switching supply for heaters

[Lindz]

I thought I would try a small switching supply for the preamp in a high gain build I am working on just for kicks. Something like the rs-25-12 listed in the link below is only $15

http://www.meanwell.com/search/RS-25/RS-25-spec.pdf

I assume 2.1 amps is plenty for 5 - 12ax7's (at 12v they will draw 150 ma/tube correct?)

Looking at a 12ax7 tube data sheet, pin 4 is my 12v, and 5 is ground correct? would one run the ground from the tubes to the main star for lowest noise potential?

Any thoughts on this?

I know I can elevate easy enough as an alternative but thought I might try this for kicks given it is not expensive

[Leo_Gnardo]

Somethin' to think about Lindz, is filaments draw a lot of current when cold then don't behave as you expect until they reach operating temp. You can read ohms on any tube you want at room temp. Do the calculation and you'll know how many amps a power supply has to deliver at the instant of power-on. (I find typically 5 to 6 times more than at temp.) If your switching supply can handle that then you have one that is (perhaps) worthy of selection. I'd hate to see your switching supply go up in smoke in the first quarter-second. Looks like the one you selected has enough beef to handle the task.

[LeftyStrat]

Nice find. Let me know how it turns out. This looks like just the thing for the 3 channel preamp I've been dreaming up.

[jazbo8]

Somethin' to think about Lindz, is filaments draw a lot of current when cold then don't behave as you expect until they reach operating temp. You can read ohms on any tube you want at room temp. Do the calculation and you'll know how many amps a power supply has to deliver at the instant of power-on. (I find typically 5 to 6 times more than at temp.) If your switching supply can handle that then you have one that is (perhaps) worthy of selection. I'd hate to see your switching supply go up in smoke in the first quarter-second. Looks like the one you selected has enough beef to handle the task.

I am not sure which supply he is going to use, but most of the modern SMPS has current limiting and short-circuit protection built-in. I use a generic off the shelf unit that was rated for 5A (may be 50% higher than the actual draw) without any issue (yet...)

[tweedeluxe]

Some Bogner amps use a switching supply for the heaters

[Lindz]

thanks for the replies and insight.. precisely why I posted since I was a little uncertain.

How about using an inrush current limiter? The math is a bit over my head but I think a pretty small inexpensive part would work ? Maybe someone can comment

I suppose I could also just power a few of the tubes with dc like many other amps do, but I thought that 2.1 amps would be enough for all 5 and save all the twisting etc for the preamp.

I could also use a larger supply easy enough for just a few dollars more but the advantage of this one is it is only 2" wide so in theory you could turn it on edge and tuck it away it on the side of the chassis inside (assuming it won't run too hot in there)

[Lindz]

OK so I found some data about cold and operating resistance for 12ax7's. I did not measure myself (not sure how - maybe someone can elighten me? across pins 4/5?)

"the cold resistance of a 12AX7 is about 13.2Ω compared to 84Ω when warmed up. At startup the inrush current into the filament can be over 2A"

assuming these numbers are correct here is what I get

I= V/R so:

12.6v/13 ohm = .96 amps x 5 tubes in my preamp = approximately 5 amps at startup

12.6v/84 ohm = 150ma x 5 = approximately 750ma operating

So if this is correct I either need a 5amp or greater supply or perhaps an inrush current limiter to add a little resistance at startup in order to use the 2.1amp supply I would like to use

Can anyone help me spec what I would use for the inrush limiter? Below is my layman's attempt at figure out how to keep below 2.1 amps at startup

12.6/30 ohms = .42 amps x 5 tubes = 2.1 amps maximum total current for the supply I want to use = I would need a current limiter that combines with the cold resistance of the tubes to somewhat above 30 ohms, and then theoretically its resistance goes down to a somewhat negligible amount when operating if I understand the theory behind them correctly

Using Mouser's search function I found this unit - 6amp rating, 30 ohm resistance cold, .31ohms at 50% operating current. less than $2 so its not an expensive addition. In series with the supply that would give me 43 ohms so 12.6/42= 1.5 amps at startup so well within the range of the supply.

http://www.ametherm.com/datasheetspdf/SL1830006.pdf

Am I on the right track here? My electronics knowledge is still limited

thanks!

[VacuumVoodoo]

I can confirm cold filament resistance in the 11-13 ohm range.
The NTC resistor will have hot resistance at 12.5% of max current somewhere between 1.5-2 Ohm and so its temperature will be ca 50°C above ambient; ambient being inside amp chassis that can reach 50°C. So just make sure the NTC is well soldered, don't the leads too short and install it so it doesn't touch any plastic stuff. NTC must run hot to do its job.

[JazzGuitarGimp]

Also, keep in mind the data sheet specifies the cold resistance as 30R +/-25%, so you may actually be looking at 22.5R when cold.

Edit: and I just did the math - 22.5R will be fine at start-up from a standpoint of current.

Also, data sheet says 40 seconds to get up to operating current, which is almost four times the 11-seconds a tube takes to get up to temp in a standard circuit. This will likely seem like forever at first, until you get used to it. Though the potential upshot is you might just get some very interesting tones out of the amp during the first 40 seconds!

[husky]

Discussed here too
http://ampgarage.com/forum/viewtopic.ph ... c&start=15

One thing though is you need to be real careful about grounding.
I have used the supply mentioned here,
http://www.cui.com/product/resource/vof-65.pdf
Bogner also uses this one but runs the power tubes in series. I had to replace one for a customer that smoked in a Shiva, probably the power tube taking it out. Also they didn't have it mounted for the optimal heat dissipation.
I wouldn't use them on the power tubes personally (I know you are not).
Tonally it really shouldn't matter switching or not but something about a linear supply now sounds better to me compared to switching and I have installed at least 12 switchers so it isn't a one time experience. Totally mojo since I haven't tried measuring sweep or Harmonic distortion after installing one.

[Lindz]

Hey John

Thanks for commenting!

perhaps you can elaborate on your grounding remark.. anything in particular that you suggest I do or watch out for? I am still pretty green so the layman's explanation is appreciated if you have a minute..

Any speculation as to why a linear supply might sound better?

Anyway, I was going down this path partially because I have struggled a bit to keep the couple high gain builds I have quiet hum wise. So any grounding comments are greatly appreciated both in relation to the dc supply and what you like to do for really high gain builds in general.

My first high gain build (SLOclone 100 watt pcb build) is pretty quiet on the crunch channel even when dimed, but the lead channel is very noisy when it is really wound up - the amount of hiss is not surprising given all the gain, but the hum level seems too high

I cannot make up my mind if I have filament noise or a ground loop (or if it simply exhibits that much hum because it has 50,000x gain on that channel - ha ha) I have tried 10 slightly different variations on the grounding scheme - moving certain sections to certain stars, different chassis ground points, etc. and none really quiet it right down. I was thinking it might not be my grounding but instead be some filament noise creeping into the high gain channel, hence me exploring using the dc supply.

[martin manning]

I cannot make up my mind if I have filament noise or a ground loop...

Have you tried powering the filaments from a battery? This is simple and quick compared to installing another power supply that may bring noise of its own into the mix.

[Lindz]

No though that would make sense to try. I suppose I should in any event just to compare noise levels to the switching supply and existing elevated AC.

I suppose I need a decent sized lantern battery to generate enough current - Frys should have something like that

However, I am really also curious about trying the switching supply just for kicks as well as to learn about its implementation so I am going to try it in any event

[DonMoose]

I'd like to just pop in to mention that switchers tend to have minimum loads at which they're stable (lowest noise). If the current load is too low, the output drifts up and you wind up in a burst mode.

You can maybe manipulate the soft-start timer (lets the output go overcurrent long enough to charge caps (or warm up filaments) before it starts paying attention).

Recommend an LC output filter, too.

Hope this helps!

[JazzGuitarGimp]

Recommend an LC output filter, too.!

Good idea. The data sheet doesn't specify the switching frequency, but I'd guess it's in the 20KHz to 100KHz range. You won't really need a large L or C to effectively shunt whatever switching artifacts that may to present in the output to GND.