18Watt.com

There was a recent post on another forum that reminded me about a modification that every 5AR4 should have. A quick look at the 36W EL34 Superlite TMB in archives here indicates it's not incorporated in the schematics. The modification essentially consists of the addition of two rectifier diodes in series with the anodes of the 5AR4. This adds the PIV rating of the diodes to that of the 5AR4, helping to eliminate the primary cause of failure, which is internal arcing. This was never an issue with original 5AR4s made in the U.S. and Great Britain, and it's one of the reasons Western 5AR4s continue to command such high prices on the used and NOS market. Russian 5AR4s, such as Sovtek, are generally pretty good in terms of current capacity and voltage drop. However, for whatever reason (possibly a softer internal vacuum), they tend to arc internally. The higher the PS voltage, the more likely this is to occur, either initially or after some period of use. The modification to forestall these failures is illustrated below. Diodes can be inexpensive 1N4007 types. There is no downside to this change other than the additional 0.6V forward drop of the diodes themselves.

Credit must be given to Eli Duttman of hi-fi fame for first publishing this "preventive care" solution within the DIY community.

Jack

Personally, I would really recommend using UF4007 rectifiers over 5AR4 (aka GZ34)s. No need to waste good glass on this menial job!