Tubescreamer transistor substitution

As part of the Tubscreamer clone project, I ordered parts from Mouser based on the list provided by the DIY Effects build documents.  They listed the MPSA18 as the transistor of choice, but at the time of my order Mouser had none, and I substituted a PN2222ATF. But, I realised much later that I didn’t have even the slightest clue as to whether this was a good thing.

From the Geofex site, there are some recommendations on choosing components:

6. Transistors: When in doubt, use a 2N5088. Or a 2N3904 – or a 2N4401 – or a BC549 – or… find cheap, available NPN and PNP devices that you can get easily and use them.

From a Tonepad customer on a forum:

Tonepad site indicates using a 2N3904. This is certainly acceptable, but you may have a little more oomph on the output with a 2N5089 or an MPSA18. 2N3904 are fine units for a lot of purposes, but in this case not so much. You’ll likely be more satisfied with MPSA18 or 2N5088/5089. Stock TS9s come with 2SC1815 transistors which tend to be higher hfe than 3904s, though not quite as high as the A18s or 508Xs.

So it’s all quite ambiguous as these all come across as “opinions” rather than an objective assessment. Time to learn something about transistors, as right now all I know is:

  • They’re all black plastic things on 3 legs
  • See previous point.

Now of course I know that tubes aren’t readily substitutable without knowing more about the application. You can’t blindly replace a 12AX7 with a 12AT7, even though they’re both 9-pin glass bottles, without knowing that the AX has a gain of 100 but the AT is more like 20. Searching around for answers led me to a document that has been plagiarised all over the inter-tubes:

Understanding the transistor data will definitely helps you to find the right and correct part number for substitution.

So, comparing the various parameter’s ratings as written in the “official” Fairchild PDF documents linked above, I see that the PN2222A is actually a pretty good fit for the MPSA18 in terms of VCEO, VCBO, IC etc. But when I look at HFE, the DC current gain, I see:

  • MPSA18  (VCE = 5.0 V, IC = 1.0 mA ) = Min 500
  • PN2222A (VCE = 10V, IC = 1.0mA) = Min 50

This seems to be off by an order of magnitude (even if you adjust for the current difference), almost like the 12AX7/12AT7 tube swap I hinted at before. So it seems the PN2222 is NOT a good fit. I’m going to ask a very knowledgeable friend about this to confirm my guesstimate.

Until I find a good answer, RadioShack has 2N3904 and 2N4401 transistors on their website, so I’m going to wander over to their store at lunchtime and buy a few of each.

UPDATE: RadioShack has little packets of 15 “assorted transistors” for about $2.50. Guess what’s in there? 2N4401, 2N2222 and 2N3904. So I bought a pack each of NPN and PNP. On the back of the pack it describes the NPN types as:

  • Typical HFE: 200
  • VCE 30V
  • IC 800mA
  • Power dissipation: 1.8W
  • …designed for high-speed, medium-power switching and general-purpose applications.

UPDATE AGAIN: Here’s the post with all the details of the final build.

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5 thoughts on “Tubescreamer transistor substitution

  1. SLW says:

    The transistors are used as input and output buffers. As you can see in the schematic it is a plain emitter follower. It is used to provide a high input impedance and a gain of 1(no voltage gain). The buffer transistors in the 808 and TS9 were cheap high gain, low noise transistors. I don’t think that substituting another high gain, low noise transistor would make any difference in sound. Just be sure that the pins are the same. With that said I have only built the circuit with MPSA18.

    SLW

  2. […] like. These things are legend and I’d never had one in my arsenal before. As mentioned in the previous post, I had an issue getting hold of the prescribed MPSA18 transistors and in the end found an […]

  3. Ockham says:

    While we’re at it and correcting little errors, please be aware that the 12AT7 gain is not “about 20”. The 12AT7 has a gain of about 60, with a transconductance of about 5500 umho at 250vdc on the plates. They are considered high gain high transconductance tubes, according the the RCA tube handbook. You’re probably thinking of the 12AU7, which has a gain of 17 at 250vdc, and a transconductance of 2200 umhos.

  4. Ockham says:

    But, tube gains aside, the real reason I posted is this is a plaintive cry for a list of the correct quiescent voltages in a properly operating stock TS-808. Searching the web, I can find lots and lots and lots of mods, suggestions, component substitutions, “this the secret sauce” claims and so forth – but not one single place where anyone can tell me the correct quiescent voltages at each and every active component (the 1815’s and the SK30 JFETs, and the JRC chip.

    Sure would be nice if someone would post them right on a copy of the OEM schematic…

    just sayin’

    🙂

  5. simonallaway says:

    Does anyone really have a stock TS-808 anymore? 😉

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