REVERSE MODULATION

[KB5MD]

I just completed a dual 811a, grid driven final for plate modulating.  The audio is great and the output is about 160 watts carrier.  The only thing is the scope pattern shows the modulation
to be in reverse (the carrier decreases when modulated rather than showing an increase in amplitude as normal.)  I've tried changing the loading and that doesn't make any difference.
What am I missing here?

[w4bfs]

hi Roy .... how are you monitoring your modulated waveform ? .... there have been several past threads about this in the past and the need for a rfc ...73 ...John

[W2PFY]

Four things come to mind.

Soft or weak tubes?
Not enough drive?
Not hard into class C?
Improper bias voltage?

I'd crank up the drive first and see what happens. What is your drive current?

[WD5JKO]

Four things come to mind.

Soft or weak tubes?
Not enough drive?
Not hard into class C?
Improper bias voltage?

I'd crank up the drive first and see what happens. What is your drive current?

 Add one more to the list:

* Hi Mu tubes? if so driving deep into class C is important. Might have to modulate the drive to get the positive peaks to go up there. Given the choice of 811 or 812 type tubes, the 812 will be much easier to modulate upward due to the lower Mu..With a lower Mu, the peak RF grid voltage will have less to go into positive territory for the same peak plate current as compared to a hi mu tube.

* I once tried to plate modulate a pair of 572B's in Push Pull RF. I had similar results. and ended up with 8005's which worked great. Later I tried 805's, and they too modulated well. The 572B's worked great as a linear amplifier, but with class C plate modulation, I could see big challenges ahead, so I went to more favorable tubes for plate modulation.

Jim
WD5JKO

[W2PFY]

If you are seeing idle current without drive, your probably in class B. Adjust your bias so you see no idle current without drive and then hit the drive and go though your tests again. Are you using a grid leak resistor? If not, try a 1000 ohm resistor in series with the bias supply to the grid. Adjust your bias to cutoff and try again. Do not use any bypass caps at the grid end of the resistor.

[steve_qix]

I just completed a dual 811a, grid driven final for plate modulating.  The audio is great and the output is about 160 watts carrier.  The only thing is the scope pattern shows the modulation
to be in reverse (the carrier decreases when modulated rather than showing an increase in amplitude as normal.)  I've tried changing the loading and that doesn't make any difference.
What am I missing here?

Ok, it sounds as if the final consists of dual 811As.  What are you using for a modulator?

[Opcom]

Need lots of bias for 811 in class C.  An electric bias supply for this tube has to be pretty stiff, despite any grid leak bias added. If the grid current's 45mA per tube, that's 90mA. The bias supply regulation can be improved with a heavy bleed, like another 90mA for the bleeder, so be capable of 180 (200)mA.

Maybe it all seems excessive but the 811 is hard to drive for class C and so a series grid leak shown was an economical way parts wise, to get the bias, assuming plenty of drive is available to power this scheme as some is lost in the resistor, which should be non inductive so as not to spoil the drive current. It might be worth it to find some way to measure the bias and the peak grid voltage, if this cant be sorted out.

Speaking of driving this, the grid impedance of the 811 is a bit low as evidenced by the 90mA grid current stated for a pair of them. It may mean a different grid tank than would be usual if the tube was a lower mu type like the 812 mentioned. The grid tank being designed for 1.5x higher current than usual; 2/3 of the impedance. -this is just assumed from the TT-4 manual, can't vouch for the 812, never had one to use 8-(.


812: bias -120V, 240V peak grid, 30mA, 6.5W drive,  grid leak 4000 Ohms
811: bias -120V, 250V peak grid, 45mA, 10W drive, grid leak 2000 Ohms
These drive Wattages from the manual are usually at the grid.
They sometimes do not include what is lost in the resistors (3.6 and 4.05W respectively). maybe someone knows better about that.

811 plate modulated service,
per tube:
plate volts                1000V   1250V
Bias*                         -55V   -120V
from grid resistor of   1200     2700 Ohms
peak RF grid voltage   150V     250V
DC plate current         115       140mA
DC grid current           45        45mA
drive                         6.1W    10W
power out                  88W      135W
* bias obtained with grid resistor of value shown or from a combination of grid resistor with either fixed supply or cathode resistor.

Only a little increase in plate volts requires 2x the bias, by the book. No harm will be done by having more bias (fixed or resistor) up to the max of -200V, as long as there is enough drive to get the grid current. My opinion is that a fixed bias supply would be as well (no grid resistor) if it is stable over the current range 0 to 90mA and avoids complications of driving through a grid resistor.

[W2PFY]

I just completed a dual 811a, grid driven final for plate modulating.

Well Roy, how did you make out?

[KB5MD]

One of those projects where the more you look for problems, others occur.  Somewhere along the line, the mod transformer gave up.  So, I am placing this thing on a big
shelf in a dark corner and forgetting about it for a while.  Time to talk on the radio rather than work on one. Thanks for all the advice anyway.