I bought a broken McMartin LT-300A mic/line amp, most of the germanium transistors test as dead. I bought the SAMS Photofact and posted a hi-res schematic pic to technical docs:
https://groupdiy.com/threads/mcmartin.90217/#post-1193464
McMartin of Omaha, NE (1955-1985) was fairly well-known in US radio (aka "McMartian"). I'm interested in hearing this more or less as it was with germanium transistors, of course these are hard to find and a short-lived technology soon replaced by silicon. I'm certain silicon replacements would be quieter, broader bandwidth, and longer-lived. And it would be interesting to find out what silicon replacements would be recommended. I'm mainly interested in getting some germaniums for it when I still can.
Q1-Q5 are SFT337 black top hats in my unit. I believe these are PNP. I was thinking of replacing these with 2N508s? The voltages are on the schematic very helpfully, Vce/Vcb no more than 12V apparently.
Q6 is a Ti R691, I believe TO-5 in a Thermalloy 2211 heatsink. I could find no documentation on this transistor, even the TI 1969 catalogue didn't list it, this may have been a special run item. I would figure it's Germanium with Vcb/Vce of over 48V given the schematic markings. A driver of the interstage transformer that balances the speaker drivers. I have no ideas on this one? The germanium 2N1375 has its voltage rating just under the ~45.5V in the schematic. Soviet MP26B? I'm thinking silicon 2N3503 might be the best bet here, the germaniums may just burn up. But maybe the 2N3503 has too much gain?
The big TO-3 speaker drivers are Ti 3031's with the whole back of the unit a pair of heatsinks for them. They might even still work, but I found several potential replacement options not sure of the benefits. I did find a datasheet on Ti3031. I will try Motorola 2N1548 for this if they need replacing, the Vce is only about 40V from what I can tell on the schematic, the 2N1548 has a healthy hFE of 75 so should have plenty of headroom.
With some nice iron already on board and germaniums, this could end up sounding pretty cool. I might also want to add a line-level output, I'm not sure exactly where to tap that from the schematic. I suppose I could adapt the phono input as DI for bass or guitar amp use, run into a mic channel. This may be cool for reamping vintage keyboard VI's.
Do we have a germanium expert who could make substitute suggestions based on what's available? Thanks so much in advance.
https://groupdiy.com/threads/mcmartin.90217/#post-1193464
McMartin of Omaha, NE (1955-1985) was fairly well-known in US radio (aka "McMartian"). I'm interested in hearing this more or less as it was with germanium transistors, of course these are hard to find and a short-lived technology soon replaced by silicon. I'm certain silicon replacements would be quieter, broader bandwidth, and longer-lived. And it would be interesting to find out what silicon replacements would be recommended. I'm mainly interested in getting some germaniums for it when I still can.
Q1-Q5 are SFT337 black top hats in my unit. I believe these are PNP. I was thinking of replacing these with 2N508s? The voltages are on the schematic very helpfully, Vce/Vcb no more than 12V apparently.
Q6 is a Ti R691, I believe TO-5 in a Thermalloy 2211 heatsink. I could find no documentation on this transistor, even the TI 1969 catalogue didn't list it, this may have been a special run item. I would figure it's Germanium with Vcb/Vce of over 48V given the schematic markings. A driver of the interstage transformer that balances the speaker drivers. I have no ideas on this one? The germanium 2N1375 has its voltage rating just under the ~45.5V in the schematic. Soviet MP26B? I'm thinking silicon 2N3503 might be the best bet here, the germaniums may just burn up. But maybe the 2N3503 has too much gain?
The big TO-3 speaker drivers are Ti 3031's with the whole back of the unit a pair of heatsinks for them. They might even still work, but I found several potential replacement options not sure of the benefits. I did find a datasheet on Ti3031. I will try Motorola 2N1548 for this if they need replacing, the Vce is only about 40V from what I can tell on the schematic, the 2N1548 has a healthy hFE of 75 so should have plenty of headroom.
With some nice iron already on board and germaniums, this could end up sounding pretty cool. I might also want to add a line-level output, I'm not sure exactly where to tap that from the schematic. I suppose I could adapt the phono input as DI for bass or guitar amp use, run into a mic channel. This may be cool for reamping vintage keyboard VI's.
Do we have a germanium expert who could make substitute suggestions based on what's available? Thanks so much in advance.
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