[Boatanchors] 6146 vs 6146A vs 6146B or 6146W

[Glen Zook]

Being the person referred to:

The 6146B/8298A tube does interchange with the 6146 and 6146A/8298 in a number of transmitters designed for the earlier tubes.  However, and that is the "gotcha", many transmitters just do not "like" the 6146B/8298A.

RCA originally said that the 6146B/8298A was completely backwards compatible with the 6146 and 6146A/8298.  Unfortunately, this was not shown to be true.  The specifications for the 6146W (originally the military equivalent of the 6146A/8298) were changed in 1964 and all contracts (multiple companies) were changed to the 6146B/8298A specifications.  However, there was no change in the nomenclature and all of the tubes manufactured to the new specifications were still labeled 6146W.

When the new versions of the 6146W made their way into the "food chain", problems were immediately noticed, especially in the Collins S-Line and KWM-2 series equipment.  In the Collins equipment the final amplifier neutralization capacitors started "burning up" with the new tubes.  In those units the neutralization capacitor is a ceramic trimmer and the trimmer would fail in only a very few minutes of operation.  This happened in well over 90% of the units.  Collins had to redesign the neutralization circuit which included the changing of the neutralization capacitor to an air variable and then changing the value of 2 fixed capacitors and 1 resistor.  With these changes, any version of the 6146 family will work fine.  When the changes were made, field modification kits were provided to the military to change the neutralization circuitry in the units that were already "in the field".  From the date that the neutralization design change was implemented all
 units were shipped from the factory with the new circuit.  These changes happened in "mid stream" for the 32S-3, 32S-3A, KWM-2, and KWM-2A.  The only way to tell for sure is to look at the neutralization capacitor that is in the final amplifier cage.  If it is a ceramic trimmer, then do NOT use the 6146B/8298A.  If it is an air variable, then any of the 6146 family of tubes can be used.

Motorola used the 6146 and 6146A/8298 in quite a number of their tube-type commercial FM equipment.  They used the 12 volt equivalents (6883 and 6883/8032) especially in the HHT Motrac series.  The later, "E" model HHT, LHT, and MHT series, were designed to use the 6883B/8032A/8552 tubes which are the 12 volt equivalent of the 6146B/8298A.  Now commercial FM equipment uses fixed neutralization and NO changes can be made to the circuit lest "type acceptance" of the unit be nullified.  In the first half of the 1970s Motorola decided to streamline their parts system and started putting 6883B/8032A/8552 tubes in boxes marked for the 6883A/8032.  At the time I owned the Motorola reconditioned equipment for the south-central United States.  We started using the later type tubes in the equipment designed for the earlier versions.  Within a week we started getting complaints that units were arriving with broken final/driver tubes.  This had never happened before.

Since the Motrac series equipment has the final amplifier and driver tubes enclosed in a black heat sink it is not possible to see the tubes when the unit is tuned.  However, we did various things to allow us to see the tubes when in operation.  It turned out that, because of the neutralization circuit, the 6883B/8032A/8552 tubes were breaking into oscillation and the plates were glowing "red hot".  This heat was annealing the glass and the forces placed on the units during shipping resulted in the breaking of the glass.

At first Motorola insisted that we had gotten a "faulty batch" of tubes.  However, within a period of less than a month, Motorola started getting similar claims from all over the country.  Motorola had to pay the claims for the failing tubes from every one of the radios.  Then, they had get in a rush order of 6883A/8032 tubes.

I definitely agree that many people have not had any problems replacing the earlier versions with the 6146B/8298A.  However, I have worked on identical transmitters, with serial numbers within 10 of each other, in one of which the 6146B/8298A worked fine and in the other two they did not.  It is just a matter of how the tolerances of the various parts used in the unit "added up".  

The problems with using the 6146B/8298A are worse in transmitters that do not have neutralization adjustments and this includes a lot of transmitters such as the Heath DX-100/DX-100B.  The best way to see if the later tubes work in a particular transmitter is to install the tubes and then try to neutralize.  Many times the tubes just will not neutralize and the later tubes should not be used in that transmitter without modification.

Then, if the tubes do neutralize, watch the performance.  In many cases the neutralization will drift within less than an hour.  If this happens, do not use the 6146B/8298A without modifying the transmitter.

If the tubes hold neutralization, then go ahead and leave them in the unit.

RCA, because of the problems with the 6146B/8298A, had to retract their statement that the 6146B/8298A were completely compatible with the 6146 and 6146A/8298.  The result was that RCA had to continue to manufacture the 6146A/8298 along with the 6146B/8298A.  This fact alone, that both the 6146A/8298 and the 6146B/8298A were manufactured simultaneously, is absolute proof that, in many situations, the 6146B/8298A is a completely different tube.  If the original claim that the tubes were interchangeable was true, then RCA would have stopped manufacturing the earlier tubes.  Unfortunately, this did not "pan out" and RCA had to continue to make both versions.

Again, I definitely believe that many people have had absolutely no problems with the 6146B/8298A.  However, the number of people who have had problems greatly exceeds the number who have not had problems.  As such, I definitely recommend that the above recommendations about observing the neutralization be followed.

Now, there is one thing that is virtually assured of causing problems.  That is to "mix" a 6146B/8298A with either a 6146 or a 6146A/8298.  You can "mix" a 6146 with a 6146A/8298 because the primary difference in the tubes is the "dark heater" in the 6146A/8298 (the "dark heater" tubes were designed for mobile operation and the performance of the tube is not affected by a heater voltage change of over +/- 10%).  But, when mixing the 6146B/8298 with an earlier version almost always results in very unstable operation.

Often, the transmitter can have the neutralization circuit modified to use the 6146B/8298A.  But, this does require circuitry changes and many operators do not want to make these changes or feel that their abilities do not allow such changes to be made.

Another thing is the grid current.  RCA originally did say a maximum of 4 mA per tube.  However, they eventually reduced that first to 3.75 mA, then to 3.5 mA per tube.  But, running the grid current at, or even near, the maximum will result in reduced tube life.  Experience has shown that running the tubes at about 2.5 mA grid current per tube is pretty much optimum.  Most of the 6146 family will produce full output with as little as 1.75 mA grid current per tube.

I agree that, on paper (spec sheets), there apparently is basically no difference among the tubes.  That is the primary reason that RCA originally stated the the tubes were fully compatible.  Unfortunately, that did not "pan out" and RCA had to retract that statement.  Again, if the tube types were fully compatible then RCA would have ceased manufacture of the 6146A/8298.  Since the manufacture of both types continued in parallel, that is proof enough that there are difference.

I have no idea as to what it is within the physical construction of the tubes that causes the problems with the 6146B/8298A.  But, since the 6146B/8298A is designed to perform with 33% more power, there definitely are changes in the internal structure and those changes are most likely causing the problems.

Glen, K9STH

Website:  http://k9sth.com


--- On Tue, 7/5/11, lee <pulsarxp at embarqmail.com> wrote:

I just scrutinized the spec sheets for the above mentioned tubes.  They are 6146, 6146A, and the 6146B or newer 6146W tubes.  The spec sheets show the 6146B and 6146A  interelectrode capacity to be virtually the same and the 6146 tube to have 1/2 pf more input capacitance.  So for practical matters, the interelectrode capacitances are the same for all these tubes.
 
The tube spec/charts also show they all can be run as plate modulated class C amplifiers with 600 volts on the plate, 250 volt grid voltage on grid 2, and -150 volt negative voltage on grid number 1.  Yes, the 6146B shows the glass envelope can run at a higher temperature then the others and the plate current can be increased from 125 ma to 250 ma. when using the 6146B or W. The number 2 grid still has a max of 2 watts and the grid current is the same for all.  That is 4 ma.
 
I have plugged in many 6146B tubes in my old boat anchor rigs which originally were supplied with 6146 or 6146A tubes.  I have never had a lick of a problem when ever I have done so.  Since my transmitters were designed for the 6146 power ratings,  I have never "pushed the rigs to give the max 6146B power output by increasing the plate current.  I think that would be unwise.  (The exception to this is my Collins S-line equipment which were built for 6146B tubes or at least came out of Collins with the B tubes already installed.
 
That said, to be honest,  I think the talk about B tubes not working in A tube transmitters is a bunch of BS or falsehood as I have never experienced the horror story myths I have read about B tubes from time to time.  I base this upon the spec sheets and the fact I have never had a problem substituting an A tube with a B tube.  As I mentioned earlier, I have never "pushed" the rig beyond the A tube ratings after installing a B set of tubes.
 
I am well aware of the WARNING given about using a B tube in place of A tubes by one individual.  (I am not trying to slam him but I do disagree  strongly with his paper.  I don't know anything about using these tubes in VHF applications but only in HF applications).  All I
> can say is, the spec sheets point out you can replace an A tube with a B or W tube and I have never had a problem doing this.  I've done it enough times without a problem and based on the spec sheets, they indicate no reason for having a problem. Another good ham friend of mine has used the B or W tubes enough times too and he too has never had a problem with the substitution.
 
Today, in 2011, new 6146B and 6146W tubes are sure a lot easier to find then 6146 or 6146A tubes.  I've also been able to buy NOS military 6146W tubes for less money then NOS 6146 or 6146A tubes.  I can buy new W tubes for about the same or less money then used 6146 or 6146A tubes.
 
I am writing this so others who may have a fear of using a B or W 6146 in place of a 6146 or 6146A tube should try it and find out for yourself what is fact and what is fiction.
 
Am I alone with never having a 6146B substitution problem?  I'd sure like to hear from others who have done the switch.  Have you personally ever had a problem doing this?  The spec sheets say you should have no problem and I have had none myself.