What does this schematic symbol mean (BJT with hash lines)?

[glebert]

Have an Eden WT300C with distortion, seems to be a problem with the bias circuit. Looking at the schematics some of the BJT symbols have hash lines on the BC leg, can't say I've ever seen that before. I don't know if it means something about the transistor properties or something about collector current or something else. Maybe isn't necessary to know this, more curious than anything. This is obviously a hand drawn schematic and I don't know the conventions they might have used. Thanks in advance.

Greg

[Dave H]

maybe means should have heatsink?

I think you are right. Perhaps it means they are all on the same heatsink because the bias transistor also has lines?

[Enzo]

I have never seen that, but if I had to guess, I'd start with it means install a ferrite bead on that transistor leg. That fights instability and parasitics.

[nevetslab]

Actually, it indicates the three power transistors....TIP 31C TO-220 and the two 2SC3858 MT-200 power transistors are mounted to a heat sink.

[glebert]

Actually, it indicates the three power transistors....TIP 31C TO-220 and the two 2SC3858 MT-200 power transistors are mounted to a heat sink.

Yep, that has to be it. Also the MPS8099 in the bias circuit that is mounted in the large heatsink. Is that a form of temp compensation?

[g1]

Yes. The bias transistor thermally tracks the heatsink. It's current will alter slightly dependent on temp. and the bias will be adjusted to compensate. This is done to prevent what is referred to as 'thermal runaway' of the output devices.

[J M Fahey]

Never saw it before either but David Nordschow is known for following his own path and he might very well write his own conventions.
That said:
a) all hashed transistors are mounted on the heatsink, so .....
b) I would not be surprised at some "schematic note" written somewhere stating so.

Just curious: what are voltage rails there?
Intrigued at seeing a humble TIP31 as driver, probably a TIP32 on the bottom half.
Would have expected higher voltage ones such as MJ15030/31/32 and such although TIP transistors are robust and well made and actual samples often stand well beyond the datasheet generic value.
Might find a note stating something like: "use only Factory selected parts".

[glebert]

Juan, the rails are 72V, but the TIP transistors don't see that voltage as far as I can tell. I haven't dealt with this sort of output circuit before, but it seems like the TIP transistor collectors "float" 5.6V above/below the speaker output as set by the zener diodes. Or something. This is actually the problem I am dealing with as the positive side TIP collector is at about 0.7V and there is 200-300mV DC on the output no matter where the bias pot is set. Haven't deep dived into it yet.

[Enzo]

The bias adjustment is not there to set offset. DC offset is handled back at the diffy pair.

The rails are 72v? And when the output swings down to -70 with signal, what voltage is then across those transistors?

[glebert]

The bias adjustment is not there to set offset. DC offset is handled back at the diffy pair.

The rails are 72v? And when the output swings down to -70 with signal, what voltage is then across those transistors?

I"m assuming that is what the diodes referenced to ground are to take care of, but hey, I didn't design the thing. As I said, this is not a circuit I have dealt with or really looked at closely.

[J M Fahey]

Mmmmmhhhhh, interesting circuit.
As I mentioned before, Eden designer Nordschow is an intelligent guy and comes up with own ideas instead of endlessly doing what all others do.

In the normal case, when bottom transistors saturate (speaker out swings down to the - rail), top transistors have essentially emitter to-V and collectors to +V, thus standing 2 x rail voltage, 144V in this case.
And in all other amps , drivers do the same ; but here as you noticed , driver collectors swing following speaker out voltage so they only see from -72V to +5.6V , well within ratings.

Clever chap

[glebert]

He may be clever, but I wish he would have used some software to create his schematics (for legibility) and put component designators on the schematic and the board. They have screen printing on the PCB, with some part numbers, so it's not like it would have been extra cost. Minor quibble obviously, but we are off topic anyway.

[glebert]

Here's the full schematic sheet in case anyone wants to see all the components.