Re: [AMRadio] Matching a 100 watt PA am to drive 833'S
Good points Geoff! Yes I suppose I should give code another try, but I have tried in earnest several times and, well I have problems with languages and ciphers. Never been able to learn any computer language, and my Spanish is nonexistant Donde estas el banjo?. I pass the written stuff just fine. Basically I have given up on code, and will just wait it out, or, when the rigs are ready, I can get a buddy over who has a better license and I can 'air out' the rigs. Really it's kind of silly, to me, to have to learn to use a digital code (Morse) in order to be allowed to use an analog computer (An AM transmitter).. :P Patrick From: Geoff <[EMAIL PROTECTED]> Subject: Re: [AMRadio] Matching a 100 watt PA am to drive 833'S Patrick, with your collection of parts and transmitters, and knowlege of the same,it's a shame you're sitting there with a Technician class license. You need to put those rigs on the air. They're old enough. The need to be HEARD! :-) 73 = Best Regards, -Geoff/W5OMR (descrete transistors are -not- evil. Evilness lies in microprocessors!)
Re: [AMRadio] Matching a 100 watt PA am to drive 833'S
Patrick Jankowiak wrote: Better than installing or connecting an evil transistorized driver to it. He asked for suggestions on how to make it work, so the field is open. As to why, I can't say. No one said anything about drilling holes (mutilation) but hopefully that won't happen. Patrick -- From: "Donald Chester" <[EMAIL PROTECTED]> Subject: RE: [AMRadio] Matching a 100 watt PA am to drive 833'S I think it would be a terrible mistake to mutilate a high quality transmitter by going from a design as superbly engineered as the BC-1F, and converting it to something as jury-rigged as using a power transformer for a driver transformer, or one output transformer feeding a second reverse connected output transformer to feed the grids. Patrick, with your collection of parts and transmitters, and knowlege of the same, it's a shame you're sitting there with a Technician class license. You need to put those rigs on the air. They're old enough. The need to be HEARD! :-) 73 = Best Regards, -Geoff/W5OMR (descrete transistors are -not- evil. Evilness lies in microprocessors!)
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
Better than installing or connecting an evil transistorized driver to it. He asked for suggestions on how to make it work, so the field is open. As to why, I can't say. No one said anything about drilling holes (mutilation) but hopefully that won't happen. Patrick -- From: "Donald Chester" <[EMAIL PROTECTED]> Subject: RE: [AMRadio] Matching a 100 watt PA am to drive 833'S I think it would be a terrible mistake to mutilate a high quality transmitter by going from a design as superbly engineered as the BC-1F, and converting it to something as jury-rigged as using a power transformer for a driver transformer, or one output transformer feeding a second reverse connected output transformer to feed the grids. Don K4KYV
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
Perhaps I missed it but why would you want to change the 845 driver circuit, are you missing the tubes? John, WA5BXO
Re: [AMRadio] Matching a 100 watt PA am to drive 833'S
In a message dated 1/5/05 3:49:42 PM, [EMAIL PROTECTED] writes: > I think it would be a terrible mistake to mutilate a high quality > transmitter by going from a design as superbly engineered as the BC-1F, and > converting it to something as jury-rigged as using a power transformer for a > driver transformer, or one output transformer feeding a second reverse > connected output transformer to feed the grids. > I would have to agree.. Dennis D. W7QHO Glendale, CA
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
I'll take a very wild stab at it. From the RCA TT-4 handbook, the 833's at 3000VDC require 400V peak G-G volts, that is 242 V RMS. The drive is 20W. That makes the current 82 mA. That makes the impedance 2950 ohms G-G. If you have an old output transformer such as a 6K CT to 0-4-8-16 ohm type, and were to connect the transformer's 16 ohm winding to the 8 ohm winding on the power amp, then the secondary would look like 3000 ohms, close enough. You could also do a chinsel-cheeze method and use a dual-primary power transformer (120V each primary) set up for 240VCT, and with a secondary voltage of 12VAC. Use one with at least 5 amps rating on the 12v winding. Hook the 12VAC winding to the power amp's 8 ohm output. The series'd two 120V windings of the power transformer (240VCT) now looks like 3200 ohms, close enough. You might be surprised how good a power transformer can sound when operated far below its ratings. I think it would be a terrible mistake to mutilate a high quality transmitter by going from a design as superbly engineered as the BC-1F, and converting it to something as jury-rigged as using a power transformer for a driver transformer, or one output transformer feeding a second reverse connected output transformer to feed the grids. Don K4KYV
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
Hello Don (s) I have that Gates driver deck with I believe an intact driver. I also have most of the manual possibly for that transmitter. The driver received feedback directly from the finals to help compensate the impedance shifts you mentioned Don. What might be of interest tho a bit over kill is the Gates 5kw audio driver deck that used a quad of EL34's or 6550's to drive the pair of 2500 modulators. Now that has drive! I would guess that the reason for eliminating the 845s is due to their high cost. A pair of equally expensive but more commonly found 211-VT4C triodes would work in place of the 845's to the best of my knowledge. Some of the old PA amps also had a 100 volt output winding that might be adaquate for drive. Not sure the actual Imp.or availability of center tap. Typically a mono bridged solid state stereo amp can approach 100v levels as well with some mfg's claiming 100V line capability in that configuration. If info from or a copy of the 1K manual is needed please E-mail me and I will try to get one made up thats readable. This one was worn out by many engineers use and am not sure if it's complete.I believe the schematic is intact tho taped together. I also have most of the Iron for the 1K tramsmitter with at least one LV powersupply transformer burnt. Not parting with the 10v filament transformers. need them for my 813's. Bill KB3DKS/1
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
I have a older BC 1F series gates transmitter. I would like to do away with the 845 driver tubes and drive the 833's with a 100 watt PA amp. Does anybody have any Ideas what I would need to match the amp to the driver transformer of the 833's. Don Moore W5FFK Don, I think you would be making a mistake to do that modification. The grid impedance of the 833A's varies widely over the audio cycle, and the original Gates class-B driver tansformer is wound specially to have extremly close coupling from primary to secondary and thus very low leakage reactance, in order to minimise distortion generated by the varying load impedance. You want a source of audio driving power with not only low distortion, but also negligible internal resistance. This cannot be accomplished with the vast majority of audio transformers not specifically designed for class-B driver service. Each grid presents an average of about 500 ohms load on the driver, which means that the input transformer winding to the grid would need to be rated for about 2000 ohms total impedance, and the other winding would need to match the output of your PA amp. Hams have often accomplished this by feeding the output of a PA or hi-fi amplifier into an output transformer wired in reverse, but these audio output transformers are rarely wound to the exacting specifications of the Gates class-B driver transformer because a speaker does not need such tight coupling, and such a transformer is expensive to manufacture. Therefore, with such a set-up the odds are overwhelming that the distortion would be substantially worse than with the stock circuit. They main reason hams have used this lash-up is because they could not find a satisfactory class-B driver transformer. I assume your PA amplifier is solid state. If it is tube type, that would further exacerbate the situation, because the audio drive would be coupled through TWO transformers, each contributing its share of distortion. I am curious, what do you find unsatisfactory about the original 845 class-B driver? A more satisfactory upgrade would be the solid state class-B tube driver described some years ago by WA1QIX. It presents the nearly ideal class-B driver with extremely low internal resistance. Check out his website; if the circuit is not there, e-mail him. I'm sure he would be glad to share the circuit. If you really insist on changing out the original driver, I'd be interested in your old driver transformer. I have an audio output transformer rated to match a quad of four 807's, which might work with your PA amp, but I will have to check it out because I don't remember if the secondary winding impedance is 500 ohms, or for a low impedance voice-coil. If it would work, I'd be willing to trade, but I still say you would be better to keep the stock audio driver. 73, Don K4KYV
Re: [AMRadio] Matching a 100 watt PA am to drive 833'S
Don, If the PA amp is tube type, just take the audio off the plates of the audio output tubes, and interface the the amp to the grids of the 833's with high voltage transistor emitter followers. This is what John WA5BXO created, and W5OMR does well with his Titanic. I think John can give you the link to the circuit. It is pretty simple, and high performance. If the audio amp is solid state, then you need a step up transformer with a secondary center tapped winding of suitable turns ratio. Load the secondary so that the amp sees the proper load impedance. This arrangement will work with a tube amp as well, but I personally don't like cascading transformers. In either case if the PA amp has 4, 8, and 16 ohm outputs (some solid state amps do) AC ground the 4 ohm tap, and you have 180 degree opposing audio at the common, and 16 ohm taps. The problem here is that 833's need a hell of a lot of grid swing, and this scenario when driven to maximum level (16 ohm swamping resistor across transformer entire secondary winding) will only provide about 40 volts RMS, or 56 volts peak grid to grid (E^2/R=W; so at 100 watts into 16 ohms you get 40 volts rms). This might do for a pair of high mu triodes AB2, but not 833's. Regards, Jim Candela WD5JKO --- Don Moore R Moore <[EMAIL PROTECTED]> wrote: > I have a older BC 1F series gates transmitter. I > would like to do away > with the 845 driver tubes and drive the 833's with a > 100 watt PA amp. > Does anybody have any Ideas what I would need to > match the amp to the > driver transformer of the 833's. > Don Moore > W5FFK > __ > AMRadio mailing list > Home: > http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net >
Re: [AMRadio] Matching a 100 watt PA am to drive 833'S
Don Moore R Moore wrote: I have a older BC 1F series gates transmitter. I would like to do away with the 845 driver tubes and drive the 833's with a 100 watt PA amp. Does anybody have any Ideas what I would need to match the amp to the driver transformer of the 833's. Don Moore W5FFK http://www.qsl.net/wa5bxo/driver1.html I use it here, with an old Bogen PA amp. I've run two different versions of this, with the help and aid of WA5BXO. Originally, the audio was taken from the plates of the output tubes in the PA, through some DC Blocking caps (0.6uF @ 450v as I recall) to the hi-Z side of a small'ish (6AQ5 type) output transformer to act as a choke, straight to the bases of a pair of ECG164 vertical output transistors, configured in an emitter-follower fashion. The emitter output, with bias voltage applied through some high-wattage resistors, is then fed directly to the grids of the modulators. In my case, I'm using a pair of 250TH's in the modulator, which, in normal/typical operation, require around 450vpk grid to grid drive. With 300vcc on the collectors, a maximum of around 600vpk is available. The regulated bias supply swings from 0 ~ -150VDC. 250TH's require around -75v w/3kV on the plates, to idle the tubes at around 100mA. The end effect is, no inter-stage driving transformer is required, which would introduce a loss in frequency response due to a non-linear hysteresis curve through the driving transformer. Whatever audio component is produced on the plates of the speech-amp, is what is applied to the grids of the Class B modulators. The 2nd incarnation of this circuit has the speech-amp audio coming straight from the plates of the output section, to a Hi-Z (6kOhm) 1:1 transformer, with the secondary actiing as the Hi-Z choke. What a FINE Job it does, too. Unfortunatly, I'm infected with the ham operators most dreaded disease; neighboritis. As a result, I normally only run ~100w after 6am... and on the Early-Risers AM (ERAM) session, the rig is up around 200w. Since I'm using seperate power supplies, wired in series, when running lower power, I turn off the modulator supply, to use common voltage for the final and modulator. Positive peaks suffer, but when you're talking only a couple of hundred miles, band conditions don't really factor a lot. Some QSB, but conditions to 200miles are stable all through the day here in South Texas. Long Haul stuff gets the higher B+ on the modulator, to produce positive peaks that approach 4:1. (see http://w5omr.shacknet.nu/~wa5bxo/asyam/Amplitude%20Modulation.htm) (BTW, John - I copied the files to my server, because of how slow QSL.Net has been of late) Pictures and schematics of modulator are available at http://w5omr.shacknet.nu/~w5omr/hamstuff/AM-Stuff/Titanic/modulator/ 73 = Best Regards, -Geoff/W5OMR
RE: [AMRadio] Matching a 100 watt PA am to drive 833'S
This was actually a popular approach during the pre-WWII era. So popular that UTC and Thordarson (and probably all the transformer manufacturers) made driver transformers that would match the low ohms output of the PA amp. But I have only seen these in sizes for lower wattages. The biggest one I have seen was for 60 watts. Maybe there are larger ones out there? Good luck. 73, Don Merz, N3RHT -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Don Moore R Moore Sent: Tuesday, January 04, 2005 12:11 AM To: amradio@mailman.qth.net Cc: amradio@mailman.qth.net Subject: [AMRadio] Matching a 100 watt PA am to drive 833'S I have a older BC 1F series gates transmitter. I would like to do away with the 845 driver tubes and drive the 833's with a 100 watt PA amp. Does anybody have any Ideas what I would need to match the amp to the driver transformer of the 833's. Don Moore W5FFK __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorized. If you are not the intended recipient please notify us immediately by returning the e-mail to the originator.(A)
