Advice needed for replacing old ER3055 transistor

Question

I have a transistor in a guitar amp with the label ER3055 that needs to be replaced. I asked about another transistor that I needed to replace in a post a few days ago here and I tried using NTE to find a replacement. What I found was that, while nobody seems to have a replacement for "ER3055", there are replacements for "3055" or "2N3055". Is the "ER" prefix something like a manufacturer code (for an extinct brand) or is this something that is crucial to the transistor specs? The specs I found for 2N3055 here seem to match up with with the other transistors in the amp so I think that the "ER" is not necessary but I wanted to check with other people first.

Thanks

Answer 1

The 2N3055 is quite a common output transistor for audio gear (I actually have a bunch of boards sitting right here for a preamp I am designing that use 2N3055's)

I think it was RCA who orignally made them, but now they are made by ST, On Semi, etc. From what I have recall the specs can vary very slightly between manufacturers (although I just checked the ST and ON Semi datasheets quickly and couldn't see any difference at all between them) but it's extremely unlikely to be enough to affect the performance of your amp (unless it's really being pushed to the limit)
So the answer is yes, any 2N3055 should work fine as a replacement in your amp. Note that the collector is actually the case itself in the TO-3 package, so make sure you bolt it firmly to whatever it's mounted on to get a good electrical contact (if the legs are soldered to a PCB, bolt first, then solder the legs to avoid putting strain on the solder when you fit the bolts if you were to do it the other way around) Also take careful note if the TO3's and heatsink are ISOLATED or not with mica, and/or dielectric grease, and/or the screws are PROTECTED by a insulating sleeve. Failure to note these details can destroy the amp further during re-assembly by shorting the output power circuit to its negative partner, ground or worse! (Dynaco brand units in particular)

Answer 2

One problem to keep in mind is that old variants of 2N3055 in power amp applications are not readily replaced by modern 2N3055 transistors created with epitaxial processes due to lower secondary breakdown ratings. This is particularly worrisome for partly reactive loads like speakers (and possibly more so, speakers with crossovers) that lead to combinations of high VCE and high collector current.

So if you are not sure the transistor needs replacement (they tend to be among the last blowing in case of catastrophic failure), hanging on might leave you with a more robust amp even though at face level "replacement with a new one of same type as a measure of caution" looks like a cheap and reassuring option.