[R-390] 26Z5W replacement

[Bernie Doran]

If there really is a problem with the SS rectifiers, just use a time delay 
relay after the rectifiers to hold the circuit open for a minute or so  Fuji 
ST7P and Magnecraft TDR782 series relays are very small and available in 120 
AC and assorted  DC voltages.    At the same time add in  a pair of 200 to 
250 Ohm 5 watt resistors to simulate the forward drop of the 26Z5s . The 
tube curves I looked at indicate a foreword drop of about 22V at 100MA.

The relays would likely be somewhat over spec with the B+ voltage, but I 
doubt if that would be an issue, for those that think the entire receiver 
would explode, then just add a tiny 600 V rated relay after the time delay. 
Total cost is probably under $60, probably about the same as NIB 26Z5s. 
This would also be a good time to add in a small fuse in the B+ line, might 
save a transformer if/ when the filter caps fail. Not sure what size, but 
easy to find the actual current drain by measuring the voltage across a low 
value resistor or a miliampere meter,wild guess would suggest that a 200 MA 
fuse could be about right.

Bernie W8RPW


----- Original Message ----- 
From: "Charles Steinmetz" <csteinmetz at yandex.com>
To: "390 list" <r-390 at mailman.qth.net>
Sent: Monday, September 16, 2013 9:00 PM
Subject: Re: [R-390] 26Z5W replacement


> Roger wrote:
>
>>What if we added a thermistor in with the diode?
>>It would limit inrush current.
>
> Unfortunately, it's not an inrush current problem -- it's the opposite. 
> With a choke-input power supply filter, the DC voltage when NO B+ current 
> is being drawn is much higher than when normal B+ current is flowing 
> (about 1.6--1.8 times more).  So, if the diodes are rectifying before the 
> rest of the radio wakes up (which is the case if they are SS rectifiers in 
> a tube radio), the B+ soars until the radio circuitry is drawing normal 
> operating current.  There is no problem with a tube rectifier with an 
> indirectly-heated cathode, because the rectifiers warm up slowly just like 
> the rest of the circuitry (typically a little slower -- they are designed 
> that way for just this reason).
>
> Note that this is NOT just the normal B+ sag from resistive losses, which 
> you also see with capacitor-input filters -- it is a fundamental 
> characteristic of choke-input filters.
>
> So, what we need is a dummy load on the power supply while the radio 
> circuitry warms up, or some way not to have the SS diodes rectifying until 
> the radio circuitry warms up.  Neither of these is a trivial design 
> challenge.
>
> Further, whatever solution we adopt for turn-on should also be activated 
> when the radio is switched to "Standby," because the B+ current drain goes 
> down and the voltage goes up in standby mode.  Note that the original 
> design does not address this -- if you switch a stock 390A to standby, the 
> B+ voltage goes up.
>
> Best regards,
>
> Charles
>
>
>
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