Valiant change 3B28's to S.S. sticks

[N8ETQ]

   Interesting that I "Policed up" a Mint Valiant that
was sporting 3B28's fer the 866's, The thing had been
re-capped in the recent past and Ep with the 3B28's was
measured at 706, with the solid state FAA XXXX? Ep was
measured at 703. Go Figure.

   It had all the "Typical" Valiant issues, Stupid line
cord plug, No "Back to Back" diodes across the "beautiful"
Teardrop meter and Internal LV Fuse Holder. Factory
assembled as the tube sockets are "Riveted". Nice Piece,
Wish it was Mine!

/Dan

[KK4YY]

Dan,

Although you probably meant "Go figure" rhetorically, it sounded interesting enough for me to Google it.
An good discussion on the subject was found here:
http://amfone.net/Amforum/index.php?topic=25830.0

It's funny how the best answers on the web often point right back to this board!

Don

[ke7trp]

Sometings fishy.. You must have some serious leakage on those SS plug ins....  To have more voltage drop than the 3b28 is not right.

C

[KM1H]

You guys better look at tube specs a bit closer......the 866 and 3B28 are rated at a 10-14V drop and neither is a high vacuum type. The SS is a string of diodes.

Carl

[ke7trp]

There is a big jump from 866 to 3b28.  We know that.  But from 3B28 to SS it should not go DOWN.  Something is wrong there...

C

[KK4YY]

A line voltage variation could easily account for the two different measurements, taken at different times. Maybe someone turned on the clothes dryer.

Also, we don't know the load applied when measurements were taken. RCA specs the 3B28 at 14 V peak drop at 2 amps. It may have a lower drop at lower currents, I don't know. But a 0A2 will vary by 6 Volts over 5 - 30mA and it's critical design feature is to regulate voltage. I can't find a Voltage vs. Current curve for the 3B28 but I'd bet it's not a flat line. The 14 V spec is probably meant as worst case. Heck, it's just a rectifier. Only after making good measurements will the truth be known but is anyone interested enough to generate a curve for the 3B28?

Don

[VE3AJM]

   Interesting that I "Policed up" a Mint Valiant that
was sporting 3B28's fer the 866's, The thing had been
re-capped in the recent past and Ep with the 3B28's was
measured at 706, with the solid state FAA XXXX? Ep was
measured at 703. Go Figure.

   It had all the "Typical" Valiant issues, Stupid line
cord plug, No "Back to Back" diodes across the "beautiful"
Teardrop meter and Internal LV Fuse Holder. Factory
assembled as the tube sockets are "Riveted". Nice Piece,
Wish it was Mine!

/Dan

Perhaps those particular SS plugins were purposely designed/made to have the same voltage drop as the 866As would?

My own experience with replacing 866s with 3B28s showed very little in the way of any B+ voltage increase if any, in my Valiant and Viking 500 transmitters. So some of the other posts here showing an 100 volt increase in B+ voltage are interesting when doing the same tube swap out. I wonder what would account for this.

Al VE3AJM

[WQ9E]

As Carl noted earlier your SS plug-in consists of a string of rectifiers EACH of which will contribute a .7 volt drop regardless of load so drop is dependent upon the number of individual silicon junctions in the plug-in.

MV rectifiers also provide a near constant voltage drop within their load limit.

Xenon rectifiers, like your 3B28, provide about the same drop as MV under full load but the drop is less under lighter load.

So I imagine you would find very similar voltage comparing the SS to 866A tubes under minimal load with the 3B28 being insignificantly higher but under full load the 866A and 3B28 will be nearly equal with the SS now higher since Vd doesn't change under load.

The big advantages of the 3B28 over the 866A are operation in any position without the flash over problems of 866A tubes.  But I would take the SS over either.

[W9BHI]

I replaced the 3B28's in my Valiant II with some IR 1N2637's
and the HV was about 2 volts lower.
Must be a lot of diodes in them things.

Don W9BHI

[WQ9E]

I looked up two different 1N2637 data sheets and they both showed a rated forward drop of 28 volts which is far more than the gaseous types they are intended to replace.  I thought the Motorola data sheet I looked at was incorrect but the second sheet listed the same value for Vf.

[WD5JKO]

I looked up two different 1N2637 data sheets and they both showed a rated forward drop of 28 volts which is far more than the gaseous types they are intended to replace.  I thought the Motorola data sheet I looked at was incorrect but the second sheet listed the same value for Vf.

  We went through this in 2010 in the thread that Don linked too. In that thread, the last post might open a few eyes as to why the SS option for some seems to provide higher DC output voltage even though the forward conducting 'ON' drop is more for those 1N6237's then either an 866 or a 3B28. In a nut shell, the gaseous rectifiers spend a portion of the 1/2 wave in a non-conducting OFF state, and the difference between these and SS is more a factor as the power supply DC voltage decreases..

Jim
WD5JKO

[KM1H]

I'll try it again, read the tube specs. The 866A only drops 10V, the 3B28 14V tops and the SS units are a direct replacement with no other changes for either tube.
Quit grousing over 3-4V or so.

Carl

[WD5JKO]

I'll try it again, read the tube specs. The 866A only drops 10V, the 3B28 14V tops and the SS units are a direct replacement with no other changes for either tube.
Quit grousing over 3-4V or so.

Carl

Glad you straightened it out Carl..

So, a 3B28 drops ~ 10v at 2 amps (?)
So a 866 drops ~ 15v at 2 amps (?)
and a 1N6237 drops ~ 28v at 250ma

In the great scheme of things, not a big difference really. I concur on that.

Jim
WD5JKO

[KK4YY]

"There's no sense beating a dead horse other than for the sheer joy of it."

Nice MV graphic, Jim. I like the "Varc" notation. Must be some noise associated with that as well as it not switching on near zero crossing. I never designed with MV so had no need to study them. Now, knowing that, I like them even less!

Anytime I've had the opportunity to replace a tube rectifier I just soldered diodes directly to the tube socket pins or wired them on a small perfboard mounted in an old tube base and plugged it in. When I see those S. S. sticks for sale at hamfests I always decide to let the guy selling them hold on to them a while longer. Maybe that's a mistake. Perhaps the sticks are designed to emulate the voltage drop of their tube counterparts. This would maintain the original equipment design parameters and prevent an old electrolytic from suddenly seeing a new higher voltage that they aren't formed for. I have had an electrolytic go South, and in a pretty dramatic way, after using diodes in a DX-100B. No joy in Mudville that day.

The beat goes on!

Don

[Ed/KB1HYS]

hmm,  I like 866's.  Must be a minority.  I even designed/built a full wave bridge for my GG amp using four of them and hand wound filament transformers for them.  I put them in a four square arraignment, but think four in a row with a nice window across the front would be the cats meow.

Kinda cool how the flicker with modulation peaks etc.

I did have one ham fest special that was arcing slightly. it looked pretty nasty anyway so I  Connected it up to a microwave oven transformer (even was able to use the filament winding on that) and let her cook off.  Was quite the light show for a bit. Then stable and has been since. debarnacling?

I do let them cook fils only if I haven't run them for a while, other than that, they run with no special cares.