Any Thoughts on the 757 amps?

[Holydvr]

I would say at least a 50amp overall. There are some 75amp server supplies that can be converted with little trouble that may not break the bank much. I know there is some math that can be done to determine it, but I am currently at work.

 

[Longrider in Texas]

A side question: not sure sbout pills inside but I am getting a RCI2970N3 soon and after properly being peaked it is supposed to do over 300 watts. Anyone know what size power supply I will need for this radio.

I use a Delta 60 amp to run a custom 4 pill that will make around 500 watts. I'm sure a good 50 amp supply would do you just fine.

 

[1883gatekeeper]

I used to think that, also. I grew up using tube stuff and slowly made the transition, unenthusiastically, to solid state. I remember 2 and 4 pill amps back in the day that sounded absolutely terrible while putting out 150 watts. Things are much different now and designs have drastically changed. I'm not a tech so I don't have the vocabulary to explain it, but the newer amps are able to hammer out huge power and remain acceptably clean.

As an example, I run a Texas Star DX-500V that came out of the box doing about 450 W PEP with a 35W drive. Now, this is what is expected of it. I did a simple power wire upgrade because of the instability in current draw and voltage drop. I removed the two fuse holders and ran separate 8ga power wires to each board. I now have two red and two black wires coming from the amp, terminated via Anderson Power pole. Now, with volts and drive the exact same as before, I'm getting a tad over 700W out of this amp with rave audio reviews and not one person complaining about dirt, distortion or splatter. Plus, I used to idle it at 14.5 volts, and it would drop to about 13.5v (roughly) during transmit. Now, it idles at 14.5v and drops to 14.2v under heavy load.

Back to 757, I have no idea how he gets a 4 pill to take 150W drive, let alone get 1KW out of it, but I know he's not the only one these days. The techs at Texas Star told me never to drive a 4 pill with more than 30W PEP, so that is what I do. Yet, I see BBI, Gatekeeper, Hopper, Donkey Stomper, and tons of others driving with 100W radios and pushing HUGE wattage. As I said, I can't explain it, but I know they're doing it, and doing it well. Some, better than others.

That is because it is a class c amplifier being driven with all that power. NOW a Biased Amplifier is differnt. Since it has a DC bias voltage on the base where the RF is entering the transistors, it doesn't take as much drive and doesn't need to be driven as much at this point. This is why the techs at Texas Star tell you this. -Gatekeeper

 

[groundwire]

I saw him on facebook the other day claiming his 2x2879 amps do 600w + and will last forever no problem. i replied to him saying thats absolutely ridiculous and the rf transistors will fail in no time over driving them to that point. he told me i was dumb and uneducated. hahahahaha, ok 757. what a joke

 

[linearone]

I saw him on facebook the other day claiming his 2x2879 amps do 600w + and will last forever no problem. i replied to him saying thats absolutely ridiculous and the rf transistors will fail in no time over driving them to that point. he told me i was dumb and uneducated. hahahahaha, ok 757. what a joke

Last forever? Eh? I dunno. Maybe a long time depending who made it and who matched everything.

I recently bought a 1 pill and a 2 pill 2290 hg box from davemadeamps.com. I have a lot of reasons for this. His seem to be one of the few that are well tuned, offer biasing, and stay together. I haven't seen many guys building these things nowadays that can keep them together past 14v. I saw these boxes survive at 21-23v. Yes.

Back in Toshiba days a 600w two pill/three pill was totally common. Even the ones I screwed together did that and continue to do it 15 years later.

Nowadays, I weighed the options of trying to reteach myself how to build them and finally decided I'd rather buy them. I know the builder and everything from the cases, the heatsinks that are twice a large as a Fatboy, every one is fan cooled, and the fact that they plain work, made me jump on the Dave train. I never thought I'd be a Dave DC customer but yup, I am.

 

[Greg T]

That is because it is a class c amplifier being driven with all that power. NOW a Biased Amplifier is differnt. Since it has a DC bias voltage on the base where the RF is entering the transistors, it doesn't take as much drive and doesn't need to be driven as much at this point. This is why the techs at Texas Star tell you this. -Gatekeeper

And I learned about the power wire upgrade from your videos, Mr. Gatekeeper! Much appreciated!

 

[Needlebenderonefivenine]

850 out of 4 2879s is going to be mostly garbage coming out, I can't imagine 1kw. 500W out of 4 2879s at 13.8V is a good honest number. Much more than that and you're spewing trash.

lol hard words for someone who have never uses a 757 amp.

 

[techradioman]

lol hard words for someone who have never uses a 757 amp.

Oh, and you're an authority on 757 amps ? Please tell us more about them and what makes them great. I had one, not much better built or electrically than a Fat Boy. And yes 800 watts from a 4 pill will show LOTS of garbage IF you have and know how to use the right equipment to show it.

 

[Greg T]

I had a 4 pill 757. Here is the fun part; The amp needs over 100W PEP drive to hit 1.1KW. BUT, If I drive it with the same drive I put into my Texas Star, I get roughly the same output. I drive my DX-500V with about 40 to 50 PEP and get about 600 PEP out. Swap out the TS for the 757 and see the same output. According to Texas Star, I am driving a little hot. Yet, a 757 can take 150W and nearly double the output. Something isn't right about that. I think anything over 5 or 6oo is garbage watts.

 

[9C1Driver]

The data sheets on these finals does not lie. The only way you get those kinda numbers is drive the snot out of it and bump up voltage. 800-1000 is overmodulated overdriven grabage watts that excite the watt meter into showing a big number. I bet the neighbors three houses in every direction are impressed too.

 

[Crawdad7]

With enough swamping resistors on the input, you could drive a 1 pill with 5KW. You just burn off the excess power as heat and give the transistors what they're looking for. Waste of energy, especially in a mobile setup where you're limited by your alternator. No point in running a driver to get 150W to put into the 4 pill when you're going to burn off 3/4 of that as heat. A dual final radio would get you the same result. You could use the amps the driver is eating up to keep the 4 pill fed.

Same reason why 2x4 or even 1x4 amps are silly. A 1x4 made sense in "before times" when radios struggled to make an honest 4W, but these days, pretty much any radio made in the last 20 years, tuned up properly, can drive a straight 4 pill to saturation just fine. A 1x8 makes sense on a single final radio, but a tuned up dual final radio driving a straight 8 can make almost as much power. And in the end, the receiving station wouldn't be able to tell the difference in signal strength between the two anyway, maybe 850 vs 1000w. If you have a radio with a RFX75 or something similar, that will drive a straight 8 to saturation no problem.

There's no magic recipe for squeezing grossly more power out of a transistor and the signal remaining clean. You can maximize the clean power out with things like keeping the transistor fed with solid voltage (like you did by upgrading the power wires), or feeding them with higher voltage (like competition guys running a motor maul, though I hear the Toshiba clones don't like high voltage). But in the end, anything over ~125W output per Toshiba 2879 at 13.8V is mostly garbage, not on your fundamental frequency. These days, with the price of Toshibas, you're better off cruising them than burning them up...

High Drive provides for versatility. Case in Point this video, this unit has a specific versatile purpose. Agree on the 8 Pill.

 

[AudioShockwav]

The upside to that mod is you don't need to use a space heater in the shop.
The downside is that you could knock about 40 amps or more off the draw.....that is just wasted in heat, in the power supply and amp.

Sorry
But I am not impressed.
He says:
So you don't need those 2- 2290's.
You sure paid for them.
Anyway, it's just a big waste, but I am sure it padded his beer fund.
Why not just run the proper radio with the correct amp and save a bunch of money on parts and a whole bunch of wasted power.


73
Jeff

 

[Crawdad7]

The upside to that mod is you don't need to use a space heater in the shop.
The downside is that you could knock about 40 amps or more off the draw.....that is just wasted in heat, in the power supply and amp.

Sorry
But I am not impressed.
He says:
So you don't need those 2- 2290's.
You sure paid for them.
Anyway, it's just a big waste, but I am sure it padded his beer fund.
Why not just run the proper radio with the correct amp and save a bunch of money on parts and a whole bunch of wasted power.


73
Jeff

Yes indeed on proper radio proper amp.

 

[linearone]

Yes indeed on proper radio proper amp.

What is a proper amp?

 

[Crawdad7]

What is a proper amp

What is a proper amp?

For starters a cool
efficient one.