"The Page 75" Amplifier question
 

I can't seem to get it to work right. It's made by Electronics Unlimited in
Lebanon, Tennesse. The capacitors on the primary side of the output
transformer were changed at some point and I can't find the right value. If
anyone has one of these or a schematic, it would be extremely helpful.

It has a high/low switch. If I put it on low or put in less power, the power
meter reading shoots up to 150 or so with a reflected reading of 40 watts,
but drops back to 50 watts with modulation. On high with 4 watts in, it
seems fairly normal, but not completely. It bounces around 75 watts. Is
anyone familiar with this amp? Is it class C? It has an AM/SSB switch. The
transistors are 2 MRF455A's. The bases are grounded through the input
transformer, which has a center tap to a choke to ground. I see no forward
bias. Is this what causes the erratic behavior? Any help with this thing
other than toss it in the river would be appreciated.

Chris

In .net, "Chris"
wrote:

I can't seem to get it to work right. It's made by Electronics Unlimited in
Lebanon, Tennesse. The capacitors on the primary side of the output
transformer were changed at some point and I can't find the right value. If
anyone has one of these or a schematic, it would be extremely helpful.


No schematic. The output impedance of the MRF455 is about 1.5 to 2 ohms, so
those caps are going to be fairly large -- somewhere around 500 to 1000 pF
depending on the band you want to splatter. I would suggest 680 pF to start, and
only change them if the output trimmers won't tune into a dummy load. And they
really should be silver-mica caps.


It has a high/low switch. If I put it on low or put in less power, the power
meter reading shoots up to 150 or so with a reflected reading of 40 watts,
but drops back to 50 watts with modulation. On high with 4 watts in, it
seems fairly normal, but not completely. It bounces around 75 watts. Is
anyone familiar with this amp? Is it class C? It has an AM/SSB switch. The
transistors are 2 MRF455A's. The bases are grounded through the input
transformer, which has a center tap to a choke to ground. I see no forward
bias. Is this what causes the erratic behavior? Any help with this thing
other than toss it in the river would be appreciated.

Chris



The transistors are indeed biased class C which makes your amp just another
cheap CB splatterbox, but that's not responsible for the unusual behavior. It's
very possible that you have a bad component somewhere, or that you don't have a
power supply that can handle the load. But it may also be your operation of the
amp. Each MRF455 can be squeezed for about 80 watts, but remember that rating is
peak power, -not- RMS. In other words, a pair of MRF455's will get you 160 watts
PEP which is great for SSB, but in AM you -must- reduce your input power so your
deadkey does not go above 40 watts RMS (or 1/4 of the rated peak power). Feed it
more than that and you start clipping, causing even -more- harmonic distortion &
splatter than a 'clean' class C amp.







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