How To Lower Ac Filament Voltage?

[RANDY]

Is it ok to use a small 1 ohm resistor to lower my ac voltage on the filaments?I'm getting 6.9VAC.Thanks.

[stokes]

Is the 6.9v with the tubes installed?If not the voltage should come down when the load is applied.Either way I wouldnt be too concerned with the extra .6 volts-your meter could be off that much and that .6 volts wont hurt.

[RANDY]

It's 6.9vac with the tubes installed.

[stokes]

I wouldnt worry about it,components in a tube amp are generally +/- 10 to 20% anyway.If it is a vintage amp it would have been designed based on a 110vac supply so if you take into consideration that you are now plugging into 120vac you are going to see a slight increase in all the voltages in the amp,and like I said earlier your meter could be off by a little anyway.

[R.G.]

It's 6.9vac with the tubes installed.

Heater life is reduced at anything above nominal voltage. It's extended below nominal. This is, of course, if something else like overdissipation doesn't kill the tube first.

It's entirely acceptable to reduce heater voltage with resistors. Ideally, you'd use two resistors, one in each line of half the resistance, to keep the voltage on the heaters balanced around ground.

The added resistance helps tame the power on current surge.

Note that this is not a huge effect. Your amp will still work at 6.9V on the heaters.

[MWJB]

Hi RG, I have heard that dropping AC voltage significantly below nominal also damages tubes, also won't it have a detrimental effect on tone if the heaters aren't running enough voltage?

[stokes]

Lower heater voltage is a sign that your heaters are drawing too much current for the tranny's rating.Dont know if it will extend the tubes life any but it will shorten your tranny's life.I am talking about lower heater voltage with out any mods such as resistors to lower the voltage.I dont see using resistors to lower the volts doing any harm but in this case I dont think it is worth the trouble and if your 120vac supply drops you could end up with 5v on your heaters and that aint good.I have noticed that in the summer it is common for my 120v outlets to drop to 111v.I dont think 6.9 volts is anything to worry about,if it were well over 7v,maybe.

[MWJB]

Some tube specs I've read recently (JJ?) quote acceptable heater voltages as "6.3 to 6.9VAC". Plenty of vintage Fenders run in this region (6.9+/-?) as stock.

Also, in response to Randy's original Q, will 1ohm be small enough? Due to the relatively high currents in heater circuits, you may need even smaller values?

[Alf]

old thread

Hi, I found this old thread on this subject , it might help you out.
I hope the Steve and Ray don't mind.

Best way to decrease heater voltage

Hi,

I just run into a prbl. The heater output of a x-former is 8,3V AC. The high volatge is just on the spot where it should be. What is the best way to bring the U down. The current req. is 2.5A. I tried this :

1. rectified via bridge and 10000uf to 4 times 7806 regulator parallel.
this don´t funktion, because the load was not equal, it means, 2 of them get very, very hot.

What options are there?

A Resistor of about 1R 10W with rectifier?
A Resistor without rec.?
on each tube a R ?
or a 2n3055 transistor somewhere?

Please help me, I just cut all the heater wires of the above mentioned dc supply and put it into waste.

best regards, Pepps

P.S.
If you like, you can have a short look on the amp I want to build now. The link is just the preamp, you will find old friends there. The pwrstage will be a cathode biased el84 pp.


From: Ray Ivers
Date: 3/21/2001 12:51 AM
Subject: Re: Best way to decrease heater voltage

Pepps,

A .5 ohm, 5 watt resistor in each heater lead should give you about 6 volts AC on the heaters. Two .4 ohm resistors would be ideal (6.3 VAC).

Ray


From: Pepps
Date: 3/21/2001 1:29 AM
Subject: Re: Best way to decrease heater voltage

Hi Ray,

thank you for your hints. If I understand right, the dropping of voltage is ok with a single R.
So I go with the rectified dc and drop via a R which fits.
Or should I use the R´s before the rectifier?

Thanks, Pepps


From: Steve A.
Date: 3/21/2001 12:54 AM
Subject: Re: Best way to decrease heater voltage

Pepps:

According to Ohm's law if you want to drop 2.0 volts with a current of 2.5 amps, you need to run it through a 0.8 ohm resistor. Also according to Ohm's law the power dissipated will be 5.0 watts so a 10 watt resistor would be a good choice.

If you can't find a 0.8 ohm resistor you can use a 4 ohm resistor in parallel with a 1 ohm 10 watt resistor. I believe that the 4 ohm resistor will carry only 20% of the total current so you could get with a 2 watt or 5 watt resistor there.

BTW have you measured the actual heater current? If it is not exactly 2.5 amps, all of these figures will need to be recomputed (which are for an ac heater supply, not a rectified dc one).

--Good luck!

Steve Ahola


From: Pepps
Date: 3/21/2001 1:40 AM
Subject: Re: Best way to decrease heater voltage

Hi Steve,

I didn´t messure the current, I killed the current path of my meter by messure the current of the filter caps. By mistake.

Yes, I will find the right R which get the U down,
I have a lot here...

But..
I try it with the DC supply. Is this okay? Drop before or after the rec.?
I will try it and let you know.

Steve,
thank you and best regards,

Pepps


From: Steve A.
Date: 3/21/2001 2:25 AM
Subject: Re: Best way to decrease heater voltage

Pepps:

If you are going to go with a DC supply you can also use diodes after the rectifier; at a voltage drop of 0.7v each, three 1N4007's should bring the voltage down about 2.1 volts.

BTW what circuit are you using to rectifier the ac heater voltage?

--Good luck!

Steve Ahola
From: Pepps
Date: 3/21/2001 9:47 AM
Subject: Re: Best way to decrease heater voltage

Steve,

I use a bridge rectifier which is rated for 5A with 10.000uF after it. The x-former winding is not center-tapped.
I´m free to switch to AC, since I removed all the wire yesterday night.
As you see, there are 2*ECC83 1*EF86 2*EL84 which is around 2.6A. Since the 1N4007 is rated for 1A I have to use 9 or better 12 of these. 3 times 3.

I think a R is in the end smaller than the diodes.
How hot will the diodes go with full current?

Pepps

From: Joe
Date: 3/21/2001 2:10 PM
Subject: Re: Best way to decrease heater voltage

I lowered my mesa's from 7.3 to 6.4 by fitting 2 x 0.1 ohm 5 watt resistors in series with the AC winding. Worked like a charm. Now my tubes last a lot longer.

[MWJB]

In this scenario (8VAC + at the heaters) wouldn't fitting a suitable auxiliary 6.3VAC 4A (or better) transformer be the most straightforward route?

[jaysg]

Inrush current limiters are a) good for filaments since they slow down the turn on peak, b) leave a fixed 'On Resistance' drop after power up. For AC filaments you put one on each leg. Should yeild close to perfect with the ~8Vac secondary. http://www.ametherm.com/Inrush_Current/welcome.html
hth

[R.G.]

Hi RG, I have heard that dropping AC voltage significantly below nominal also damages tubes, also won't it have a detrimental effect on tone if the heaters aren't running enough voltage?

Like so many apparently simple things, the background is complex.

Vacuum tubes work because their cathodes emit electrons. The electrons are sucked to the plate by the plate's positive voltage. The grid can deflect them back to the cathode by being pulled negative enough. In normal tubes, the cathodes emit so many electrons that they form an electron cloud between the cathode and grid. Even when the grid is at 0V to the source/cathode, the emission is sufficient that there is still a cloud of electrons there.

That's important because any gas molecules left in the tube that bump into the plate have electrons stripped off, and the now-positive molecule starts "falling" toward the cathode. They can't move as fast as the electrons, but they are hugely more massive, so once they get going, they carry a punch. The electron cloud tends to plaster electrons back on them, even to putting excess electrons on them, so they are repelled. As long as there is an electron cloud between cathode and grid, the molecules don't hit the cathode very much.

If they do hit the cathode, they tend to destroy the rare earth oxides there that emit electrons at low temperatures. So the space charge protects the cathode's emission by the cathode emitting a lot. This setup lets us use cathodes at only about 700C instead of the much higher temps of thoriated tungsten directly heated electrodes.

Cathode emission goes up with temperature - good for space charge/electron cloud. Cathode heater filament life goes down with temperature, not linearly but as a higher power of the applied voltage. In incandescent light bulbs, the life goes down proportionately to the thirteenth power of the applied voltage, but I don't know the exact number for cathode heaters. So a little more voltage drives the burnout life of the heater down quickly, and a little less voltage drives the burnout life of the heater up quickly.

But a little less voltage also means a cooler cathode and less protection from ion bombardment. They compete to kill the tube.

For small signal tubes like the 12AX7, the emission is so vastly more than the tube can otherwise conduct, you can run them at quite low voltage, 5.5 to 5.8V, and get massive increases in life. When you do this, the tube gain goes down some from the nominal, but is more stable over the much longer life of the tube.

For big signal tubes, you have to be sure that you get enough. If you regularly overdrive your tubes like crazy, you need all the emission you can get to keep the electron cloud there. If you don't drive them so hard, it's not quite as bad.

In any case, the tubes are **designed to work from 6.3Vac +/-10%**. From this discussion, 6.3V -10% is sufficient (that is, it's not too bad in the manufacturer's judgement). So the target for power tubes should be to drive them from 6.3Vac +0%, -10%. That ought to be as long a life as you can influence by heater voltage. 6.93Vac is +10%. 5.67Vac is -10%.

Of course, over heating and screen dissipation, gassing, venting, and all the rest of the ills may kill the tube before the heater filament would have worn out too. No way to tell. But it's certain that tubes were NOT designed to work from 7Vac +/-10%.

So - will dropping heater voltage significantly below nominal damage tubes? Yes, if that means running them under 5.67Vac. Any voltage above that to 6.3Vac is good for life.

Will it have a detrimental effect on tone to run them at lower voltages? Maybe - the gain does get slightly lower if the heater voltage is lower, but this is a minor effect. It depends on whether you are always running your preamp tubes at full warp drive. If so, maybe. It could be you like that tone, too. Tone is not a single-dimensional quantity. It might sound different or not. It may go into distortion more easily or more smoothly too. Your ears will know. Do what they tell you to.

But the tubes will last longer between 5.67 and 6.3Vac if something else doesn't kill them first.

[R.G.]

Some tube specs I've read recently (JJ?) quote acceptable heater voltages as "6.3 to 6.9VAC". Plenty of vintage Fenders run in this region (6.9+/-?) as stock.

Also, in response to Randy's original Q, will 1ohm be small enough? Due to the relatively high currents in heater circuits, you may need even smaller values?

Vintage Fenders run in this region because they were designed for AC power line voltages of 110 to 112 VAC. My power line, like most of North America, runs at 123 to 128Vac all the time. That increases the "nominal" voltage the heaters see up to about 6.9 to 7V.

[MWJB]

Indeed, but my point is that vintage Fenders with 7VAC heaters
don't suffer heater related premature tube failure and they sound great.

My own experience with running heaters under voltage is that tone can go mushy, with poor dynamics...but as you say, let your ears be the judge.

In some cases, you can fine tune heater voltages by employing different power tubes that draw more heater current (assuming any other relevant modifications are undertaken). However, ensure that your heater voltage (with the stock PT, fed with the line voltage it was designed for), doesn't drop below 6.2VAC or you could burn up your PT.

[swt]

can remember who told me, but i did this and it works. Just put a pair of back to back rectifier diodes on one side of the transformer. it won't rectify anything because they're ack to back, but the voltage drops 0.7 volts...