I've recently been testing a 14Mc power amp circuit, comparing several output transistors for 5 watts (45 Vpp) into a measured 50 ohm dummy load. I put a scope probe on the output and adjust for 45 Vpp with bench power? supply set to 12.8 volts. I've found that a good transistor will give 5 watts out at that frequency at 12.8 volts. My reference has been a genuine 2SC5739. I adjust the input power from the function generator to give 45 Vpp output with the reference, then substitute other transistors to see how they compare. For instance the 2SC2078 is a popular transistor used in many CB radios as the final transistor. I have several genuine 2078s as pulls from old CBs (all over 10 years old) found in thrift shops. Most of them have a blown output transistor, but the few that are good I save the transistors. A good 2078 will give 46 Vpp or sometimes a little more output compared to the reference 5739. If you look on eBay there are literally hundreds of sellers of the 2078--the majority of them from China. I found a US seller offering 10 of them for a reasonable price and bought them to test. Every single one was a fake counterfeit with 30-35 Vpp output at 14Mc! When I complained, I was given a full refund. Someone sent me two 2078s to test, bought from a reputable US seller that most of us have bought from. Both of these too were fake counterfeits with 30-35 Vpp output. My last purchase of 2078s was from another US seller on eBay--his ad shows testing them on a Peak meter--he tests them for beta (gain) at the low currents any of these test meters can only test at. I corresponded with the seller, saying such a test at such low currents didn't tell much, that a real test would be for output power. He responded that he could only test the way he was showing but that previous purchasers seemed to be happy with these 2078s. I bought 5 of them to test--they finally arrived today, after 12 days! He's in Las Vegas, only a day or two mail time from me in Arizona,? so for unknown reasons he appears to be very slow in shipping. I immediately tested all 5 and I'm very happy to report they all passed--3 gave 46 Vpp output, the other two 45.7 and 45.6 Vpp--exceeding the reference 5739 set for 45 Vpp. So these are the first Chinese 2SC2078s I can recommend, if you're looking for a decent RF power transistor. is the item and seller. Again note that you'll probably wait quite awhile to get these.

73,

Steve AA7U

Thanks Steve,? exactly what I need.? One question,? is the unlabeled capacitor to the left of T2 a .1uF?

?

Doug

On 04/29/2025 3:04 PM EDT Steve Ratzlaff via groups.io <ratzlaffsteve@...> wrote:

?

?

Here is the schematic of the PA transistor test fixture I'm using, and a

picture of it with a reference 2SC5739 installed.

?

It's easy and fast to simply bend the base and collector leads out at

right angles, solder the base and collector to the appropriate leads,

then solder the (unbent) emitter lead to the ground plane--and easy to

remove it to test another PA transistor.? I use a 12 watt QRP dummy

load, such as the one qrpguys.com sell, and mine measures 50.06 ohms, so

the formula (Vpp squared/400 = watts) is appropriate. 45 Vpp gives a

solid 5 watts output. I set the bench supply to 12.8 VDC for the test.

Note the initial power seen on the oscope is the peak value as the

transistor will immediately start to heat and power will decrease--only

key down as long as it takes to read the voltage on the oscope. My

oscope (Siglent DSO) shows the Vpp instantly as soon as I press the

"key" button switch, so I'm looking at the oscope to catch the initial

peak voltage. Once you've set the RF input to give the 45 Vpp output

with the reference transistor, remove the supply voltage, remove the

transistor and install another transistor to compare to the reference.

Of course don't change the RF input level from the setting used for the

reference transistor.

?

If you have not tried to purchase a QRP-power level RF PA transistor

recently, be aware that most of them on eBay and Amazon are fake

counterfeits and will not perform anywhere close to what a genuine

transistor of the same type will. That includes ones such as 2N3866,

2N5109, 2SC5739, 2SC2078, 2SC2166, 2SC1969--and it doesn't matter who

the seller is. All the eBay sellers from China, in my experience, their

transistors are all fake counterfeits.

?

73,

?

Steve AA7U

?

On 4/28/2025 5:11 PM, Steve Ratzlaff via groups.io wrote:

I've recently been testing a 14Mc power amp circuit, comparing several

output transistors for 5 watts (45 Vpp) into a measured 50 ohm dummy

load. I put a scope probe on the output and adjust for 45 Vpp with

bench power? supply set to 12.8 volts. I've found that a good

transistor will give 5 watts out at that frequency at 12.8 volts. My

reference has been a genuine 2SC5739. I adjust the input power from

the function generator to give 45 Vpp output with the reference, then

substitute other transistors to see how they compare. For instance the

2SC2078 is a popular transistor used in many CB radios as the final

transistor. I have several genuine 2078s as pulls from old CBs (all

over 10 years old) found in thrift shops. Most of them have a blown

output transistor, but the few that are good I save the transistors. A

good 2078 will give 46 Vpp or sometimes a little more output compared

to the reference 5739. If you look on eBay there are literally

hundreds of sellers of the 2078--the majority of them from China. I

found a US seller offering 10 of them for a reasonable price and

bought them to test. Every single one was a fake counterfeit with

30-35 Vpp output at 14Mc! When I complained, I was given a full

refund. Someone sent me two 2078s to test, bought from a reputable US

seller that most of us have bought from. Both of these too were fake

counterfeits with 30-35 Vpp output. My last purchase of 2078s was from

another US seller on eBay--his ad shows testing them on a Peak

meter--he tests them for beta (gain) at the low currents any of these

test meters can only test at. I corresponded with the seller, saying

such a test at such low currents didn't tell much, that a real test

would be for output power. He responded that he could only test the

way he was showing but that previous purchasers seemed to be happy

with these 2078s. I bought 5 of them to test--they finally arrived

today, after 12 days! He's in Las Vegas, only a day or two mail time

from me in Arizona,? so for unknown reasons he appears to be very slow

in shipping. I immediately tested all 5 and I'm very happy to report

they all passed--3 gave 46 Vpp output, the other two 45.7 and 45.6

Vpp--exceeding the reference 5739 set for 45 Vpp. So these are the

first Chinese 2SC2078s I can recommend, if you're looking for a decent

RF power transistor. is the item

and seller. Again note that you'll probably wait quite awhile to get

these.

73,

Steve AA7U

>

>

>

?

?

Steve,

?

Thanks for the link and schematic. I was going to order some but it looks like he is out of stock.

?

We can fall back to Dan's Small Parts.?

?

Mike N2MS?

On 04/29/2025 5:00 PM EDT Steve Ratzlaff via groups.io <ratzlaffsteve@...> wrote:


Hi Mike,
Yes, I see folks bought his remaining stock after I posted about it. But he emailed me today that he has 100 on order, but didn't say when they're expected. First person to notice his ad shows more, please let us all know. :)
Unfortunately, I cannot recommend buying 2SC2078's from Dan's Small Parts--the ones I recently tested are fake counterfeits with very low output.... This is the first time I've had bad parts from him.

73,
Steve AA7U



On 4/29/2025 1:45 PM, Mike N2MS via groups.io wrote:

Steve,

Thanks for the link and schematic. I was going to order some but it looks like he is out of stock.

We can fall back to Dan's Small Parts.

Mike N2MS

Steve,

Thanks for the update. Sorry to hear about Dan's Small Parts. Maybe his suppliers are drying up. Did you ever evaluate his RCA4013 transistors? I have a handful of them.

73 Mike N2MS

On 04/29/2025 8:50 PM EDT Steve Ratzlaff via groups.io <ratzlaffsteve@...> wrote:


Hi Mike,

No, I have not tried any of Dan's other RF power transistors, just the
2SC2078. If you wanted to send me one of your other ones I'd be happy to
check it out and return it to you.

73,

Steve AA7U

On 4/29/2025 5:36 PM, Mike N2MS via groups.io wrote:

Steve,

Thanks for the update. Sorry to hear about Dan's Small Parts. Maybe his suppliers are drying up. Did you ever evaluate his RCA4013 transistors? I have a handful of them.

73 Mike N2MS

Steve,

Thanks for the update. Sorry to hear about Dan's Small Parts. Maybe his suppliers are drying up. Did you ever evaluate is RCA4013 transistors? I have a handful of them.

73 Mike N2MS