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    Modification to rectifier tube



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    Join date : 2012-03-19

    Modification to rectifier tube

    Post by wildiowa on Fri May 18, 2012 9:58 am

    I left a Mark III at a repair shop long ago after being told it would cost several hundred $$ to repair...with my new interest in Dynas I picked it up and found that he had done some mods and had it up and running....this photo shows his mod to the recifier tube which looks resourceful but not sure if it's OK or approved by the community here...has he simply added diodes to replace the rectifier tube here? Any analysis or advice appreciated...thx. IA

    Bob Latino

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    Re: Modification to rectifier tube

    Post by Bob Latino on Fri May 18, 2012 10:24 am

    What he did was a common "mod" although it's not really a mod. He used two diodes in an old tube socket shell to create a solid state rectifier in place of a GZ34 tube rectifier. The only issue with this practice is that B+ high voltage comes on almost instantaneously. There is no slow warm up like a GZ34 tube rectifier. Some say that using a solid state rectifier can shorten output life? Whether that is true or not, I can't really say.

    Also - the use of solid state rectifier will also cause a higher than normal B+ high voltage. On your Mark III pins 3 and 4 of each output tube should be 470 to maybe 490 volts. I would be willing to bet that with that made up solid state rectifier pins 3 and 4 are looking at 500+ VDC. Higher than normal voltages can cause shorter tube life.

    If you do want to use a solid state rectifier in a Dynaco amp, what I would do is pull that made up solid state rectifier out of there and replace it with a Weber WZ68 Copper cap solid state rectifier. The Weber's have a short 3 to 5 second delay built in and a resistor (or two?) inside the copper cap to give you normal B+ voltages. The Weber's are not too expensive ($22 + shipping) and work well in Dynao amps that are supposed to use a GZ34 tube rectifier.



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    Re: Modification to rectifier tube

    Post by dynacojoe on Wed Aug 22, 2012 7:50 pm

    Good advice from Bob! Instantaneous B+ does not have the benefit of plate current flow, for the first few seconds after turn on, to moderate B+ levels. Solid state rectifiers, with no delay provision, will cause over 400 volts to appear on the 7199 triode plates briefly. These plates are rated for 330 volts max. Some tubes will take the abuse, others may not. I learned this lesson the hard way...


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    Re: Modification to rectifier tube

    Post by dunwichamps on Fri Aug 24, 2012 11:59 am

    just to elaborate, higher plate voltage is usually not an issue if the tube is rated to handle the extra voltage, its really the larger than normal screen voltage which is the culprit. If the bias is not sufficient to bring the idle dissipation back down to its previous power level with a tube recto then you would be running the tube harder and thus yes shorter life on average.

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