[Central-Electronics] Fwd: 20A 6AG7 screen grid resistors

[Jim Candela]

----- Original Message ----
From: k6jek <k6jek at comcast.net>


>How many gray hairs did you get puzzling out that mixer decoupling business?  
>I'll not mess with that unless I'm sure it's kafluey.
>Jon


Reply by Jim, WD5JKO:

Well, a lot of time was spent on this issue. I converted two 20a's into higher 
power, one 65 watt, another about 50 watt rigs. In both cases I need more AC 
drive signal to the final tube(s). These rigs, and posts about them are in this 
reflector archives.One place to go was to remove the shunt resistor across the 
mixer tank coil. The gain goes way up, but to insure stability, and linearity, I 
must de-tune the mixer tuning capacitor about 20% on the high frequency side.

On the second rig, I did it different. I still removed the shunt resistor across 
the coil. But here I played with the mixer tube B+ bypass capacitor. I used a 
combination of polypropylene capacitors in parallel (something like .01 and 
.1uf). These caps have very low ESR, so stage gain was extremely high, and 
tuning very narrow and touchy. Then I added a series resistance with the 
capacitors, starting with 2 ohms ( I had a bag of 2 ohm, CC 1/2 watters). The 
gain went way down, but was stable, and linear. Then I added more resistors in 
parallel with that resistor until I had a compromise between the gain I needed, 
with acceptable linearity. As it turned out, I was now able to peak the tuning 
capacitor and still get good linearity.

The other thing I did was to boost the 9 Mhz oscillator drive by raising the 
plate voltage to the pentode section. Problem here was the 6U8 already runs hot, 
and tube life was short. I found that the 6MU8 is similar, but has beefier 
ratings (pentode section), and shares the same pinout. The 6MU8 is cheap, 
readily available, and in my opinion a good substitute for the 6U8 in any 20A.

Jim
WD5JKO