Transistors for Emerson Custom Paramount (and EM Drive)

[sliberty]

I've built both of these pedals, and really like them. Easy build, and great tone. The diagrams for the EM Drive indicate a 2N5088 transistor, and the diagrams for the Paramount indicate an MPSA13. They are virtually the same circuit, with only a tone pot added on the Paramount. Also, some threads I've read have suggested the 2N3904 as another good alternative. Yet, the 2N5088 is the only transistor I have had any luck with. I've tried the 3904 and the MPSA13, and the pedals made no sound at all with them. And then when I reinstalled the 5088, it worked again. (yes, I installed them with the correct orientation of the pins).

Do these transistors need to be biased differently from a 5088 in order to work in these circuits? Or is there something else going on?

[PRR]

The should be similar enough to "work".

How can you be *sure* of the pinouts? There are variants sold under the same type number. A ohmmeter can identify a Base, which rules-out some confusion. 12V supply and resistor can prove a Silicon Emitter, though low-price transistor testers do it easier.

[sliberty]

Thanks for the reply. You sparked a thought - I had an old meter buried somewhere that can test transistors. So I went and dug it out. It is an old Radio Shack Digital Multimeter with an hFE test socket. Woohoo!

The 2N5088 has an hFE of 450.
The 2N3904 has an hFE of 151.
The MSA13 may be bad - the meter shows type (NPN) and pin layout, but isn't displaying hFE.
The MSA18 has an hFE of 850 (I have not tried this on in the circuit yet).

If the MSA13 is bad, that certainly explains no sound.
As for the 2N3904, with the hFE at 151, is that possibly too low to get the circuit working?

BTW, your comment about getting the pin layout right was certainly a fair one, but the meter has shown that I did have the orientation correct (coincidently).

Any thoughts on other transistor choices that would be worth trying?

[Renegadrian]

as per the datasheet, the MPSA13 has a very high hfe, a lot higher than your regular transistor. common cheap testers may read hfe up to 1k, hence the tester recognizes the transistor but cannot read its hfe.

as per the datasheet, the 2n3904 has a hfe within the indicated range.

You may want to try some BC series transistors (327, 547, ecc) - these transistor need to be reversed 180° compared to 5088 ecc

[duck_arse]

mpsa13 is a darlington, so it might be confusing the meter some. it should have REALLY high gain. can we see the circuit diagrams?

[midwayfair]

Yet, the 2N5088 is the only transistor I have had any luck with

These circuits were not designed with the ability to do anything but select transistors that actually work at all gain points. One can charitably describe these as lousy engineering even by the lazy crap standards that we generally get away with for pedals. The Paramount has marginally better biasing.

There are a couple really easy solutions that won't appreciably change what a single-transistor gain stage sounds like and will let you use almost any NPN transistor you can get your hands on:

1) In the Em Drive, fix the biasing so that the base is referenced to the collector at least and preferably also to ground. Look at the LPB-1 for an example. Or the SHO, which although it's a MOSFET and not a BJT, works every time no matter what N-channel MOSFET you plug into it. Fancy that.

2) Change the collector resistor to a trimpot. Now you can bias every transistor to its optimum point. Man, that would have been a good idea with the originals. Think of the human hours they could save by not having to sort transistors!

[sliberty]

Thanks for the suggestions. Regarding referencing to ground: would I use a 2M resistor to ground? ie same value as the existing resistor from Base to Collector? Thats how the SHO is setup (differen value of course), but not the LPB1.

[midwayfair]

Thanks for the suggestions. Regarding referencing to ground: would I use a 2M resistor to ground? ie same value as the existing resistor from Base to Collector? Thats how the SHO is setup (differen value of course), but not the LPB1.

It might be fine. But note that the SHO has the other resistor base-collector while the LPB1 and the EM Drive have Base-9V. There is a difference in how that affects the biasing, so you can't necessarily do exactly the same thing in the two different pedals you've built.

You will usually need a smaller base-ground (smaller than the -> 9V) resistor with the LPB-1's method to bias near the center. However, you could just change it to the SHO's biasing method in the EM Drive as well and you'd be better off overall. I don't actually recommend the LPB-1's values necessarily since it reduces the input impedance pretty severely. (You won't be able to get the SHO's high input impedance at low gains because it's a BJT and not a MOSFET, but the biasing is reliable even for BJTs.)

Also, while I'm complaining in my previous post about the engineering, it's more about the fact that they build a bunch of these. You're a DIYer and you can afford the extra time on a single build or on your breadboard to try out different things for each variation.