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Title: What affordable audio preamps work well with a Gates BC1G? Post by: K2DUV on October 19, 2015, 11:06:37 PM The Gates BC500G (ex WRAR) that I am restoring uses a pair of 807s (in Class AB2??) followed by 807 Cathode Followers to drive the Class B 833 modulator grids. The input to the BC1G is applied to a 600ohm primary with split 600ohm-0-600ohm balanced secondary transformer that according to Gates requires about 16 dB of input power. I believe that for olden broadcast audio purposes these dB are referenced to 6mw=0dB. {RCA Radiotron Designer's Handbook, Section 19.1, Table 4.} So 16dB would be 10^1.6 or 39.6 So the input power required would be 6*39.6= 239mw . Drive voltage would be E=((239*0.001)*600ohm)^0.5=11.97 vRMS or thereabouts. So..what are successful Gates BC1G AMers using to get this 12vRMS of audio across the 600 ohm input of the balanced input transformer required for 100% modulation? I am trying to figure out how to affordably develop the 16 db of audio specified by using a 35 watt PA amplifier with a 4/8/16 ohm and 70.7V output. I might be able to use a split primary 480V center tap control transformer with 24V secondary hooked in reverse to develop a balanced output, but frequency response might be horrible. PP plate to voice coil transformers like UTC LS-series have become too pricey. The Gates STA-LEVEL far-far exceeds my pay grade! Forum member's advice will be appreciated. Thanks, Floyd Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: W7TFO on October 20, 2015, 04:15:06 AM The standard used by Gates then was the current 0dB = 1mW/.77V, not the old standard.
The input transformer has a split input winding so will work at 600 or 150 Ohms, series/parallel. You can drive it with any decent amp, but you'll want a good compressor/limiter or overmod will be a real problem as that TX can do more audio-wise than any ham rig with just a microphone. Do yourself a favor and spend the $ for an ex-broadcast limiter and not some Chinese junk from Mackie. A scope plumbed in properly for a trapezoidal display (not just looking at the RF off the air) will be a good idea as well. Again, a real used modulation monitor is an easy way to stay legal. 73DG Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: DMOD on October 20, 2015, 07:28:18 AM I have a Shure 4 mic preamp that puts out a miniumum of 6.5 volts into 600 ohm balanced.
Phil - AC0OB Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: KL7OF on October 20, 2015, 09:49:07 AM I have been using a Behringer VX2000 with condensor mic into a harris limiter....direct to the 600 ohm input...Lots of audio...the two boxes were less than $100 used....good luk
Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: W9BHI on October 20, 2015, 04:47:52 PM I am using a Symetrix 528E with my Gates BC1-G and can modulate the heck out of it.
W9BHI Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: w4bfs on October 20, 2015, 06:13:21 PM The Gates BC500G (ex WRAR) that I am restoring uses a pair of 807s (in Class AB2??) followed by 807 Cathode Followers to drive the Class B 833 modulator grids. The input to the BC1G is applied to a 600ohm primary with split 600ohm-0-600ohm balanced secondary transformer that according to Gates requires about 16 dB of input power. I believe that for olden broadcast audio purposes these dB are referenced to 6mw=0dB. {RCA Radiotron Designer's Handbook, Section 19.1, Table 4.} So 16dB would be 10^1.6 or 39.6 So the input power required would be 6*39.6= 239mw . Drive voltage would be E=((239*0.001)*600ohm)^0.5=11.97 vRMS or thereabouts. So..what are successful Gates BC1G AMers using to get this 12vRMS of audio across the 600 ohm input of the balanced input transformer required for 100% modulation? I am trying to figure out how to affordably develop the 16 db of audio specified by using a 35 watt PA amplifier with a 4/8/16 ohm and 70.7V output. I might be able to use a split primary 480V center tap control transformer with 24V secondary hooked in reverse to develop a balanced output, but frequency response might be horrible. PP plate to voice coil transformers like UTC LS-series have become too pricey. The Gates STA-LEVEL far-far exceeds my pay grade! Forum member's advice will be appreciated. Thanks, Floyd I believe that 16dbm is a maximum input level .... most am broadcast sites fed by ma Bell dedicated lines for the audio and you would expect 0 dbm, properly equalized Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: W7TFO on October 20, 2015, 06:38:34 PM Most BC house standard level has been +4dB (reads 100%) for some 40 years.
I've seen some @ +6dB, but not many. This is on a standard VU meter cal @ 0=.77V/1mW. 73DG Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: KL7OF on October 20, 2015, 08:36:48 PM have you tried the 70V output of the PA amp direct to the 600 ohm input???
Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: kg7bz on October 20, 2015, 09:44:40 PM have you tried the 70V output of the PA amp direct to the 600 ohm input??? OUCH!!! 0dbm = 1mw @600 ohms, 0.775V. If that input accepts +4dbm at 600 ohms, that's 1.23V! +16dbm is 4.89V Nowhere near 70 volts. Maybe use the 4 or 8 ohm tap? Title: Re: What affordable audio preamps work well with a Gates BC1G? Post by: WB4AIO on October 21, 2015, 12:43:54 PM Get a mic preamp and equalizer of decent quality AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
(this one works well: http://www.ebay.com/itm/Behringer-XENYX-X1204USB-Premium-12-Input-2-2-Bus-Mixer-w-Mic-Preamps-Compress-/291594787068?hash=item43e46758fc:g:cswAAOSwPe1UFkvQ ) and then feed it into this as the final peak limiter: http://www.ebay.com/itm/APHEX-720-DOMINATOR-II-Stereo-Multiband-Peak-Limiter-with-manual-/111796709673?hash=item1a079a6929:g:Ug0AAOSwKIpWALsK It will drive the Gates beautifully and work and sound like a dream. 73, Kevin, WB4AIO. |