lots of HEATER current

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rafafredd

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Jun 3, 2004
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Rio, Brazil
I´m working on the Langevin AM864 varimu comp and I would like to use DC heaters as I´m doing a PCB for the project. So, I would like to know if my calculations for heater current are right.

It uses two 6SK7 (300mA Ih each=600mA), one 6SN7 (600mA Ih) and one 6SQ7 (300mA Ih) that gives a total of 1.2A heater current at 6.3 VAC. But as I´m using 12 VDC, I think I´ll need two times this current (am I right?), and this is a lot. 2.4A just for one channel? UGH!

I was thinking about using a big transistor for the heater supply, with a 12v/1w zener driving the base, but I´m open to suggestions. What would you do?
 
No, half the current. W=VxA, the W is constant, the V is doubled, the A is halfed..

Yes, I'd do that zener/transistor thing too. Pick a big ass slow TO-3. Those are cheap. Maybe an MJ15003. I've used that up to 12A :thumb:
 
Check that those tubes actually can run at 12V. I would not have the heater wires on the PCB, if you do make the traces real thick. Why not P2P it?

Doesn't the original run on AC?
 
2.4A is not lot's of heater current. My 845 PP amps use 20 amps :green: & my micpre with dual 6AS7G uses 10..

The 6SN7 is definitely 6V only, but you can get a 12SN7 instead.
 
I'm assuming that when you say you're going to use 12VDC, you intend to run the heaters in series, as a bunch of pairs. Those tubes all have 6-volt heaters (hence the "6" in the type numbers).

A 12V zener on the base of an NPN power transistor will give you 11.4V output. Use a 13V zener instead. Better yet, don't even bother with DC heaters, since it's not really necessary for this project. Just use a 6.3VAC heater supply like the original. Unless the layout is super-sloppy, DC heaters are generally a waste of time except in very low-level preamps.
 
Well, I know it´s a very simple circuit, but as I´ve got many PCB octal sockets and no chassis mount sockets, I´ve decided to go PCB on this one to make it a very simple build, with onboard PSU´s etc... So, I´ll have to use DC heaters in PCB, no?

I´m thinking about running the 2x 6sk7 heater in series for 12v. I´m also thinking of running the 6sq7 and 6sn7 in series. Now I´m thinking in a simpler localy regulated heater supply. I would use a LM317 or a 7812 for the 2 6sk7s and another one for the 6sn7/6sq7 heaters.

So, if I´m following it right, I will only need 300mA 12Vdc for BOTH 6sk7 tubes and only 450mA 12Vdc for 6SN7/6SQ7 in series.

So, if my thinking is right now, a LM317 or a a 7812 for each pair of tubes/heaters in series will do, and the traces will not need to be EXTRA SUPER tick because the current is lower with local regulation.

Sorry for the wrong math on the first post. I hope now I´m on the right path.
 
For heaters on a pcb, make sure your traces are FAT and nothing else connects to them.
 
Yeah, just provide a couple of nice pads at each socket and wire twisted pair from socket to socket, floating above the foil side of the board.

I'll say it again: it's really not worth going to the trouble of DC heaters unless it's a high-gain mic preamp. Langevin's noise figures, which were very good, were quoted with AC on the heaters.
 
> 450mA 12Vdc for 6SN7/6SQ7 in series

That will not work. A 6V 0.3A heater and a 6V 0.6A tube in series across 12V will probably pull about 0.4A, but drop about 4V across the 0.6A heater and about 8V across the 0.3A heater. The 0.3A tube will glow very bright and burn-out in weeks. The 0.6A tube will be under-heated and perform very poorly. Tube heaters in series MUST be rated for the same current. (If you are really stuck, you can use a resistor to divert excess current, but this can give trouble on cold-start and anyway is a big waste.)

Building a DC supply is more work and less reliability than just leaving pads and hard-wiring AC heater power with twisted leads up above/under the PCB.

> the current is lower with local regulation.

Only if you are looking at building TWO regulators. Which goes against "make it a simple build". And gives you more to debug, and more to go wrong. And bringing the AC or raw DC onto the PCB carelessly is liable to leak more buzz than well-twisted off-PCB AC wiring.

But AC was good enough for Langevin, and it can be good enough for you.
 
[quote author="Rob Flinn"]If your using PCB valve bases you can still run wires for the heaters ![/quote]

alot of the big tube guitar amp manufactures still do this.


toby
 
So, I will just include some terminals on the PCB´s for the AC heater and make the twisted wires reach it. That´s fine for me.
 

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