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PostPosted: July 16th, 2016, 4:03 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 686
The problem is a simple one and one that regularly occurs. Forgive me for restating these things for those of you with an understanding of the problem but here goes. Just because a device may be better in many of its characteristics as solid state rectifiers are, that superiority can come with unintended consequences. In this case, the very low forward voltage drop of the SSR even at high currents in a circuit designed for a lot of forward drop of a vacuum tube rectifier results in an output voltage that is higher by the difference in drop of the SSR and the VTR. This usually adds extra stress to the following circuit elements and can result in catastrophic failure.

Another problem is that the SSR conducts immediately where a VTR needs time to warm up for conduction to occur. The immediate conduction of the SSR will put high voltage on the tubes before they are warmed up as well as resulting in a higher B+ until the tubes conduct lowering the voltage by an amount dependent on circuit impedance.

Unless you are using a time delay circuit to prevent the HVAC entering the rectifying circuit until the tubes warm up, using an SSR with a limiting resistor does not solve the problem of instant voltage on a cold tube and can also result in over voltage on the filter capacitors. Until the following tubes warm up and draw current there is almost no drop across the resistor because circuit current draw cold compared to after the tube warms is usually quite low. If a circuit is designed to use a SSR instead of a VTR, the transformer secondary will have fewer turns resulting in a lower AC voltage when rectified that will be within the ratings of the other circuit elements.

Tom


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PostPosted: July 16th, 2016, 4:11 pm 
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Joined: July 8th, 2016, 4:34 pm
Posts: 145
Thank you.


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PostPosted: July 17th, 2016, 7:26 am 
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Joined: January 13th, 2016, 9:14 pm
Posts: 388
Also if the Power Transformer is an original, check the tube filament voltages.

In my early '70s guitar amp with my wall power of 126VAC, the filament voltages were over spec for the ECC83/12AX7's and EL34's. I use a Variac and bring it down to 110VAC and it is at the correct voltages. Much longer tube life is a good thing.


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PostPosted: July 17th, 2016, 4:41 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 53
justinis wrote:
Am I being dense when I ask why not use solid state silicon rectifiers in the power supply since in these day ones with appropriate ratings exist?


I'm using a tube rectifier for mostly aesthetic reasons. More tubes = more awesome. :idea: 8-) But also so I don't have to worry about the problems tomp described.


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PostPosted: July 20th, 2016, 6:06 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 53
Just to close the loop here, I modeled and tested with a 220 ohm resistor before the first cap. Worked like a champ. Thanks all!


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