Amp blows resistor between output transformers.

[AudioShockwav]

There's a Huge Difference between MRF453's and the SD1446's

I disagree with that, the 453 and the 455 are identical, PO, gain, input/output impedance same spec`s the only difference is the case size, go look at the data sheets.
And the 1446 is a common replacement for the 455.
Aside from minor differences, and the fact that the 453`s have slightly higher gain, they are designed to work @ 30 Mhz were the 1446 is actually made to work @ 50Mhz they are often used in place of 455/1446/sd1074 devices.

I've seen shorted caps, open resistors, bad bias circuit components or high bias current... You gotta use your print and check everything

I agree, the amp died the first time from excessive heat and may have taken other things with it.
These basic design of these CB style amps gives lots of room in replacement devices, and I have used 1446/455/453/sd1074 in several amps with out having them go bang .
Here is a perfect example:
ZETAGI B300P - TRANSISTOR REPLACEMENT -MRF455. - YouTube

If you are trying to optimize the amp for maximum power output, yes you can spend some time and improve the input/output match for a few more watts.
But the are accepted replacements.

73
Jeff

73
Jeff

 

[mackmobile43]

Where's my B-550 bob

 

[mackmobile43]

Poor Fella!.. Your combiner balancing should always use a 100 ohm/ 2 watt flameproof resistor... It "smoked" because the amp blew-up, again... Something may be wrong with the amp besides the outputs and now the resistor... I've seen shorted caps, open resistors, bad bias circuit components or high bias current... You gotta use your print and check everything!..The common misperception among amp users is that when the devices "pop", you can just replace 'em with similar devices that fit the hole... I rebuilt an old, ugly Varmint mobibile amp from the 70's, I believe it was the first one by four ever commercially made... The ancient low gain devices were no longer available, and I had a new quad of 1446's laying around, so I used them...Simple right ?.. Four Hours Later(sigh!) I discovered I had to retune the transformers, rebuild the biasing circuitry, and I had to strap across the driver, they were too drive sensitive!..There's a Huge Difference between MRF453's and the SD1446's...Even if you restore it with original parts, but can't determine the original problem, the results will be catastrophic failure, again... The best option you have is to find a tech that Knows His Amps to repair it, if you can't figure it out... I know you don't want to hear this, but I'd hate to see you blow up another Quad of Devices at $175 Bucks a Pop!.. These things aren't just light bulbs to change out... Sorry about that, brother... I see you've posted on the CBRT forum as well, I hope out of all these guys you get the help you ask for... Good Luck... CBSUPERTECH

I agree with bolded text especially for the input transformers to control input gain for different type devices.

Mr. fix you should look at pictures of amps using 2290's and 2879's as drive deviced for quad stage amps and count the wraps on those input transformers as the same thing goes for the quad stage amps when different devices are used.

 

[bob85]

Where's my B-550 bob

not had a broken 550 through here since you asked, had a few b750's for repair, i picked up a kl800 to play with..

 

[mr_fx]

Mr. fix you should look at pictures of amps using 2290's and 2879's as drive deviced for quad stage amps and count the wraps on those input transformers as the same thing goes for the quad stage amps when different devices are used.

that's a good idea. I'm glad you are back. A few days ago it said you were BANNED, I was like, damnit MAN. I need that guy's knowledge

 

[bob85]

i have used 455, 453, sd1446, srf3749 (mrf454) as drop in replacements,
in order of preferance,
srf3749 or mrf454's same thing different badge,
sd1446
mrf453
mrf455

when replacing transistors you should countersink/deburr the screw holes to make sure the transistors sit flat to the heatsink, virtually all rm and zetagi amps will have raised edges/distortion around the holes that needs fixing,

when using used transistors its important to make sure the tabs are not bent like mack describes,
often re-used devices are bent and won't touch the sink where they should even when you make the heatsink flat,

you can lap the tabs in with fine emery paper on plate glass.


use good thermal paste and apply VERY sparingly, you are not buttering bread.

I disagree with that, the 453 and the 455 are identical, PO, gain, input/output impedance same spec`s the only difference is the case size, go look at the data sheets.
And the 1446 is a common replacement for the 455.
Aside from minor differences, and the fact that the 453`s have slightly higher gain, they are designed to work @ 30 Mhz were the 1446 is actually made to work @ 50Mhz they are often used in place of 455/1446/sd1074 devices.

I agree, the amp died the first time from excessive heat and may have taken other things with it.
These basic design of these CB style amps gives lots of room in replacement devices, and I have used 1446/455/453/sd1074 in several amps with out having them go bang .
Here is a perfect example:
ZETAGI B300P - TRANSISTOR REPLACEMENT -MRF455. - YouTube

If you are trying to optimize the amp for maximum power output, yes you can spend some time and improve the input/output match for a few more watts.
But the are accepted replacements.

73
Jeff

73
Jeff