7806 heater issue

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bal704
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7806 heater issue

Post by bal704 »

I'm building an amp with one 12AX7, one 6SN7 and two 6V6's. I've been reading that the 6SN7 (and 6SL7) can be noisy, and I"m trying to implement DC heaters for the 12AX7 and 6SN7.

I'm using my 5V rectifier tap, through a volage doubler, into the 7806. With no tube installed, I'm getting about 10.5 VDC into the 7806, and about 6.1 VDC out. However, once I install one or both tubes, the output voltage drops to roughly zero. I've ohmed out the heater string feeding the two tubes, and it's reading about 3 ohms. That's not much, and I'm wondered if the 7806 needs to see a bigger load. I installed a 470 ohm resistor in series with one of the heater leads right after the 7806. Again with the tubes installed, I saw about 6.1 VDC on the 7806 side of the resistor, and 0 VDC on the tube side of the 470 ohm resistor.

I'm sure I'm missing something simple here.....can anybody point me in the right direction?
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xtian
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Re: 7806 heater issue

Post by xtian »

Adding more load is not the solution. What’s your entire heater circuit? Do you have a reservoir cap after the voltage regulator?
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bal704
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Re: 7806 heater issue

Post by bal704 »

I put a 0.1uf cap between the in and ground on the 7806, and a 0.1uf cap between the out and ground of the 7806. It didn't help.

I have a twisted pair running in series to the 12AX7 and 6SN7. This terminates at the output and ground of the 7806.

Where do you recommend I put a reservoir cap?
Ten Over
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Re: 7806 heater issue

Post by Ten Over »

That seems like a lot of current for a voltage doubler. What is the voltage going into the 7806 with the heaters connected to the output of the 7806?
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xtian
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Re: 7806 heater issue

Post by xtian »

A reservoir cap would go from output to ground of regulator output, and would be high value, like 1000uF.
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pdf64
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Re: 7806 heater issue

Post by pdf64 »

I think that really, you need to sketch the whole circuit, from 5v winding to the heater load.
eg 'through a voltage doubler' could mean a lot of things, 'a bigger load' could mean both higher or lower load current.
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Re: 7806 heater issue

Post by Stevem »

Unless your building a amp with 3 preamp gain stages octal tubes are really not any worse then you have been hearing about in terms of Heater hum!
Screw the D.C. Regulated heater supply and just bias up the center tap of the heater supply with some 35 to 40 volts of D.C. Off of the B+ string and call it a day.
You will need a filter for this if you pull of the D.C. From the OT feed node.
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M Fowler
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Re: 7806 heater issue

Post by M Fowler »

I agree with Steve and you may not even need the elevated DC as he has suggested I know I don't.
bal704
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Re: 7806 heater issue

Post by bal704 »

I have put both 6SL7 and 6SN7 tubes in this, and some hummed a bunch, others not much. This is a cathode bias amp btw...

1) first thing I tried was taking my AC heater reference from ground to my output cathode (about +30 VDC). That had no effect on the hum. That led me to the DC heaters.

2) I removed the 470 ohm resistor, and added a 220uf cap between the 7806 output and ground. That was the biggest cap I had handy. Didn't help. With no tubes in, I have about 10.5 volts feeding the 7806, and about 6.1 volts out. With one tube in, the input into the 7806 drops to 4 volts in, with 0 out. One tube draws anywhere from 300 mA to 600 mA, depending on which tube I'm using. It's hard for me to believe that tiny amount of current is too much for a voltage doubler, but perhaps it is.

3) I'm going to try a couple of other things, then it's back to the AC heaters, and rolling tubes.
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martin manning
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Re: 7806 heater issue

Post by martin manning »

Without knowing all the details, I think the underlying problem is getting enough current out of the doubler. Most DC heater schemes only aim to provide the 300 mA needed for one 12AX7, where you are looking for 900 mA. Using a doubler, the 5VAC winding will have to supply 3-4 times that much. Putting the 470 ohm in series increases the load impedance and lowers the current draw to around 10-12 mA, and 470 ohms dwarfs the filament resistance, which would be 7 ohms at 6.3V, and much less when cold. Consequently, (almost) all the supply voltage is being dropped across that resistor, and even if you had unlimited current the heaters wouldn't get enough power. I'm not surprised that the 220u cap added at the output didn't help. The regulator would take the ripple down to a negligible level, but you just aren't able to keep the input voltage high enough for it to function properly.
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Re: 7806 heater issue

Post by sluckey »

Reservoir cap needs to be on the INPUT of the regulator chip, not the output. It's OK to put an extra cap on the output but you must have one on the input. There should also be some big caps in the doubler circuit and these can serve as a reservoir cap if they are big enough.
R.G.
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Re: 7806 heater issue

Post by R.G. »

The heater current of a 6SN7 is specified at 600ma hot. Cold, it's probably four or more times that.the 12AX7 heater current in 6V/parallel is 300ma. Best case is that you are trying to pull 900ma out of a 7806, which is nominally a 1A regulator. It might or might not start the loads you have on it, especially cold.

Also as noted before, you need 2-3V headroom for the 7806 to regulate. Lower than 8.5-9V at the input pin will let the output voltage sag out of regulation, a couple of volts lower than the input.

Here's a thought: will the 7806 run just the 6SN7 when fed from some other power supply of 9-11Vdc? Perhaps a car battery? If one 7806 won't supply the startup current of the 6SN7, you're out of luck with the 7806. Maybe it will. Maybe it will go into current limit, sag the output voltage till it's running at 1A, and have the 6SN7 ramp up to hot v e r r y s l o w l y.

There is also the possibility that you'r experiencing a forehead slapper - missing ground connection on the 7806, bad connection to load, etc.

If I were doing this, I would test out the voltage versus load current ability of the DC supply before the regulator, test the regulator separately, then try to marry them. Divide and conquer.

Just as I hit 'submit' it occurred to me that the LM317 in the TO-220 will usually do 1.5A before limiting if it's not also overheating. It only takes two more resistors than a 7806, and you get to set the voltage to >anything< you like above 1.25V and below two volts less than the raw DC supply. Just sayin' ...

Dag nabbit! I have to put off hitting 'submit'. You could also use one 7806 per tube, or parallel up two or three of them. This is not always a good idea from the power-sharing standpoint, but it's cheap information about the problem.
bal704
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Re: 7806 heater issue

Post by bal704 »

Thanks for the replies...

The 470 ohm resistor is gone. I added a reservoir cap to the input of the 7806, to no improvement.

For now I'm going back to AC heaters and cherry-picking tubes....
Ten Over
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Re: 7806 heater issue

Post by Ten Over »

That capacitance multiplier thing that Sovtek did might be an option. I made one that will drive 600mA at 6.15Vdc with a 7.5mV ripple. It's not regulated, but it isn't going very far.
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martin manning
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Re: 7806 heater issue

Post by martin manning »

That circuit needs a volt or two of headroom too, for the 390R Resistor and Vbe drop. I like RG's suggestion to find out what the VI characteristic of the doubler supply is, and I'm betting it won't provide the required current, but it's probably ok for one tube. There's a resistor missing from that scheme too, see below (and better yet, replace that with a 6.8V Zener, as discussed recently in another thread).
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