[R-390] R-390A Alternative to 6DC6 RF amp

[Larry H]

I've been testing various tubes over the past few weeks to see if there is an alternative type that would work as well as a 6DC6, without any circuit modifications.   Here's the list of the ones I started with: 6AW6, 6BJ6, 6BZ6, 6CB6, 6CF6, 6DE6, 6DK6, 6GM6, 6HQ6, 6JH6, 6662, 6676, 7694, 7732.  These have the correct basing diagram.  Unfortunately, I do not have all of these types to test.  Here's the ones I have and the quantity:  6BZ6 (2), 6CB6 (3), 6DC6 (8), 6GM6 (5), 6JH6 (2).  As you probably know, some of these are 'sharp' cutoff and 'remote' cutoff designs, with the grid 1 (control) cutoff at about -6 volts for sharp and -20 for remote.  The 6DC6 is designed at -12.5.  

The first thing I did was to go through all of my 6DC6's to find the best one and document the range of quality I had.  The range was not as great as I thought it would be, about 16%.  But, the best one I found was 7% better than the one I was originally using.  I was pleased about that.

I had heard a number of times that the 6GM6 was very good and that the Equivalent Noise Resistance (ENR) of it (490/480) was considerably better than the 6DC6 (1,400/1,800).  Well, I had high hopes for them, but after testing the 5 that I had, I was quite disappointed.  None of them were close to the 6DC6, perhaps due to the plate resistance being half of the 6DC6.

I measured all the tubes 2 ways, noise level and signal gain.  I started with my best 6DC6, injecting about a .4 microvolt unmodulated signal  into the balanced ant connection.  I measured the diode load voltage in mgc mode with max RF gain, and adjusted the line gain for -5 on the line meter.  The reason I chose .4 uv, is at this level the noise is elevated above the noise level with no signal and it's quite a ways from the quieting threshold.  The input and output alignment was tuned for each tube.  I recorded each tubes 'noise level' and diode load voltage.  This established a base for comparison of the rest of the tubes.  Keep in mind that these measurements were made in mgc mode with RF gain at max, so the bias on the control grid is kept at 0.  This means that the tubes cutoff level is not a factor here.  To eliminate errors, I measured all the tubes twice.

To make a long story short, only 3 tubes had a better 'noise level' than my best 6DC6, BUT they had less gain.  Only 1 6GM6 had a better noise level, but its much lower gain made it less effective.  There were 2 6CB6's that had better noise, but only 1 had sufficient gain to make it about equivalent to the 6DC6.  Since many of these tubes were sharp cutoff, I tested the effect with agc on.  Although the agc voltage was only slightly less, the noise level at low signal levels was higher at certain low levels.  The agc issue could be fixed with a simple change to the RF tube bias wiring.  The sharp cutoff tubes also affected the RF gain linearity, and I don't see an easy fix to that.

And the winner is: 6DC6.  But, a 6CB6 could be used in a pinch.  My 3rd and 4th choices would be a 6GM6, and then a 6JH6 or 6BZ6.  Of course, any in the 1st long list will work in an emergency.  The following tubes are remote cutoffs and would not be an issue when using agc (as we normally do): 6GM6, 6HQ6, 6JH6, 7732, 6BZ6, 6662, 7694, 6BJ6.

Happy listening.

Regards, Larry