PL700B's...White Oak or Pine Box
- Thread starter Peter S
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- Halfbiass...Electron Herder and Backass Woof
- Joined
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- 74,244
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
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- 74,244
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- Halfbiass...Electron Herder and Backass Woof
6-1.4. Current sharing test. This test is necessary to verify that ALL outputs are operating properly, ensuring a permanent repair. Although the amplifier will probably meet all specs without all outputs operational, increased current loading of the remaining outputs will result in a significant reduction in reliability.
a) With an 8 ohm {dummy} load connected to the output terminals of the left channel, drive the left input with a 200 Hz signal to obtain 53 V RMS output. {using a Variac} Turn the line voltage down to 75 VAC. {prior to this step drop the amplifier Faceplate and Control Board forward onto a soft cloth to gain access the the back wall of the amplifier}
b) Using {a} DC voltmeter, measure the voltage drop across the {.33 Ohm} emitter resistors in each {left channel} output bank. Verify that there is between 150 mV and 180 mV DC across each {emitter} resistor.
c) Replace any output transistor whose emitter resistor reading varies more than 20% from the typical; verify first that the resistor itself is not damaged. {Repeat this test for the right channel}
From the PL700 Series II Service Manual (text in brackets was added by the technician)
a) With an 8 ohm {dummy} load connected to the output terminals of the left channel, drive the left input with a 200 Hz signal to obtain 53 V RMS output. {using a Variac} Turn the line voltage down to 75 VAC. {prior to this step drop the amplifier Faceplate and Control Board forward onto a soft cloth to gain access the the back wall of the amplifier}
b) Using {a} DC voltmeter, measure the voltage drop across the {.33 Ohm} emitter resistors in each {left channel} output bank. Verify that there is between 150 mV and 180 mV DC across each {emitter} resistor.
c) Replace any output transistor whose emitter resistor reading varies more than 20% from the typical; verify first that the resistor itself is not damaged. {Repeat this test for the right channel}
From the PL700 Series II Service Manual (text in brackets was added by the technician)
Thanks, so while attempting to check the bias, there was a pop and a rail fuse blew without an input signal. Replaced fuse and powered up with no load. Left output was 70 Volts DC with DBT. Pulled left chan outputs and drivers; still 11 Volts on speaker terminal (no load or input connected). Tested (in circuit) transistors on PL20B, all good. Compared voltages Left to Right. But then, all reverted to normal...Put drivers and outputs back and the amp seems to behave normally, 430 Watts into the dummy load. Is this the first PL to ever heal itself? Has anyone had an intermittent issue like this?
Carrying on to current sharing, my variac does not have sufficient capacity so hoping the following would show some indication; Using full line voltage, a 200 Hz signal drove the amp to 40 VAC output into 8 ohms. The worst column of output emitter resistors measured from 136 mV to 156 mV. I replaced the transistor with the lowest reading and now that Re measures 143 mV.
Beginning to have doubts about this bridging project! At the very least, temperature controlled fans would be a must. Are the emitter resistors capable of handling full output into a 4 ohm load? I recall the White Oak backplanes using higher precision E resistors which I would imagine would insure better current sharing but are they designed for a 4 ohm load?
Carrying on to current sharing, my variac does not have sufficient capacity so hoping the following would show some indication; Using full line voltage, a 200 Hz signal drove the amp to 40 VAC output into 8 ohms. The worst column of output emitter resistors measured from 136 mV to 156 mV. I replaced the transistor with the lowest reading and now that Re measures 143 mV.
Beginning to have doubts about this bridging project! At the very least, temperature controlled fans would be a must. Are the emitter resistors capable of handling full output into a 4 ohm load? I recall the White Oak backplanes using higher precision E resistors which I would imagine would insure better current sharing but are they designed for a 4 ohm load?
“Beginning to have doubts about this bridging project! At the very least, temperature controlled fans would be a must. Are the emitter resistors capable of handling full output into a 4 ohm load?”
You should build an amp with all new components that are designed to be functional. Using all that old junk like carbon emitter resistors and old drivers probably will not live.
You should build an amp with all new components that are designed to be functional. Using all that old junk like carbon emitter resistors and old drivers probably will not live.
- Joined
- Jan 14, 2011
- Messages
- 74,244
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 74,244
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
I've had similiar experiences after replacing electro's on stock boards. It's not a rare occurance..
https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/
https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/
I guess that means it could happen again at anytime. I found Joe's explanation;
https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/
Will the component numbers correspond to the PL20B control board?, thanks again.
https://forums.phxaudiotape.com/thr...-up-tendency-in-phase-linear-amplifiers.2307/
Will the component numbers correspond to the PL20B control board?, thanks again.
Still weighing pros and cons of bridging;
Here is an interesting excerpt from "Designing Audio Power Amplifiers" by Bob Cordell regarding power supply loading in bridged amplifiers.
Cooling fans will be a necessity of course, but 730 watts into 4 ohms (Phase Linear's spec) is 13.5 amps. How can the amplifier deliver this power when the maximum allowable ( PL manual ) fuse size is 8 amps? Is this because the rail fuses are only conducting for half of each cycle?
Is 4 ohm operation ( bridging with an 8 ohm load) threatening the SOA of MJ21196's? Attached is SOA curve(s) for an MJ15024 from D. Self's book "audio power amplifier design"
Here is an interesting excerpt from "Designing Audio Power Amplifiers" by Bob Cordell regarding power supply loading in bridged amplifiers.
Cooling fans will be a necessity of course, but 730 watts into 4 ohms (Phase Linear's spec) is 13.5 amps. How can the amplifier deliver this power when the maximum allowable ( PL manual ) fuse size is 8 amps? Is this because the rail fuses are only conducting for half of each cycle?
Is 4 ohm operation ( bridging with an 8 ohm load) threatening the SOA of MJ21196's? Attached is SOA curve(s) for an MJ15024 from D. Self's book "audio power amplifier design"
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