[AMRadio] mike hu-m-m-m-m problem
I'm having tons o' fun with my new Collins 32V-2, BUT, I've got a hum problem. According to several listeners, my Heil Classic microphone -- while sounding great otherwise -- may be picking up EMI from the nearby transmitter iron. The mike has no internal humbucking coil. Has anyone else experienced this problem with dynamic mikes and heavy iron? If so, how did you solve it, please? Many thanks and 73, Craig W3CRR
Re: [AMRadio] mike hu-m-m-m-m problem
Craig, If the problem is the mic element picking up stray hum, then moving the mic around will vary your symptoms. Do you see that? If so, you can use the mic to zero in on the source of magnetic flux. Steel, or preferably Mu-metal (the stuff around oscilloscope CRT's) can offer shielding from stray flux. Here is a silly thought; remember an old photo in the ARRL handbook where a car distributor cap was shielded with a large tin can, and then copper braid was added to shield each plug whire, and was soldered to the same tin can? Doing something similar to your prize mic might help, or go back to an Astatic D-104. That crystal element is immune to this problem. :-) More seriously, maybe you can find the source of flux, and eliminate it at it's source. For example with my QRO 20A I had hum in the nulled carrier (120 hz) that was much worse after adding a filter choke (open frame)to the power supply. The flux from the choke was coupling through the metal chassis, and into the two modulation transformers. The fix was to replace the choke with another that was enclosed in a steel case. Problem was solved. I could have also tried to change component orientation, but limited real estate to do so did not allow this option. I still had some hum in the carrier null but this was 60 hertz. I added a hum neutralizing circuit to buck out the hum in the proper phase, and magnitude. In the end with a 6.3 vCT filament circuit, I used 20 ohms to inject current into the balanced modulator diode support bracket. The carrier null was now without a measurable 60 hz component. I hope you resolve the huuu issue with that nice Heil microphone. Regards, Jim Candela WD5JKO --- W3CRR [EMAIL PROTECTED] wrote: I'm having tons o' fun with my new Collins 32V-2, BUT, I've got a hum problem. According to several listeners, my Heil Classic microphone -- while sounding great otherwise -- may be picking up EMI from the nearby transmitter iron. The mike has no internal humbucking coil. Has anyone else experienced this problem with dynamic mikes and heavy iron? If so, how did you solve it, please? Many thanks and 73, Craig W3CRR __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami
Re: [AMRadio] mike hu-m-m-m-m problem
Many thanks for your very detailed and useful reply, Jim. I was sorta hoping this could be dealt with with a wad of aluminum foil, but :-) . I just read a broadcast engineering publication review of a couple of Heil mikes. Sonic quality and value for money ratings were high -- but the reviewer did register a complaint that these Heil models did not incorporate a hum bucking coil and were, consequently, prone to noise pickup from nearby power supplies. Simply moving the mike far enough away from the hum source cured the problem, but that's not always feasible. I do have a roll of 4-inch wide Mu metal, so I could fashion shielding, if need be and I'll look into the more complex solutions you recalled if I must. However, I did, in fact, buy a good old D-104 from an eBayer this morning just in case all else fails :-) . Every vintage shack should display one anyway. 73, Craig W3CRR
Re: [AMRadio] Mike connectors needed
Thanks, I will order tomorrow. I hadn't thought of them for mike conns just RF type. Healthfully yours, DON - Original Message - From: [EMAIL PROTECTED] To: Discussion of AM Radio amradio@mailman.qth.net Sent: Tuesday, August 02, 2005 9:04 PM Subject: Re: [AMRadio] Mike connectors needed Hi Don, The best place I have found to buy the mic connectors you are needing is from The RF Connection (link below). They have the best prices around. Click on microphone connectors. 73, Mark http://users.erols.com/rfc/index1.htm Quoting Rev. Don Sanders [EMAIL PROTECTED]: I am refurbishing several rigs and need microphone connectors as follows. 2 each - Amphenol 2 pin chassis mount like on theHeath HW series 1 each - Amphenol 2 pin mating connector for above that goes on the end of the mike cable. 2 each - Amphenol or other 4 pin chassis mount like on the TS520/530 series for mike connector 1 each - 8 pin chassis mount mike conn like on the IC735 and similar rigs If you have any or all of these please contact me direct with quantity and price. [EMAIL PROTECTED] Thanks. Healthfully yours, DON W4BWS Healthfully yours, DON __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net __ 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] Mike connectors needed
Rev. Don Sanders wrote: I am refurbishing several rigs and need microphone connectors as follows. 2 each - Amphenol 2 pin chassis mount like on theHeath HW series 1 each - Amphenol 2 pin mating connector for above that goes on the end of the mike cable. 2 each - Amphenol or other 4 pin chassis mount like on the TS520/530 series for mike connector 1 each - 8 pin chassis mount mike conn like on the IC735 and similar rigs If you have any or all of these please contact me direct with quantity and price. [EMAIL PROTECTED] http://www.tannerelectronics.com/ They got that. --- 73 = Best Regards, -Geoff/W5OMR
[AMRadio] Mike connectors needed
I am refurbishing several rigs and need microphone connectors as follows. 2 each - Amphenol 2 pin chassis mount like on theHeath HW series 1 each - Amphenol 2 pin mating connector for above that goes on the end of the mike cable. 2 each - Amphenol or other 4 pin chassis mount like on the TS520/530 series for mike connector 1 each - 8 pin chassis mount mike conn like on the IC735 and similar rigs If you have any or all of these please contact me direct with quantity and price. [EMAIL PROTECTED] Thanks. Healthfully yours, DON W4BWS Healthfully yours, DON
Re: [AMRadio] Mike connectors needed
The Amphenol connectors are available from Allied. They are now manufatured by Wire Pro. Bob Macklin K5MYJ Seattle, Wa.
Re: [AMRadio] Mike connectors needed
Hi Don, The best place I have found to buy the mic connectors you are needing is from The RF Connection (link below). They have the best prices around. Click on microphone connectors. 73, Mark http://users.erols.com/rfc/index1.htm Quoting Rev. Don Sanders [EMAIL PROTECTED]: I am refurbishing several rigs and need microphone connectors as follows. 2 each - Amphenol 2 pin chassis mount like on theHeath HW series 1 each - Amphenol 2 pin mating connector for above that goes on the end of the mike cable. 2 each - Amphenol or other 4 pin chassis mount like on the TS520/530 series for mike connector 1 each - 8 pin chassis mount mike conn like on the IC735 and similar rigs If you have any or all of these please contact me direct with quantity and price. [EMAIL PROTECTED] Thanks. Healthfully yours, DON W4BWS Healthfully yours, DON __ 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] Mike
John, I been busy trying to reassemble my shack. Getting 2 tons of junk in a 1/2 ton of space is a challenge! I need to get a ladder to the attic for my collection of tubes, capacitors, etc. I still got a 10 X 20 rented storage room that is nearly 50% full. Might be time to trim the fat... Anyway the grounded plate pentode making a triode has been used before. I seem to recall it being used with a 6AU6, and used as a low noise hi mu triode where the signal levels are low, and s/n ratio is important. Maybe somebody else has a better idea about this. I think I once read about this on a VT audio reflector. Regards, Jim -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Coleman, ARS WA5BXO Sent: Tuesday, March 15, 2005 9:28 PM To: 'Discussion of AM Radio' Subject: RE: [AMRadio] Mike Jim, That is an interesting circuit with the plate grounded and output taken from the screen grid as an anode. I don't quite see the advantage other than fewer parts. The gain seems to be about 10 db less if I read it right. Perhaps the screen to grid capacitance is much lower than an equivalent triode. As I see it a triode especially one with a lot of plate to grid capacitance is going to represent a large amount of miller effect capacitance to the crystal cartridge, similar to what you and I experimented with by placing the 1000pf caps from plate to grid of the 12AX7 input. This must have been equivalent to placing a .01 from grid to ground except it did provide some gain and special control. I would guess that a 10 meg grid leak resistor and 1 to 2 pf of plate to grid capacitance and a 12AX7 with a gain of about 40 would be about 100 pf to ground. I not sure how the math works on all that, but I do know that the higher the input resistance, or the high the gain is, the more miller effect is present. This could be a desirable thing for limiting high freq response while boosting the low freq response do to a greater RC time constant from the crystal representation the Bob described in another message. On the other hand a pentode connection with less plate to grid capacitance and miller effect would not attenuate the high frequency components and the full amplification of those signals would be realized with less phase distortion, not that anyone cares much about phase distortion at 10 - 15KCs. John Coleman WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim candela Sent: Tuesday, March 15, 2005 9:33 AM To: Discussion of AM Radio Subject: RE: [AMRadio] Mike Steve, I just was reading about this in my Radiotron Designers Handbook where a simple preamp using a 6SJ7 was used with a crystal microphone. In short you can use a higher grid resistance to change the frequency response (more lows), but the text describes the limitations involving reverse grid current, dc operating point shift, signal to noise ratio, dynamic range, and so on. There also is some alternative circuitry shown to give you some ideas if you want to do some extensive modifications. I find it interesting that they show the 6SJ7 in triode mode using two different techniques. I scanned in several pages, and have posted them on the net at the following link. The text is sideways, and you can turn image 90 degrees CCW with your Acrobat reader. Hope this helps. http://pages.prodigy.net/jcandela/Preamps/Mic_Preamps.pdf Regards, Jim WD5JKO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: Tuesday, March 15, 2005 8:25 AM To: amradio@mailman.qth.net Subject: [AMRadio] Mike Thanks Guys: This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved into Senior Residence. The speech amplifier/modulator schematic is right from August 1947 QST. I got the rack and the 5 sections directly from the flat bed trailer at the estate sale...and now just about finished doing what very little fixin required. The article cites a crystal mike; it feeds directly into the grid of a 6SJ7 with no grid leak resistor. As I understand these things, the mike - what type of cartridge - needs to put out 'so many' microvolts necessary for the tube into which it feeds. And the impedance needs to 'match'. I understand there plenty of gain after the 6SJ7...and one of the 6SN7's has a tone control potentiometer in the grid. Personally, my voice is a little higher from center in pitch..and I'd prefer more lower than higher freq. response...which is generally a problem due to low freq. rolloff in the mike curves...that's why I'm eager to try this 'tone control'. I guess the first question is: what impedance number is required for a 6SJ7 grid? Your generous help is always appreciated. Regards, Steve WA2TAK __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post
[AMRadio] Mike
Thanks Guys: This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved into Senior Residence. The speech amplifier/modulator schematic is right from August 1947 QST. I got the rack and the 5 sections directly from the flat bed trailer at the estate sale...and now just about finished doing what very little fixin required. The article cites a crystal mike; it feeds directly into the grid of a 6SJ7 with no grid leak resistor. As I understand these things, the mike - what type of cartridge - needs to put out 'so many' microvolts necessary for the tube into which it feeds. And the impedance needs to 'match'. I understand there plenty of gain after the 6SJ7...and one of the 6SN7's has a tone control potentiometer in the grid. Personally, my voice is a little higher from center in pitch..and I'd prefer more lower than higher freq. response...which is generally a problem due to low freq. rolloff in the mike curves...that's why I'm eager to try this 'tone control'. I guess the first question is: what impedance number is required for a 6SJ7 grid? Your generous help is always appreciated. Regards, Steve WA2TAK
RE: [AMRadio] Mike
First of all there should never be an element of any tube that is left open as far as DC is concerned, and typically, crystal microphones have no measurable DC leakage so it is an open circuit to DC. If a Crystal microphone is to be used on this circuit the tube would soon go to cut off or thermal grid emission run away, depending on the tube condition. If the tube has no cathode resistor for bias then I would recommend a few mega ohms of grid leak resistor for contact bias and a coupling capacitor in case a microphone such as a dynamic or some type of microphone that has a DC path to ground is used. If the tube has a cathode resistor, then a lower value of grid leak could be used as a load for the microphone. A 5 Meg potentiometer is useful here with the wiper to the grid like a rheostat to ground. Some microphones do better with a lower load resistor for damping but crystal microphones will lose all low frequency response if loaded too heavy with out equalization. Matching impedances is confusing. If you match the source impedance with the load impedance, maximum power will be transferred from source to load. With microphones, we are not concerned in transferring power. It is the response fidelity and ringing factors that we are concerned with. There is a best load that is needed but it is hardly ever the impedance of the source. See http://www.qsl.net/wa5bxo/power.html allow a lot of time for the picture to finish loading. Good Luck John, WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Tuesday, March 15, 2005 8:25 AM To: amradio@mailman.qth.net Subject: [AMRadio] Mike Thanks Guys: The article cites a crystal mike; it feeds directly into the grid of a 6SJ7 with no grid leak resistor. As I understand these things, the mike - what type of cartridge - needs to put out 'so many' microvolts necessary for the tube into which it feeds. And the impedance needs to 'match'. I guess the first question is: what impedance number is required for a 6SJ7 grid? Regards, Steve WA2TAK
RE: [AMRadio] Mike
Hi Steve, What's the finals on that rig, 813's? I think I have a rig just like that here. Can you describe the rig? Is a schematic available? TIA, 73, Ed, VA3ES -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: Tuesday, March 15, 2005 9:25 AM To: amradio@mailman.qth.net Subject: [AMRadio] Mike Thanks Guys: This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved into Senior Residence. The speech amplifier/modulator schematic is right from August 1947 QST. I got the rack and the 5 sections directly from the flat bed trailer at the estate sale...and now just about finished doing what very little fixin required. The article cites a crystal mike; it feeds directly into the grid of a 6SJ7 with no grid leak resistor. As I understand these things, the mike - what type of cartridge - needs to put out 'so many' microvolts necessary for the tube into which it feeds. And the impedance needs to 'match'. I understand there plenty of gain after the 6SJ7...and one of the 6SN7's has a tone control potentiometer in the grid. Personally, my voice is a little higher from center in pitch..and I'd prefer more lower than higher freq. response...which is generally a problem due to low freq. rolloff in the mike curves...that's why I'm eager to try this 'tone control'. I guess the first question is: what impedance number is required for a 6SJ7 grid? Your generous help is always appreciated. Regards, Steve WA2TAK __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net
[AMRadio] mike
I appreciate all your time and help, guys. Excuse me for not mentioning the cathode resistor for bias. From what little I have found...both the Collins 310A exciter and WRL Globe King 275 use a 6SJ7 for mike input...in which case, a hi-Z dynamic should work. So...back to square #1 - what is hi? Mouser has cheap ( $4 ) crystal elements whose impedance range is from 9K to 25K which doesn't look hi to me. I've also read that a ceramic element - acting in the same fashion as the piezoelectric - can be used. Which means that my Electro-Voice 729 is a candidate which is already on my shelf...sparkling since I polished all that cast metal to high sheen. The modulators are 2 x 807's going into an RF deck of 2 X 1625's having link coupled input from a 6L6 oscillator or 6L6 doubler stages...fortunately, I can get the 3.5715 Mc FT 243 type rock from PR Crystal for my 20 meter AM freq 14.286...only $15. Working on this restoration is a great learning experience for me...and again, I sincerely appreciate you sharing with this rookie. Regards, Steve WA2TAK PS...the 813 homebrew restoration is in the workshop patiently waiting for me...another 1x3 - now SK - in North Carolina made it.
Re: [AMRadio] mike
In a message dated 3/15/05 9:31:31 AM, [EMAIL PROTECTED] writes: Which means that my Electro-Voice 729 is a candidate which is already on my shelf...sparkling since I polished all that cast metal to high sheen. Steve, Your EV 729 should work fine. I would add a grid leak resistor, though. Anything 1 to 10 megs FB. Dennis D. W7QHO Glendale, CA
Re: [AMRadio] mike
[EMAIL PROTECTED] wrote: I appreciate all your time and help, guys. Excuse me for not mentioning the cathode resistor for bias. From what little I have found...both the Collins 310A exciter and WRL Globe King 275 use a 6SJ7 for mike input...in which case, a hi-Z dynamic should work. So...back to square #1 - what is hi? a hiGH Impedance miCrophone is around 50+kohms. Mouser has cheap ( $4 ) crystal elements whose impedance range is from 9K to 25K which doesn't look hi to me. The only way to find out for sure if it will work, or not, is to -try- it :-) Nothing ventured, nothing gained. 73 = Best Regards, -Geoff/W5OMR
RE: [AMRadio] Mike
Jim, That is an interesting circuit with the plate grounded and output taken from the screen grid as an anode. I don't quite see the advantage other than fewer parts. The gain seems to be about 10 db less if I read it right. Perhaps the screen to grid capacitance is much lower than an equivalent triode. As I see it a triode especially one with a lot of plate to grid capacitance is going to represent a large amount of miller effect capacitance to the crystal cartridge, similar to what you and I experimented with by placing the 1000pf caps from plate to grid of the 12AX7 input. This must have been equivalent to placing a .01 from grid to ground except it did provide some gain and special control. I would guess that a 10 meg grid leak resistor and 1 to 2 pf of plate to grid capacitance and a 12AX7 with a gain of about 40 would be about 100 pf to ground. I not sure how the math works on all that, but I do know that the higher the input resistance, or the high the gain is, the more miller effect is present. This could be a desirable thing for limiting high freq response while boosting the low freq response do to a greater RC time constant from the crystal representation the Bob described in another message. On the other hand a pentode connection with less plate to grid capacitance and miller effect would not attenuate the high frequency components and the full amplification of those signals would be realized with less phase distortion, not that anyone cares much about phase distortion at 10 - 15KCs. John Coleman WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim candela Sent: Tuesday, March 15, 2005 9:33 AM To: Discussion of AM Radio Subject: RE: [AMRadio] Mike Steve, I just was reading about this in my Radiotron Designers Handbook where a simple preamp using a 6SJ7 was used with a crystal microphone. In short you can use a higher grid resistance to change the frequency response (more lows), but the text describes the limitations involving reverse grid current, dc operating point shift, signal to noise ratio, dynamic range, and so on. There also is some alternative circuitry shown to give you some ideas if you want to do some extensive modifications. I find it interesting that they show the 6SJ7 in triode mode using two different techniques. I scanned in several pages, and have posted them on the net at the following link. The text is sideways, and you can turn image 90 degrees CCW with your Acrobat reader. Hope this helps. http://pages.prodigy.net/jcandela/Preamps/Mic_Preamps.pdf Regards, Jim WD5JKO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: Tuesday, March 15, 2005 8:25 AM To: amradio@mailman.qth.net Subject: [AMRadio] Mike Thanks Guys: This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved into Senior Residence. The speech amplifier/modulator schematic is right from August 1947 QST. I got the rack and the 5 sections directly from the flat bed trailer at the estate sale...and now just about finished doing what very little fixin required. The article cites a crystal mike; it feeds directly into the grid of a 6SJ7 with no grid leak resistor. As I understand these things, the mike - what type of cartridge - needs to put out 'so many' microvolts necessary for the tube into which it feeds. And the impedance needs to 'match'. I understand there plenty of gain after the 6SJ7...and one of the 6SN7's has a tone control potentiometer in the grid. Personally, my voice is a little higher from center in pitch..and I'd prefer more lower than higher freq. response...which is generally a problem due to low freq. rolloff in the mike curves...that's why I'm eager to try this 'tone control'. I guess the first question is: what impedance number is required for a 6SJ7 grid? Your generous help is always appreciated. Regards, Steve WA2TAK __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net -- No virus found in this incoming message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.7.2 - Release Date: 3/11/2005 -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.7.2 - Release Date: 3/11/2005 __ 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] Mike
Personally, my voice is a little higher from center in pitch..and I'd prefer more lower than higher freq. response...which is generally a problem due to low freq. rolloff in the mike curves... Steve, this makes me think that your best bet would be to close-talk a unidirectional mike. A low-cost ball-head dynamic unidirectional stage mike would probably be your best bet. The proximity effect of these microphones give a voice much more depth, and for some reason it is really rare for this effect to be duplicated well by electronic frequency response equalization. If you can find an old Shure SM58... Of course, that's a low-Z dynamic mike, so you would want a matching transformer to give the grids more signal level. Some of these microphones have a high Z output, though, which is what you want with a tube audio chain. But one problem with dynamic mikes in ham stations is magnetic hum pickup. So you might do well with a low cost unidirectional condenser mike. For some reason, these mikes usually sound muddy and require more midrange and high boost, though. 3 KHz boost is still a good thing, too. With the bass boost you'll get from proximity effect, you'll probably want a lot of 3 KHz boost. As long as you have enough deep low end, this kind of high-boosting does not make your voice sound higher pitched, it makes it sound clearer. And this frequency range is usually best equalized with electronics. There are a number of ways to do it. One simple trick is to remove two or three cathode bypass capacitors in the low level audio chain and put in relatively small capacitor values. Divide 210 by the cathode resistor value and use about that many uF of cathode bypass capacitance. So with a 1K cathode resistor, use a 0.22 uF bypass cap. The bypass effect works strongly at higher audio frequencies, but only weakly at lower frequencies, so the effect is a high frequncy boost. However, this boost comes at the cost of gain that you would otherwise have had in the middle and low frequency range. If you get into resonator design, you could take a 2 or 4 KHz peaker from a graphic equalizer, and move it to 3 KHz, and experiment with the Q of the resonance. You want a fairly low Q (broad) peak at about 3 KHz or just a little lower (like 2.9KHz). Bacon, WA3WDR
RE: [AMRadio] Mike Cable Capacitance
I don't think I will spend much time trouble shooting, since I went to a whole new setup, I am now going balanced between the mike preamp (berringer) and the power amp (symetrix and soon a samson). The old nikko power amp had unbalanced inputs. I doubt it was rf getting into the audio chain, but its possible. I was quite sure things were much better after just changing the rg8m out, I had put phono ends on it, and had an adaptor (phono to mono 1/4 inch jack) into the berringer. That should have unbalanced the output of the berringer, which is supposed to compensate for the change in level output with some magic circuit. There were (are) no built in bits in the cables, I DID have an T pad (600 ohm I think) to attenuate one input on the nikko. The new amps have level controls for each channel, so I chucked that. I was also driving two inputs with one (berringer) output. I just thought, since BOTH nikko inputs were bypassed with .047uf caps, that is almost a .1!!! The wrong cable types, add on T pads, excessive bypassing, running two inputs off one output, lots of not so great things that can cause problems. Its good that I have cleaned up this mess. Another nice thing with the new amps is that they have headphone outputs, so I can hear the audio feeding the modulators without doing anything with the 8 ohm outputs. Brett N2DTS
[AMRadio] Mike Cable Capacitance
John Coleman, WA5BXO, posted: (Regarding 50-ohm coax attenuating the high audio frequencies when used as a microphone cable). I feel there must be some other thing that is overlooked about the connector or cable. 15 ft of cable would not normally cause any noticeable effect on audio unless the terminating Z was greater that a few mega ohms as it would be for some of the older equipment made for Xtal mike input but I can't imagine a modern day line input being greater than 100K ohms The impedance to be concerned with in this case is not the terminating impedance, but rather the source impedance of the microphone itself. In the case of the D-104, although the impedance of the microphone is specified at a nominal 10K-ohms, its actual source impedance is much higher. It is a well-known fact that the element must be terminated in a resistive impedance of at least 10-Megohms to obtain adequate low-frequency response - indicating a source impedance of at least that amount. Additionally, since there is no DC path through a crystal acoustic transducer, one would be suspicious that whatever the actual source impedance is, it would contain a capacitive reactance as a series component. The best solution to the problem is that advocated by several other posters here - incorporate a preamplifier very close to the microphone element (inches, not feet, away) having an imput impedance in the tens of megohms and capable of driving a 500-ohm termination at unity gain. Jim Bromley, K7JEB Glendale, AZ James E. BromleyTel: 623-848-8711 5128 N. 69th Ave. E-Mail: [EMAIL PROTECTED] Glendale, Arizona 85303
RE: [AMRadio] Mike Cable Capacitance
I replaced a 10 or 15 foot piece of rg8m with a 20 foot radio shack regular audio cable. This went between the berringer on the desk, to the nikko power amp in the rack (now empty and out of the way). The rack will hold the homebrew pair of 4d32 rig in the future. The level was the same except for the extreme highs, which jumped from 20% modulation to close to 100% with the radio shack cable. This was from making loud s sounds into the mike. Doing more checking, I found I had put bypass caps in the audio power amp, both on the input, .047 uf, and the 8 ohm output .01 uf. These were likely to large, and along with the coax, reducing the highs. There was a large change between the coax and the regular audio cable when I tried that first, but its very likely it interacted with the bypass caps and pushed the roll off frequency down to 3000 to 4000 hz??? I changed the caps to .003 in that amp, and also bought 2 new amps, a one rack mount space 20 watt per channel symetrix amp, that seems to need no RF bypassing at all, and a samson 60 watt per channel amp, also one rack mount space (nice and small). It was mail order, and has not got here yet. The 20 watt symetrix (pawn shop special) works fine on the 811a mod deck, and the 4x150a mod deck (ab1), but does not have enough power to drive the push pull parallel 100th mod deck, it gets up to about 90% mod at the symetrix clipping (20 watts). I likely need to change taps on the driver trans to get more voltage, 4 100th tubes should take less than 20 watts drive... My ears are no good, but I think it sounds much better than it used to, much less high boost for the same sound (in my ears). I guess the 120 watt per channel nikko amp will be a ham fest special, its worked great for 20 years of abuse, has nice analog meters on it, but its quite large and heavy for the shack. I like having all the audio stuff close together on the desk, the berringer mike amp/compressor/eq/etc, the power amp, the tape deck, and the receiver audio amp (marantz). With a one rack space berringer, and a one rack space audio amp or two, you can drive a bunch of modulator grids in a very small space. High quality audio in about 4 inches of total rack space! I paid $90.00 for the symetrix, $180.00 for the (new) samson 60 watt per channel amp. And yes, I still have the good tube audio in the 32v3 transmitters, so I am not a total sell out. I hope to build up a nice tube mike and line preamp/mixer/eq/ compressor/power amp to drive the various homebrew rigs. A tube compressor that works well might be a bit tough though. I have bunch of nice balanced 600 ohm to grid hi fi type transformers that need to go in something. A pair of KT90 tubes will do 160 watts in AB1, very low distortion. Tone control circuits are in the back of the RCA receiving tube manual, along with the mike preamp and mixer circuits. That and the homebrew RF vfo/exciter/driver are all that is left to have a completely home brew station from mike jack to antenna. (can you home brew a mike?) I heard the first part of the article on the second homebrew receiver is in Electric Radio this month, but have not got my copy yet. I have no idea what pictures they used, I sent in a whole mess of photos of various things in the shack. Brett N2DTS
RE: [AMRadio] Mike Cable Capacitance
I use an outboard audio amp for my 75A-4 receiver with a system of adaptors that requires no modification to the receiver itself. I pull audio directly from the grid pin of the 12AT7 1st audio stage (with the tube removed). This is a high impedance point, 470k if I recall correctly. To avoid high frequency loss in the 5 ft. or so of shielded cable between the tube socket and amplifier, I built up a cathode follower stage to replace the 12AT7. It is a plug-in module that plugs right into the 12AT7 socket, and gets its B+ from the screen grid pin of the 6AQ5, which was also removed to conserve drain on the power supply and reduce unneeded heat in the receiver. I can remove the plug-in units and replace them with the original tubes, and the receiver will be completely stock once again. -k4kyv _ Learn how to choose, serve, and enjoy wine at Wine @ MSN. http://wine.msn.com/
[AMRadio] Mike Premus, W2OY article from AM Press/Exchange
From the November, 1987 issue - files 3 and 4 are pictures: www.qsl.net/w3nu/w2oy-1.jpg www.qsl.net/w3nu/w2oy-2.jpg www.qsl.net/w3nu/w2oy-3.jpg www.qsl.net/w3nu/w2oy-4.jpg www.qsl.net/w3nu/w2oy-5.jpg www.qsl.net/w3nu/w2oy-6.jpg
