FireGuruMN
Active Member
Oh no another W5M restoration!
I had a pair sitting on the bench with a whole pile of expensive tubes and replacements parts sitting next to them for about a year of so. I thought it would be wise to learn how tube amplification works before tearing into these amps. I was originally thinking I only had enough usable bits to make one working amplifier but after checking all the transformers other "iron" I figured I had enough to get 2 working amps. One has an "off brand" output transformer instead of the grey Peerless but the wire resistances match almost exactly so I decided to give it a try after loosing out on numerous overpriced replacement on epray. Did you see the NOS one that went for $350!
So I filled my brain with 1950's schematics and manuals this winter. It was actually pretty enjoyable after living in the semiconductor world for so long. I also read through several excellent posts on AK. Thank you very much.
I had a few stumbles along the way. One 470k ohm resistor when it should have been 470ohm. A few cold solder joints. Dirty tube sockets. blah blah.
But when I was all done I brought them up on a variac and nothing smoked. But they are ohhhh soooo quiet. I would think that 25 watts would be enough to rattle some plates and annoy my wife at least. Don't get me wrong, they do put out a decent amount of sound. and it sounds very good. I just was expecting more. I have a 35 watt AA100 and a few other tube amps and they do much better.
I have verified that the first 12AU7 is amplifying the signal and splitting the phases correctly. The second 12AU7 is raising that signal a lot more and the KT66's are boosting it quite well. But something is not right. Both amps have identical output as much as my scope and hears can tell.
Here is a picture for those that like that sort of thing. Not as pretty as some that I have seen.
I originally had these hooked up to a DAC on the RCA connectors of the DAC. I moved it over to the headphone connectors and it got a bit louder. But nothing drastic.
So is there some little resistor that I can swap out to get the gain up a little? What is the best way to measure the output that I am getting? Obviously I don't want to melt the expensive output transformers. I have the amp driving a pair of Boston Acoustic A400's (4 ohm load). Can I just measure the RMS voltage and figure the power out from that resistance? They are not too efficient but not the worst either. I attached a schematic also that has voltages that are nice for troubleshooting. That helped me find my wrong resistor.
I also have heard of "Williamson modifications" out there to make this even better sounding. But nothing specific for this amplifier. Any thoughts?
And here are some lessons learned:
Replace all the hookup wire. I though I could get off easy by leaving it in but many of the connections were not good because of the solder flux or melted plastic on the old wire.
Replace the tube sockets. I really wanted to keep my looking original on the outside so I kept mine but I believe there are brown sockets out there that would have been a better choice. But mine cleaned up well enough to work. But I can get some "scratchies" until everything heats up.
Specify the manufacturer on NOS tubes if you want matching amps. I ended up with different brands of rectifiers. They both fine work but I wanted matching amps.
Add a CL80 in series with the power circuit to slow down surges on startup. It's not much of a change but seems to help.
Order replacement mounting plates for replacement multi cap cans. it is pretty much impossible to get the old ones off the old cans.
Replace the 300ohm pots. one of mine was replaced at some time and it is easier to adjust than the other.
I can put up a parts list if anyone in interested.
Things I would love to have for parts:
Replacements for the 120volt accessory outlets. Two of mine are damaged and dangerous.
Anyone got a 16458 transformer laying around? I kinda like the look of the big black one I have but I really want a matching set.
And one of the amps goes "pop" "pop" exactly twice every time I power it up cold. It is not in the speakers, it is coming from the amp. Maybe the capacitor cans shorting? The other one is just fine on startup.
Off to a Sansui 1000a next. . .lots more spaghetti in that pot.
I had a pair sitting on the bench with a whole pile of expensive tubes and replacements parts sitting next to them for about a year of so. I thought it would be wise to learn how tube amplification works before tearing into these amps. I was originally thinking I only had enough usable bits to make one working amplifier but after checking all the transformers other "iron" I figured I had enough to get 2 working amps. One has an "off brand" output transformer instead of the grey Peerless but the wire resistances match almost exactly so I decided to give it a try after loosing out on numerous overpriced replacement on epray. Did you see the NOS one that went for $350!
So I filled my brain with 1950's schematics and manuals this winter. It was actually pretty enjoyable after living in the semiconductor world for so long. I also read through several excellent posts on AK. Thank you very much.
I had a few stumbles along the way. One 470k ohm resistor when it should have been 470ohm. A few cold solder joints. Dirty tube sockets. blah blah.
But when I was all done I brought them up on a variac and nothing smoked. But they are ohhhh soooo quiet. I would think that 25 watts would be enough to rattle some plates and annoy my wife at least. Don't get me wrong, they do put out a decent amount of sound. and it sounds very good. I just was expecting more. I have a 35 watt AA100 and a few other tube amps and they do much better.
I have verified that the first 12AU7 is amplifying the signal and splitting the phases correctly. The second 12AU7 is raising that signal a lot more and the KT66's are boosting it quite well. But something is not right. Both amps have identical output as much as my scope and hears can tell.
Here is a picture for those that like that sort of thing. Not as pretty as some that I have seen.
I originally had these hooked up to a DAC on the RCA connectors of the DAC. I moved it over to the headphone connectors and it got a bit louder. But nothing drastic.
So is there some little resistor that I can swap out to get the gain up a little? What is the best way to measure the output that I am getting? Obviously I don't want to melt the expensive output transformers. I have the amp driving a pair of Boston Acoustic A400's (4 ohm load). Can I just measure the RMS voltage and figure the power out from that resistance? They are not too efficient but not the worst either. I attached a schematic also that has voltages that are nice for troubleshooting. That helped me find my wrong resistor.
I also have heard of "Williamson modifications" out there to make this even better sounding. But nothing specific for this amplifier. Any thoughts?
And here are some lessons learned:
Replace all the hookup wire. I though I could get off easy by leaving it in but many of the connections were not good because of the solder flux or melted plastic on the old wire.
Replace the tube sockets. I really wanted to keep my looking original on the outside so I kept mine but I believe there are brown sockets out there that would have been a better choice. But mine cleaned up well enough to work. But I can get some "scratchies" until everything heats up.
Specify the manufacturer on NOS tubes if you want matching amps. I ended up with different brands of rectifiers. They both fine work but I wanted matching amps.
Add a CL80 in series with the power circuit to slow down surges on startup. It's not much of a change but seems to help.
Order replacement mounting plates for replacement multi cap cans. it is pretty much impossible to get the old ones off the old cans.
Replace the 300ohm pots. one of mine was replaced at some time and it is easier to adjust than the other.
I can put up a parts list if anyone in interested.
Things I would love to have for parts:
Replacements for the 120volt accessory outlets. Two of mine are damaged and dangerous.
Anyone got a 16458 transformer laying around? I kinda like the look of the big black one I have but I really want a matching set.
And one of the amps goes "pop" "pop" exactly twice every time I power it up cold. It is not in the speakers, it is coming from the amp. Maybe the capacitor cans shorting? The other one is just fine on startup.
Off to a Sansui 1000a next. . .lots more spaghetti in that pot.
I guess it has never mattered with all the 100 watt amps I've been messing with.
. The sacrifices we make for science. . .
. That makes a lot of sense when you think about the readouts on modern amps going past "0db" on their volume scales. Anything past 0db is a signal boost. But I would still like to increase the sensitivity so I don't need to introduce another component into the system. I'm considering using the preamps that came with these W5M's but something tells me that is going to be a big hit in signal quality. And I only have one preamp at this time. Yea, I know I am taking this "authentic" thing too far. 
The windows are all still closed here in Minnesota.
.
