Reduced Ac line voltage and tube rectifier filament voltages

[68strat]

Data sheets for tube rectifiers spec the filament - heater voltage. 5 volts for one example.

With reduced Ac line voltages, when is a voltage lower than 5 volts to where it becomes a concern as to proper function of the tube rectifier. Is 4.7 or 4.3 volts ac going to lend to arcing or malfunction causing the rectifier tube to fail?

5ar4 indirect heater. 5u4 direct heater. 5y3. Using standby. Not using standby switching, all scenarios. any tube rectifier.

How low can the heater voltage go for the Rectifier Tube?

We built 3 R.G. Keen line voltage bucker boxes 12.6 volt with center taps at 4 amp transformers. 123v to 113 and 106 volts reduced. Ac led line voltage meters on each bucker box.

Using on Fender, Gibson, Silvertone, and an Allen amp.

[martin manning]

Checking some GE vacuum rectifier data sheets I'm seeing the same +/-10% as for triodes and pentodes, but directly heated cathode types (5Y3 and 5U4GB) don't include a range. I would expect more voltage drop as heater voltage is reduced.

[68strat]

Thank you very much Martin Manning

Why indeed, I do now see on GE 5ar4 tube data sheet, the 10 percent plus / minus 5 volt heater tolerance spec.

Two Falcons 5y3 and a Deluxe Reverb gz34 mid 1960's are not my amps.

It was my suggestion to build keen buckers. Later it became a concerned about low filament voltages especially on the rectifier tubes.

I'd like to rephrase my original question.

Is a tube rectifier running at less than optimal Filament voltage more susceptible to damage from voltage spikes due to hot-switching B+ or other anomalies such as 120 Ac Line voltage dropouts or spikes?

Thanks again

[R.G.]

You're probably fine.
The whole point of heater voltage is to get the heater up to hot enough to boil off enough electrons from the cathode. That "enough electrons" was designed in so that a heater voltage at the tube of nominal voltage minus 10% is "enough". They had to do that because the heater voltage was just line voltage transformed down, and reduced a bit by the PT's internal resistances. The only regulation of heater voltages was done at the electric company by supplying line voltage, and the transformers not making it too much worse.

Power tubes do need a lot of electrons; you're right that starving them too much is not good. Below the specified heater voltage range, there aren't enough electrons boiled off to keep an electron cloud above the cathode to neutralize any stray gas ions that eventually damage the cathode surface. Keep them within specified range. But you don't need to sweat the spikes.

[68strat]

Dear R.G. Sir, we Thank You so much for being so kind in sharing Your top level information with us and the world.

[TUBEDUDE]

Good info.
In my location the issue is overvoltage. The mains is frequently 124Vac and sometimes more. My old Fenders have greater High voltage and heater voltage.
I tried to use a diode array to drop the heater voltage on one. It worked, but contributed a little noise.
A variac is definitely recommended for amps designed to operate on 115V.

[R.G.]

My AC runs 125 normally and I've seen 129. It's scary when you're dealing with stuff designed for 112Vac.

There are a few ways to deal with this. A variac is one, although it's expensive for a big enough one. The fixed voltage version is the autotransformer, if you can find one with the right taps.The "Vintage Voltage Adapter", my take on the bucking transformer connection is actually a modifiable version of the autotransformer that is configurable. Diodes work for just dropping voltages, but it takes four diodes for every 1.4V you drop, so you need a LOT of diodes, and they change the voltage by wasting the difference as heat. Normal diodes have the "slam off" effect that causes a line frequency buzz. This can be cured by snubbing them with caps or RC networks, or by using FRED type diodes that don't generate the spikes to start with.

For me, the set of pluses and minuses settles down to the bucking transformer. A 120V to 12Vac trannie at 2A to 4A is cheap, and converts 124V to 112Vac. These are usually small enough to tuck away into the amplifier chassis somewhere, and effectively convert the amp itself to modern voltages. But there are pluses and minuses to all of the options.

[martin manning]

12V CT secondary gives you the option of knocking off either 6 or 12V.

[68strat]

We are seeing about 2 to 3 volts less than expected with our line buckers.

The transfomers we used are not the Organ brand name.
They are off shore 12.6 volt ct. Listed as 117 volt input. 4 amp
But the data sheet say 115 volt input 4 amp iirc

Using tramsformers spec'd at 115 volts input on 120 to 123 volt and above Ac line voltage.
Is the transformer bucking differently resulting in lower voltage than expected?
Is a transformer used for bucking rated at 4 amps verses 2 amps bucking effect stronger?

The Math, Physics, clevernsess and kindness. Thanks again R.G. for helping us with your Vintage Voltage Adaptor to keep vintage amps them running where they want to be.

[martin manning]

We are seeing about 2 to 3 volts less than expected with our line buckers.

The transfomers we used are not the Organ brand name.
They are off shore 12.6 volt ct. Listed as 117 volt input. 4 amp
But the data sheet say 115 volt input 4 amp iirc

Using tramsformers spec'd at 115 volts input on 120 to 123 volt and above Ac line voltage.
Is the transformer bucking differently resulting in lower voltage than expected?
Is a transformer used for bucking rated at 4 amps verses 2 amps bucking effect stronger?

The voltages specified for the transformer are at full load. Unloaded, the secondary voltage will be higher, maybe 10% or so. If your amp is only drawing 1A, indeed a 4A rated transformer will very likely have a higher secondary voltage than a 2A rated one (working at a 25% load vs. 50% load), and you will get more voltage reduction. On top of that, if you are supplying more than the spec input voltage (120+ vs. 115), the secondary voltage increases proportionally, resulting in more voltage reduction. Combined, these two effects could account for the additional 2V reduction you are seeing.

[R.G.]

Absolutely correct, Martin.