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 1 
 on: Yesterday at 11:38:45 PM 
Started by K8DI - Last post by Steve - K4HX
At least one (big IMHO) consideration is modulation linearity.

From the "Radio Handbook", 1940 edition, page 160:

Class C Plate Modulation. The characteristic of a class C amplifier which makes it linear with respect to changes in plate voltage is that which allows such an amplifier to be plate modulated for radio telephony. Through the use of higher bias than is required for a c.w. class C amplifier and greater excitation, the linearity of such an amplifier may be extended from zero plate voltage to twice the normal value. The output power of a class C amplifier adjusted for plate modulation varies with the square of the plate voltage. Since this is the same condition that would take place if a resistor equal to the voltage on the amplifier divided by its plate current were substituted for the amplifier, it is said the stage presents a resistive load to the modulator.

 2 
 on: Yesterday at 11:28:51 PM 
Started by Steve - K4HX - Last post by Steve - K4HX
.

 3 
 on: Yesterday at 11:27:37 PM 
Started by KD1SH - Last post by Steve - K4HX
Lots of good Qs with VVI on 160 meters in years past. RIP

805s are a great tube. They remind me of an 810 without the grid cap. When I used the Derb Homebrew rig for a year, it had a pair of 805 mods pushing tow 812H in the final. No lack of audio!



K4HX said:
Quote
Very interesting transmitter Mike. Where did you come across such a rare bird?
I bought this off of Ray, WD2AFJ. It became a want the first time I laid eyes on it. He told me it originally belonged to my good friend Eric, WB4VVI(SK). Originally, Ray modulated it with a pair of 572Bs and an ART-13 mod transformer. Later, he installed an audio choke on the screens and built a larger separate modulator using 805's. This past year the B+ transformer shorted(again!) and I went in and completely overhauled the power supply. It currently uses an RCA uses a K900763-001 mod transformer. I'm thinking about replacing it with either a VM-4 or a CVM-5 that I have and put the that transformer back in the ET-4336F and restore that box.
One of the things to note. Although they aren't rare, Ray gave me the original Mullard 813s that came with it. As you can see, the plate is what I call 'geodesic' and looks as though as it were cast instead of machined like that of a typical RCA carbon plate.

 4 
 on: Yesterday at 11:22:13 PM 
Started by W1RKW - Last post by Steve - K4HX
That's a beautiful antenna! They should do well on 40 meters. It's great band for both local and DX work.

 5 
 on: Yesterday at 08:03:21 PM 
Started by KD6CXW - Last post by KD6CXW
Thanks Wayne , I went to the sites and found others as well .
Waiting for parts and time to work on the VFO , I'll  post as the project progresses.

73's
John

 6 
 on: Yesterday at 07:46:40 PM 
Started by KD6CXW - Last post by KD6CXW
to many things going on, hard to find time for projects .
I have been reading Steve's site on AM PWM transmitters and decided  to go from tubes to solid state . Started collecting parts , will need more though . The fabrication is no problem that's what I do for work.
Finding time may take some work.
Need to order boards and parts from Steve then work on the DDS VFO , making sure it acts the way it should and with a touch screen display with options I would like to add.

 7 
 on: Yesterday at 04:47:50 PM 
Started by KW6T - Last post by KW6T
I am starting a new thread since it is more about repairing the RF deck and reworking the Invader 200 power supply to match the 500.  The old thread: http://amfone.net/Amforum/index.php?topic=48992.

The status at this point is the RF deck is "sort of working", so I decided to go forward with the Invader 2000 power supply conversion.

On January 23rd, the order for a new PW Dahl (Hammond) modulation transformer was placed.   The following day, the seller, Jameco Electronics, advised the lead time would be eight weeks.  That was fine, so the order was confirmed.

Today,
A postman appeared at my front door carrying a heavy box with HAMMOND MANUFACTURING marked on its side.  The modulation transformer is here! 20 days from the order date to my doorstep.  

Photo attached.

Russ



 8 
 on: Yesterday at 04:22:59 PM 
Started by W3JKO - Last post by DMOD
First time post here.  I have a DX-60B on the bench and am stumped on what audio levels I should expect in the controlled carrier circuit. First off here is what has been done to date:

- Replaced all electrolytics
- Replaced a burnt R9
- Replaced R34 which was open
- Replaced the milky white 6146 which lost vacuum.
- Tested all tubes.  All check out as good on my tester.

Everything powers up fine and all voltages are in the ballpark per the schematic. Mic audio is distorted at pretty much all levels.  Connecting my signal gen into the mic input and bringing it up there appears to be positive clipping at V5 pin 1 as soon as the modulation becomes slightly audible and of course getting worse as the level increases. Without having an reference point I don't believe this is normal.  Again all component values check out and all in circuit voltages are close to the print.  I want to get it to be stock before I try any audio mods but I want to know if this is "stock".  Any feedback is appreciated.


Have you checked the DC voltages on Pins 6 and 1 of V5?

Each V5 stage has a voltage gain of about 60 so if you input 10mVp-p into pin 7 of V5A then Pin 6 should show about 500mVp-p, more with C31 lifted. The reduculously low value of R19 prevents a full voltage gain of 60. (The mic gain pot. should be 1Megohm).

With 500mV out of stage 1, stage 2 Pin 1 should show about 25Vp-p.

I have often found C33 goes leaky and reduces the plate voltages.

The p-p output voltage of the cathode follower V4 Pin 9 will be about 0.8 X the p-p voltage at the grid, Pin 2 V4.

V4 Pin 9 DC voltage has to be about 135Vdc to produce 65Vdc at V3 Pin 3 with no modulation.

Phil - AC0OB

 9 
 on: Yesterday at 11:24:59 AM 
Started by K8DI - Last post by K8DI
So, question for those who know more than I..

In a given grid driven plate modulated class C amplifier, ignoring melting the grid/excess grid power dissipation for the moment, and also assuming some kind of protective bias.

What happens to amplifier output and level of modulation if the grid drive is much higher than normal (say double) datasheet values?  Does this cross a point where it starts to lower RF output at no modulation? does this get to a point where modulation is less linear, that is, more audio power is needed to push it to zero, or hit 100%? Besides cooking the grid, is there a limit to high grid drive?

And, what happens if grid drive level is under spec by a lot (say half of data sheet values)? Is unmodulated carrier level affected? Does it make it impossible to achieve 100% positive (or negative) modulation?  Besides high plate current due to low self-bias, is there a limit to low grid drive?

Ed

 10 
 on: Yesterday at 10:41:40 AM 
Started by KD1SH - Last post by W3SLK
K4HX said:
Quote
Very interesting transmitter Mike. Where did you come across such a rare bird?
I bought this off of Ray, WD2AFJ. It became a want the first time I laid eyes on it. He told me it originally belonged to my good friend Eric, WB4VVI(SK). Originally, Ray modulated it with a pair of 572Bs and an ART-13 mod transformer. Later, he installed an audio choke on the screens and built a larger separate modulator using 805's. This past year the B+ transformer shorted(again!) and I went in and completely overhauled the power supply. It currently uses an RCA uses a K900763-001 mod transformer. I'm thinking about replacing it with either a VM-4 or a CVM-5 that I have and put the that transformer back in the ET-4336F and restore that box.
One of the things to note. Although they aren't rare, Ray gave me the original Mullard 813s that came with it. As you can see, the plate is what I call 'geodesic' and looks as though as it were cast instead of machined like that of a typical RCA carbon plate.

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