ejp, I did see an ebay listing but you are right, nothing now. They where made only for the 303 correct?
BD140 is what I have readly available. The pin's are in a different order so it will be an awkward fit.
Mooly, what other transistors do you suggest me buying? The likes of BD140 are cheap so I can order a few.
BD140 is what I have readly available. The pin's are in a different order so it will be an awkward fit.
Mooly, what other transistors do you suggest me buying? The likes of BD140 are cheap so I can order a few.
If this were me I would try the BD139/140 pair. Always buy from authorised distributors though. The pin out is no problem, just carefully preform the leads to fit directly. Do not bend them at the package but use pliers and bend only the leads away from where they enter the package. Fit a couple of T0126 heatsinks. Heatsinks on these are not needed for testing as they run cool at anything other than really high output levels.
Are we sure both outputs are OK? I've seen 2N3055 mentioned for those. 2N3773 was another I though of.
Are we sure both outputs are OK? I've seen 2N3055 mentioned for those. 2N3773 was another I though of.
They were selected for the 303, as were the power transistors (that’s why going back to the base part 2N3055) is a backward step).
When you say "BD139/140 pair" you mean replacing both output transistors right?
If it means more stability no problem also replacing Tr106 even if it's good.
I will order the parts
If it means more stability no problem also replacing Tr106 even if it's good.
I will order the parts
A meant BD139/140 for the drivers TR105 and TR106. Ideally they should be a complementary pair.
If the output transistors are OK then retain them but if not you have to look for alternatives. The 2N3055's manufactured today are very different in the details compared to the original 1960's types so it really is all an unknown. In most designs it's just not an issue but the 'triple' configuration used by Quad partly relies on the 'slow' speeds of the original devices for stability of the design as a whole.
If the output transistors are OK then retain them but if not you have to look for alternatives. The 2N3055's manufactured today are very different in the details compared to the original 1960's types so it really is all an unknown. In most designs it's just not an issue but the 'triple' configuration used by Quad partly relies on the 'slow' speeds of the original devices for stability of the design as a whole.
I just order two pairs of BD139/140 with heatsinks 😉
2N3055 - these are the 5 big boys in the back right? I will try to check them. Let's pray they are ok!
What could happen with an unstable design?
2N3055 - these are the 5 big boys in the back right? I will try to check them. Let's pray they are ok!
What could happen with an unstable design?
The early 2N3055's from eg RCA, were hometaxial, with an 800kHz fT. Modern 2N3055's are epitaxial with an fT of 2.5MHz minimum. Spooning in a modern 2N3055 into a 303 might well result in instability.
The other thing to watch is that manufacturers are progressively obsoleting TO3 metal canned power transistors for plastic encapsulated.
The other thing to watch is that manufacturers are progressively obsoleting TO3 metal canned power transistors for plastic encapsulated.
It can behave as an oscillator running at many hundreds of kHz.
This is a simulation of the 303 pushed into instability. The output is supposed to be a 1kHz sine at 2.83vrms (1watt/8ohm loading).
And it should be like this:
The power transistors usually fail short circuit C to E. If the 67v reg is correct then that one is OK.
If you have to change 303 power transistors, use MJ15003 for the audio channels and MJ802 for the regulator.
Assuming the same pin layout of the 2N3055 and measuring with them in place I got the same reading from the four 38494:
B-E 70mV
B-C 550mV
C-E 570mV
Seems they are all ok?
We don't want none of that nasty oscillations 😆
B-E 70mV
B-C 550mV
C-E 570mV
Seems they are all ok?
We don't want none of that nasty oscillations 😆
They likely are OK based on those readings. The '70mv' is actually way to low but probably because they are in circuit.
You got b/e of 70mV because of the 68R resistors. That doesn’t measure the transistor at all. You need to disconnect for that. However the important thing is no c/e shorts, so they look OK.
The problem with even modern 2N3055s is their Vceo of 70V, which isn’t adequate under the fairly common fault condition of Tr3 s/c, and they start to leak around 60V, which is even less adequate.
The problem with even modern 2N3055s is their Vceo of 70V, which isn’t adequate under the fairly common fault condition of Tr3 s/c, and they start to leak around 60V, which is even less adequate.
Let's hope we'll not be needing the 2N3055s or any other type 😉
So... I just recieved the new transistor pairs! Excited to try this out.
Just to make sure I don't screw this up:
Does this look right?
I will remove Tr106 and replace both Tr105 and Tr106 with BD140 and BD139.
I will then power the amp through the DBT and see if things look good. Will report after 😉
So... I just recieved the new transistor pairs! Excited to try this out.
Just to make sure I don't screw this up:
Does this look right?
I will remove Tr106 and replace both Tr105 and Tr106 with BD140 and BD139.
I will then power the amp through the DBT and see if things look good. Will report after 😉
I don't like working off pictures 🙂 but that does seem correct. Make sure you form the leads correctly to get the different pin out to fit neatly.
I love the form factor of this amp and use one casually here and there for e.g powering some dipole woofers. But you can see it's a really dated design if you test against a modern transistor amp. But what's the draw here? You can buy something for very little money that will out perform it.
It was passed down to me by my grandfather, it has great sentimental value. I hope to keep it running for many years to come, together with the JBL4311 and the Thorens TD145 🙂
Now to the big update!
I changed both transistors and I'm seeing 67V across 1-9, 33.5V across 5-9 and I'm now able to adjust the quiescent to 5mV between 4-6 😀
Everything went smoothly apart from a strange behavior in the beginning. After turning it on for the first time the DBT flashed and went silent. Then after about 20 seconds the light started pulsating with a flash followed by about 3s of no light. After messing with RV101 for a bit it stopped so I guess that's the culprit. As someone suggested earlier I probably should change the pots, multiturn ones if I recall.
That all sounds very promising 🙂
I wouldn't necessarily use multi turn pots tbh as there should need for that.
Just a small detail regarding bias current... looking at the circuit and the recommended bias current is 5 to 10ma. The current in the 0.3 ohm resistors is actually the sum of the currents in the output transistor plus the driver/s.
The current in the 100 ohm resistor is so low as to be discounted, the current in the 10 ohm can be either approximated (good enough) or calculated exactly by measuring the voltage across it. Approximation is fine here though.
The upper NPN output needs around 0.5 volt across base and emitter to just begin to conduct a little and so we can say that I=V/R =0.5/10 = 5 milliamps is flowing. When you measure the voltage across the 0.3 ohm/s you are measuring 5 milliamps PLUS the wanted bias current in the output transistor.
So lets say we want 8 milliamp bias current. That means we need 13ma flowing in each 0.3 ohm or around 8 millivolts across the pair of them.
If you replace the preset be sure to start with it on maximum resistance as before. It might be worth you giving it a clean and whizzing it to and fro a few times and seeing if that fixes the odd start up behaviour you noted.
I wouldn't necessarily use multi turn pots tbh as there should need for that.
Just a small detail regarding bias current... looking at the circuit and the recommended bias current is 5 to 10ma. The current in the 0.3 ohm resistors is actually the sum of the currents in the output transistor plus the driver/s.
The current in the 100 ohm resistor is so low as to be discounted, the current in the 10 ohm can be either approximated (good enough) or calculated exactly by measuring the voltage across it. Approximation is fine here though.
The upper NPN output needs around 0.5 volt across base and emitter to just begin to conduct a little and so we can say that I=V/R =0.5/10 = 5 milliamps is flowing. When you measure the voltage across the 0.3 ohm/s you are measuring 5 milliamps PLUS the wanted bias current in the output transistor.
So lets say we want 8 milliamp bias current. That means we need 13ma flowing in each 0.3 ohm or around 8 millivolts across the pair of them.
If you replace the preset be sure to start with it on maximum resistance as before. It might be worth you giving it a clean and whizzing it to and fro a few times and seeing if that fixes the odd start up behaviour you noted.
Yeah that is what I did and it seems to have solved the problem. I will not be replacing it for now.
If I understood you correctly I should I be going for 8mV between 4-6 right? That way we set the bias to 8mA.
I will be connecting the other channel and fine tuning the pots to give me precise values after warmup.
What should be the next step? I'm tempted to try it out with the Quad33 (my next project) and some speakers but something tells me that is not very wise 😆
That sounds about right. It works like this. The 8ma is the wanted bias current. The 7ma flows from the driver into the 0.3 ohm and so does the 8ma. That gives 15ma flowing in the 0.3 ohm which gives a total value of 9mv across the combined value of 0.6 ohm. Its not exact because we estimate the 0.5 volt and we also have a small base current into the 2N3055 but it gets us close.
The official way is to break the connection to the collector and measure the current but that can be risky as its easy to slip or make a bad connection. Another way would be to break the connection and add a 1 ohm in series and then set the voltage across the 1 ohm for 8 millivolts. That would be a true reading of the current.
The official way is to break the connection to the collector and measure the current but that can be risky as its easy to slip or make a bad connection. Another way would be to break the connection and add a 1 ohm in series and then set the voltage across the 1 ohm for 8 millivolts. That would be a true reading of the current.
The 303 is speaker safe because it is AC coupled so no problem trying it for real. If you do then only use low volume with the bulb in place. If it seems OK then link the bulb out.
Remember to check the 67 volts is still correct on full mains... it should not change much but check.