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 Post subject: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Sun 19, 2020 2:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 66
Location: Grand Island, NY, USA + AZ
Well, as much has been written about this unsafe bugger, I can still add some things of substance. So that is what this post is all about.

First, for safety - remove the original AC cord, and replace it with a polarized two wire 18 or 20 AWG line cord. Remove the "safety" capacitor (0.05uF) between the negative side of the can electrolytic and to the chassis, and replace the 0.05uF capacitor with a wire. Now, your chassis and the outside of the pair of RCA jacks are directly connected to the neutral side of the AC mains, AS LONG AS YOUR OUTLET IS PROPERLY wired. IF IN DOUBT, USE AN OUTLET TESTER TO CHECK THE OUTLET FIRST! This procedure leaves the set as safe as an AC/DC set without an isolation transformer could be. I now do this with ALL my AC/DC set restorations, and caution the customer about checking their outlet first.

Second, I read many of the on-line/internet articles about the KT-195 as I could easily find. One article in particular caught my interest (being an amateur radio operator). This was rewiring the set to have plate modulation rather than screen grid modulation (sometimes called scratchy grid). So following a very well drawn schematic I found at a web site, I rewired my "transmitter" for plate modulation. I'll put any credits at the end of this article. I had fully repaired and tested the KT-195 as built and actually found it quite pleasing in tone, if not in range. Using an oscilloscope, and my interpretation of the readings, I found I maxed out at 15 to 20% modulation. Not really much for a "hi-fi signal, but nonetheless still pleasing to the ear. I used a GE Super Radio II as the receiver.

So third, I rebuilt the transmitter to the exact schematic I found on a web page. (except for my direct AC connection to the chassis). Wow! Instant disappointment! The audio was very tinny, even though I could achieve 100% modulation with ease. Well, I thought, I want my rig to sound good and work good. So I studied the schematic, and undid many of the recommended changes as follows:

Fourth: To begin, I replaced C1, C4, and C5 with 0.02uF ceramic disk capacitors. I left J1 connected to the new 0.02uF capacitor and to pin 1 of V1a. This couples more low frequency audio in the amplifier section than 0.01uF capacitors do. I removed Cb, a 47pF capacitor between the input grid (pin1) and cathode (pin 8) of V1a. I "readded" the 3.3 mega-ohm grid leak from the grid (Pin 1 of V1a) to the chassis or negative power supply rail. I then connected the "High" (J2) Input through another 0/02uF capacitor to the plate (pin 9) of V1a. This essentially couples whatever is connected to the High RCA Jack to the input grid of tube V2a. I then added a 5uF electrolytic capacitor in parallel with R5 (1500 ohm). Again, this helps biasing and low frequency response to tube V2a. Next I added a 10uF electrolytic capacitor in parallel with R7 in the cathode of V2b. Again, somewhat improving low frequency response of the audio output stage. In both cases, the plus side of the capacitor connects to the respective tubes cathode. Finally, I removed R10, a 47K ohm resistor from the grid (pin 7) of V1a. I replaced this with a 150kilo-ohm resistor to the plus side of C12C.

My net result: Fantastic audio with the switch SW2 in the amplify position, and equally good audio in the transmit SW2 position still using plate modulation. As received on the GE Super Radio, the audio sounded just as good as a professional AM broadcast station! If the volume control pot is advanced too far clockwise, you end up over modulating the signal, and cause feedback and a terrible signal. I found with the volume control pot pointer at the 1:00 position, I was achieving 70 to 80% moducation peaks, which sounded excellent with no feedback or unwanted oscillations.

Author unknown, but an excellent site for information on the KT-195 (https://people.ohio.edu/postr/bapix/LA23.htm)
Bob KA8FVW (http://heartandhoof.com/mram/KT-195) Designer of the Plate Modulation Version. Bob has the KT-195 original, and as he modified it. Excellent all around!

I take no credit for the above two author's work, or their website content, but highly recommend the sites as VERY informative on the Lafayette KT-195 Amplifier/Broadcaster. I played a CD for input into j2 for all of my tests, except for a very brief test of J1 with a crystal microphone. Hope you found this article interesting enough to try out my ideas! Good luck and 73 - KB2NFQ
P.S. I proofread, but it is possible I did not edit out all of my typos and technical errors :-)

Picture of originally neatly wired chassis (I hope!)


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Lafayette KT195 chassis.JPG
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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Sun 19, 2020 5:00 pm 
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Good work! Sounds like without mods, the best thing about this device is the case!


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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Tue 21, 2020 5:37 pm 
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That's a good read, the schematics provided in your link brings it all together.

Mebby one day I'll try similar with my Knight Broadcaster.

I did plate modulate 6AQ5, 6AS5, etc using a PP 6DE7 amp I built(also a single ended 6BQ5), those worked very well.

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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Tue 21, 2020 9:12 pm 
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At the prices the Knight Broadcaster is going for I don't think I would modify one.

DM


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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Wed 22, 2020 12:21 am 
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devilsmist wrote:
At the prices the Knight Broadcaster is going for I don't think I would modify one.

DM

Mod such at made to the Lafayette would no doubt a major improvement for a KB. As orig, they are fair on voice & lousy for music.

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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Thu 23, 2020 6:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 66
Location: Grand Island, NY, USA + AZ
Ah-h grasshopper,
Plate Modulation much better Scratchy Grid Modulation. Sound from record much improved!

My attempt at some humor. If you have the schematics (I posted where to find them) this IS s very worthwhile mod, especially the polarized line cord for safety!

It also would adapt to the Knight Broadcaster with similar effort. Actually, since I have 2 KB's, I will for sure mod one in the future. Right now, too many projects on or around my limited area bench!

Kind regards,
Don H.


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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Thu 23, 2020 1:47 pm 
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Problem with plate modulation is finding a good transformer( at least inexpensively), most have a limited low freq range. Also to fully modulate, driver amp should have at least 25% headroom. For short range units(home broadcaster), 70% modulation is more than adequate.

Screen grid can be OK. A voltage divider(requires a little experimenting) and fairly large cap from plate of audio amp works quite well. A modulation transformer can be used but we're back to finding good iron. Using the voltage divider & cap method, beam power types modulate far more easily than power pentodes.

For home use, suppressor grid modulation(via a dual control pentode 6888, 6GY6 etc), can easily match either of the above and require far less parts in the process. No specialized iron or audio output section required. A 6AV6 will fully modulate either tube mentioned.

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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Fri 24, 2020 3:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4498
Location: Boston, MA USA
KB already uses Heising Plate Modulation, just like the Westinghouse 50-HG 50kW transmitter I dealt with years ago.

-David


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 Post subject: Re: Lafayette KT-195 (LA-23) AM Radio Broadcast Amplifier
PostPosted: Jul Sat 25, 2020 1:06 am 
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Posts: 9467
Location: Beaver Falls, PA. USA
The author of that article is Rich Post, KB8TAD. His website is very nice!

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