[R-390] Two new tutorials posted on the R390A FAQ site

Wed Jul 24 19:02:06 EDT 2019

Hiya Alex,

I like to be clarified about applying HV before the filaments.  I just can't see this being a bad thing at the voltages we're dealing with.  I've heard of cathode stripping, but I can't see that occurring under the conditions we see in these units - <450V, low possible currents.  Cathode stripping could occur due to corona, but unless the tube is already gassy, I can't see it actually happening.  With no heat, no thermionic emission can occur and the plate/cathode/grids are just metal with a voltage on it in a vacuum.  Vacuum is good, partial vacuum is bad with regard to corona.  I see delayed application of B+ often in audio amplifiers, but I can't help but wonder if the fear of cathode stripping is just an urban myth.

The use of solid state rectifiers predates the R390 and R390A.  Indeed, the selenium rectifier has the same issues as a silicon diode and the selenium rectifier has extensive heritage with VT circuits.

I continue to use rectifier tubes in my R390, R390A and all my other antique radios for originality and to avoid issues with high B+.  I have designed a circuit that emulates the characteristics of the 26Z5 as a full-wave rectifier, but have never built it.  It works in PSPICE for what that's worth.  If I ever go with SS, I'd use a filament transformer wired in buck to lower the line voltage and make a box to house it.  I may do that anyway since line here is 122V which equates to about 25-30V or higher unloaded B+ in my R390A.  Using my Variac is an unwanted nuisance for casual use.

As a fellow EE I'd be very interested in hearing the physics behind the HV before filament concern.

John

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of alex kosman
Sent: Wednesday, July 24, 2019 3:08 PM
To: w1nzr at cs.com
Cc: Perry Sandeen <sandeenpa at yahoo.com>; r-390 at mailman.qth.net
Subject: Re: [R-390] Two new tutorials posted on the R390A FAQ site

Hello, I absolutely  agree,  those old input AC  filter make the "differential relay" mounted in many modern houses to turn off the electric supply.
If someone wants to open the old filter and replace the caps it will work, but its not electronics but archeology !!
And, applying HV to cold tubes is a bad idea too. I used in many cases in the 60' a delayed relay  (rectified  heater voltage can be used to add a
555 or else) with relay that connect the HV after 10 sec or more. Did that in many receivers, automation circuits and more (miss those days).
Another  story, for those who insist to use rectifier tubes, the circuit has an engineering  problem.
I dont have the schematics in front of me but I remember that the circuit is grounded at one side and at the cathode  is under HV, that means that there is hv between the cathode and the filament, the  26Z5 in this case is made to resist but in many cases the isolation between the cathode and the filament fail. In many countries a separate floating coil was made in the main transformer just for the rectifier tube to avoid this stress. if you use solid state rec. you eliminate also this problem.

many best regards to all,
Alex
Kosman Alex
Electronic Engineer
Mobile:  972-52-2812797
alexkosman10 at gmail.com

‫בתאריך יום ד׳, 24 ביולי 2019 ב-20:14 מאת ‪Brown Beezer via R-390‬‏ <‪ r-390 at mailman.qth.net‬‏>:‬

> Thank you Perrier
>
>
> -----Original Message-----
> From: Perry Sandeen via R-390 <r-390 at mailman.qth.net>
> To: r-390 <r-390 at mailman.qth.net>
> Sent: Tue, Jul 23, 2019 10:16 pm
> Subject: [R-390] Two new tutorials posted on the R390A FAQ site
>
> Yo Bubba Dudes!,
> Al has posted two new tutorials I've written.
> The first is how to replace the OEM large and dangerous AC input 
> filter,
> FL101 with a modern computer input style filter.  They are smaller and 
> work better than the OEM and best of all won't shock the dickens out 
> of you if there is no 3 wire ground ans you are grounded when touching it.
>
> The second is an electronic regulated PS circuit.
> Now why would you really want to use it.
> I'm so glad you asked.
> Besides good B+ regulation there is IMNSHO a much more important feature.
> Almost all have given up 26Z5's because of cost and use SS diodes.  
> And here lies the problem.  When first turned on before the filaments 
> have a chance to warm, up full B+ is applied to the tubes and filter capacitors.
> Both aren't happy with this. This causes tubes to be stressed and 
> causes premature failure.
> Now they're not making the tubes we use anymore and the prices are 
> slowly rising so prolonging their life has direct economic benefits for us.
>
> This wasn't a problem with filament rectifiers as B+ would slowly rise 
> as the tube warmed up.
>
> So the circuit I came up with has a RC delay for a slow B+ rise.
> An additional benefit that is new to this is the LR8 three terminal 
> programmable HV IC. With just two resistors it can be set from 12 to 450V.
> This can be used on any B/A receiver.  Used alone it can replace a 0A2 
> or any other gas regulator. What is so very helpful is that one 
> doesn't need an input dropping resistor.
> All the details are on the tutorial.
> Regards,
> Perrier
>
>
>
>
>
>
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--
Kosman Alex
Electronic Engineer
Mobile:  972-52-2812797
alexkosman10 at gmail.com
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