2 is the center (common) pin
(It seems to me):
1-2 = 0.68 E
2-3 = 0.68 E
1-3 = 1.36 E
E is used for Ohms here on resistors.
From the looks, it may be a fusible resistor.
Check before powering up.
(It seems to me):
1-2 = 0.68 E
2-3 = 0.68 E
1-3 = 1.36 E
E is used for Ohms here on resistors.
From the looks, it may be a fusible resistor.
Check before powering up.
His amp was already running - just concerned that the bias may be off in one channel because somebody previously put TIP output trannies in one channel.
Once you get the voltage across a single emitter resistor much above one or two millivolts gross crossover distortion will disappear. Optimal bias for an EF2 stage is theoretically 26 mV across the resistor, which is 38 mA. Most manufacturers compromise this to keep the heat down, as 10 to 20 mA is plenty for most amplifiers. With TO-220 outputs I’d certainly do this. Even TIP41’s have flat gain down at 10 mA (Old 2N3055’s, beta might be in the single digits at low current). I’d use the channel that had the Sanken outputs as a guide/sanity check to see where they set it. Run the TIP side there, or perhaps a bit higher. The 0.68 ohm is a fairly large value as emitter resistors go. Probably the reason they used it instead of the usual 0.22 or so is to keep heat down when running closer to optimum bias. And it provides more negative feedback in the thermal path, which is compromised somewhat by using TO-220 packaged transistors.
If you DID retrofit this with higher powered TO-3P’s I’d run it all the way up to 38 mA, since there would be no reason NOT to.
Once you get the voltage across a single emitter resistor much above one or two millivolts gross crossover distortion will disappear. Optimal bias for an EF2 stage is theoretically 26 mV across the resistor, which is 38 mA. Most manufacturers compromise this to keep the heat down, as 10 to 20 mA is plenty for most amplifiers. With TO-220 outputs I’d certainly do this. Even TIP41’s have flat gain down at 10 mA (Old 2N3055’s, beta might be in the single digits at low current). I’d use the channel that had the Sanken outputs as a guide/sanity check to see where they set it. Run the TIP side there, or perhaps a bit higher. The 0.68 ohm is a fairly large value as emitter resistors go. Probably the reason they used it instead of the usual 0.22 or so is to keep heat down when running closer to optimum bias. And it provides more negative feedback in the thermal path, which is compromised somewhat by using TO-220 packaged transistors.
If you DID retrofit this with higher powered TO-3P’s I’d run it all the way up to 38 mA, since there would be no reason NOT to.
An old favorite still available is:
https://www.onsemi.com/pdf/datasheet/bd809-d.pdf
A little slow but that may be a good thing depending on the circuit.
A larger package could be mounted on the other side of the heatsink, with added wires, or, double the transistors with an extra pair or emitter resistors.
Or, I added a set of 2n3773s after the existing OPs using 2 Ohm base resistors to a Sansui 4700 and it is now indestructible.
https://www.onsemi.com/pdf/datasheet/bd809-d.pdf
A little slow but that may be a good thing depending on the circuit.
A larger package could be mounted on the other side of the heatsink, with added wires, or, double the transistors with an extra pair or emitter resistors.
Or, I added a set of 2n3773s after the existing OPs using 2 Ohm base resistors to a Sansui 4700 and it is now indestructible.
I made the measurements (with the cold heatsink, the volume potentiometer at minimum) and I have the following values (I hope my multimeter is good):
DC offset -25mV and -27mV
emitter voltage:
Sanken channel: 16mV on each resistor> 23.53mA
Channel TIP: 1mV on each resistor> 1.47mA
How do you think I'd better? try 38mA on both channels, or with the Sanken setting?
Does it make sense to test at 100mA (for fun)? In your experience, will it bring significant improvements in sound, as the world speaks on forums? 🙂
Thanks again for your very accurate answers.
Your fun might blow up those Sankens, which are no longer made!
wg_ski posted above:
"If you DID retrofit this with higher powered TO-3P’s I’d run it all the way up to 38 mA, since there would be no reason NOT to."
That is a LOT less than 100 mA for a much more powerful device.
wg_ski posted above:
"If you DID retrofit this with higher powered TO-3P’s I’d run it all the way up to 38 mA, since there would be no reason NOT to."
That is a LOT less than 100 mA for a much more powerful device.
"And if you blow something up, don't tell anybody, just dispose of the dead body."
The Sanken setting is probably enough. The higher you run it the more prone to thermal run away it will be. Remember that Tj is higher than Tc, both are higher than Tsink, and that thermal compensation diodes can only sense Tsink. Crank the TIP side up to 23 mA, and you will likely get rid of any residual difference between channels. No need to go higher than 38 on either - the difference between 23 and 38 will be FAR less than the difference between 23 and 1.5. And going to 100 will be even smaller in addition to risky. At 100 mA, that’s 2.5 watts for a 25 volt rail. With 4 of them that’s 10 watts. 10 watts will make that aluminum radiator run HOT. More if your rails are any higher.
Higher power devices have a lower difference between Tj and the heat sink temperature. That’s why they are less prone to thermal runaway. And you can beat on them with 4 ohm loads.
Higher power devices have a lower difference between Tj and the heat sink temperature. That’s why they are less prone to thermal runaway. And you can beat on them with 4 ohm loads.
😀
Then all our effort would have gone to waste.
Philips were good quality units, audiophile grade they were not, but to the DIN 45500 standard, they certainly were.
Such an old set in good shape is rare, so do not destroy as an experiment.
Enjoy the music.
Then all our effort would have gone to waste.
Philips were good quality units, audiophile grade they were not, but to the DIN 45500 standard, they certainly were.
Such an old set in good shape is rare, so do not destroy as an experiment.
Enjoy the music.
More bias current isn't always better!
In fact, just from the distortion standpoing, too much bias curent, is just as bad as too little. (And it can burn out your transistors, too, as you have been hearing here.)
In this type of output stage (class B), there is a "hand off" between the two output devices, as the output signal goes through its zero-crossing. One device (usually the PNP one) handles negative half-cycles, the other device (usually the NPN one) handles positive half-cycles. But right in between there is a very small overlap, where both devices are working together, as the signal goes from positive to negative and vice versa, through the zero-crossing region.
The whole point of biasing the output transistors is to provide a smooth transition through this overlapping region, when both transistors are working together.
If the output devices have too little bias current, there is a "flat spot" in this area, and you get crossover distortion.
If the output devices have too much bias current, you get the opposite problem: instead of a flat spot at the crossover, you get a too-steep region. And this also creates crossover distortion!
So you do not gain anything by simply increasing bias current beyond the optimum value. Instead, you get more crossover distortion, along with a high risk of burning out and destroying the output transistors.
Electronics forums are a mixed blessing. Sometimes you get wonderful help from kind people who really know the subject, and that is wonderful. 🙂
But sometimes you get advice from people who really don't know the subject. And that can be a very bad thing, as it can cost you wasted time, money, and damaged equipment.
For a newcomer to the subject, it's very hard to tell which advice is good, and which advice is bad. That is the bad thing about trying to learn electronics from online forums - there's nobody ensuring the quality of the information. If possible, it's safer to learn electronics from an old textbook written by engineers, and proof-read by editors, and cross-checked by other engineers.
That said, you have been getting good information from several people in this thread.
But that other forum, where someone told you that more bias current is better? That person (or persons) is wrong, and their advice should NOT be followed. 🙁
-Gnobuddy