bc550c and bc560c

[Wagener]

I will be using the BC550c and the BC560c instead of the MPSA05 and MPSA55. The original transistors used was MPS8099 and MPS8599. They are not available anymore. This is for the leach amplifier

Is there anything I should be concerned about?

[Workhorse]

Originally posted by Wagener
I will be using the BC550c and the BC560c instead of the MPSA05 and MPSA55. The original transistors used was MPS8099 and MPS8599. They are not available anymore. This is for the leach amplifier

Is there anything I should be concerned about?

I think u are moving on right track!

with regards
ampman

[Netlist]

Hmm, I don’t know the Leach amp but here are some details for the different devices:
MPS8099/8599: NF, 80V, 0,5A, 0,625W, >150MHz
BC550C/560C: Uni, ra, 50V, 0,1A, 0,5W, 300MHz
A bit risky if you ask me.

Your best bet would be the MPSA06/MPSA56: NF-Tr, 80V, 0,5A, 0,625W, >100MHz
Don’t know if they are easily available.

Other replacements might be:
BC639/640: NF-Tr/E, 100V, 1A, 0,8W, 50MHz
2SC3939/2SA1533: Uni, 80V, 0,5A, 1W, 120MHz

Leach amp gurus could have different opinions

/Hugo

[Wagener]

Why do you say it is risky? Because the voltage rails are more than the transistor voltage?

[djk]

The current Leach is cascoded, not to worry about the voltage on the diff inputs.

The original Leach was not, MPSA06/A56, or 2N540X/550X needed.

WATCH THE PIN_OUT.

[Eva]

BC550C and BC560C are nice small signal transistors but have only 45V Vce blocking capability. Anyway, Leach Amp uses cascodes on the input stage so full voltage rails are not applied to these transistors

Reliability may be enhanced by properly selecting D13-D16 voltages [ie: 12V+15V instead of 20V+20V] in order to dump half the supply voltage over each BC550C/BC560C. Adjusting R13-R16 values will be required in order to mantain the same bias currents

Q5 and Q6 may suffer voltage stress if supply rails rise too fast [ie: no soft start circuit] since C4 and C5 require some time to charge through R13 and R14 in order to provide the right voltage rail splitting between transistors

Since Q5 and Q6 are cascode transistors, you may use here other devices with higher Vce blocking capability but worse specs, like BC546C/BC556C, 2N5401/2N5551, MPSA06/MPSA56, BD139/BD140, etc...

As an alternative, you may use BC546B/BC556B everywhere since they have better specs than MPSA06/MPSA56 and have still enough Vce blocking capability to work reliably without adittional circuit modifications

[Netlist]

As stated before, I don't know the Leach amp nor its rail voltage.
Mr. Leach must have had a reason to select the MPS8099/8599 's.
To me it seems that the best replacements would be the MPSA06/56. Just to stay in the SOA. Hfe is probably quite important too. BC550C has a much higher value. 450-900 vs. max. 100 for MPS8099 and max. 50 for MPSA06.

/Hugo

[Eva]

The Leach Amp uses about 56V supply rails. Mr Leach specifies only american Motorola transistors since they are cheaper and much easier to find on USA than european ones

In contrast, In Europe and Asia, american transistors tend to be very expensive, hard to find, and usually fakes. Some years ago I bought some MPSA06 and MPSA56. A06 turned to be useless fakes due to very bad specs [Hfe = 5 to 25 for Ic=1mA!!!!], A56 were original devices from Motorola but have also poor specs compared to similar european devices. I paid for them about 6 times the money I usually pay for BC series and got mostly unusable crap. Wasted money. I ended using these transistors for LED and relay driving applications...

MPS8098 and MPS8598 are the preferred devices by Mr.Leach and have better specs than A06 and A56, but they are really hard to get in Europe and allways at the risk of paying 5-10 times more money for useless fakes

I think half the low sales-volume or rare transistors sold in Europe are actually fakes

[richie00boy]

Doesn't Leach specify low gain (50-150) devices in the interests of gain margin and stability? I'm sure I read that on his website.

BCxxx devices also have reversed pinout to MPSAxx devices.

[Eva]

The gain of the input devices only affects ouput offset and input impedance [the more gain, the better]. The overall gain of the long tailed pair is limited by emitter resistors so open loop gain is not increased. Placing faster transistors in the LTP will actually increase the phase margin of the circuit

The amplifier has already a pole estabilished by VAS miller capacitance and the next higher pole is introduced by output transistors, so the rest of the poles should be at frequencies high enough where there is no longer enough feedback for oscillation. Using slow transistors in LTP, VAS, etc..., actually increases the risk of unstability since it may place the LTP pole at too low a frequency

Using BC550C/BC560C for LTP and faster devices for VAS and predriver stage may even allow to increase gain bandwith product of the circuit without danger of oscillation. Remember that Leach Amp was designed long time ago and better bipolar transistor are now available