Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 2 [3] 4 Next   Go Down
  Print  
Author Topic: Which AM Transmitter?  (Read 38261 times)
N2EY
Member

Posts: 5085




Ignore
« Reply #30 on: October 04, 2011, 10:04:49 AM »

I get a kick out of reading boatanchor audio websites where people at negative limiting by stopping an anode supply from going negative with a diode and think they won't splatter.

Well, that can work if there's an adequate splatter filter to soften the diode conduction point. But yes, a diode alone will do nothing but save the modulation transformer.

They add negative feedback within the audio stages and remove driver transformers while the THD of the modulation system is 10% or more because of the modulated PA stage non-linearity in response to modulated voltage.

I still remember the push-pull 450TLs at W3FDY....(sigh)

These are cheap Ham rigs that served a good purpose in their day. It didn't matter in the 1950's if they were -25 dB 10 kHz out on each side because no one had good receivers anyway.  :-)

They weren't cheap in the 1950s! And there were some good receivers, but they were usually down the band a bit.

73 de Jim, N2EY
Logged
VA3AEX
Member

Posts: 22




Ignore
« Reply #31 on: October 04, 2011, 01:49:52 PM »

K7DYY offers a new D class am transmitter, called the Super Senior, with 375w carrier that covers 160/80M for $1,400 (k7dyy.com).  Haven't heard anything on it yet but heard positive things about the prior model called the Senior.

The guys at Amfone web page will a great help if you decide to build or restore a BA TX, and there's a link to the E class TX builders forum if your looking to build a digital version.

73 Alex
Logged
VE3LYX
Member

Posts: 814




Ignore
« Reply #32 on: October 04, 2011, 06:55:03 PM »

I just finished building one. Screen grid modulated with companion recvr tuned off the same dual gang variable cap. Because there are limits to what you can do with screen modulation and there is a balance between modulation and carrier it is a tricky deal but it works . I used the same osc circuit as the recvr so both would tune the same. Different osc but exact same components and values except tx circuit has lowervalue gridleak resistor.  I am in the process of building a PA for the transmitter. It isnt a world beater and it doesnt have a lot of power but it was fun to design and build and it works.  Circuit used two 12sl7s and two 50l6 tubes. Oscs (regen detector)are hartley which seem to be stable with no sign of change due to hand capacity.
I used a carbon mic (WW2 vintage)with a battery and a audio transformer for the front end of the modulator. That drives 1/2 the 12sl7 amp which drives the screen of the 50l6. I too am tired of donald duck sounds.  Next one will be plate modulated but I wanted to try this.
Don VE3LYX
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #33 on: October 04, 2011, 07:36:16 PM »

I get a kick out of reading boatanchor audio websites where people at negative limiting by stopping an anode supply from going negative with a diode and think they won't splatter.

Well, that can work if there's an adequate splatter filter to soften the diode conduction point. But yes, a diode alone will do nothing but save the modulation transformer.

No. While it won't work without a splatter filter ever, it also won't work with a splatter filter with tetrodes.  A tetrode goes 100% negative while HV to the tube is well above zero volts. That threshold voltage varies with loading, tuning, grid drive, and everything else.

A negative peak limiter and filter at the modulation transformer output only works with low mu triodes. We could make one work with tetrodes, but the threshold would have to be variable (and well above zero volts) and reset every time any parameter affecting the PA stage changed.

73 Tom
« Last Edit: October 04, 2011, 07:41:51 PM by W8JI » Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #34 on: October 04, 2011, 07:39:03 PM »


 It seems to me that on the negative cycles of audio, you will start diverting a substantial amount of current to the screen. That suggests that there needs to be a series resistance to the supply if there isn't going to be over dissipation in the screen grid, and that as a result, screen volts will drop on negative half cycles, increasing the modulation depth on the negative half cycles, but not the positive.

Screen mod in combo with plate works well when properly balanced in level with the anode.

When the anode goes less positive, so goes the screen. When the anode goes more positive, the screen follows.

No problems if setup OK.
Logged
G3RZP
Member

Posts: 1284




Ignore
« Reply #35 on: October 05, 2011, 12:46:31 AM »

Tom,

I was talking specifically of suppressor grid modulation. Although where you would find an old RF power pentode in good condition is another matter.
Logged
KX5JT
Member

Posts: 217




Ignore
« Reply #36 on: October 05, 2011, 03:40:00 AM »

Well, that can work if there's an adequate splatter filter to soften the diode conduction point. But yes, a diode alone will do nothing but save the modulation transformer.


Saving the modulation transformer is an admirable endeavor!!  How many new modulation transformers are even being built these days?
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #37 on: October 05, 2011, 04:42:13 AM »

Well, that can work if there's an adequate splatter filter to soften the diode conduction point. But yes, a diode alone will do nothing but save the modulation transformer.


Saving the modulation transformer is an admirable endeavor!!  How many new modulation transformers are even being built these days?

The problem is the diode clamp people traditionally advocate in old boatanchor Ham gear does NOT save the transformer, it actually makes life a whole lot rougher for the transformer.

There isn't even one positive reason to add the series diode commonly advocated by some in "plate" modulated boat anchors because:

1.) 100% negative peak is reached while significant positive voltage remains on the anode of a tetrode. That exact -100% modulation envelope occurs at a positive anode voltage that varies greatly with tuning and loading and grid drive, but is never near zero anode volts.

2.) The diode encourages people to run past -100% because they "think" they have installed a negative modulation peak limiter.

3.) The diode shuts off and totally unloads the transformer on negative peaks. Since the transition point of -100% envelope is at some varying positive voltage, it is impossible to build a fixed clamp that loads the transformer with a resistance.

Nothing good comes from adding that diode, including improved transformer life. It actually makes things worse in every aspect.

The entire thing about modifying audio systems in old transmitters is so full of flaws that half the mods correct problems caused by other mods. Generally two or three capacitors do everything measurable without hacking the rig all up and needlessly stressing components.  

73 Tom
« Last Edit: October 05, 2011, 04:46:12 AM by W8JI » Logged
KC8Y
Member

Posts: 572




Ignore
« Reply #38 on: October 05, 2011, 05:59:45 AM »

YES , AM modulation IS very much back again...
My old friend & I, both go into ham radio-while in high school (back in the late 60's)...
My friend just acquired a Heathkit DX-60b w/HG-10 VFO...
I just acquired an Icom IC-7410 (which also does AM)...
He got me interested in AM, again...i love it on 40 & 80M !!

KC8Y
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #39 on: October 05, 2011, 06:09:05 AM »

Tom,

I was talking specifically of suppressor grid modulation. Although where you would find an old RF power pentode in good condition is another matter.

I never looked at suppressor modulation. I did modulate the signal grid  (G1) in a 6BE6.  :-)
Logged
G3RZP
Member

Posts: 1284




Ignore
« Reply #40 on: October 05, 2011, 10:04:56 AM »

Tom,

You're not old enough, then! Neither am I, but I have a lot of old radio books.....

It is strange how many things keep re-appearing after the passage of years, and how many things that were understood years ago suddenly aren't. Such as flashover in PA valves which was well covered in the early 1930s - the 'Rocky Point' effect.
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #41 on: October 05, 2011, 12:53:11 PM »

Tom,

You're not old enough, then! Neither am I, but I have a lot of old radio books.....

It is strange how many things keep re-appearing after the passage of years, and how many things that were understood years ago suddenly aren't. Such as flashover in PA valves which was well covered in the early 1930s - the 'Rocky Point' effect.

I know. Now the amplifier forum is nothing but blue noise.
Logged
N2EY
Member

Posts: 5085




Ignore
« Reply #42 on: October 05, 2011, 01:02:37 PM »

MY BAD!

I was thinking of a shunt diode, not a series diode, with adequate filter and bias set so that it conducts well above zero volts.

73 de Jim, N2EY
« Last Edit: October 05, 2011, 02:14:03 PM by N2EY » Logged
WA1OOA
Member

Posts: 9




Ignore
« Reply #43 on: October 05, 2011, 01:39:39 PM »

Every once in while you do see the old Collins and Gates 1kw Broadcast transmitters but one has to wonder about the quality of the power supply and the finals are wicked expensive.  They range in price between $1500-$4000 or so.  And they weigh over 1000 pounds!  So they have to be local.  Check out the small local am stations maybe they need to upgrade and are just waiting for someone to part with their money.

For me the 2nd and 3rd choice would be the Heath DX-60 or Eico 720/730.  I used to run them back in the late 60's and they were a lot of fun as well as cheap.  Today though my first choice would be the Drake T-4 series with the Drake L-4B linear.  A good audio train, very stable vfo and problem solved.  If you need the receiver then again the Drake R-4C is still quite arguably one of the best receivers ever built.  Modernise them with better filters and they are very good indeed.
Logged
W8JI
Member

Posts: 9748


WWW

Ignore
« Reply #44 on: October 05, 2011, 04:53:35 PM »

MY BAD!

I was thinking of a shunt diode, not a series diode, with adequate filter and bias set so that it conducts well above zero volts.

73 de Jim, N2EY

A shunt diode from the PA stage modulated B+ feed to "ground" to prevent exceeding -99% only works if it has a very stiff reference voltage to hold it off. In the case of a Ranger loaded to 35 watts it would be about 50 volts of stiff + bias. The diode could be arranged to provide + minimum voltage to the PA feed that prevents going more - than -99%.

Even then, and with a low pass filter, it would not work well unless readjusted often. Modulation voltage for -100% varies far too much with tuning, loading, and grid drive.

I just can't understand why no one figured that out in the boat anchor community, because there are some pretty sharp people. But one of the common mods is just a series diode, not even with a filter. I tried almost every mod off and on, and very few (including the driver transformer changes) do much more than a few simple capacitor changes.

I changed the low level clipper in my Valiant so it only clips negative peaks. That works well, because it is followed by a low-pass. I can set the - mod peak at anything I like with a front panel control, and the + peaks just go where the gain takes them.
Logged
Pages: Prev 1 2 [3] 4 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!