Debunk (or not) my little theory - rect tube socket

[pjd3]

Happy 4th all,

While venturing into my "tube design station" thing, I've come upon having the primary coil fuse (2A) of the PT blow at various times but so far haven't been able to figure out why.
Note: - unused PT coil wires are always terminated with heatshrink

However, the other day while testing out my RobRob's bias trim and balance circuit that I built on a dedicated 2" x 3" board, all was going fine, but then 2 fuses blew back to back. Currently, there are no heaters wired up, no HT load except for a 22uF 1st cap and another 22uF after the choke. Pretty normal I would think.

The second blown fuse occurred even after removing the GZ24 rect tube. But then, I removed the 2 .5A fuses that are in series with the legs of the HV coil. There have been no fuses blown following this move. Question.

Would a badly soldered rectifier socket be cause of over current in the high voltage circuit? Reason I say this is that the rect socket was removed from a printed circuit board from my old Bogner Alchemist. I soldered the wires to the sockets little pcb tabs and wasn't happy with the state of my work but decided to go with it. I also used flux on the tabs and wire so I could hold the wire to the tab with one hand, and bring in the soldering iron with the other (with a big blog of molten solder on the tip). I am certain that the flux was boiling and spitting away and migrating who knows where. The socket was already mounted to the wooden board so, it was nearly impossible to see the quality (or lack off) of soldering underneath the socket.

Where there are big voltages present coming into the rect socket (690vac p-p and 470dcv out unloaded) do you think that this could be the source of over current that was blowing the primary PT fuse? Can burnt or residual solder flux be conductive enough to create that level of leakage current? I'm probably answering my own question here but, I usually take deep pride in every single solder joint I do, but, not this time! I've not had to really ever contend with arcing tube sockets or shorts anywhere in my builds so far - which I suppose I"m grateful for in one respect.

Thank you for showing up. And I did just order some brand new Belton sockets with nice solder lugs. It really sucked trying to "stick/tack" wires to tiny pcb tabs.

Best,
Phil D

[Stevem]

Let me see if I have what you posted right.

So you have had a 2 amp primary fuse blow. Plus a 2.5 amp fuse on each leg of the HT on the secondary side of the PT, and these two 2.5 amp fuses blew at a time when there was no recto tube in the socket, do I have that right?

With no filament voltage being applied to the recto socket then you would have had no dc voltage to arc over the socket and short out to the HT winding .

I think there maybe a combination of things going on here.

1) I think that if testing shows that you have no short or low resistance to ground showing up at your first filter node then your recto must be shorted.

2) If you replace that blown 2 amp primary fuse with a lower amperage fuse ( even as low as 1/2 amp ) then disconnect everything on the secondary side and you still blow that primary fuse when you power the amp on, then your PT is shorted.

Once again if I have your discription right I need to ask what the heck your doing with those massive 2.5 amp fuse on each leg of the HT, and why not just one fuse on the center tap of that winding if it has a center tap?

[Phil_S]

I may be headed the same place as Steve. Something isn't adding up. And just to check the obvious, you are using slow-blow fuses?
It certainly won't hurt to swap out the suspect rectifier socket. Your description of it makes me want to pull it and throw it away. There is a point of diminishing returns with respect to frugality.

[pjd3]

Yes Phil S.

I'm happy to admit that I may be getting just what I deserve. Not too proud of that job. Amplified parts order to the rescue.

I believe the fuses I"m using are fast blow (little thin strand of fuse element). I will however check the .5 amp HV fuses more closely. The 2 amp primary fuse definitely has a thin strand-like fuse element.

Phil D.

[sluckey]

Sounds like your solder job could be a factor. My question is... Why are you even messing around with a rectifier tube if you are just testing a bias circuit?

[pjd3]

Hi Sluckey,

I just left the GZ34 in since it seemed to be doing what it suppose to as far as outputting an expected DC voltage. The secondary has a 70vac bias tap so, it didn't bother me that the rect tube was in-circuit as I checked out the bias boards ability to properly trim and balance the negative bias voltage. I was also trying different resistor values in the bias board to set a good working range negative voltage. I'm planning to try this circuit with EL34's as a starter.

To my knowledge, it was OK to allow the rect tube to be there. Perhaps it is not with in a no-load state?

Phil D

[pjd3]

And just to say, the amp will not be turned on again, until a proper socket has arrived and installed to the usual quality of how I dress and solder things.

Phil D.

[pjd3]

Also,

Those were .5 amps fuses in line with the legs of the HT coil, not 2.5 amp.

Thanks!

Phil D

[Stevem]

Ok, sorry I misunderstood.
So you have a .5 amp in each leg of the HT, right?

If so that’s 1 amp total and a 100 watt 4 output tube amp uses a 1 amp fast blow, yet you only are running two 34s?

You can power it up again to get your bias circuit tuned in, just leave any wiring that would go to that recto socket unhooked.

Also if your not using a current limiter bulb set up I would only for now and with the output tubes removed run a 1 amp fast blow fuse on the Pt primary side.

[pjd3]

Thanks Stevem,

It "will" be running a pair of EL34's in PP but, there is nothing hooked up at this point. The 6.3 heaters as well as the HT have been in a total NO Load state. Only the 22uF main filter caps are connected to the DC output of the rect tube. So, there really has never been any current draw to speak of so far. I'm working from the power supply -out.

Thats a good idea. I will look to see about a one amp fast blow for the primary. After all, for now with no loads, why would I even want a two amp primary.

And I will not have any connections to the rectifier socket until the new right one comes in, in a few days.

This is a funny PT transformer. (Edcor XPWR096) The HT is 345-0-345 with a 70vac tap, plus it has three 6.3vac coils - 6.3 @ 4A, 6.3 @ 4A, and 6.3 @ 2A with CT. For the GZ34 heater I installed a .68 large wattage resistor in one of the 6.3/4A coils to eat up 1.3 volts so that the remaining 5 volts feeds the GZ34 heater. I was very skeptical if this could be safely done but, alot of research and a few threads on the topic led me to believe this was fine to do, as long as I kept the resistor away from the actual GZ34 dc output, which I did. I really don't believe this was the source for any fuse blowing.

Best,
Phil D

[sluckey]

For the GZ34 heater I installed a .68 large wattage resistor in one of the 6.3/4A coils to eat up 1.3 volts so that the remaining 5 volts feeds the GZ34 heater.

Be aware that there will be B+ on that resistor when the GZ34 is plugged in. You don't have a center tap on that winding that's connected to ground do you?

[pjd3]

Nope, no center tap on that coil, just a 6.3 @ 4 amps coil by itself. And I placed the resistor on the non-output side of the heater circuit.

Phil D

[sluckey]

And I placed the resistor on the non-output side of the heater circuit.

Still gonna have B+ on it when the GZ34 is plugged in.

[pjd3]

I hadn't thought about that or measured for it, only measured for AC across both the GZ34 filament and the resistor - and did the math out of course, first. It did nicely divide the 6.3 vac into 5.0 and 1.3 volts, filament and resistor, respectively, but didn't think about the B+.

I'll want to go back now and just measure around with the Fluke on DC volts.

What would you consider the ramifications of the B+ on the resistor? My initial thought is that it is Ok but, that's just an initial thought. It looks as though some fairly popular amps did this with no purported issue. Hope that is the same in my case.

Thanks,

Phil D,

[sluckey]

I hadn't thought about that or measured for it

What would you consider the ramifications of the B+ on the resistor?

No ramifications. You just need to be aware and treat it with respect.