WCAP Update

[Dan.Strassberg]

Back in college, more than 60 years ago, we studied the efficiency of 
high-level-modulated AM transmitters. If I'm not mistaken, the most modern 
designs of the day used something called Doherty modulation. The maximum 
efficiency at 100% modulation was surprisingly low, although I can't 
remember how low. Low or not, the efficiency as a lot higher than that of 
low-level modulation (also called grid modulation). Also, the analysis made 
no provision for compression or limiters ahead of the transmitter in the 
audio chain, enabling what was, in effect, modulation greater than 100%.

Suppose the efficiency of the high-level modulated unit at 100% modulation 
was 40% (I don't know whether that figure is high or low) and that of a 
solid-state transmitter of modern design is 85%. If so, the 
high-level-modulated 5-kW transmitter dissipated 7.5 kW as heat and the 
solid-state unit dissipates approximately 880W as heat. Back in the day, FCC 
rules stipulated that an engineer had to be on duty at the transmitter site 
whenever the station was on the air. The 7500W that the high-level-modulated 
transmitter dissipated as heat was likely adequate to keep the transmitter 
building comfortably warm for the transmitter engineer on the coldest day. 
The 880W dissipated by the modern transmitter is probably also adequate 
because there is normally no engineer to keep warm inside the transmitter 
building.

-----
Dan Strassberg (dan.strassberg@att.net)
eFax 1-707-215-6367

----- Original Message ----- 
From: "John Francini" <francini@mac.com>
To: "Dan.Strassberg" <dan.strassberg@att.net>
Cc: "Boston Radio Group" <boston-radio-interest@lists.BostonRadio.org>
Sent: Monday, September 02, 2013 11:09 AM
Subject: Re: WCAP Update


So then, out of curiosity, for a 5kW transmitter, how much input power is 
needed for a typical vacuum-tube unit versus its solid-state equivalent? 
Would a 5kW transmitter require 10kW from the mains (thus dissipating 5kW in 
other ways (mainly heat))?  Would a solid-state unit draw significantly less 
(like, say, around 6kW for the same output power?

john



On 2 Sep 2013, at 7:43 , Dan.Strassberg <dan.strassberg@att.net> wrote:

> Economics dictate that vacuum-tube-based AM transmitters remain in use,
> especially at financially challenged stations like WCAP. In the short run, 
> a new tube and all of the sturm und drang associated with replacing the 
> tube will still probably cost Poulten way less than a new solid-state 
> transmitter would cost. It wouldn't surprise me to learn that, even with 
> all of the fuss over getting the tube from Florida and installing it in 
> the existing transmitter, the tube is costing Poulten only about 10% of 
> what a new 5-kW solid-state AM transmitter would cost. Over the long term, 
> however, the solid-state rig would probably be cheaper--especially here in 
> the Northeast, where power bills are high--because the power bills for the 
> solid-state unit would be a lot lower.
>
> -----
> Dan Strassberg (dan.strassberg@att.net)
> eFax 1-707-215-6367
>
> ----- Original Message ----- From: "Paul B. Walker, Jr." 
> <walkerbroadcasting@gmail.com>
> To: "A Joseph Ross" <joe@attorneyross.com>
> Cc: "Boston Radio Group" <boston-radio-interest@lists.BostonRadio.org>
> Sent: Monday, September 02, 2013 12:49 AM
> Subject: Re: WCAP Update
>
>
>> Uh, yeah.. station transmitters still use them.
>>
>> It's not un heard of even in this day.
>>
>>
>>
>>
>> On Mon, Sep 2, 2013 at 12:45 AM, A Joseph Ross <joe@attorneyross.com>
>> wrote:
>>
>>> On 9/1/2013 7:08 AM, Mark Watson wrote:
>>>
>>> The Lowell Sun reports that WCAP is still on the air at very low power
>>>> while the station awaits the arrival of a tube from Florida. Owner Sam
>>>> Poulten claims that those extremely close to the transmitter site can
>>>> hear
>>>> them over the air. They are still streaming on the WCAP website and
>>>> smartphone apps.
>>>>
>>>
>>> a,,, TUBE???
>>>
>>> --
>>> A. Joseph Ross, J.D.| 92 State Street| Suite 700 | Boston, MA 02109-2004
>>> 617.367.0468|Fx:617.507.7856| http://www.attorneyross.com
>>>
>>>
>