know only gives me 8uf there, but this is a test so the capacitance value shouldn't be all that important right now. However, when I turned this on it immediately popped the 470 ohm sandstone resistor. I tried a second one thinking the first one might have been defective and it popped that one too.

Is 505v too much for that type of resistor? And why am I getting 505v (DC) instead of mid-high 400v with a 5Y3? I realize this is unloaded but my pre-rectifier voltage is only 373v per leg with the CT going to ground so it seems a bit high.

An input of 373VAC would give you well over 500VDC rectified, unloaded. Most likely voltage is not the problem, its current draw (a short) that is causing it to open.

TM

[gui_tarzan]

Looks like that was it. I replaced cap #2 with cap #3 and it didn't blow the resistor this time. I measured resistance on cap #2 with the leads both ways and have zero resistance but not an open. It also didn't show a rise and fall in resistance when I did that like it does on others I've tested so I'm thinking that one is shorted.

Thanks guys!

[vibratoking]

The recommendation for capacitance directly hanging on the output of a 5Y3 is 20uF. You have 30uF. You should add a series dropping/isolation resistor between the series caps and the rest of the filters. Further, the caps look like shorts when you apply power with all the caps discharged. This means you have 500V through that 470 Ohm resistor directly to ground for a short time. That's 1A of instantaneous current and 542W of instantaneous power. Try something like a 5K resistor in series. The actual value will depend on the real load and what you are going for.

With no load I would expect 373*1.414 = 527V. I think you are in the ballpark.

[gui_tarzan]

Ok, correct me if I'm wrong but doesn't the capacitance cut in half when they're in series?

[eniam rognab]

Ok, correct me if I'm wrong but doesn't the capacitance cut in half when they're in series?

if they are the same value, yes. old adage i learned about resistors in parallel and caps in series is the final outcome will be "less than the least"

so a combined say 47 and 22 uf in series will be less than 22

[vibratoking]

Yes only if they are the same value, but what you have is 16uF in series with 16uF which is 8uF. Then you have that 8uF in parallel with 22uF, which is 30uF.

BTW, it sounds like you may be making capacitance measurements on caps that have charge in them. Make sure you are safe and discharge those caps before handling and making measurements.

[gui_tarzan]

Oh no, I discharged them first. I don't like getting shocks and I certainly don't want a shock with high voltage. I also have a fuse between my IEC connector and the on-off switch. That's not on the drawing but that's the only thing that's not there.

And you are correct, I don't have a choke between the first pair of 16uf and the 22uf. I will put that in there.

[gui_tarzan]

Here's the schem added to the layout. I will re-draw the schem with one of the programs available but this is directly from my circuit so far.