kc0hfl
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« on: April 21, 2021, 08:50:09 AM » |
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Need help with a speech amp and modulator for a single 813 transmitter. Any proven designs or suggestions would be appreciated.
Kc0hfl
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w0lpg
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« Reply #1 on: April 21, 2021, 11:40:41 AM » |
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The usual modulator for an 813 running 2000 volts is a pair of 811A triodes with a bit of negative bias voltage to keep the idle plate current down to within ratings.. The main problem is getting the Class B grid drive to be clean enough to sound good. I use push-pull triode connected 6Y6 drivers through a step down transformer. (may be hard to find now days). The other problem is a good modulation transformer. My first one (175W Merit. shorted winding to core after awhile) Now have a 325 W Potted one , working fine.
The pictures of the rig is on my QRZ page. Schematic can be sent later.
Val W0LPG
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kc0hfl
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« Reply #2 on: April 21, 2021, 12:34:59 PM » |
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thank you. Please share the schematics. ai have mod iron options it’s the driver transformer getting me.
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w9jsw
Two shots of Whisky
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« Reply #3 on: April 21, 2021, 02:43:45 PM » |
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On QRZ.com in the Amplitude Modulation forum is a thread called "Simple Modulator". The thread may be informative. They are using a 20W audio transformer driven backwards to drive the grids of 811As. I am planning to do this with a pair of 813's in triode mode. The K1JJ transmitter uses this audio driver method also. The key I think is to use a few diodes in the cathode CT to get the bias right.
John
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KA2DZT
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« Reply #4 on: April 21, 2021, 02:57:57 PM » |
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thank you. Please share the schematics. ai have mod iron options it’s the driver transformer getting me.
You can use a small modulation xfmr for a driver xfmr. You need a big step down ratio for the driver. Fred |
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG
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« Reply #5 on: April 21, 2021, 03:00:33 PM » |
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Look up what the GATES transmitter company did as a driver, no drive transformer.
It's called Cathode drive.
You can substitute a center tapped choke for the resistors used by Gates.
It works better.
You only need to change the stage that uses the usual driver transformer (although
that's not terribly difficult to find) from the driver transformer to a cathode follower -
the drive comes from the cathodes.
There have been several threads here on the topic.
_-_-bear
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w0lpg
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« Reply #6 on: April 21, 2021, 03:01:48 PM » |
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The Modulator shown was given to me by a ham , now a SK. Originally had PP 6B4G driver but they were missing I happened to find the data curves for triode connected 6Y6s and they check out very similar to the 6B4 tubes. The speech amplifier looks very similar to something in an earlier handbook. More modern miniature tubes can be more available. Someone may pop in with a different driver, cathode output drive, or even solid state ! I used what was already in the chassis.
W0LPG
Val
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kc0hfl
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« Reply #7 on: April 21, 2021, 05:38:04 PM » |
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I was hoping to use the Art 13 mod iron I have with some 811's/811A if all possible I have larger iron if need be. Anything around the Art 13 iron that might be suggested? Thanks for the reply's With running the single 813 what wattage of modulation should I be shooting for?
Bob, Kc0hfl
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W7TFO
WTF-OVER in 7 land Dennis
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IN A TRIODE NO ONE CAN HEAR YOUR SCREEN
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« Reply #8 on: April 21, 2021, 07:19:32 PM » |
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ART-13 mod iron is all but bulletproof. Many have been used for more power than you need for a single 813.
73DG
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Just pacing the Farady cage...
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Chuck...K1KW
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« Reply #9 on: April 22, 2021, 04:24:54 AM » |
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See my previous post on 70 volt transformers. Use a pair of 811's or 811A's for the tubes and an ART-13 mod transformer. You could do the whole modulator for under $100 with a little diligence.
Chuck
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73, Chuck...K1KW
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kc0hfl
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« Reply #10 on: April 22, 2021, 08:37:54 AM » |
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Thanks Chuck I like that idea. Any chance you have a schematic of your set up I think I will go with it. One question is the input of the amp board do you direct drive it with a mic or do you add a preamp stage. Thanks for the ideal
Bob
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Chuck...K1KW
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« Reply #11 on: April 22, 2021, 12:40:56 PM » |
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It doesn't get much simpler than this! The amp board is the one with the link to Ebay in the 70 Volt transformer thread I posted Also, use the same 70 volt transformer or one similar. There are a lot of them around. The mod transformer can be from an ART-13 or any multi match for the power level.
The dBx 286 takes mic input and will provide the 2V or so the amp board needs. It's an excellent preamp and processor for voice with a compressor and EQ.
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73, Chuck...K1KW
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kc0hfl
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« Reply #12 on: April 22, 2021, 12:59:08 PM » |
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Thank you very much it is was I was thinking it would look like. I am going this route already ordered the amp and transformer.
Bob
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KC2ZFA
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« Reply #13 on: April 23, 2021, 10:39:13 AM » |
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interesting audio board the one shown by k1kw (and cheap !)
what sort of current rating for the power transformer it it is, say, 48vct ?
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