[Amps] tube replacement ?

[R.Measures]

>In a message dated 11/18/03 7:13:14 PM Central Standard Time, r at somis.org 
>writes:
>> 
>> >> ** The 4-400A and the 3-500Z use the same 5V/14.5A cathode and have the 
>> >> same peak-emission (c. 1.3A) , thus, the output Z of a 4-400A and a 
>> >> 3-500Z are pretty much the same if the anode supply potentials are equal.
>
>^I don't know where you got your peak emission numbers from, but the 
>characteristic curves published by Eimac for both tubes show Ip as high as 
>2.2 
>amperes.
> 
>^ The only difference is that the 4-400A needed 500 volts on the screen to 
>do 
>it.
>^ (The 4-400 did the 2.2 amps Ip with an Ig of 200ma and an Iscn of 100ma.)
>> 
>> **  Applying screen V to a tetrode with both grids RF-grounded does not 
>> change the  amplification because screen-amplification only occurs when 
>> the the grid's RF potential differs from that on the screen.
>
>^ Wouldn't you consider the RF riding on top of a DC potential of 500 volts 
>(or 750 volts) on the screen grid to be at a different RF potential than 
>the RF riding on the Grid which is at ground potential or somewhere very 
close to 
>it? 

**  With the screen at RF-ground, there is no RF potential on the screen. 
 RF-ground can be either connecting the screen direct to chassis-gnd 
(tetrodes with handles), or bypassing the screen to chassis-gnd with a 
low-L capacitor (4cx 600J, 4cx1500B, on and on)
>
>> > If he has less than 300 watts of drive available, (say only 50 to 100 
>> >watts), his power output would positively increase with 3-500Z's being 
>> >used
>> 
>> 300W would drive 4 - 5, 4-400As in g-g with a 3500V anode supply.
>
>^ Probably more like 6 or 7 if you use the 40 watt Max Sig drive figure 
>posted both by the ARRL and by Bill Orr for those tubes running in a 
>Ground Grid  configuration.

**  I tried running 4-400As in a SB-220 at 70W PEP drive per tube.  
Anode/plate current was c. 350mA per tube instead of the rated 400mA per 
tube.  My guess was that it takes c. 3600v to make 4-400As play fairly 
well in G-G.
>
>^ I'm saying that he would need 25% more drive to obtain 30% less output 
>power using 4-400s than using 3-500s.
>
**  With more Mu, less driving potential is needed at the cathode to 
achieve the same anode current.

>^  If he only has 50 to 100 watts of drive available he should be able to 
>see 1500 watts peak from a pair of 3-500Zs at 3KV. That number would drop to 
>less than 1000 watts, (probably more like 750 watts), if the same drive was 
used 
>on 4-400's running G-G at the same plate potential.

**  I doubt this.  The main rub with 4-400As in G-G at 0v-bias is that 
the ZSAC is not enough unless c. 3500VDC is used.  The same problem 
happens when a 3cx3000A7 is operated at 1500VDC and 0v-bias.  
>
>> 
>> With = anode voltages, a 4-400A in g-g has less ZSAC than a 3-500Z 
>> because a 4-400Ain g-g has more Mu.  
>
>^ I cannot argue that point because I have never seen any published figures 
>from Eimac or any other source stating what the Mu was for a 4-400A with 
>both grids tied together.

**  It ain't rocket science.  The higher a triode's Mu, the lower the 
0v-bias ZSAC.  
>
>^ All I can say is that my own experiences have proven contrary to your 
>statements and are reinforced by what I have seen published by Eimac, 
>Arrl, and Bill Orr.

**  Bill Orr is much revered, but I have a copy of a Bill Orr letter the 
ARRL has in its files that is not exactly supportive of his reputation 
for electronic knowledge.  Most people seem to forget that Mr. Orr was 
principally a publisher of other folks construction projects.
>
>^ My personal experiences with replacing 3-500's with 4-400's in grounded 
>grid amplifiers have always resulted in significantly less power output for 
the 
>same drive levels and plate potentials, and I had to double or triple the 
>drive level to obtain the same power output I had with the 3-500Zs.
>
>^ If what you stated is true, and the 4-400's had higher gain (Mu) and the 
>same plate voltage and peak Filament/Cathode emission capabilities, please 
>explain to me why Heathkit (and everyone else) switched to the more 
>expensive 3-500Z tubes?
>
** OK
1.  New 3-500Zs were cheaper at the OEM price because they do not have a 
screen.
2.   4-400As require at least another 600VDC to achieve the needed ZSAC, 
which would mean that:  2-more 450V filter caps would be needed, a more 
costly/higher-V  bandswitch would be needed, a wider-spaced Tune-C would 
be needed, which would have required a Tune-C padder to cover 80m, which 
would have required another bandswitch section..  In other words, Heath 
had an OK SSB linear amp kit at the best price in town.  

cheerz, Dennis