# DC heater voltage 6 volts or 12 volts?

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#### speakerfritz

I googled around and found a split verdict. Some tube amps using DC are running at 6 volts, some tube amps running DC are running at 12 volts.

The spec sheet for a KT-88 says heater voltage 6.3 volts DC or AC.

Doesn't say anything about using 12 volts DC.

#### bob91343

You can't use 12 Volts on a 6.3 Volt heater. It requires 6.3 V within about 5% but it can be AC or DC.

You can run 12 V if you put two 6.3 V heaters of the same current in series. The advantage of this is lower current, as if you put them in parallel.

The main advantage of DC is reduced likelihood of hum, but that applies more to low level amplifier tubes than to output stages.

#### Brit01

As Bob said, tubes in series must be of the same current (preferably same type of tube).

I've always used AC and never had hum problems.

Also you can use higher voltages depending on the application.

I have 4 x 6080 tubes that consume a whopping 2.5 amps each.

That would require a very high amperage 6.3 volt tranny, so I use a 28 volt 3-4 amp tranny. Easier and sometimes cheaper.
I got 4 of these 28 volt transformers for just 30 USD.

I get just over 25 volts with them in series (slight drop). Perfect 6.3 volts per tube.

#### DF96

Twice the voltage means half the current. Half the current means half the magnetic induction of charging pulses into audio circuitry. This could be an argument for going higher in voltage. The snag is that the higher the voltage the more it looks like a series current heater supply rather than a parallel voltage supply, and a series supply needs to be more tightly regulated and surge limited because of the way valve heaters change their resistance with temperature.

I would guess that 25V is about the limit for a voltage-based supply.

#### piano3

I used a passive 25V@ 450mA supply for a RIAA amp I built for my Dad with 2 sections of RC smoothing. That shouldn't be too hard on the valve heaters,should it?

#### Brit01

The snag is that the higher the voltage the more it looks like a series current heater supply rather than a parallel voltage supply, and a series supply needs to be more tightly regulated and surge limited because of the way valve heaters change their resistance with temperature.

Thxs for the info. I wasn't aware of that.
So maybe a simple resistor and switch in parallel for the surge issue.

#### DF96

Don't panic! The figures often quoted are that a voltage (parallel) heater supply should be within 10% of the correct figure, and a current (series) supply should be within 5%. The reason for this is that if a valve heater is a little too hot then it will increase resistance, which reduces power from a voltage supply but increases power from a current supply. This is not a huge effect but it is not negligible either.

Valve heaters can withstand some surge. The problem comes if a series string has valves with different size heaters or different heating times. The hottest ones can grab too much power and overheat, if their resistance is too small a part of the total to reduce the current. Valves intended for series heaters (such as 300mA TV valves) are designed to cope with this. In the US this is sometimes shown by an A suffix to the valve name. The problem can be reduced by ensuring some series resistance, independent of temperature. A voltage supply does not have this problem.

Above 12V I would use a fairly small reservoir capacitor followed by RC smoothing. Together these will add some series resistance. Probably not necessary to have a switched surge limiter; this would be more useful with a 6V DC supply using a fat capacitor, where the aim would be to limit the charging surge rather than the heater surge.

#### barrymagrec

Leevers Rich tape Machines always used 12 volts DC for their valve heaters. This was partially a throwback to the days they were run off a car battery for location work but it also made sense to halve the current through the then expensive rectifier and easier to smooth the higher voltage / lower current. Switch on surge is not really an issue with an RC circuit as the volts come up relatively slowly as the heaters come up to temperature.

Barry

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