I'm assembling a parts list to restore an integrated amplifier from the 1960's, I haven't tested the preamp & driver transistors yet but I would like to have backups on hand to avoid starting and stopping in the event one is bad.
With the exception of the outputs, all others will need to be substitutes since the originals are now obsolete.
The outputs are 2N3055 and have a fT of .8 MHz (min) to 6.0 MHz (max). Do the replacement transistors in the preamplifier and driver stages need to fall in this range?
With the exception of the outputs, all others will need to be substitutes since the originals are now obsolete.
The outputs are 2N3055 and have a fT of .8 MHz (min) to 6.0 MHz (max). Do the replacement transistors in the preamplifier and driver stages need to fall in this range?
I may have answered my own question. According to an old RCA Transistor Manual, one of the original driver transistors, 40361 has a fT of 100 Mc/s, I assume that's the old fashioned way of saying MHz
I have extra 2N3055's on hand if needed. It's the smaller transistors that are giving me a hard time.
The fT’s will be a lot higher in all the smaller transistors. You could likely use any modern TO-126 driver pair, including BD139/140. Good transistors were expensive in 1960, they are not now.
Very early 2n3055 had Ft about 400 khz and were partially responsible for the bad reviews (1966) of the transistor amps of the time versus vacuum tube amps. When epitaxial TIP3055 and 2n3773 became available, existing design bureaus upgraded. Cymbals and top octave pianos sounded more real. At the same time 50 mhz drivers like 40410 were replaced by 6 mhz TIP31c/32c. I thought this would be good enough, but direct compaison proved me wrong. Surviving 1970 RCA TO5 transistors, likely 2n5320/5322 marked as house brands, sounded much better on high frequency containing tracks than TIP31c/32c. . BD139/140 from philips and telefunken had high Ft specs, but the fairchild and On clones never have these published. Many people assert these copies are fine, but I have never used any due to the low wattage rating. My first transistor amp contained heatsinks on TO5 drivers, and TO126 just does not make that wattage. Lots of threads of 1970's gear discuss blown drivers, so I have first used 20 mhz D44r2 from GE with heatsinks which had 20 mhz.. The NTE drivers available to those without a sales tax number in the 80's had abysmal Ft. 2n5320/5322 totally disappeared. Now they are back. TO220 drivers GE d44r2 (not the same as european D44)now superceded by by On MJ15028/29 for 50-70 watt appliances. If you are running 150-500 w PA gear, go directly to MJ15031/32.
TO-126 is 1 watt no heat sink, 12.5W infinite heat sink. TO-5/39 is 1 watt no heat sink, 10 watt infinite heat sink. Actually comparable. Practical heat sinks make either good for 2-3 watts. Don’t trust OnSemi BD139’s? Toshiba makes an 80V version of their TO-126’s with 200 MHz fT and good quasi-saturation characteristic. I forget the exact part #s -TTA/TTC something-or-other. Their 160 and 230V series are fine too, if you can deal with the beta falling off below Vce=5. Really just depends on your rail voltages, but 1960’s gear using 2N3055’s won’t be above maybe 75V (single supply). Could be a lot lower, and then you want low knee voltages on the drivers and VAS to get the most power out of it.
2N5320’s are going to be second-sourced, from the likes of Central Semi, Mospec or CDIL, and be overpriced compared to anybody’s current production audio drivers. Gonna pay thru the nose for that little metal case. Still want it that bad?
PS. Anyone using TIP31/2 as drivers in EF2 or QC deserves their fate. TIP31/41 in EF3 with a high speed TO-92 out front and the outputs biased optimally is actually quite acceptable.
2N5320’s are going to be second-sourced, from the likes of Central Semi, Mospec or CDIL, and be overpriced compared to anybody’s current production audio drivers. Gonna pay thru the nose for that little metal case. Still want it that bad?
PS. Anyone using TIP31/2 as drivers in EF2 or QC deserves their fate. TIP31/41 in EF3 with a high speed TO-92 out front and the outputs biased optimally is actually quite acceptable.
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