[R-390] Re: 6C4? 6C4W? 6C4WA?

[Flowertime01 at wmconnect.com]

Glen,

I have found any and all 6C4 of any variety that would test OK in a tube 
tester would
work in and R390 or R390/A.

But as you say "Of course, there are other factors to take into consideration 
besides the tubes suffix.  Such as Manufacturer and Date of Manufacture, etc. 
"

Once you get past the tube works and you go on to signal to noise and 
receiver sensitive all bets from the tube tester results are off. I have tubes with 
good high tube tester reading that are very noisy in the circuits of the 
receivers. Likewise a low reading is no clue to tube noise either. First find tubes 
that test good in the tube tester as their is no reason to burn out R390 
resistors as a means of checking tubes for shorts. Then as higher reading tubes in 
the tube tester are though to be better tubes, start subbing them into a 
circuit and measure the signal to noise. Start comparing the tubes you own in the 
same circuit. Consider the signal to noise ratio. You can adjust the final 
receiver gain in several stages. Pick from the tubes you have in hand the best one 
as rated on signal to noise ratio. Put the best one forward in the circuit.

Different tubes from the same manufacture have whole ranges of signal to 
noise. I have new tubes with very bad signal to noise. I have brought know pulls 
and used tubes at swap meets and found some to be better than new tubes. Not 
all and not
every one. But I cannot tell a good tube by looking at the item.

6C4 are not the only tube in this class. Every tube in the R390's come in the 
full range of noise flavors. Every tube circuit in the receiver will benefit 
from tube selection on the signal to noise test. Some circuits in the receiver 
(forward circuits) provide a better place to test tubes than other circuits 
(back end) but always rank all the tubes. Install the bet you have and place 
the best ones forward.

If you do not own enough tubes to do signal to noise testing and own enough 
good tubes so that you do not need to think about which circuit to place the 
tubes into you do not own enough tubes. You will just have to listen to what you 
own until you find some more good tubes. Real life is that you will likely 
buy more bad tubes before you find more good tubes. I have purchased brand new 
tubes that had more noise than the tubes I already had. Going off to the store 
is no sure bet that you will come home with a better tube. 

So a good many folks not knowing the variation of tubes from manufactures, 
dates and batches have found a good tube marked as and a bad tube marked as and 
just declared all tube marked as are not as good as the other flavor. In fact 
is was just two random of 12 random tubes compared and a declaration make. 
Another run from the same manufactures may have completely different results.

Back in the 68 - 75 era I would get new JAN tubes out of supply and find bad 
tubes. Likely if we found a new tube to be noisy, every thing we had in stock 
would also be from the same production run and also bad. Life was hell then. 
You just could not toss 20 tubes in the trash and ask for more. There was no 
way to send them back.
We had to find a circuit to use them in where we could get enough good tubes 
around then to make the receiver pass minimum acceptable tests. Other than 
6DC6's you can like with a noisy tube type some where in the receiver.

Roger.