Can't find a BD370D BJT equivalent (old transistor 100 Volt)

[dariodario]

I need an equivalent BJT to replace a BD370D. It is an old transistor used in a guitar amplifier. I attached the schematic (TR7 in the final stage).

Now I mounted a BD370C I had (80V vs 100V) and work fine, but I have to deliver the amp to the customer and I'm not sure about it: the rail-to-rail voltage is 90VDC.

Can you help me?

From what I understand is not easy to find an equivalent, I wrote in two other forums with no luck.

Thank you in advance.

[mzzj]

TO-237 is pretty much history but one of these should be electrically close match:
https://www.digikey.fi/fi/products/filter/yksitt%C3%A4iset-bipolaaritransistorit/276?s=N4IgjCBcoGwJxVAYygMwIYBsDOBTANCAPZQDaIALGGABxwDsIAuoQA4AuUIAyuwE4BLAHYBzEAF9CdAEyIQKSBhwFiZcDQrTqIQlop0ADDpDSAzDWkUKx6QFZ65mMzadIPfsLGSTYAxGjyaFh4hCSQ5LYGBgAEALYAgsYU0YmEpgB0tinGptm6eeCZBWDp0sUFNFFxqSD0VQnGMPWJLCAcXACqQgLsAPKoALK46NgArny4Erq%2BpnIKSiGq4SBwMQBqxr7rm77RG4Rg0tsHdnubpsfgFJdgkWcHTffg9Fn76jc0rzZVG63tbl0ev0hiNxpNvFoDAgAvNgiowuQwNEAOqbdLJVEHIqYkwxVF-VwgQF9QbDMYTKYgAC0shhUH4o3halszHEELUrCErFZQA

[Gyro]

From that list, the MJE253G does look like a reasonable physical match. It's a little slower (f_T 40MHz vs 50MHz) and the minimum h_FE a bit lower (40 @200mA vs 63 @ unspecified). I didn't confirm the pinout.

EDIT: The 2SA1859 is a closer electrical match though but bigger and pinout reversed compared to the MJE253.

[wraper]

You can measure its actual VCEO if concerned. Most likely it's quite a bit over 100V. Also this transistor will not see full rail-to-rail voltage.

[dariodario]

Thanks, I will see that BJTs you proposed.

Sorry wraper, just to understand better...

I can measure the VCEO with the amp powered and multimeter? It's OK? Even if no audio signal is present?

You say that the VCEO I measure "is a bit over 100V", and after you say that the transistor (TR7) will not see full rail to rail voltage, that in the circuit is -45 -- +45=90V. Can you please explain me better? 

[Kleinstein]

One could check how much voltage the transitor can acutually withstand and this can be better than the specs.
One may need some extra head room for operation at a higher temperature or do the test at elevated temperature.

With large transients ( unplugging conections) the transistor may see the full supply (transistor turned off all the way) and since this is an unregulated +-45 V this voltage can be even higher than the nominal 90 V - maybe 100 V when mains is on the high side and with low load.
With the capacitor for bootstrapping there may even be a small chance that transients could even reach a little beyound the supply.

[dariodario]

Kleinstein, no chance for the BD370C at this point. The unregulated voltage is 46.2 for side.

I will try the 2SA1859 suggested by Gyro.

It is incredible that a 1985 BJT don't have a substitute in the 2023 in that TO-92 package. 

[Kleinstein]

I would not go with a TO92 case, but better a bit larger, like a TO126 case. Chances are the trisistor will run a bit warm.  Just looking at the circuit my first crude guess would be a BD140.
If really desparate for a TO92 this would be the BC640 with AFAIK essentilly the same die as the BD140.
Even without a heat sink the TO126 is a little larger than TO92 and this could help with the heat.


P.s.  The expected about 600 mW (15 mA*40 V) are to much for a TO92, unless mounted to a heat sink.

[dariodario]

Kleinstein OK for the TO126. But the VCEO of BD140 is 80V. Isn't that a low value given the circuit? (46+46=92 Volt)

Thank you.

[Kleinstein]

The 80 V VCE rating for the BD140 could indeed be a problem, though one might argue that the base is not open and VCB may apply instead. Still better to have some reserve.
So no such simple solution. Some of the Japanese SA.... series should be OK.

[David_AVD]

Does TR7 say A56 next to it? If so, that would ve MPSA56 which is 80V 500mA 0.625W TO-92.

[dariodario]

Yes, I don't know why they call it A56 in the schematic when in the original circuit I found a BD370D (never repaired amp). Even in the final stage the darlington are labeled different from the real one (MJ3001 and 2502, the original mounted by marshall).

Anyway the MPSA56 is rated 80V and Ic=500mA, different from the 100V Ic=1A requested. At this point I could leave the current BD370C I already mounted, 80V, Ic=1A. What do you think?

I checked the real C-E voltage present in the circuit across the BD370C turning the amp on and playing some music with different volume. The strange thing (for me) is that I measure about 45 volt in any condition between C-E. So when the 46+46=92V should appear? When the base current Ib=0? So when?

Regards.

[Kleinstein]

The full voltage would come up at high volume all the way to clipping or during transients (e.g. unplugging or connecting the input). This is when the ouput is pushed all the way to the negative side.
In therory this may even go below the negative supply due to the BS capacitor. An added diode to clamp to the negative supply may be worth considering.

The idele / low volume case would be half the supply - that is normal. The voltage rating is needed for the worst case conditions, not the average case. Ideally this includes some headroom for a high than normal mains.

[dariodario]

Thanks a lot Kleinstein.

At this point I'm tempted to deliver as is the amp to the client with the BD370C. No replacement is possible for the BD370D, so if it should burn again I will repair it with no charge.

I will mark the client to have care to plug and unplug all the jacks (input, headphone, send, return) with the amp turned off (that is a normal correct use).