Bad diode?

[Diverted]

I just powered up a new build for the first time. It started right up, so I played around with it for a while and all was good. Voltages right on, etc. everything working as it should. Turned it off and came back later, turned it in and it blows a fuse.
Weird. I hadn’t touched the circuit. I Disconnected the first filter and choke from the rectifier (two diodes), replaced the fuse and started it again at low voltage. It immediately blew.

I started checking the transformers, all good. Removed 6.3 winding and all tubes from circuit, replaced fuse, It blows again.

Finally removed the HV secondary wires from the diodes, tested each half to ground AC. They were fine, 6.3v winding was fine too. So by the process of elimination it had to be the rectifier. I took the diodes out, replaced them and the fuse, powered up and all is well again.

I have never seen a diode fail like this. Maybe I left the iron a little too long on one or both? They were 1kv/3a 1N5408 diodes. I did not expect that!

[PRR]

1000V diodes are barely good enough for 500V of B+. Are you in that zone?

[Diverted]

B+ is around 365.
Transformer is a Hammond, 275-0-275@104ma.
The amp is a single ended Princeton 5F2 circuit (with choke) and Fender two-tube reverb. This is the third one of these I’ve built and it’s pretty much the same board with a few small changes here and there.

I’ve cycled the amp on and off a bunch of times since replacing the diodes and am checking voltages, bias and current draw. Everything is right where it was before the diodes went and the voltages are all about right. Amp currently biased at 90 percent w/504 ohm cathode resistor. 20.5 volts across it.

I just am at a loss to understand the diodes going.


One question. The 6v6 screen grid is around 5-6v higher than the plate... schematics I’ve seen for this amp online show this as common, or maybe 2-3 volts above plate. Should I put a grid stopper and screen resistor in?

[sluckey]

I would not bother with grid or screen resistors. Build it exactly IAW this schematic and play it like a student amp.

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2.pdf

[Diverted]

Looking at that original schematic, both the screen and plate are fed by the same node.

My power supply looks like the other schematic. Dropping resistors are 1K and 10K ... and this is the first time I’ve tried a choke in this circuit.

Should I try increasing that first dropping resistor, leaving it as is or switching to the same-node arrangement as shown in the old schematic? With a resistor instead of the choke, my other two builds had lower screen voltage w/respect to the plate.
Thanks.

[Latole]

I'll put a 470 ohms 1 watt resistor at the screen like Blackface amps

[PRR]

Don't obsess about screen-to-plate voltage relationship. This is not 1928.

[Fresh_Start]

The voltage difference is simply choke vs. dropping resistor.  You have better filtering with the choke.

It's been a long time, so I had to find the schematic for my 5F2-A derivative.  I put the choke before the plate node with a "reservoir" filter cap, then a 10k/1W dropping resistor between the plate node and the node for the screen grids.  470 ohm screen grid resistor and 1.5k control grid resistor.  Is all that overkill?  Probably but it was easy enough to do and the amp is pretty darn quiet.
Cheers,Chip

[Diverted]

GAAAAHHHH! Here's a video:

Well this amp continues to vex me. At this point I'll try anything. I thought the buzz was gone, but it persists (I incorrectly said/hoped/wished it was gone after listening to/playing the amp in a noisy basement that drowned out the noise).

The video should illustrate the issues:
1. The buzz starts right off at idle, volume 0. It increases until I hit about 9 on the volume, and then it vanishes.
2. Meanwhile, the volume pot does not have a smooth increase in volume. It's very staticky and jumpy.
3. Meanwhile, the hiss is also apparent at idle, volume 0, and gets quite loud dimed.

I have done as many things as I could (but apparently not?) to root these issues out. I swapped out the volume pot, added a 47K grid stopper to V1B as suggested here, reworked the 6.3v heater wires between V1 and V2 to separate them more from upper leads. I tied all the power grounds to one lug, as opposed to the earlier configuration, in which they were all tied to a ground that ran along the bottom of a metal-clad terminal strip the diodes are attached to. I've tried many tubes. I've chopsticked, deoxed, tightened the grounding points at the transformer and input jack, etc etc etc. The crappy noise persists.
What am I missing?

[Keppy]

Whatever else is going on, it sounds like there's DC on the volume pot. Have you metered for voltages there? If there's DC on that pot, that would indicate a bad coupling cap from the previous stage or a fault in the following tube stage.

[Diverted]

Currently, at volume 0 I have no DC on the wiper. Dimed , I have about 100mv. I don’t know what is acceptable for leakage but I’m thinking 1/10th of a volt is low?

Thank you.

[pdf64]

I suspect oscillation.
Try fitting a 1k5 grid stopper to pin 5 of the output valve socket.

[Diverted]

Thanks. After I took this video I did that, with no change in the issue. Thanks though!

[pdf64]

Hmm, how about if one end of the 22k feedback resistor is lifted?

[sluckey]

Currently, at volume 0 I have no DC on the wiper. Dimed , I have about 100mv. I don’t know what is acceptable for leakage but I’m thinking 1/10th of a volt is low?

Temporarily put a .1µF cap between the treble wiper and the top of the volume control. If that stops the scratchy volume control then one of the tone stack caps is leaky, most likely that 250pF treble cap. If still no joy clean the pot or replace it.

[Diverted]

Hmm, how about if one end of the 22k feedback resistor is lifted?

Thanks for the suggestion. I did try removing the feedback resistor, no luck.

[Diverted]

Temporarily put a .1µF cap between the treble wiper and the top of the volume control. If that stops the scratchy volume control then one of the tone stack caps is leaky, most likely that 250pF treble cap. If still no joy clean the pot or replace it.
[/quote]

Thanks, I tried that the cap, no luck.
I am now on my second volume pot. I replaced it a few days ago, thinking/hoping that was it. But it's not the pot, unless I got two bad CTS pots in a row.

I have replaced or paralleled new caps across every capacitor in the amp, hoping that one of them was leaky and that that would be overcome by tacking in a good cap.This goes for filter caps, cathode bypass caps, ceramics and film caps. Crazy! :(

[Latole]

Show us a good picture of your wiring

[sluckey]

You cannot test for a leaky cap by putting another cap in parallel.

Two new CTS pots. Squirt some De-Oxit in it.

[Diverted]

You cannot test for a leaky cap by putting another cap in parallel.

Two new CTS pots. Squirt some De-Oxit in it.

Sorry, I’m not trying to be a know it all.. I did de-ox both of them, a few times.
I thought paralleling two caps would at least reduce overall leakage, presuming the other one was good. 

[sluckey]

Well let us know this stuff and maybe we can avoid stupid suggestions.   

[Diverted]

Gotcha. Sorry, I just assumed it was such a commonplace thing, I didn't mention it. I'll try to avoid leaving out those details! Thanks, the help is appreciated as I don't know where to turn from here.

Anyway, someone asked me about my current wiring. Here is the most recent wiring (with the 100uf first cap) and a photo I posted here at the beginning.
I have lots of wiring shots, but if you look at the imgur link at the top of this thread, you can find them. The pots haven't changed layout-wise, but a few things have been changed on the sockets etc. If anyone would like to see anything else, please let me know!

https://imgur.com/a/76RJfJu

[Latole]

Gotcha. Sorry, I just assumed it was such a commonplace thing, I didn't mention it. I'll try to avoid leaving out those details! Thanks, the help is appreciated as I don't know where to turn from here.

Anyway, someone asked me about my current wiring. Here is the most recent wiring (with the 100uf first cap) and a photo I posted here at the beginning.
I have lots of wiring shots, but if you look at the imgur link at the top of this thread, you can find them. The pots haven't changed layout-wise, but a few things have been changed on the sockets etc. If anyone would like to see anything else, please let me know!

https://imgur.com/a/76RJfJu

Now I remember where I see those pictures ; https://el34world.com/Forum/index.php?topic=27591.0