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? For Silicon, maybe the 2N3503? The germanium 2N1375 has its voltage rating just under the ~45.5V in the schematic. Soviet MP26B?
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? For Silicon, maybe the 2N3503? The germanium 2N1375 has its voltage rating just under the ~45.5V in the schematic. Soviet MP26B?
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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