Looking for info on Texas Star 350DX

[BC Coyote]

A friend has one I may borrow and / or buy. I'm very curious about whether these have A/B bias or if they are just another shitty class C amp ?
Does anybody out there have a definitive answer ? I don't want to spend a ton of money only to have crunchy sound........
Lack of an LPF is no big deal as I have an external one I can use.

I let the magic smoke out of my ancient Palomar 200 today, and buying another amp will allow me to procrastinate fixing the Palomar until later.

 

[Xracerx13]

Texas Star Amplifiers are claimed to be AB biased

 

[brandon7861]

Schematic from CBTricks. I don't see any AB bias circuitry in this schematic, just a switch that adds some delay to the relay. Unless I have the wrong schematic, that's class C correction: class B.

Edit: you can tell by the transistor's bases. They are tied together. Class AB has circuitry to provide about 1.2v between the bases so that they are both on the verge of turning on to limit crossover distortion.

Edit: class C would be with the transistors in parallel turning on at the same time...

 

[BC Coyote]

Thanks for the schematic Brandon ! That's pretty much what I suspected......
The 455 driver is properly biased but not the two SRF3222's. So they kind of did it half-right......

The Palomar emitted some smoke while I was talking, power dropped about 25%, and now the audio is crunchy through it, so I suspect it is a biasing issue too. I rebiased it A/B about 15 years ago or so. Not really all that many components in the bias network, just a relay, a couple resistors, couple electrolytics, couple diodes and some inductors.
I just checked and I'm only getting 0.7 volts on the transistor bases, which seems kind of low......

Now I'm running my 5555N2 with just the power out the back and I'm still making contacts no problem............ phew !

 

[Eldorado828]

I'm no amplifier guru by any means but can tell you that I have used Texas star amplifiers throughout the years and have had nothing but good on the audio side of things. I've received transmissions from many as well and have listened to lots of fantastic audio from Texas star amps.
@Ranch55 is a Texas Star guru and could probably better explain them much better than I ever could.

Just my thoughts.

 

[BC Coyote]

@Ranch55 is a Texas Star guru

That's promising ! Paging Ranch 55 ........ Hopefully he'll pop in....

 

[AudioShockwav]

That is a schematic of a old Texas star bi lateral amp that used a mrf455 driver, that design went away a long time ago.

The one your friend has most likely has two Toshiba 2879's unless it was built later with DEI transistors.
See attached photos
The bias network is right behind the meter.
You can see is not regulated bias like the TX line of amps were, its fixed so changes in input voltage can effect it, its not the best design but it works.
As long as you feed it with about 14 volts, dont drive the snot out of it and put a fan on it they are good amps.
I have a 500 is 20+ years old that regularly gets used on 10 and 12 meters with no issues.
I run it easy, 340/350 out and it has served me well.

These are push/pull amps, even the two transistor class c comp amps are push/pull, they just have no bias.

 

[9C1Driver]

For decades the DX350 and 350 HDV are all 2 pill amplifiers with 2879 finals. They are what I have heard people say is a passive class AB that when heated up drifts into more of a class B operation.

Some of the older models before they were called Texas Star said Bi-Lateral on them. If the video below is what you have that is a very old amplifier for sure. The one in the video is a 1 driving 2 which I have never actually seen they are so old.

 

[BC Coyote]

I just got more information from my friend who has the amp. It is a DX350HDV, so it's actually a high drive version.
So if I understand correctly this is the model with 2879's in it ? So what kind of drive power does it need ? He says he drove it with under 20 watts and got about 200 out of it.

He's going to mail it tomorrow. By the time Canada Post gets it here it will be early next week, and then we'll see how well it works........

 

[AudioShockwav]

I just got more information from my friend who has the amp. It is a DX350HDV, so it's actually a high drive version.
So if I understand correctly this is the model with 2879's in it ? So what kind of drive power does it need ? He says he drove it with under 20 watts and got about 200 out of it.

He's going to mail it tomorrow. By the time Canada Post gets it here it will be early next week, and then we'll see how well it works........

That's fine for output, it should live a long life at that level with a fan.
It has the variable on the front, leave it open and control drive with the radio.
Enough drive to get 200 on ssb and limit the carrier out of the amp to 60/70 on am.
They will do more but its a very safe level to run and cleaner if you are using them on the ham bands.

73
Jeff
On edit

Helpful link

Texas Star DX 250HDV-DX 350HDV Service Manual

As for the high drive part

Output Power with 4 watts input

Note that the HDV models produce the same power out at the same drive levels.

 

[999]

Schematic from CBTricks. I don't see any AB bias circuitry in this schematic, just a switch that adds some delay to the relay. Unless I have the wrong schematic, that's class C correction: class B.

Edit: you can tell by the transistor's bases. They are tied together. Class AB has circuitry to provide about 1.2v between the bases so that they are both on the verge of turning on to limit crossover distortion.

Edit: class C would be with the transistors in parallel turning on at the same time...

What schematic is that? That's no DX350 that I've ever seen.

This is a standard DX350 with 2 2SC2879 transistors.

 

[AudioShockwav]

What schematic is that? That's no DX350 that I've ever seen.

This is a standard DX350 with 2 2SC2879 transistors.

View attachment 68002

The schematic Brandon posted is a old model of Texas star that is no longer made, they used a driver in front of the two finals.

73
Jeff

 

[BC Coyote]

This is a standard DX350 with 2 2SC2879 transistors.

I believe the one I am getting is this arrangement. I will know early next week when it gets here.
At this point I am borrowing it with a possible option to buy later if it meets my high expectations....... so nothing to lose really !