Offboard Wiring Tube Preamp

[pokus]

Hey there,
I'm about to build a tube preamp. Well I'm nearly done but I have a problem with the "offboard wiring" because + and - of my DC input are connected. I assume it's because of the connected heater pins. But doesn't the layout says one goes to + the other to - ? Or could it be something else? Thanks for help!

[bluebunny]

You can connect your 12V supply across pins 4 and 5 for the heater(s).  It doesn't matter which way round these go.  Pin 9 (heater centre-tap) remains unconnected.

[pokus]

Ok that's clear so far. But my multimeter shows that there is connection between 5 and 4 so that there are only a few ohms in between and also a connection 3 to 6. Wouldn't all of that cause a short circuit?

[bluebunny]

There should be a connection between pins 4 and 5 - it's the heater (it's a piece of wire, more or less).  But you shouldn't have any connection between 3 and 6 - cathode #2 and anode #1.

[GibsonGM]

You want to be VERY sure 3 and 6 aren't connected!  Perhaps there's a short between + and - somewhere, making it appear that they are?   Before powering on any circuit, make sure the power is going where it should, and the grounds are connected correctly.    " + " only connects to 3 places.  There should never be a dead short between plus and minus...the heater resistance may show a LOW ohms condition, tho.

The heater is very low ohms, like 40 ohms if you measure it.   It is ok no matter how you connect it, as bunny said.   As shown, your + goes to pin 5, and pin 4 is grounded (" - ").    The resistance between them is NOT a short, so there isn't any problem (altho it does take some current - 150ma - to heat the tube!  Your power supply must be able to source that, plus whatever the triodes are using, a few mA more....). 

[pokus]

Ok, thanks guys. I guess the problem is the heater because there's no resistance( 0,0x R) between pin 4 and 5. So everything that runs to ground is now connected to pin 5, 6 and R6. But how is this possible? I would assume a defect tube has no heater connection instead of a direct one.

[PRR]

> there's no resistance( 0,0x R) between pin 4 and 5.

Should be near 10 ohms, not zero Ohms.

Take the tube out of the socket. Disconnected the power supply. Is it still zero Ohms from 4 to 5? Then you have a wiring mistake.

[pokus]

I took the tube out of the socket and it said 12 Ohms between 4 and 5. So the connection probe still gives me a tone. I also tried a 27 and 50 Ohm resistor, tone at 27, no tone at 50.
That's how I wired it up, there's is an Bandaxall Eq instead of the tone stack from the schematic(it's extern Band In/ Band Out)
GND is at the 4th pin of the board connected to the pF capacitor. Anyone spots some quick mistakes?




Edit: I realized that this time I bought different DC jacks and the pin where I usually put the + is connected to the enclosure. And so is the input/output jacks gnd. That should cause my problems, shouldn't it?

[bluebunny]

Edit: I realized that this time I bought different DC jacks and the pin where I usually put the + is connected to the enclosure. And so is the input/output jacks gnd. That should cause my problems, shouldn't it?

Yep.  All those electrons go rushing straight back into the PSU where they came from, leaving none for your circuit.  This causes silence.  And perhaps, fires. 

[anotherjim]

You aren't measuring resistance with power on are you? Never do that!

[pokus]

Ok, I will wire it the other way around and try again. No I didn't measure the resistance with power on, the tone came from my multimeter when there is a connection 

[PRR]

> I took the tube out of the socket and it said 12 Ohms between 4 and 5.

That's surely correct.

> So the connection probe still gives me a tone.

There's no "precise" connect/not-connect value. 1 Ohm? 10 Ohms? 10,000 Ohms? Each of these may be "low" or "high" depending on the context.

Many jiffy-meters use "100 Ohms" as a buzz/no-buzz level, though they probably vary.

Buzzers are for QUICK continuity tests. Hooking up trailer lights. Tracing a telephone switchboard. Most of your connections will be less than a few ohms or more than a few thousand ohms. Then a 100r buzz-threshold lets you zip through the wires and find "wrong" (un-)connections.

When we have electronics in the system, 10 50 or 1000 Ohms are significantly different. That why you, the electronic technician, got a "buzzer" with NUMBERS on it.

And of course, in the end, it was something else: misidentified connector pins. (Been there, done that, been stuck for a month before I caught on.)