What affordable audio preamps work well with a Gates BC1G?

[K2DUV]

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

[W7TFO]

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

[DMOD]

I have a Shure 4 mic preamp that puts out a miniumum of 6.5 volts into 600 ohm balanced.


Phil - AC0OB

[KL7OF]

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

[W9BHI]

I am using a Symetrix 528E with my Gates BC1-G and can modulate the heck out of it.
W9BHI

[w4bfs]

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

[W7TFO]

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

[KL7OF]

have you tried the 70V output of the PA amp direct to the 600 ohm input???

[kg7bz]

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?

[WB4AIO]

Get a mic preamp and equalizer of decent quality

(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.