Looking through my collection of Sam's schematics, you have the MA-25NR (Sam's set #49, folder #12). B+ at the 6L6G plates is given as 360V, and 270 V at the screens, with 117VAC as the reference power supply input voltage. The leg of the power transformer primary that has the switch in line with it goes directly to the transformer, but on the other side where the fuse holders are, there are 2 different primary primary legs, one of them being a "tap", but each one with a fuse holder, and you're only supposed to use 1 fuse at a time, which may be where somebody screwed up: If you put a fuse in one of those fuse holders it will give you a higher secondary voltage and if you put it in the other fuseholder it will give you a lower secondary voltage, but I can't tell you which is which you'll have to experiment and use your meter. The unit isn't supposed to have 2 fuses! The mains fuse value is 1.5 amps. The output transformer has 4, 8 and 15 ohm outputs, with feedback being taken off of the 8 ohm tap, and it also has 125, 250 and 500 ohm outputs as well. After the 3 6J7 inputs comes a 6SC7 as a mixer, followed by the tone controls and another 6J7 audio amplifier stage, which is also where the feedback signal is applied, and finally another 6SC7 is used for the phase inverter.
Hello. Thank you for looking that over. I am a dummy sometimes. I should have realised there was two tappings. I down loaded some of the Sams on Radio Museum. I think there might be a page or two missing. I will check it in more detail when I start restoring it. Thanks again mate. Take care.
@@vintageguitaramp_guitarteacher , I could put up the Sam's manual on my TH-cam channel but I might run afoul of their copyright. Amazingly, ridiculously when you think about it, some corporate entity still owns the legal rights to all those Sam's schematics all these years later, despite the fact that they ceased publishing them in the 1980's. I used to be able to buy entire file cabinets full of them for like 40 or 50 bucks and I would sort through them and pull out all the tube hifi and PA amp schematics and other useful stuff like stereo hifi consoles, real to reels, portable photographs and such that could be cannibalized for transformers and tubes, and I junked all the TV schematics which virtually no one wants anymore.
Sam's published schematics and detailed service info for these. The metal 6J7 pentodes have the grid input connection on the top cap of the tube, and that's what Martyn is touching, eliciting a hum/buzz, to see which input will pass signal (only one of them so far). Maximum plate voltage specification for a 6L6G (or the original metal 6L6) is just 300 volts, although the glass versions are able to radiate some of the excess heat out through the glass, whereas the metal 6L6 tubes hold in more of the heat.
Hello. The customer asked me about the 6L6 metal tubes. He asked if they should be replaced. What do you think? My thinking is that if they test good they should be fine. Take care
@@vintageguitaramp_guitarteacher , it would depend on the amount of plate dissipation the metal 6L6's are subject to in that circuit. If they're running reasonably close to tube manual specs you might get away with it but the one thing I don't like is that you can't see them if they red-plate! 6L6G's have the same rating but they'll dissipate heat better because of infrared that passes right through the glass. If the customer elects to stay with the metal tubes then I would suggest you add a B+ fuse inside the amp.
6J7's sound great for guitar amps. They are just a 6SJ7 with the input grid on the top cap. You can either use them as a pentode preamp tube and you get more gain than half of a 12AX7, you can set it up like an early champ with grid leak biasing for less gain or you can run it in triode mode which gives you half of a 6SN7 equivalent. Sometimes I use 6J7/6SJ7 in parallel with a 6SF5 triode which is an octal triode much like half a 12AX7 wih amplification factor 100.
Love the look and construction of this amp. The two little cowled bulbs to light up the control panel are a classy touch. Not sure if you noticed Goodun's comment about the two fuse clip holders were not supposed to be populated together? One or t'other. Great content Martyn and the comments are a resource in themselves.
Hello Rob. It's a gorgeous amp. I can't wait to start restoring it. I did see gooduns comment. He's a gem. I have learnt so much from him. Take care Rob.
The grounding bolts have a greenish patina and the chassis is heavily painted underneath those bolted-down ring terminals, so you know what you have to do to those!
Can't wait to see/hear this sorted out. Jim is in for a treat!. What a beautiful example of US design and engineering. There's a pristine one of these on US eBay right now. Always a joy watching you bring these to life!.
It seems that these type of old amps can have a fair amount of transformer hum even with new caps is what I've experienced. These amps can sound really good especially with those massive transformers.
Hello Ron. 16uf is a bit low for an amp this size. Maybe with 47uf it would be a little quieter. I will do a bit of testing when I do the video. I sent you the shematic for that tone stack. Did you get it?
Hello, thank you. I think not being on the normal bench has affected the sound a little on this one. I must have stood slightly further away from the camera. Thanks for watching and take care.
My kind of amps! Your channel deserves much praise, thanks for sharing with us Martyn
Hello Ron, thank you. Yes, it's a lovely amp. Transformers are so good. Take care Ron.
Looking through my collection of Sam's schematics, you have the MA-25NR (Sam's set #49, folder #12). B+ at the 6L6G plates is given as 360V, and 270 V at the screens, with 117VAC as the reference power supply input voltage. The leg of the power transformer primary that has the switch in line with it goes directly to the transformer, but on the other side where the fuse holders are, there are 2 different primary primary legs, one of them being a "tap", but each one with a fuse holder, and you're only supposed to use 1 fuse at a time, which may be where somebody screwed up: If you put a fuse in one of those fuse holders it will give you a higher secondary voltage and if you put it in the other fuseholder it will give you a lower secondary voltage, but I can't tell you which is which you'll have to experiment and use your meter. The unit isn't supposed to have 2 fuses! The mains fuse value is 1.5 amps.
The output transformer has 4, 8 and 15 ohm outputs, with feedback being taken off of the 8 ohm tap, and it also has 125, 250 and 500 ohm outputs as well. After the 3 6J7 inputs comes a 6SC7 as a mixer, followed by the tone controls and another 6J7 audio amplifier stage, which is also where the feedback signal is applied, and finally another 6SC7 is used for the phase inverter.
Hello. Thank you for looking that over. I am a dummy sometimes. I should have realised there was two tappings. I down loaded some of the Sams on Radio Museum. I think there might be a page or two missing. I will check it in more detail when I start restoring it. Thanks again mate. Take care.
@@vintageguitaramp_guitarteacher , I could put up the Sam's manual on my TH-cam channel but I might run afoul of their copyright. Amazingly, ridiculously when you think about it, some corporate entity still owns the legal rights to all those Sam's schematics all these years later, despite the fact that they ceased publishing them in the 1980's. I used to be able to buy entire file cabinets full of them for like 40 or 50 bucks and I would sort through them and pull out all the tube hifi and PA amp schematics and other useful stuff like stereo hifi consoles, real to reels, portable photographs and such that could be cannibalized for transformers and tubes, and I junked all the TV schematics which virtually no one wants anymore.
Sam's published schematics and detailed service info for these. The metal 6J7 pentodes have the grid input connection on the top cap of the tube, and that's what Martyn is touching, eliciting a hum/buzz, to see which input will pass signal (only one of them so far). Maximum plate voltage specification for a 6L6G (or the original metal 6L6) is just 300 volts, although the glass versions are able to radiate some of the excess heat out through the glass, whereas the metal 6L6 tubes hold in more of the heat.
Hello. The customer asked me about the 6L6 metal tubes. He asked if they should be replaced. What do you think? My thinking is that if they test good they should be fine. Take care
@@vintageguitaramp_guitarteacher , it would depend on the amount of plate dissipation the metal 6L6's are subject to in that circuit. If they're running reasonably close to tube manual specs you might get away with it but the one thing I don't like is that you can't see them if they red-plate! 6L6G's have the same rating but they'll dissipate heat better because of infrared that passes right through the glass. If the customer elects to stay with the metal tubes then I would suggest you add a B+ fuse inside the amp.
6J7's sound great for guitar amps. They are just a 6SJ7 with the input grid on the top cap. You can either use them as a pentode preamp tube and you get more gain than half of a 12AX7, you can set it up like an early champ with grid leak biasing for less gain or you can run it in triode mode which gives you half of a 6SN7 equivalent. Sometimes I use 6J7/6SJ7 in parallel with a 6SF5 triode which is an octal triode much like half a 12AX7 wih amplification factor 100.
Hello. that's really interesting. I will give that some thought. Thank you. Thanks for watching and take care
Love the look and construction of this amp. The two little cowled bulbs to light up the control panel are a classy touch.
Not sure if you noticed Goodun's comment about the two fuse clip holders were not supposed to be populated together? One or t'other.
Great content Martyn and the comments are a resource in themselves.
Hello Rob. It's a gorgeous amp. I can't wait to start restoring it. I did see gooduns comment. He's a gem. I have learnt so much from him. Take care Rob.
The grounding bolts have a greenish patina and the chassis is heavily painted underneath those bolted-down ring terminals, so you know what you have to do to those!
Hello. Yes, I spotted that. It's going to be a big job. I am looking foward to getting this on the bench
Thanks for the ride ! I'm learning bits here and there.
Looking forward to hearing some licks after some changes.
Hello Lu, thank you. Take care.
Can't wait to see/hear this sorted out.
Jim is in for a treat!.
What a beautiful example of US design and engineering.
There's a pristine one of these on US eBay right now.
Always a joy watching you bring these to life!.
Hello Keiran. Yes, it's a stunner. I am looking forward to this one . take care.
Thanks, this was sick!
Hello, thank you. Take care.
It seems that these type of old amps can have a fair amount of transformer hum even with new caps is what I've experienced. These amps can sound really good especially with those massive transformers.
Hello Ron. 16uf is a bit low for an amp this size. Maybe with 47uf it would be a little quieter. I will do a bit of testing when I do the video. I sent you the shematic for that tone stack. Did you get it?
The Zabriskie Point reference. 👍
Hello Paul, indeed lol. Take care
Great video as always! For some reason, the volume is very low in this video, might want to check your camera settings. Keep up the great work!
Hello, thank you. I think not being on the normal bench has affected the sound a little on this one. I must have stood slightly further away from the camera. Thanks for watching and take care.