a.c. heaters

[ruffrecords]

When I started designing tube mic pres some years ago I initially used ac heaters and got into all sorts of trouble with hum. I quickly moved to dc heaters, raised the voltage to 12.6V dc for easier, more efficient  regulation and used tubes that either had a 12.6V heater or could be wired in series to operate on a 12.6V heater supply. Since then I have never had hum problems caused by heaters.

The current range of designs all use this 12.6V dc heater supply. However, because the main output tube used is the 6922, and this only has a 6.3V heater, they always have to be used in pairs. Recently there have been a couple of occasions when it would have been very convenient to use a single 6922 on a board but to be compatible with the 12V dc heater system you would need to use a wasteful heat generating dropper resistor/regulator of some sort or provide an additional 6.3V heater supply. There is a 12.6V heater version of the 6922 but it is extremely rare and very expensive and therefore not a viable option.

As a result of all this, I began to wonder whether 6.3V heaters, even ac ones, might be viable. For a completely unrelated reason I had been looking at the schematics of the Altec 250SU tube console and in particular its power supply, the  535A. The power supply can provide 6.3V roughly smoothed dc at an incredible 13 amps with a ripple of only 1.5V pp (this is better than 20dB below 6.3VAC). This supplies a mixer whose mic pres achieve a specified EIN of -126dBu, so hum is clearly not an issue. It looks like the ancients knew a thing or to.

I therefore decided to test this out on my standard Eurocard mic pre. I modified the heater wiring on one I had been using on the lunch box project so it could be powered from 6.3V. I then connected it to an old power supply I built a long time ago that has my standard HT supply and just a 6.3VAC heater winding output. I took no special precautions; the extra heater wires were just strung across the underside of the PCB and the PCB just sat in the open on my bench (no screening at all). The heater wires from the power supply to the board were not even twisted.

I powered it up and measured the gain from mic input at maximum gain to transformer balanced output of the first amplifier. the results were:

Gain:  51,4dB into 600 ohm load
Noise at max gain, input shorted: -69dBu

This means the EIN is -120.5dBu

I had measured this same board the day before in the screened lunch box with dc heaters and under the same source, gain and load conditions and the output noise was only 1dB better at -70dBu. I conclude that using 6.3V ac heaters seems to make no real difference to this board. This is surprisingly encouraging.

There are a couple of downsides. Running a  6.3V ac hefty current along the back plane of a mixer or lunch box could possibly induce interference insensitive inputs if proper attention is not paid to screens In that event, the use of roughly smoothed 6.3V ac might be better. The Altec 13A supply uses 70,000uF of smoothing to achieve its 1.5V pp ripple at 13A. Today, 47,000uF 16V capacitors are readily available so providing 100,000uF of smoothing should be straightforward. For something like a lunch box which would take only 4A at 6.3V, this amount of smoothing would reduce ripple to 0.4V pp.

This makes for a really simple power supply.  The only problem with this simplicity is that there is no regulation so the voltage would vary with load. Altec overcame this by having a series of primary taps on the heater transformer so the output voltage could be adjusted. I am seriously toying with the idea of moving the Mark3 to 6.3V heaters.

Cheers

Ian

 

[joaquins]

First Q to mind is why don't you use 6.3AC for your output tube which is already much less sensitive to noise and DC with the others which can go at 12.6V?

I've seen some old fender twin inside and others of the same age, the wiring is pretty ugly and they doesn't have hum, looks like the hum leakage has more to do with the wiring and lay out than using DC heaters, since I've also seen some amps with DC heaters and pretty ugly hum. Since spikes in the smoothing caps for those sizes is a pretty hefty source of noise looks possible, elevated heaters only would make sense to avoid noise inside the tube, in the outside of them it has nothing to do. at 6.3 the balanced ground reference makes sense, I don't know if it works in 12.6V heaters, when there are 2 6.3 in series.

I think with a proper layout and elevating heaters if there is some noise problem inside the tubes (outside of them is been taking care by the good layout) AC heaters should be just fine, DC heaters always looked like brute force, of course with 100dB of gain in a high gain guitar amp there may be a problem or two added to the usual problems in <60dB of a mic preamp...

Good to know about your experience... thanks for sharing.

JS

 

[CJ]

worked on a Silvertone shock box yesterday,

kind of a weird heater circuit>

 

[ruffrecords]

Heater wiring certainly seems to be something of a black art; sometimes neat stuff hums and ugly stuff works fine. When I first started design mic pres I was not using elevated heaters because there was no need from the basic design point of view. Later when I started using mu followers and SRPP stages it became necessary. One hidden benefit of elevated heaters is that it reverse biases the cathode relative to the heaters so there can be current flow from the bare heater to the cathode. I am pretty certain this does a lot to prevent heater hum being induced into the circuit. I must say was was very surprised there was no hum in the rough set up on my bench and I am sure elevated heaters has a lot to do with it.

I suppose an interesting experiment would be to test the same board without elevated heaters to get an idea of how effective they are.

Cheers

ian

 

[Chris_V]

And what about a step-down switching converter, they are quite cheap (specially for low voltages and at least you don't need tons of F) and efficiant.
Did you test it ?
Regards,
Chris

 

[ruffrecords]

And what about a step-down switching converter, they are quite cheap (specially for low voltages and at least you don't need tons of F) and efficiant.
Did you test it ?
Regards,
Chris

Yes, I investigated these for use in external 12V supplies for mixer projects.  You need to be careful to account for heater inrush current which can put them into foldback current limiting from which they never recover because the limit current is not high enough for the heaters to actually start to warm up and hence increase in resistance. I found one that delivers 12V at 15amps and cost me about £10. works fine up to about half the rated output. I have not tried looking for ones that will deliver 6.3V - good tip - thanks.

Cheers

Ian

 

[Chris_V]

Yes you need to be carefull with inrush currents. A lot of converters have a tracking or soft-start pin that can be used for slow ramping the output voltage.
The other option is to use a current regulated converter, they are intensely used in led lightning but it is more tricky to implement since current have to be adjust for each configuration and it can be problematic when you mix tubes with very different heaters power.

 

[merlin]

I modified the heater wiring on one I had been using on the lunch box project so it could be powered from 6.3V. I then connected it to an old power supply I built a long time ago that has my standard HT supply and just a 6.3VAC heater winding output.

What sort of ground reference did it have? Grounded centre tap, or what?

 

[ruffrecords]

I modified the heater wiring on one I had been using on the lunch box project so it could be powered from 6.3V. I then connected it to an old power supply I built a long time ago that has my standard HT supply and just a 6.3VAC heater winding output.

What sort of ground reference did it have? Grounded centre tap, or what?

The heaters are elevated to about 25% of the HT via a decoupled pot divider.  There's a 500 ohm WW pot across the heaters with its wiper connected to the elevation point.

Cheers

ian

 

[Matador]

I did a very similar experiment a few years ago:  I tried an elevated-center-tap method on a guitar amp that lacked a center tap on the heater connections.  The previous design DC rectified and filtered all tube supplies (2 12AT7's and a single EL84 for the power section - yes, very low wattage).  Just two 100ohm resistors to the elevated point, which was set to about 50V and tapped from the B+ supply via two resistors, and filtered with a single cap.

I was shocked that I couldn't hear much if any difference between background noise/hum - if anything, using full-wave rectification made the 120Hz hum more noticeable than the afterwards 60 Hz hum.  Very eye-opening indeed.

 

[ruffrecords]

I was shocked that I couldn't hear much if any difference between background noise/hum - if anything, using full-wave rectification made the 120Hz hum more noticeable than the afterwards 60 Hz hum.  Very eye-opening indeed.

I have not found any learned  reference to the hum improving properties of elevated heaters. I seem to remember Norman Crowhurst publishing some work on tits improvement in valve lifetime, but that is about all. As you say, eye opening.

Cheers

Ian

 

[PRR]

> Silvertone shock box
> kind of a weird heater circuit

Transitional.

There were true hot-chassis amps, like radios, except the radio user does not hold on to the chassis (a radio chassis can be totally enclosed against finger-contact).

These could be lethal, so you find the input jack stood-up on some significant impedance. Here it is R5 C3 between user-jack and line. Maximum shock current a bit over 3mA. Hardly ever lethal.

But now what if V1 heater-cathode insulation breaks down? This was a concern. The original 7-pin ACDC tube like 50A5 was quickly replaced with 50B5 which moved the heater pins for better safety. Even that was eventually deemed not-enuff for guitar amps. T1 gives a totally isolated heater power. Making T1 a 1:1 12V:12V minimizes its size/cost and keeps some drop in the 35V+50V string.

I hope you have bought/made an isolation transformer for that shocker.

 

[gridcurrent]

I have not found any learned  reference to the hum improving properties of elevated heaters.

RDH4 page 1197.
RCA Receiving Tube Manual RC-30 page 82.

 

[buildafriend]

worked on a Silvertone shock box yesterday,

kind of a weird heater circuit>

I don't get it

 

[bluebird]

Back in the day...Some tube stuff would not have a power transformer and actually just run straight off the wall voltage. they would string the heaters of the tubes up in series so the total heater voltage was about 120V. Like PRR said its a good idea to put a 120V:120V isolation transformer in the amp...

Safety first!

 

[CJ]

yes we installed a 120:120 iso xfmr in there, and this model did not have a fuse, so we stuck one in there also, not that a human could blow the fuse, unless they ate a large pepperoni and had a lot of salt in their blood,

this amp was for Bob Dylan's pedal steel player's girlfriend, so we were thinking that maybe the boyfriend might steal the amp from the girlfriend because it sounds so good, then  Bob would try it out, did not want to be on the hook as the guy who curled Dylan's hair, 

 

[joaquins]

Back in the day...Some tube stuff would not have a power transformer and actually just run straight off the wall voltage. they would string the heaters of the tubes up in series so the total heater voltage was about 120V. Like PRR said its a good idea to put a 120V:120V isolation transformer in the amp...

I have to say, maybe I'm just clumsy but I get shocked all the time while working on my tube stuff. I never feel like I almost died or anything. Just a sharp zing to make me feel like I'm alive again. I have a lot of respect for electricity but I think people over exaggerate the danger of a lil' ol' tube amp.

I mean... I don't play guitar in the shower or anything...

Sadly they can be if something fail, some live chasis amps wouldn't have that safety RC mentioned earlier and you need to plug it in the right way or the chasis will be connected to the mains live directly, then the guitar player when touches the strings also is, if he doesn't touch anything he will be fine, if he touches a microphone or the guitar plugged to a different amp which are properly grounded ban, mains from one hand to the other, right through the players heart...

When working on high power amps which you still don't know if the caps are charged or not, or working on it plugged in while tweaking some internal trims or measuring voltages, is always a good thing to have one hand in the pocket, standing up with proper shoes, so in case you get zapped it's only through your hand and you only get to rice your voice a little and remember your neighbor's family.

Be careful and try to avoid those ugly shocks, they can be lethal if they find a path through the heart, otherwise painful as much. I got a few shocks working on my first tube amp and I learned the lesson, once I was on my father's boat in a electric storm with the wheel in my hand, something happened and I find my self half a meter in the air, then I heard a noise and took me a few seconds grab the wheel again. The wheel had some leather cover but it had a hole in it, long story short I just ended looking at my finger for two days not knowing if it will move again, slowly it recovered by it self. It was much worse than what a tube amp can make to your hand, but still I rather avoid the pain.

JS

 

[Kingston]

I have to say, maybe I'm just clumsy but I get shocked all the time while working on my tube stuff. I never feel like I almost died or anything. Just a sharp zing to make me feel like I'm alive again. I have a lot of respect for electricity but I think people over exaggerate the danger of a lil' ol' tube amp. I mean... I don't play guitar in the shower or anything...

You just gave incredibly bad advice. Completely irresponsible and ignorant. People die doing the mistakes you do. Getting shocked is not some badge of merit. Just means you were being stupid.

 

[MicDaddy]

Good reading guys, I have an old Airline amp I'm trying to wrap my head around the tube rectifier circuit.

The B+ is tapped and tied to ground (chassis)

One of the heater legs is tied to ground (chassis)

When micing this amp, I get some good shocks, is this stray capacitance through the transformer or is this having to do with a hot chassis?  I didn't think the voltage on the chassis was large enough to feel but hold a mic and touch it and it smacks not painfully but very obviously.

The B+ needs to be tied to the ground side of the filter caps to complete the circuit I believe, but the one side of the heater I have lifted from chassis.

Considered doing the quasi center tap with a pair of resistors but now reading maybe I don't do that.  Any sense rectifying the heaters or do I just leave it like they did in '65 and string up the run of ac heaters?  (having removed half of the leg from chassis)  do I need to reference any part of the ac/heater circuit to common/ground/earth/chassis?  In some builds there are 100ohm resistors from ac heaters to ground... necessary?

The amp is a simple 5Y3, 12AX7, 6V6. 

 

[Bill Wilson]

Hi All: In 64 I built a 6 input tube mike mixer for location mastering direct to my Ampex 351-2 recorder.
The circuit used all 12ax7's, except for the 12au7 cathode follower outputs. Heater wires were twisted between tube sockets & laid against the chassis. At the power supply (which was in a separate steel case) I installed a 150 ohm 10 watt wire wound pot across the 6.3vac filament line, with the center tap wiper to chassis ground. I listened to the mixer thru my monitor system w one mike pot wide open & tuned out the slight bit of residual hum. You could never hear any ac hum in my recordings.

I get a chuckle when I see others obsessing over dc filament voltages; it is not necessary!