Drop DC heater voltage by 1 volt, what resistor

[Guitarist]

Hi, I just added a Mesa style DC making circuit for three preamp tubes. They all draw about 1A. Voltage is currently at about 7.2V. I don't want a regulator. It sounds great right now but I hope to preserve the old, quality tubes.

What resistor value in Ωs and watts can I insert, if you don't mind. Thanks.

[Diablo]

V=IR, so R= 1 volt / 1 amp = 1 ohm.
Power = VI = 1 watt.
So to be safe, double the wattage rating, or triple it.
1 ohm - 3 Watt resistor, wire wound should do the trick.

[Chuck H]

Maybe you could just add a couple of diodes to the rectifier? That should drop a volt.

[R.G.]

Diablo and Chuck have the right idea. Resistors are all about V = I * R in all its permutations. Diodes have a kind of fixed voltage. Silicon diodes drop about 0.6 to 0.7V at the kinds of currents you're talking about. You might consider using two silicons, giving 1.2 to 1.6V drop. This will give you 6.0 or slightly under. Running preamp tubes with mildly lower filament voltages helps with long life according to some research I read from the "Golden Age".

Also be aware that P = I * V (power equals current times voltage) applies for all parts, not just resistors. A diode which drops 0.7V will dissipate 0.7W when 1A is running through it. It's not just resistors that generate heat.

[Chuck H]

A diode which drops 0.7V will dissipate 0.7W when 1A is running through it. It's not just resistors that generate heat.

I did assume that but had no direct knowledge to convey. You completed my suggestion well, thank you.

[soundguruman]

Lower heater voltage shortens tube life. It does not prolong it.
It applies to broadcast tubes and audio tubes, alike.

Broadcast has known this for a long time, simply because, they pay a lot for tubes.
They pay $395 for one output tube, they sure as heck want it to last longer.
But, we pay $9 for 12AX7, and the same rules apply. Still has heater and cathode, just like broadcast.

If 12AX7 ran acceptably on 5-6.3 volts, it would say so in the spec sheet.

Now if your heater voltage is 7.2 volts, it is understandable why you need to drop it to 6.3 - I can go with that for sure.

[Guitarist]

OK guys that it extremely helpful, thank you. I just checked my larger wattage resistor stache and have some 2W 1Ω resistors which I bought for output tube biasing. A resistor or diode needs only go along the +'ve side (preferred), vs both, —'ve as well, balanced? I'll get it close to 6.3v...

This Kent amp had no heater twisting and has become rather high gain so the DC really works great. The noise floor was pretty high prior.

[pdf64]

'Lower heater voltage shortens tube life. It does not prolong it.
It applies to broadcast tubes and audio tubes, alike.'

Did this viewpoint ever get confirmed? ie that audio signal tubes age more quickly if the heater voltage is lower than the normal lower limit of 90% of 6.3V = 5.67; eg 80% of 6.3 = 5.04V.

The various mechanisms of tube aging are rather over my head.

However, my 'tubes for dummies' understanding is that transmitting tubes tend to be run flat out whereas by comparison audio signal tubes just tootle along, so I can envisage that because their operating conditions are so different, so too might their significant constraints / aging mechanisms differ.
Pete

[Chuck H]

Indeed! And if it turns out that a little high IS better than a little low one could drop the 7.2V to 6.5V or 6.6V with a single diode added to the string (observing it's dissipation of course, thanks R.G.).

I've never read anything definitive on cathode stripping or cathode poisoning as it relates to tubes in consumer electronics. Some of what I've read suggests that it's so insignificant in these devices that it can be ignored. Digging through the information super highway (I call it the information pile) is just a mash of mixed reviews and opinions.

I've safely run tubes a little high for many years because I use Hammond transformer a lot. Off the shelf Hammonds have a 115V primary and I live in a 120V country (where the actual voltage is often 125!).

[daz]

My heaters are at 7.1vac. Should i be worried about this? I'm told hammond PT's, at least some, are modeled after older ones when wall voltage was lower.

[Chuck H]

Any spec I've seen has +/- 10% for filament voltage. 10% of 6.3V is .63V so it would seem that, by spec anyway, staying over 5.67V and under 6.93V is ideal. That said, my filaments are often at 7.1V or 7.2V depending on my wall voltage de jour. I haven't had any problems I can pin on the filament voltage. Ever.

[g1]

I have heard extra noise from excess filament voltage. This is in critical applications like tube mic's. With adjustable heater voltage in the mic supply you could actually hear the noise increase with higher filament voltage.
Not sure if you would hear it in a guitar amp but it's worth noting.

[loudthud]

The gain of a tube is higher when the heater voltage is high. The first thing you do when you look at the curves on a Tektronix 570 curve tracer is adjust the heater voltage. There is a meter and variac to do this. You can watch the curves move as you adjust the voltage.

It seems like there was a graph of tube life vs heater voltage as a percent in one of those old texts like the RDH.

[daz]

The gain of a tube is higher when the heater voltage is high. The first thing you do when you look at the curves on a Tektronix 570 curve tracer is adjust the heater voltage. There is a meter and variac to do this. You can watch the curves move as you adjust the voltage.

It seems like there was a graph of tube life vs heater voltage as a percent in one of those old texts like the RDH.

Is there a simple and safe way to drop it?

[Chuck H]

Is there a simple and safe way to drop it?

Yes.