811 project

[K9ACT]

I got too depressed wasting hours calling CQ and listening on 7290 and only making a few contacts over the course of the five days since I got my new rig on the air.

So yesterday, I wound new grid and tank coils for 80 and fired it up now it doesn't work at all.   Well, it sounds aweful.... loud buzzing, distorted audio.

Even  with the modulator out of the circuit, driving with the Kenwood and a dummy load so it's in the amp somewhere but I went to bed unhappy last night.

I have checked all the obvious possibilities and nothing occurs to me but to put it back the way it was.

What is very strange is that there is nearly zero signal on the output when I put a sig gen on the input.  This was not the case when it was working.  I was getting all sorts of feed through but it neutralized properly.

It still neutralizes properly and there is no indication of self-oscillation other than the horrible sounding carrier.

Any ideas?  I don't expect any but just wanted to vent my spleen.

js

[KB5MD]

Do you have an rf bypass capacitor from the hv to ground?  I had the same problem  and a 500 pf doorknob from the hv to ground cured it.  Good luck

[AB2EZ]

I agree that, when you go from 7.3 Mhz to 3.9 MHz, all of the bypass capacitors have twice the impedance... and may not do a good enough job of bypassing the rf. Likewise, the plate choke has half the impedance, and may not be doing a good enough job of isolating the power supply from the rf. Beefing up the plate B+ bypass capacitor at the point where it attached to the plate choke (or inserting one there, if one is not already present), and beefing up the filament rf bypass capacitors sounds like a good thing to try.


Good luck
Stu

[W9GT]

Hello Jack,

I also heard you on 75M last night talking to K9EYE.  Your rig sounded like it had multiple problems.  First..you had a loud hum on the carrier.  It sounded like, perhaps, a power supply problem.  You also had some drift and varying frequency...not in one direction but it wandered back and forth a bit.  Your signal also sounded like there was some sort of oscillation or instability present....you might have some parasitic gremlins that have crept into the picture.  I also noted that you were FMing noticably when you modulated.  Much of this stuff could be due to some orientation of components and resulting oscillation or instability.

I would first rule out the power supply...check for bad caps or rectifiers.  Then...if you changed the physical layout at all when you put in different coils...change it back to the way it was.  Also..what kind of plate choke are you using?  Maybe you need a larger one for 75M/80M?

Just some suggestions....I do not wish to sound overly critical.  I know how frustrating de-bugging can be sometimes!

73,  Jack, W9GT

[AB2EZ]

Hum:

1. Check to make sure that the rectifier is acting symmetrically on both halves of the AC cycle (i.e., all diodes still working properly) so that the output ripple of the power supply is at 120 Hz (and harmonics of 120 Hz), not 60 Hz.

2. For a capacitor-input supply, the size of the capacitor needs to be at least (I/120) /(.03V)
where I is the average plate current at carrier, and V is the B+ voltage. This will ensure that the 120 Hz hum modulating the output of the amplifier will be more than 40dB below the carrier level (i.e. the peak-to-peak ripple is .03 x the B+ => the rms ripple ~ .01 x the B+

Example: If I= 200 ma = 0.2 amps, and V=1000 volts, then the filter capacitance between B+ and ground must be at least (0.2/120) /(.03 x 1000) = 55 uF.

If you can live with the 120 Hz hum being only 30dB below the carrier, then the capacitance can be 1/3 of the value calculated above (18 uF in the example)

3. Check to be sure that any bias being fed through the filament (no separate cathode in an 811A) is being fed through a center tap on the filament transformer. Make sure that all filament bypass capacitors are symmetrically placed between each side of each filament and ground. Otherwise, the filament voltage can add to the bias voltage... and cause hum.

FM-ing during modulation:

Usually caused by either

a) The modulation is causing the voltages supplying the VFO to change (take steps to better regulate the VFO voltages, including the VFO filament voltage if it has tubes in it)

b) RF from the amplifier is getting into the VFO and adding to the RF signal on the grid of the VFO. This changes the frequency at which the gain around the loop is unity and zero degrees in phase... resulting in a change in the VFO's frequency to bring the gain around the loop back to unity and zero phase. As the level of the rf from the amplifier moves up and down with the modulation, the frequency at the output of the VFO changes as well... causing FM-ing. (Possible fixes: Do a better job of shielding the VFO and bypassing all of the power supply leads coming into the VFO compartment. Make sure that the VFO compartment is well-grounded, and use a shielded cable with a common mode choke between the VFO and the input to the amplifier.

Best regards
Stu

[W9GT]

Well Jack    You left us hanging......did you get it fixed or resolve any of the problems?  Triode amplifiers can sometimes be challenging to get settled down, but their simplicity makes them attractive to build.  Hope you are having success with your rig.

73,  Jack, W9GT

[K9ACT]

Well Jack    You left us hanging......did you get it fixed or resolve any of the problems?  Triode amplifiers can sometimes be challenging to get settled down, but their simplicity makes them attractive to build.  Hope you are having success with your rig.

My apologies to all.  I seem to have forgotten about this thread and never read any of the responses until just now.  Shame on me!

I think the problem is that I had two threads by the same name discussing different problems and I forgot to check the notify box.

Furthermore, I gave up in disgust and went back to 40 meters with it. Where I have no such problems but had a modulation problem that I just resolved tonight but can't find anyone to talk to on 40 to confirm it.

I am waiting to go back to 80 after the hamfest on Saturday at which I hope to get some needed parts.  I had the feeling it might be too low Q in the grid tank but now after reading all these ideas, I think I will make trying some of them my project for tomorrow.

I have to go through the numbers for the power supply cap but I did add a 2 mf to the rectifier side which now makes it a cap input.  The only effect it had was to raise the voltage about 200 volts.  I have a limited supply of these old oil filled caps so they are on my list for the fest.  I would like to run it at the higher voltage.

Anyway, I will keep you posted when I learn something,

Thanks,

js