DC Tube Heater Supply Help

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UnlikeKurt
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DC Tube Heater Supply Help

Post by UnlikeKurt » Fri Mar 13, 2009 6:43 am

Hey guys
I am working on a guitar amplifier and am sorta stumped.
Sorry if this is "too much info" but I wanted to provide all I could.

I'm trying to run DC Heaters on two 12AX7 preamp valves.
Here's the schematic for the supply:
http://img26.imageshack.us/img26/7462/s ... dchtrp.jpg

At first my filaments weren't even lighting up. When I checked V at pins 4/5 & 9 I found that I was getting +4.12DCV on each when referencing ground. I started thinking about it and remembered that with tubes in those pins would be connected to one another. I pulled the tubes and remeasured. I found 4.12DCV at pin 9 and nothing at pins 4/5.

I then checked the DB101 and read 3.35VAC on each leg going in and equal Neg & Pos DCV coming out. I found that the negative dcv lead from the PS wasn't showing continuity with pins 4/5. I soldered the joint at the pin and things started looking up. 2.68VDC on pins 9 & -2.68VDC on pins 4/5. Plugged in the tubes and like magic we had a functional guitar amplifier.

I then put down the guitar and grabbed my meter and a spreadsheet I made for my voltages and started going through. When I had finished measuring everything (and everything looked really good within maybe a few mV) my DC Heater filaments had gone out. I checked the voltages and it was weird.

-3.77VDC and +3.93VDC

Checked my AC source and it was still right 3.35AC on each lead.
Checked the DB101 again and found -3.77VDC and +3.93VDC

The other thing is that I am now getting a continuity reading from my DMM when I have the positive lead on the NEG DCV leg and black lead on the POS DCV leg. If I re-arrange the orientation I get no continuity.

Is my DB101 fried? If so, how do I determine what caused it so that I can prevent it from happening again?

If you have any ideas you can toss my way I'd really appreciate it.

PS: It works with AC Heaters so that isn't an issue.

Thanks for any help
James

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Post by The Scum » Fri Mar 13, 2009 9:30 am

From your description, I'd venture that everything's fine.

Looking at the schem, the heater lines are built floating, mainly only with reference to each other, not the amp chassis ground. If you measure VDC from one leg to the other, you'll see ~7VDC...and hopefully no AC.

You got that cap at the bottom of the schem installed correctly, right? It's on the negative leg, thus it's drawn upside-down WRT ground. If it were in wrong, it would have probably 'sploded by now.

Even with AC heaters, the heater circuit is often floating WRT the rest of the circuit - it's just the twisted green wires...you can really only reference the one to the other, and sometimes they do funny things if you try to reference the rest of the circuit.
The other thing is that I am now getting a continuity reading from my DMM when I have the positive lead on the NEG DCV leg and black lead on the POS DCV leg. If I re-arrange the orientation I get no continuity.
Yer measuring the diodes in the rectifier when you do that. They only conduct one way. If you meter has a diode checker, try it, and you'll measure roughly 1.4V.

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Post by Electricide » Fri Mar 13, 2009 10:47 am

duuuuuuuuhhhhhh!

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Post by UnlikeKurt » Fri Mar 13, 2009 10:53 am

Hmmm.
Well it sure ain't heating up those tubes.

As for the caps. I'm pretty sure I have it drawn the way it is on the PCB.

~- to C1- & R1 to C2- to C3- to DC- Output / C3+ to ground

~+ to C1+ & R2 to C2+ to C4+ to DC+ Output / C4- to ground

So the first two caps go between the pos/neg sides
and the 3rd cap goes from neg with positive side to ground
and the 4th cap goes from pos with negative side to ground

??

why duhhhh?

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Post by The Scum » Fri Mar 13, 2009 11:38 am

So you're saying it worked for a bit, then you went in to measure things, and it stopped working?

Let's try a couple things:

With the tubes in, measure the voltage drop across the resistors...one lead on one side, the other lead on the other side. Those are some kinda big power resistor, right?

Then, with the tubes out, measure the voltage at the heater pins of the socket...between 4 and 9, and between 5 and 9.

So what was the genesis of this design?

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Post by UnlikeKurt » Fri Mar 13, 2009 12:10 pm

Yes it worked. Then I was measuring and writing down all my voltages against the amp schematic. Put down my pad, filaments out.

The design was on the power supply pcb.

I will perform the measurements on sunday morning when I get home.

Thank you for helping!

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Post by The Scum » Fri Mar 13, 2009 12:38 pm

So what was the genesis of this design?
The design was on the power supply pcb.
What I'd like to know is who designed it? And did they leave any documentation, theory of operation, etc?

I find it a little bizarre that they'd split up the voltage to be ~+3.5V and ~-3.5V, rather than just ground and +6V. I'm wondering if there's any insight around.

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Post by nclayton » Sat Mar 14, 2009 3:31 pm

The Scum wrote:
So what was the genesis of this design?
The design was on the power supply pcb.
What I'd like to know is who designed it? And did they leave any documentation, theory of operation, etc?

I find it a little bizarre that they'd split up the voltage to be ~+3.5V and ~-3.5V, rather than just ground and +6V. I'm wondering if there's any insight around.
Probably they did it that way so that the unrectified voltage (shown going off to V3-V5 on the schematic) would stay voltage balanced. I'd think you'd be best off grounding the center tap, though. If it's in a regular guitar amp it's probably already that way.

What's a DB101?

What wattage of resistors did you use? If you're powering 2 heaters you'll be dissipating about .36 watts through a 1 ohm resistor. That's steady state, but cold heaters and the caps will pull a lot more current when you power on, so best to go with 1 watt resistors. If you used quarter watt, one of the resistors almost definitely died. It also looks like that circuit was designed with 1 tube in mind and you might be dropping the voltage too much. You might want .47 ohm or to just use 1 of the 2 resistors shown.

Ned

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