I will stop. There is problem is with the power supply. The voltage coming out of the power transformer is 450V instead of what should have been 360V. I bought the amp from Japan. The amp is labeled as 120V, but I suspect the previous owner modify it to use with Japan 100V electricity. Since 100V supposed to get you 360V, 120V would be right around the 440V - 450V. I think that where my problem is, unfortunately, not quite sure how to fix it. The transformer is original, there is a 0 and 120V label lead. Right now it is being hooked up to the one next to it. Is it as simple as returning it to the labeled position? Also, the starting capacitor in there is 0.1uF instead of 0.022uF in the schematic, does it need to be changed? I am thinking about removing all the tubes. Then switch the AC input and measure the voltage coming out of the power transformer. Is that ok?
My apology for this critical oversight. There was a miscommunication between me and the seller that led me to assume the amp is set up for 120V operation.
My apology for this critical oversight. There was a miscommunication between me and the seller that led me to assume the amp is set up for 120V operation.
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7.6V on AC filament. I can try that and see if I can get the right voltage. Let me go grab my variac.
I did it. Switch one wire to the input on the power transformer.
For now, input voltage is 123VAC (my area is a bit higher than the rated 120V). Output voltage at the 360V feed is 384VAC. I think it is acceptable. Is there any risk with the voltage 20V higher than what it said??? I really like Air Tight for having everything labeled so clearly. Unfortunately, I have no time to check the bias now since wife wants to go see the cherry blossom.
For now, input voltage is 123VAC (my area is a bit higher than the rated 120V). Output voltage at the 360V feed is 384VAC. I think it is acceptable. Is there any risk with the voltage 20V higher than what it said??? I really like Air Tight for having everything labeled so clearly. Unfortunately, I have no time to check the bias now since wife wants to go see the cherry blossom.
emk2....20 v in input v of main ac transformer is risk.....h=7.5 v high voltage go to more than 500....
More words would help understanding. What means “h=7.5 v high voltage go to more than 500....” ? 500 what?
f or h =filament or heter of tubes..... v = voltage ........ high votag in atm-1 amp =460 v so when recive to 500 and more...... means 500 voltage...... . 500 volts is an important number in tube amplifiers. because the power supply capacitors are usually not available at a voltage higher than 500 v. And if a defect causes such an increase, it is dangerous for them.
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Thank you. I agree that both heater supply voltage AND HV supplies are important. @DK1327 ‘s measured his power transformer output at 384VAC. It is probably acceptable as I assume that was measured without the tubes installed, and under load it would come down. I recall that @iWozza measured 480 Vdc for his B+ as I mentioned, but @DK1327 had not reported yet his B+ with the 120Vac connection, or what his heater voltages are now. (Btw, what was it with the 110 V connection?)
Heater voltage right now is 6.5VAC. It is .2 higher than the rated 6.3. After I switch the tap on the power transformers, all voltage is more in line with the schematic than before. I was able to adjust the balance to the bias closer to suggestion. For the OPT side with 76.3ohm I was able to adjust both side to 3.04VDC, the OPT sides with 66.7 ohms I was able to adjust one to 2.64 while the other one is hover lower at 2.35VDC. I can't get the last tube to get to higher voltage so I gave up and take the result.
I also checked all the components again, one side of the 50/50uF capacitor has a higher ESR than before so maybe it is damaged (I operated the amp at 120V for about 3 hours before making the correct switch).
After all the adjustment, I let the amp ran again for 4 hours. It is quiet, no noise, no hum like before and thankfully this time, the power transformer is warm but not hot to touch.
I also checked all the components again, one side of the 50/50uF capacitor has a higher ESR than before so maybe it is damaged (I operated the amp at 120V for about 3 hours before making the correct switch).
After all the adjustment, I let the amp ran again for 4 hours. It is quiet, no noise, no hum like before and thankfully this time, the power transformer is warm but not hot to touch.
Just saw the post and I went and measure.
Heater voltage is 6.5VAC, B+ (I assumed at pin 3) is 485VDC.
I measure voltage at my outlet is 123VAC.
Very good news! The heater voltage is within ~3%, and I consider that good-enough. With your B+ now at 488 B+ is at your OT center tap “test point”) with the amp running, it is a bit high. I suggest you play the amp a while and watch the output tube dissipation and redplating as well as monitoring the power transformer temp.
For this and other reasons you may want to install an Inrush Current Limiter (https://www.mouser.com/ProductDetail/Amphenol-Advanced-Sensors/CL-80?qs=VwrmENAm9QmVgx79fWpdUA==) in the AC line, the slight drop will bring your heaters closer to 6.3vac and you B+ down a few volts. If you install this be aware that it will dissipate some heat and keep away from heat sensitive parts.
That one tube output tube (with 2.35 V bias measurement) may be just tired. Have you tried the matched quad EL34s with the new power supply voltages?
For this and other reasons you may want to install an Inrush Current Limiter (https://www.mouser.com/ProductDetail/Amphenol-Advanced-Sensors/CL-80?qs=VwrmENAm9QmVgx79fWpdUA==) in the AC line, the slight drop will bring your heaters closer to 6.3vac and you B+ down a few volts. If you install this be aware that it will dissipate some heat and keep away from heat sensitive parts.
That one tube output tube (with 2.35 V bias measurement) may be just tired. Have you tried the matched quad EL34s with the new power supply voltages?
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I still recommend installing the 10 Ohm precision resistors between cathode and ground to measure the actual cathode currents. That way you can know exactly what current each tube is conducting.
The Inrush Current Limiter seem to be a popular choice. Do I install it in series before or after the fuse? Also for one side of the AC main voltage line or both?
The 10 ohm resistors are in order, 1/2 watt with 1% tolerance ????? They are wired from Cathode (pin 8) to ground right?
That one output tubes that have 2.35V actually tested strong and equal to its pair. I am currently don't have a matched quad EL34 at the moment since I suspect the quad tube that came with it is damaged (with that EH quad EL34 in the amp, I can't bias properly at all). I don't want to install my vintage EL34 yet after I am 100% sure it won't blow it up, they are hard to come by nowaday.
Another thing someone recommended was a bucking transformer, so that is something I will explore.
The 10 ohm resistors are in order, 1/2 watt with 1% tolerance ????? They are wired from Cathode (pin 8) to ground right?
That one output tubes that have 2.35V actually tested strong and equal to its pair. I am currently don't have a matched quad EL34 at the moment since I suspect the quad tube that came with it is damaged (with that EH quad EL34 in the amp, I can't bias properly at all). I don't want to install my vintage EL34 yet after I am 100% sure it won't blow it up, they are hard to come by nowaday.
Another thing someone recommended was a bucking transformer, so that is something I will explore.
Since you have HV to spare I suggest one CL-80 per leg on the AC line-in. Exact location is not critical, more dependent on good, safe place to install them minding heat dissipation from them.
Based on the schematic the 10 ohm resistors go functionally between the junction of pin 1 and 8, and junction of ground and 68k resistor. Practical placement depends on convenience of access.
I’m puzzled by your experience of the output tube with the 2.35 V reading. This implies you are unable to bring the negative bias close enough to zero. To me this indicates reduced emission, but that tube should not measure normal then. Have you switched pairs, and within pair to see what happens?
After the ICLs your heater voltages will probably be close to perfect, and HV still little high, but “tameable“ by the tube current you bias for. It will likely be acceptable as long as you keep plate dissipation somewhere under 25 watts. So, I expect the bucking transformer will not be necessary. But let’s see what you have once you amp is operating at a stable point.
Based on the schematic the 10 ohm resistors go functionally between the junction of pin 1 and 8, and junction of ground and 68k resistor. Practical placement depends on convenience of access.
I’m puzzled by your experience of the output tube with the 2.35 V reading. This implies you are unable to bring the negative bias close enough to zero. To me this indicates reduced emission, but that tube should not measure normal then. Have you switched pairs, and within pair to see what happens?
After the ICLs your heater voltages will probably be close to perfect, and HV still little high, but “tameable“ by the tube current you bias for. It will likely be acceptable as long as you keep plate dissipation somewhere under 25 watts. So, I expect the bucking transformer will not be necessary. But let’s see what you have once you amp is operating at a stable point.
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True, but I was talking about B+ only, that's what I understood from the OP, after he soldered the mains wire on another tap of the primary PT
After I rewire the power transformer, the heater voltage is 6.5V, which is acceptable.
Unfortunately, during assembly, I broke one of the EL34. That prompts a search for a replacement. Finally find 2 matching pairs that tested 38/40 and 40/42 (not the same type, but both fat bottle EL34). I was able to get them to bias pretty closely. One set was 3.04 and 2.67, the other was 3.1 and 2.8. It seems to be holding for the time being. I let the amplifier run for a few hours, and everything seemed okay. The output transformers are just a bit warm, one is more so than another but it isn't too much different. The power transformer is warm to the touch, but not too hot. The temperature seems to be the same as my ATM-2 after extended listening so I am not worried about it. The ATM-2 bias process is sooooooooo much easier though.
About those CL-80. I will try to find a safe spot to tuck them in. There seems to be space near the power transformer that I can find a way to isolate them safely. Most likely I will replace the electrolytics since they seem to be accessible and easy to do. That one cap that has a high ESR worries me so there is no point keeping it.
Thank everyone again for your help, and I am looking for a scope so I can do more of my repair.
Unfortunately, during assembly, I broke one of the EL34. That prompts a search for a replacement. Finally find 2 matching pairs that tested 38/40 and 40/42 (not the same type, but both fat bottle EL34). I was able to get them to bias pretty closely. One set was 3.04 and 2.67, the other was 3.1 and 2.8. It seems to be holding for the time being. I let the amplifier run for a few hours, and everything seemed okay. The output transformers are just a bit warm, one is more so than another but it isn't too much different. The power transformer is warm to the touch, but not too hot. The temperature seems to be the same as my ATM-2 after extended listening so I am not worried about it. The ATM-2 bias process is sooooooooo much easier though.
About those CL-80. I will try to find a safe spot to tuck them in. There seems to be space near the power transformer that I can find a way to isolate them safely. Most likely I will replace the electrolytics since they seem to be accessible and easy to do. That one cap that has a high ESR worries me so there is no point keeping it.
Thank everyone again for your help, and I am looking for a scope so I can do more of my repair.
Heat shrink on the leads are fine, but do not put heat shrink on the NTC body! It needs to dissipate heat.
Yup, 6.3 V heaters are perfect. Is 6.5 V too much (for long term use)? Not really. The specs says +-10%.
Here is what “The Valve Wizzard says”: The heater voltage should be kept within +/-10% of its nominal value for optimum valve performance. For 6.3V heaters that means 5.7V to 6.9V, though it is even better to stay within +/-5% if you can (6V to 6.6V)”
Yup, 6.3 V heaters are perfect. Is 6.5 V too much (for long term use)? Not really. The specs says +-10%.
Here is what “The Valve Wizzard says”: The heater voltage should be kept within +/-10% of its nominal value for optimum valve performance. For 6.3V heaters that means 5.7V to 6.9V, though it is even better to stay within +/-5% if you can (6V to 6.6V)”