No.15
Amateur Radio
572B
TECHNICAL TOPICS
Article as first appeared in RadCom Magazine
(October 1996)
A
letter from George Badger, W6TC, of Svetlana notes that the Yaesu FL-2100
series of linears, using the 572B high-mu high-power triode, has proved
very popular in the UK. He points out that the new Svetlana 572B has slightly
higher gain than the original. When fitted in the FL-2100, Fl-2100B, FL2100B
and FL2100F linears they sometimes oscillate in the standby mode because,
as Yaesu has confirmed, the cut-off bias is only slightly higher than cut-off.
If the amplifier 'takes off' as an unloaded TPTG oscillator it can wreak
havoc.
W6TC bought a used FL-2100B and modified the bias circuit: Fig 5 shows the original arrangement and Fig 6 the modification, incorporating a voltage-doubler configuration to increase the stand-by cut-off bias. After modification, the amplifier becomes unconditionally stable when fitted with the higher-gain Svetlana 572B.
Fig 5: Stand-by bias arrangement as used in the Yaesu FL-2100B linear amplifier.
Fig6: How W6TC modified the stand-by bias in his FL-2100B to increase the bias voltage by using a voltage-doubler arrangement in order to ensure unloaded stability when the higher-gain Svetlana 572B valve is fitted.
The Heathkit SB-200 which uses a similar circuit has 100V cut-off bias and is stable when fitted with the Svetlana 572B. This applies also to the new Yaesu FL-2100Z.
The Svetlana 572B features a massive graphite anode for high peak overload capability and an average anode dissipation of 160W. Typically, as a linear Class B grounded-grip amplifier with 2.4kV HT and 50W drive, it can provide a single-tone useful output power of about 300W. It can be used as a drop-in replacement for the 811A to provide up-graded performance.
Svetlana Electron Devices, Inc (3000 Alpine Road, Portola Valley, CA 94028, USA) can also supply a 4CX400A tetrode for linear amplifier service and also a relatively inexpensive Svetlana SK2A ceramic socket for this valve. In the AB2 mode with 2.5kV EHT, it can produce over 600W PEP with low intermodulation distortion. A pair can produce 1.2kW PEP or CW at up to 500MHz.
**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.