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Phase Linear 400 driver transistor substitution

premiumplus

premiumplus

Vintage Junkie
Hi guys,
Any Phase Linear experts here? I have a 400 amp that has a bad output driver transistor. I know that it is a bad thing to substitute output transistors in these old amps, but can you tell me if it would be okay to get a functional substitute for the driver transistor? It's a Motorola SJ2741 or an RCA410. I probably have a transistor in my stock that would meet the basic ratings, but can you tell me if it could cause instability like mixing the output transistors will?
Thanks!
 
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premiumplus said:
Hi guys,
Any Phase Linear experts here? I have a 400 amp that has a bad output driver transistor. I know that it is a bad thing to substitute output transistors in these old amps, but can you tell me if it would be okay to get a functional substitute for the driver transistor? It's a Motorola SJ2741 or an RCA410. I probably have a transistor in my stock that would meet the basic ratings, but can you tell me if it could cause instability like mixing the output transistors will?
Thanks!
Click to expand...

You need Lee !"Mr. PhaseLinear" ! Laatsch55...:thmbsp:

http://www.audiokarma.org/forums/member.php?u=33651
 
You can mix drivers, but not outputs. RCA 410's are the better sounding drivers. Even with MJ 21196's in the 410's are still the best. I have seen 410, SJ 2741's, MJ 1800's , MJ15024's, as drivers. When you start to get oscillations it's usually running a faster driver than outputt. I do not recommend a 2SD555 driving an MJ 15024. That is a terrible combination....
 
I've invested quite a bit of time reworking these 400's, and I want to get at least one of them to the next level, so to speak. I've got original PL909 outputs in both of my amps. Can't remember which was the bad lot (green script or black?) but the ones I have are the "good ones." I have replaced all the electrolytics (including the large power supply filters) and checked resistors for tolerance, etc. The one on my bench right now has SJ 2741's for drivers.
They sound good, but not like I would like. One amp has the PL-14 and the other has the PL-400 driver board. Would changing all the drivers to RCA 410's improve the sound? Are they available anywhere?
One last thing...I just bought some Dahlquist DQ-10's :banana: that I want to drive with the 400. They sound awesome with my rebuilt SX-1250...would the PL-400 be a good match for the DQ-10's?
 
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If the 10's are 4 ohm and you plan to push them hard,, a fan would be wise. Running MJ21196's would be wise if you are going to push also.The 93's,94's, 95's and 96's are dedicated audio p[arts the xpl 909's , MJ 15024's are GP and quite a bit noiser.
If you are going for the ultimate in sound a White Oak Rev "D" driver board along with the niw backplane boards in fully comp mode is an awesome upgrade-----check out this upgrade http://forums.phxaudiotape.com/showthread.php/4038-Russ-Phase-Linear-400-Ser-I-TO-BE-WOPL-d
 
If you keep the original boards run a 470uf/25 volt audio grade cap for C6. On the foil side of the board run 2 1N4148's back to back accross the leads of C6, that will keep the amp from latching up to the positive rail...

The 470uf cap will increase the low frequency response tremendously..
 
Thanks! Since I last posted, I changed all the drivers to ON Semiconductor MJ15022G. It looks to be a good substitute, and I ran a frequency sweep on the amp, outputs unloaded, and saw no oscillations. I don't have a resistive load big enough, power wise, to run a loaded test, but it's been running some old Klipsch KG-2's for a couple of hours and sounds good.
I'll change those caps to 470 and put the diodes across as advised, and post back.
Thanks so much...another thing I want to do before I hook it up to my Dahlquists is get a DC protection board from d3imlay. I understand those are the best.
 
The 15022's don't have the SOA that the 96's do. But with a 400 you're not gving up that much. In fact in the 400's running Quasi I'll run the MJ21194's, in fully comp mode O'll run the 94's with their complement the MJ21193's.
The DCP's on Don's are all I use. They require no drilling, mount directly to the speaker binding posts and have the Phoenix connectors to make hookup a snap.
So if you're capable of running a frequency sweep, then you should be able to graph a frequency response?? If you can, run one prior to changing out the C6 cap and one AFTER, you'll be amazed at the inmprovement. At 10Hz it's only down .1DB. Stock is 5-10DB down at 10 HZ. That's a pretty flat response at 10HZ. One tenth db is world class. Joe has also got residual noise down to 110 MICRO volts UNWEIGHTED and Signal to Noise is now 110DB unweighted and 122 IEC A weighted. That is also world class. White Oak has done some wonderful things for the old PL's.
 
The bass is better indeed. I changed the C6 caps and put the diodes in parallel with each other, back to back, across the caps. Fired it up on the DBT and the bulb stayed lit, and I got a fairly loud hum from the speakers. Powered down and double checked my work, AOK. Disconnected the speakers, and it came right up. Checked the speaker dc resistance, and reconnected them with the amp still on the DBT; no problem. So I powered it down again, waited a couple of minutes and turned it on again. Once more the DBT stayed bright (100 watt bulb) and the speakers hummed. I unplugged the speakers and the bulb immediately dimmed. Plugged them back in and it played fine. The bulb stayed dark except on musical peaks. The speakers are around 4 ohms dcr, and I suspect that the protection circuit in the amp was activated when the amp was powering up on the DBT and didn't have enough juice to stabilize.
Have you ever seen this happen?
At any rate, the amp sounds great now and I'm listening to some Joe Cocker as I type. It's biased at 385 mV. Now to order the DC protection boards and get them installed!
 
Sounds like you had the anode running with the other anode. That's a textbook description of the amp latching up. The 1N4148's need to be opposite anode and cathode. So on one lead of C6 you should nave 1 anode lead and 1 cathode lead, the other lead of C6 should be the opposite. When you powered down it bled all energy from the caps, and that's when the latchup occurs. Were you monitoring the speaker outs for DC when you were powering up?? The protection circuit becomes effectiive around 50-60 volts. Do you have a DCP mounted up already??
 
Re-read your post. No DCP yet. Can you take a pic of the foil side of the drivr board?? Is this the amp with the PL400C board or the 14 A?B??
 
No DCP yet, I am just ordering it. I installed the diodes just as described. I checked them with my meter after the latchup, and they both show around .5 volts drop. After all that, I took it off the DBT, and ran it for a good three hours without problems. It doesn't do this unless it's on the dim bulb tester. Strange.
 
OK, when you load the amp with the speakers , then limit current with the DBT it's going to try and draw more amperage to get the same power. The first time you fire it up after working on it should be UNLOADED. If everything checks out you can then take the DBT out of circuit. Did the current sharing test go well??
 
Yeah, everything checks out fine...no current hogging, and it sounds great. Bias is rock solid. It seems to like the new driver transistors fine, and they sound better than the old Motorolas did.
BTW, in the picture, your avatar...is that an old Kraft Gold Medal transmitter that you're flying with?
 
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That's been my experience with em too. If you do the 400C board C6 470uf cap swap, on that board it will be C8 that you change and leave the diodes off, the 400C board is not known for latching up.
 
No, when I was flying RC I had a Heathkit 5 channel radio. In that pic I'm at my favorite fishing hole with my 9 ft, 7weight , Powell Freestone flyrod.

I also had a Kraft KP-5C radio.
 
I can't measure loaded clipping; I don't have a suitable load. I did check it for unloaded clipping. It's symmetrical...I don't remember what the voltage was, but it seems to me that it was up around 180-200 vpp, does that make sense? It was way up there.
After it ran a couple of hours I ran the amp up to where my soundl level meter was reading 110dB on a pair of old Klipsch KG4's I have in the basement. It was loud and proud.
Had a friend that flew Heathkit. It was a nice radio...this was back in the late 60's. I used to work for K&B manufacturing when they were in Downey, Ca. I fixed all the engines that everybody stuffed into the ground!
 
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