eicoeico on October 6, 2012 at 10:29 AM said:

A problem caused by directly heated or filamentary cathode rectifiers is known as cathode stripping. Cathode stripping is a highly technical issue, but the basics of it are this: Filamentary Cathode rectifier tubes like 5R4, 5U4, 274B etc can deliver full DC levels only FIVE seconds after powering up an amp in which they are installed. In contrast, it takes TWELVE seconds, more or less, for the input tubes (12AX7, 12AU7, 6SN7 etc) and the output tubes (KT88, EL34 etc) to even BEGIN conducting this DC voltage.. so there are about 7 seconds where full DC levels are applied to the power supply capacitors and the audio input and output tubes… and these tubes are not ready to use/conduct this DC voltage. Thus, when the audio tubes do begin to conduct current at ca. twelve seconds after the amplifier is turned on, the 5U4/5R4 type rectifiers have already been charging the entire circuit with full DC current/voltage levels for 6-7 seconds. This difference in warm-up time causes a huge transient surge of current and electron emission in the cathodes of the input and output tubes as these audio tubes begin emitting electrons and thus conducting current— The audio tubes essentially are “slammed” by the full DC levels that have been present for several seconds before they were able to conduct current. These extreme transient currents and emission levels at the moment when the audio tubes begin to emit electrons and thus conduct current causes cumulative damage to the cathode. This cumulative damage gradually begins “stripping” precious cathode emulsion away as small amounts of the cathode emulsion are literally boiled off the cathode during the brief but intense current spike as the audio tubes first reach operating temperature some 7 seconds later than a filamentary cathode rectifier reaches operating temperature. The damage is rather small each time, but over the course of hundreds of turn-on cycles when the amplifier is powered up it cumulatively contributes to an early death for expensive audio tubes. Cathode Stripping is NOT an issue with triode output tubes like type 45, 2A3, 300B, 6B4G etc because these early triodes are filamentary cathode/directly heated tubes just like 5U4/5R4 etc.. these early triode output tubes reach operating temperature at the same exact time a 5U4 or 5R4 will. To eliminate cathode stripping, indirectly heated/cathode sleeve-type rectifiers like 5V4, 5Z4, GZ30, GZ34, GZ37, 6X5 and many others were developed by tube designers. These tubes, being indirectly heated, take 12 seconds to even begin delivering DC voltage/current, and are only able to deliver maximum DC voltage and current levels to the audio tubes after 18-20 seconds. This is known as a “slow ramp up” of the DC voltage applied to the circuit by a rectifier, and this characteristic of delayed delivery of DC voltage to the audio tubes in an amplifier eliminates the intense current spikes that can stress power supply capacitors and cause cathode stripping in audio tubes. This is why indirectly heated rectifiers such as the legendary Mullard GZ34 are so much in demand. Slow ramp-up of the DC avoids huge transient current spikes, and thus prolongs the life not just of the expensive audio amplifying tubes but also the power supply capacitors and the power transformer itself. In the next section, I’ll go deeper into the indirectly heated rectifiers like GZ34/5V4/5Z4 etc, comparing and contrasting their electrical parameters, and referencing these abstract electrical data to real-world performance Finally, a large percentage of modern tube equipment uses solid state diodes to rectify DC; a diode can deliver full voltage and current in less than a second after turn-on. Well-designed modern tube equipment uses a variety of methods to delay the instantaneous voltage/current delivery of these diodes and thus it is not generally a problem. Likewise, there are delay devices available that allow slow ramp-up of DC levels with filamentray cathode rectifiers like 5U4 for audiophiles who prefer the sound of the directly heated type rectifiers.