Bias Adjustment on the White Oak Audio Design PL14_20 Board
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
The nose was pungent with a hint of Marlboro and without any sweet citrus overtones. The bouquet was tart like an early disco fever but finished with a boot scoot boogie on the palate. Color was dark and unattractive with cat hair and road dust mixed into the unfiltered container. The overall impression was skunky due to excessive aging and the audacious presentation, I would have to give it no stars, and a recommendation to not buy.
The nose was pungent with a hint of Marlboro and without any sweet citrus overtones. The bouquet was tart like an early disco fever but finished with a boot scoot boogie on the palate. Color was dark and unattractive with cat hair and road dust mixed into the unfiltered container. The overall impression was skunky due to excessive aging and the audacious presentation, I would have to give it no stars, and a recommendation to not buy.
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
Some tips on bias setting and warm up characteristics. This is in response to a question I received about bias current behavior after power on and warm up.
Normally on power up whatever bias setting is set by the bias pots usually starts out on the low side, then increases and eventually stabilizes and then settles back down a bit. Q6 is basically a heat sensor and compensator and takes a while to perform its intended job due to the thermal mass of the amplifier heat sink structure. The bias current in the output predrivers (RCA410) causes these TO-3 devices to warm up slightly reducing their Vbe and in turn, increasing the bias current flowing. Eventually the heat from these transistors heats up the aluminum of the heat sink which conducts into the Q6 bias transistors. Once these Q6 transistors start to heat up to a similar temperature to the output drivers their Vbe also reduces stabilizing the rising bias current through the output devices. It is a feedback-control mechanism. This process takes several to 5 minutes to regulate and stabilize. As long as you see the bias current plateau and come into control, you should be OK.
I recommend you start with the bias pots turned fully counter-clockwise until you get the bias settings right. Once the warm up process completes, then adjust to the final value between 0.300-0.400V Even with 0.500 bias voltage, the bias current is only ~50mA and the output stage is only dissipating about 8W per channel or 4W per transistor device on a PL400 (more on a PL700B). This gives you a little perspective.
Some tips on bias setting and warm up characteristics. This is in response to a question I received about bias current behavior after power on and warm up.
Normally on power up whatever bias setting is set by the bias pots usually starts out on the low side, then increases and eventually stabilizes and then settles back down a bit. Q6 is basically a heat sensor and compensator and takes a while to perform its intended job due to the thermal mass of the amplifier heat sink structure. The bias current in the output predrivers (RCA410) causes these TO-3 devices to warm up slightly reducing their Vbe and in turn, increasing the bias current flowing. Eventually the heat from these transistors heats up the aluminum of the heat sink which conducts into the Q6 bias transistors. Once these Q6 transistors start to heat up to a similar temperature to the output drivers their Vbe also reduces stabilizing the rising bias current through the output devices. It is a feedback-control mechanism. This process takes several to 5 minutes to regulate and stabilize. As long as you see the bias current plateau and come into control, you should be OK.
I recommend you start with the bias pots turned fully counter-clockwise until you get the bias settings right. Once the warm up process completes, then adjust to the final value between 0.300-0.400V Even with 0.500 bias voltage, the bias current is only ~50mA and the output stage is only dissipating about 8W per channel or 4W per transistor device on a PL400 (more on a PL700B). This gives you a little perspective.
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
Only the predrivers (RCA410) are on by the bias setting Lee. The predrivers are more than enough to drive the sub watt output that occurs in the crossover region. The main outputs pick up the load seamlessly in both directions on any excursion that drives more than about 0.5 Vp-p at the output (about 0.1W). The predrivers drive the small signal load through the 10 ohm bias resistors. With a bias setting of 0.35V the main drivers are approximately 0.25V short of coming on.
Only the predrivers (RCA410) are on by the bias setting Lee. The predrivers are more than enough to drive the sub watt output that occurs in the crossover region. The main outputs pick up the load seamlessly in both directions on any excursion that drives more than about 0.5 Vp-p at the output (about 0.1W). The predrivers drive the small signal load through the 10 ohm bias resistors. With a bias setting of 0.35V the main drivers are approximately 0.25V short of coming on.
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
I had some time today so I changed the Q6 bias transistor to a new 2N3403 to see if it would make a difference on the Left channel. It actually made it worse. The bias on the Left channel is 620 mV now. I wanted to see if I had a bad Q6 transistor and I think all of the 2N3403s I have are tired. I will probably just replace the Q6 on both channels with the 2N5088s that Lee sent me. I understand the 2N5088 pinout is different than the 2N3403. It that documented anywhere on this thread?
I had some time today so I changed the Q6 bias transistor to a new 2N3403 to see if it would make a difference on the Left channel. It actually made it worse. The bias on the Left channel is 620 mV now. I wanted to see if I had a bad Q6 transistor and I think all of the 2N3403s I have are tired. I will probably just replace the Q6 on both channels with the 2N5088s that Lee sent me. I understand the 2N5088 pinout is different than the 2N3403. It that documented anywhere on this thread?
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- Joined
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
So I tested all the 2N4303s I had and just as Joe has stated before, the gain is low. It was also all over the place. They ranged in hFE from 208 t0 361 (T=18). The one I pulled out today measured 437, so it was likely replaced by a device with lower gain (arrrrggg).
A sample test of the 2N5088s that Lee sent me (T=10) gave hFE readings of 550 +/- 5. Much higher and consistent forward current gain.
I will be replacing the 2N3403 (ECB connected to Brown Red Black) with 2N5088 (EBC connected to Brown Black Red).
So I tested all the 2N4303s I had and just as Joe has stated before, the gain is low. It was also all over the place. They ranged in hFE from 208 t0 361 (T=18). The one I pulled out today measured 437, so it was likely replaced by a device with lower gain (arrrrggg).
A sample test of the 2N5088s that Lee sent me (T=10) gave hFE readings of 550 +/- 5. Much higher and consistent forward current gain.
I will be replacing the 2N3403 (ECB connected to Brown Red Black) with 2N5088 (EBC connected to Brown Black Red).
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
You have some suspicious looking 2N3403s there Mark. AFAIK the 2N3403 was always a top hat design so you would see the flange hanging out the bottom of the metal strap. I only see that in a couple of your pics (the ones with the marking on the strap and the low gain
).
You have some suspicious looking 2N3403s there Mark. AFAIK the 2N3403 was always a top hat design so you would see the flange hanging out the bottom of the metal strap. I only see that in a couple of your pics (the ones with the marking on the strap and the low gain
).
Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
Joe, Those oddballs have 2N3403 stamped on the front of the band in tiny print. I don't know where Lee got them from, maybe they are a Chinese copy. They also have a mechanical crimp and a healthy dose of glue on the top. I may tear one apart and see if the transistor is labeled.
Joe, Those oddballs have 2N3403 stamped on the front of the band in tiny print. I don't know where Lee got them from, maybe they are a Chinese copy. They also have a mechanical crimp and a healthy dose of glue on the top. I may tear one apart and see if the transistor is labeled.
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Re: Bias Adjustment on the White Oak Audio Design PL14_20 Bo
The 2N3403s were $5.00 more than they were worth? Anyway, my bias on the PL 700B is fixed - 350 mV on both sides after idling for over 4 hours. Turn on bias readings are about 362 mV and then it climbs to about 370 mV and then settles down to 350 +/- 1 mV warm. This is with the MJ15024 drivers. The RCA 410s were not running that much difference between turn on and warmed up, but they drifted a bit more too.
I should have pointed out in the picture that the two devices on the bottom were 2N5088s that I slipped into the heatsink straps. Those are the ones I installed in the amp. But you probably figured that out with the higher gain readings.
The 2N3403s were $5.00 more than they were worth? Anyway, my bias on the PL 700B is fixed - 350 mV on both sides after idling for over 4 hours. Turn on bias readings are about 362 mV and then it climbs to about 370 mV and then settles down to 350 +/- 1 mV warm. This is with the MJ15024 drivers. The RCA 410s were not running that much difference between turn on and warmed up, but they drifted a bit more too.
I should have pointed out in the picture that the two devices on the bottom were 2N5088s that I slipped into the heatsink straps. Those are the ones I installed in the amp. But you probably figured that out with the higher gain readings.
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