[Fitz]

Quote:

Originally Posted by ethan961 /img/forum/go_quote.gif I believe it is rated for 100V, so there should be no worries. TomB didn't warn me of it (I am having the same problem) and I believe it is on one of his sites at DIYForums.org. The tubes normally run at half voltage, so I wouldn't be worried. I can't currently check the resistance on my amp but I still cannot see anywhere there might be a short. I might get some pics up when I get home tomorrow evening.

The plate is normally run at around 100V (max 300V), but runs at half that in the SSMH. The filament is absolutely not rated to 100V, and running it above 19V is a very good way to damage it. Even going only slightly over the rated filament voltage will rapidly diminish tube life.

 

[hitman47]

Quote:

Originally Posted by ethan961 /img/forum/go_quote.gif Something must have shorted to ground, possibly damaging something.

Is it possible that it might have come from the back side (the metal part) of one of the caps? Cause those are quite close to the casing in my build, I gotta get a layer of plastic in there

 

[Nonchalance]

Quote:

Originally Posted by Fitz /img/forum/go_quote.gif The plate is normally run at around 100V (max 300V), but runs at half that in the SSMH. The filament is absolutely not rated to 100V, and running it above 19V is a very good way to damage it. Even going only slightly over the rated filament voltage will rapidly diminish tube life.

Shoot... Oh well I'm lucky that the tube's even alive.

I still have no idea why it's 48V on the right tube pin 3... I don't think it's the resistors because the circuit wouldn't even go through if a resistor was messed up. Which leaves the MOSFET, but how can a messed up MOSFET change the voltage?

I'm not sure what test to do at this point.

 

[tomb]

Quote:

Originally Posted by ethan961 /img/forum/go_quote.gif I believe it is rated for 100V, so there should be no worries. TomB didn't warn me of it (I am having the same problem) and I believe it is on one of his sites at DIYForums.org. The tubes normally run at half voltage, so I wouldn't be worried. I can't currently check the resistance on my amp but I still cannot see anywhere there might be a short. I might get some pics up when I get home tomorrow evening.

I'm not sure what it is I didn't warn you about, but I'm absolutely certain I never said that more than 19V was OK for the heaters.

Fitz is absolutely correct -
Triode/double triode tubes have two voltage limitations that we are concerned with: the plates and the heaters (filaments). In the 19J6, the plates are rated for 100V and will go much higher (300V) without too much of an issue. As you state, in the Starving Student design - the plates are running at less than half of the smaller voltage spec.

None of that counts for the heaters, though. Those are on a different circuit - they are used to "turn the tube on" and are what makes a tube glow. In the 19J6, those heaters are run at 19V (spec'd at 18.9V). Most (if not all) tubes have very strict limits when it comes to exceeding heater voltage - like 1 or 2 volts before tube life is severely reduced.

 

[scompton]

Quote:

Originally Posted by hitman47 /img/forum/go_quote.gif Is it possible that it might have come from the back side (the metal part) of one of the caps? Cause those are quite close to the casing in my build, I gotta get a layer of plastic in there

Quote:

Originally Posted by Nonchalance /img/forum/go_quote.gif Shoot... Oh well I'm lucky that the tube's even alive. I still have no idea why it's 48V on the right tube pin 3... I don't think it's the resistors because the circuit wouldn't even go through if a resistor was messed up. Which leaves the MOSFET, but how can a messed up MOSFET change the voltage? I'm not sure what test to do at this point.

My first build, I had lots of long leads on resistors and caps. First time I powered it up, everything worked fine. When I closed the lid, stuff got pushed together and I had a short. It fried a mosfet which caused the tube to glow very bright.

 

[Nonchalance]

Oh my god that sounds exactly like what I did. Same symptoms too! I'll grab a new MOSFET from Radioshack tomorrow...

Have to cut off the nice heat shrink though, haha

Thank you so much for the info, cheers

 

[hitman47]

Quote:

Originally Posted by scompton /img/forum/go_quote.gif My first build, I had lots of long leads on resistors and caps. First time I powered it up, everything worked fine. When I closed the lid, stuff got pushed together and I had a short. It fried a mosfet which caused the tube to glow very bright.

I thought of that too, but my tube doesn't glow brighter than usual, the one on the other side just doesn't glow. That's what makes me think it's not one of the FETs, but I can't be sure. I just need to know what I should replace and where I should check voltages and currents to figure it out...

 

[kuroguy]

Regarding the 48 volts at pin 3, I'd bet your mosfet is shorted from pin 2 to 3 and pin 1 is open. Remember Ohm's law: E=IR. for there to be no voltage drop from the node marked 48 volts to pin 3 of the tube there would have to be no current flowing through the mosfet from pin 1 and no drop (resistance) through the mosfets at pins 2 and 3.

Of course, this is only a guess, and I am a mechanical engineer as opposed to an electrical engineer.

 

[Nonchalance]

I got my amp to work! Thanks everyone, it was the MOSFET after all.

It sounds really good, but there's a slight hum and a really weird noise every 10-20 seconds or so. Wonder if I can fix this. (I have this problem both with the sources connected and disconnected).

 

[hitman47]

Quote:

Originally Posted by Nonchalance /img/forum/go_quote.gif I got my amp to work! Thanks everyone, it was the MOSFET after all. It sounds really good, but there's a slight hum and a really weird noise every 10-20 seconds or so. Wonder if I can fix this. (I have this problem both with the sources connected and disconnected).

I had that when my amp still worked, I think the problem in my case was that I didn't have enough ground resources, try using an additional copper plate or something like that...

BTW, anyone have any help for my amp? Should I just assume it's one of the caps and replace them all on that side?

 

[Llama16]

Wow, I lost this thread a while ago

I tried reading through all of it but got to p200 and then I gave up

.

My mouser order is holding me back, especially the backorder for the p2p build. I just read that the 19j6 tubes got disconnected and saved for and additional 25kits.
Keep us updated if they'll become available again seperately because I want to do the p2p as well

the PCB seems too easy, and it doesn't allow for any basic mistakes that occure in p2p and from which you can learn.

I'm currently doing the mini3, some cmoys and the pcb SSMH (hopefully the p2p as well someday)
Any other recommended (useful

) basic builds that I could order for as well.
Shipping costs and VAT cost a fortune :S

 

[Nonchalance]

The slight buzzing should be ground, but the regular noise is what's bothering me.

 

[Nonchalance]

I got rid of the buzzing by doing more connections to ground on my panel components, but the slight noise at a regular interval is still there.

 

[dsavitsk]

Quote:

Originally Posted by Nonchalance /img/forum/go_quote.gif I got rid of the buzzing by doing more connections to ground on my panel components, but the slight noise at a regular interval is still there.

It sounds like something is oscillating -- are there stoppers on the pins of the fets and tubes?

 

[tomb]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif It sounds like something is oscillating -- are there stoppers on the pins of the fets and tubes?

R14 and R15 are the grid stoppers here:


and R3 and R9 should be attached directly to the Pin 3's on the MOSFETs - right?