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Post subject: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 6:50 pm
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Notice how Fender is placing sinter-glass, solid-state diodes (like BYT52M) inline with the 5AR4's anode plates? This mod is not noted in their schematic. However, it allows the use of Sovtek 5AR4 with little loss of tube tone. Much longer tube life.

A fix recommended for all newly made 5AR4/GZ34.



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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 7:42 pm
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BMW2002Ti wrote:
Notice how Fender is placing sinter-glass, solid-state diodes (like BYT52M) inline with the 5AR4's anode plates? This mod is not noted in their schematic. However, it allows the use of Sovtek 5AR4 with little loss of tube tone. Much longer tube life.

A fix recommended for all newly made 5AR4/GZ34.


Slower start-up?

Arjay

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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 8:20 pm
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Arjay,

No. The solid-state diodes are to help relieve the tube 5AR4 from excessive current and voltage spikes. This is what kills modern era rectifiers. Either slowly or quickly (as in arcing).

The problem is that this cure also changes the sag character of the overall PSU. So, you lose a bit of "breathing" quality. Since 5AR4/GZ34 have little sag (or voltage drop), you don't hear this as much, as with rectifiers that sag a lot (direct-heated 5U4GB & 5Y3GT). This mod is not recommended on amps powered by those tubes. Luckily, good NOS 5U4GB and 5Y3GT are still available, at a good price.


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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 8:24 pm
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These diodes simply help protect the amp in case the tube rectifier shorts. In the event of a tube rectifier short, the diodes take over and keep AC from getting past the tube into the rest of the power supply. In this event, the DC output will rise, and the difference in sound will probably be noticed, but the amp will work nearly normally until such time as the tube rectifier can be replaced. This rise in the DC supply will also increase the current through the preamp and output tubes, so the rectifier tube should be replaced as soon as possible to prevent cooking the output tubes if they are already biased hot. These diodes do nothing unless the tube shorts, so you still get the sag of the tube rectifier, preserving the response of the amp, and no they don't protect the rectifier tube in any way.

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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 8:56 pm
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Shimmy, You are correct. Solid-state takes over if cathode shorts to anode, in the tube rectifier. This stops full AC from power tranny's secondary to go into filter cap stages and blow them up.

Sorry, about the mis-information. Here's a good article:

http://www.premierguitar.com/articles/t ... fier-mod-1


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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Sat Feb 22, 2014 9:32 pm
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No problem, just a friendly reminder. :) I knew that if you thought about it for a minute, you would realize that a diode has no regulating or suppression abilities, it's just a one-way valve so-to-speak. It is very interesting to see this mod incorporated into the stock amp. I've done this mod on some vintage amps, so I got to measure the DC supply before and after. I didn't even note the 0.7 volt difference. :lol:

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Post subject: Re: Solid-state diodes on the reissued SR 5AR4 socket
Posted: Mon Feb 24, 2014 8:09 am
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After much conjecture, this is one answer I got on a DIY board, re: adding solid-state diodes to a tube rectifier:

" All of what you are saying is correct. [that ss diodes are only safety devices --- which act only when the tube rectifier shorts-out].

However you are only thinking in one direction, the forward direction. When the diode/rectifier is conducting. You liken the diode to a one-way valve, which is 100% correct. So lets go with this.

You need to think about what happens when that one-way valve is forced to work the other way and block. That happens every 1/120 of a second. The B+ cap will be charged to peak AC voltage (upwards of 1.4 times transformer secondary under no load). During the reverse cycle the rectifier/diode will need to block 2.8 times the transformers RMS value (with full wave, center tap).

An original real GZ34 has a reverse voltage rating of 1,500 volts. So the slightly over 1,000 volts one would see in a Dynaco ST70 wouldn't be a problem under most conditions. Today's GZ34's aren't nearly as robust and the real thing is getting costly. Add in the fact that AC wall voltages are higher today. Using a 1N4007 in series with today's GZ34 increases it's PIV rating to slightly over 1,000 volts. "


In other words, the solid-state diodes have no beneficial effect for the original GZ34/5AR4. But MAY assist today's rectifiers, many of which are not built to the same standards as the original.


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