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Indirectly- versus directly-heated rectifiers

Added 11-01-2015: The text below is primarily meant as a directive when designing valve amplifiers, not as a guide how to modify valve amplifiers. Most likely the choosen rectifier(s) will suit the amplifiers technical specifications. Be aware of that when changing these..

A while ago I posted on the website of Jimmy Auw about the use of valve rectifiers. More or less when to consider indirectly heated rectifiers or directly heated rectifiers. Maybe it is worthwile to dig into that a little deeper..

His post was about building an universal power supply and was considering several types of rectifiers. This is what I replied:

Interesting topic. Preheat rectifiers versus Cold start.. When using directly heated (dh) rectifiers, preheating them will result in maximum current demand from both capacitors and signal tubes when high voltage is applied. This results in a current peak and high voltage peak. Cold start (just one on/off switch) will take care of instant current demand of signal tubes and the voltage peak. When directly heated power tubes are used, operational temperature of the filaments will be reached at about the same time. So more hand-in-hand. Current draw from capacitors will remain at a too low cathode temperature. This was no problem at that time since only low-uF capacitors were available. When indirectly heated (idh) rectifiers are used, current draw from capacitors and dh power tubes will be present at a too low cathode temperature. Current draw from idh-power tubes will go hand-in-hand. My experience is that it is best to use dh-rectifiers in solo dh-tube circuits, and idh-rectifiers in solo idh-tube circuits. Use of type power tubes (dh vs idh) dictates more or less type of rectifier. So when a mix of idh-tubes and a dh-power tube is used, it is best to use dh-rectifier since these will demand highest current.

Since a lot of rectifiers are interchangeable and choice can be of importance in an amplifier, I will clarify my post a little bit and put down my recommendations.

Rectifiers
There are basically two types of rectifiers, directly heated and indirecly heated. A directly heated rectifier is operational as soon as the cathode has reached optimal temperature. Since the cathode is heated driectly, this can be as fast as 2 or 3 seconds. An indirectly heated rectifier is also operational as soon as the cathode reaches optimal temperature only the heater needs to heat the cathode, hence indirectly heated. This can take up to 10 to 15 seconds. This means that high voltage supply will be available faster when dh rectifiers are used.

Other valves..
The other valves in an amplifier are also directly- or indirectly-heated and require about the same time to reach operational temperature as the rectifiers do. Now what happens if these are mixed? To get an idea it is good to get some kind of understanding what is happening when a valve heats up.

Basic principles.
To emit the cathode needs to have a certain temperature and a plate voltage supply. There a two stages that are somehow critical:

Both result in damaging the valve.
Of course current draw from the amplifier is what completes Ohm´s law. In general we have two kinds of current draw to take into account:

Put these facts together and you are presented with some kind of dilemma: No matter how you switch on an amplifier, problems will occur.. Luckily engineers knew that as well and designed most valves to withstand (for a short period) some of these shortcomings. But it leaves us with the question what is the best way to switch on an amplifier with a certain valve configuration? I´ve seen (and used) different methods to deal with this:

Recommendations please?
Here is my shortlist..

If you decide to go for a directly heated rectifier with indirectly heated power valves, take into account that a much higher supply voltage will be present few seconds after switch on, since the rectifier is already conducting when the power valves are still heating up. This means that here is no dynamic current demand at that time so no voltage drop over resistors and/or chokes. This is especially the case if a choke input power supply is used with no effective bleeding resistor. Be sure that capacitors, resistors and valves can withstand this higher voltage!

Datasheet indicates that the EL3 can withstand 550 Volts on the plate in cold condition..