Phase Linear Model 700

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#101
anatech said:
Hi Pete,
I used to stock 2SD424 and 2SB554 as well as 2SD555 and 2SB600. Wonderful output devices. I can't say that I had any trouble with oscillation with these.

Of course I also stocked the jedec numbers, or ordered them as required.

I would suspect that the layout may have something to do with devices oscillating. Yes, the 2Sx series seemed to be faster with higher gain.

-Chris
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Not sure I'm following, did you try them in a PL700, and not see oscillations? If Phase Linear was right about only using 2SD555s it points to the MJL4281 as the right part. On the other hand, it's well documented that the PL700 output deviced were selected for LOW beta, why would they suggest a high beta replacement part? On the other hand the MJL4281 SOA is only 1A, 100V, 1 sec, whereas the 21194 is 2A, 100V, 1 sec. 21194s win from an SOA perspective and I believe it's worth figuring out if a mod is needed to make them stable.
 
#103
Hi Pete,
I went with manufacturer's recommendations where ever possible. I'll bet that I used MJ15024 / 5 in an emergency for a road amp without too many troubles. I would not generally use a Japanese part for a US one or vice versa.

I am so use to seeing amps oscillate that it would not be memorable if I had to modifiy the feedback cap or bandwidth limiting caps in some way. I would refer to a manufacturer's in print changes for other models if need be. Even C-B caps could be used.

-Chris
 
#104
Last year I repaired a 700II with the original 2SD555's in it. It had burnt the output R/C network, specifically the cap in both channels. It was oscillating. I tried the usual fix of 150pf on the bc of the predrivers to no avail. Decoupling caps, and all of the tricks I know didn't work. I switched to MJ2119X and all was well. Question of the day: Did it leave the factory like that? Who knows.
 
#107
Hi Hugo,
You are right, but they are supposed to be an approved substitute. See the Phase Linear service bulleton 1-80 concerning output transistors.

That amplifier may have been serviced at an autorized warranty center or other good shop. Bad shops generally would not use 2SD555's due to cost.

-Chris
 
#108
anatech said:
Hi Hugo,
You are right, but they are supposed to be an approved substitute. See the Phase Linear service bulleton 1-80 concerning output transistors.

That amplifier may have been serviced at an autorized warranty center or other good shop. Bad shops generally would not use 2SD555's due to cost.

-Chris
Click to expand...


Hi Chris,

As I read it the faster 2SD555 were only for the older PL700s, and the slower MJs were for the newer series II. So that would be the wrong part in that amp.

Pete B.
 
#114
I found some time to finish modeling the 700B.
Picture is the best I can produce, Transistors are what was reasonably available in Microcap except for the Ge's Q8 and Q9 which are standard models.
I can pass the model text file to anyone interested.

/Hugo
 

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#118
Gain:
With a 1Khz sine wave of 6.44Vpp at the input the output slightly clips at 189.58Vpp into 8ohm.
If my calculation is correct: VRMS = Vpp * 0.3535, the ouput power would be 561 WRMS.

/Hugo
 

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#119
sixtek,
In this simulation, I have omitted the PSU caps as this is supposed to be battery operated.
In real life, PL only use two big capacitors, not bypassed.

I can post any results you want, given that I know how to perform them in Spice.

/Hugo
 
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