What to look for in a Substitute Transistor? - Page 1
Forum section: electronics / beginners

author: kazzle101
author_rank: Contributor
timestamp: February 24, 2021, 07:41:00 pm
content: HelloIn this 1980's Guitar Amp I'm attempting to repair, the two power transistors need replacing. The heatsink isn't properly secured to the PCB only being supported by the transistors a couple of their legs have broken, and the tracks below have come way from the board (strange that, who'd of thought...)The obsolete transistors are BD537 NPN in a TO-220 package, I found a datasheet:https://www.st.com/resource/en/datasheet/cd00000915.pdfand am now looking for a substitution.I've been going on various supplier pages and plugging in the various details from the datasheet to see what matches.Is there anything that MUST match? As most of the parameters have an absolute maximum or minimum rating, such as 80V for the Collector-base voltage, if I select a transistor in the same package with better ratings then it'd work as expected as a replacement? Does that make this BD911 a suitable candidate:https://uk.rs-online.com/web/p/bipolar-transistors/1693158/ThanksKarl.

author: fcb
author_rank: Super Contributor
timestamp: February 24, 2021, 07:45:18 pm
content: https://www.silicon-ark.co.uk/BD537-transistor-by-sgs

author: kazzle101
author_rank: Contributor
timestamp: February 24, 2021, 08:02:33 pm
content: Hello fcbThat web page looks like its gone badly wrong, I checked with chrome and IE.

author: bob91343
author_rank: Super Contributor
timestamp: February 24, 2021, 09:50:36 pm
content: The only problem with substitution that I can foresee is frequency. The original circuit was presumably optimized for the original transistors. If the replacements have different cutoff frequency you may see some stability problems. If you have a fast oscilloscope you can see if it oscillates or otherwise acts unstable.

author: Refrigerator
author_rank: Super Contributor
timestamp: February 24, 2021, 10:04:35 pm
content: Are the transistors from the output stage of are they from some sort of power regulation?What i see audio people talk most about when replacing a transistor is them being a matched pair and also their linearity.Otherwise as long as the specs match or exceed that of the original i see no problem, only catch is as @bob91343 mentioned frequency response.Also make sure the gain is similar to the original.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2021, 10:11:07 pm
content: The transistor seems to be made for switching. The BD911 is quite a bit larger and thus higher capacitance and likely also slower. This may be a problem.I would consider the D44H11 transistor - likely slightly faster and similar to only slightly higher power rating.The gain is usually not a problem - a higher gain usually does not hurt. It is usually not good practice to depend on the gain, as it changes with temperature. In a modern circuit with feedback, there is little need for matching the power transistors. If there are several transistors in parallel the VBE should be reasonable similar - so mixing different types in parallel is usually not good.One may have to adjust the standing current if the base - emitter voltage changes. This may be needed even when replacing with the same type of transistor.

author: srb1954
author_rank: Super Contributor
timestamp: February 25, 2021, 10:01:59 am
content: Quote from: kazzle101 on February 24, 2021, 07:41:00 pmWhen looking at substitute BJTs in any moderate to high power application always check the safe operating area (SOA) specifications as well as maximum voltage and current ratings.The SOA specification is usually presented in the datasheet as a graph showing the allowable range of collector currents and collector-emitter voltages for safe operation. Operating a transistor outside the allowable area can result in very early failure.

author: david77
author_rank: Frequent Contributor
timestamp: February 25, 2021, 10:29:40 am
content: I find the Jaeger Semicon very helpfull for substitutionshttp://www.ko4bb.com/getsimple/index.php?id=manuals&dir=09_Components_Specs_Datasheets/10_Misc

author: Terry Bites
author_rank: Super Contributor
timestamp: February 25, 2021, 07:46:47 pm
content: It depends on what the tranistor function is in the circuit. Some paramaters can be critical. Mostly they are not!50W 80V 5A Hfe ~20 T0-220. ?? D44H11 - D45H11

author: Terry Bites
author_rank: Super Contributor
timestamp: February 26, 2021, 05:15:48 pm
content: It occured to me that if this transistor forms part of a complemtary pair, it would be better to replace both the pnp and npn designed to work together.

author: kazzle101
author_rank: Contributor
timestamp: February 27, 2021, 01:35:28 pm
content: Hello TerryThe circuit diagram shows them as being both the same:https://www.korguk.com/voxcircuits/circuits/suprtwin.jpg

author: mawyatt
author_rank: Super Contributor
timestamp: February 27, 2021, 04:07:33 pm
content: As a kid I worked my way thru school repairing, designing and modifying all sorts of audio and music equipment in the late 60s, early 70s. Probably repaired over 100 tube and solid state guitar amps back then and don't recall any particular sensitivity to the output power transistors which was the most common failure in SS amps. Just use a replacement that has the proper case, power, Vceo breakdown, minimum beta (hfe) and current handing, speed or ft really shouldn't matter. It appears from the schematic that complementary output devices npn & pnp are not used, but the equivalent of a pnp power device is created by TR6 and TR3. This is a Class AB output stage that seems to be biased more towards class B than A, which was common with guitar amps since "distortion" wasn't an issue, and often desired. The supply is ~42VDC, so a Vceo of 60 volts should suffice.BTW recall the only time I ran into trouble with replacing output devices was an obscure amp that used germanium power devices, we ended up modifying the design to accommodate silicon (2N3055 & 2N2955) since the germanium power devices weren't readily available. Fond memories from back then like the time we modified a Hammond B3 organ to allow use of the Cry Baby Wah-Wah pedal that Clapton used in White Room. The configuration allowed a separate amp, or running thru the Hammond Leslie which produced some amazing sounds!! Other fun stories, like about one of the most popular bands of all time but don't want sidetrack your thread.Anyway, good luck with your repair and nice to see folks trying to keep these old Rock relics alive!!Best,

author: fcb
author_rank: Super Contributor
timestamp: February 28, 2021, 12:52:04 pm
content: Quote from: kazzle101 on February 24, 2021, 08:02:33 pmI use firefox mostly. If you gotowww.silicon-ark.co.ukand search for your part, they have some in-stock for £0.50each. Saves the hazzle kazzle.