Don't you think maybe this is a bit overkill for a headphone amp. 
I thought I saw a capacitor between the bottom of the IT secondary and ground. Not really clear on my screen. That has to be a DC path, nix the cap. If that is a capacitor your output tube grid is floating and that will not work.
I thought I saw a capacitor between the bottom of the IT secondary and ground. Not really clear on my screen. That has to be a DC path, nix the cap. If that is a capacitor your output tube grid is floating and that will not work.
It is not uncommon for sound to come from an audio transformer. The output transformers make some sound in almost every amp that I have tested. You normally won't notice it with speakers present because the speakers make far louder sound. I have noticed some sound from most transformers when testing with a resistive load. Thansformers with metal end bells are much louder than open frame transformers. I have noticed that transformers without a load make more sound, and transformers with a short across the output are quieter. The OPT's in my 845 amp are quite loud, and one is louder than the other. They are cheap transformers from Handwound.
This observation shows that your input tube and driver are working. You have current flowing through the 845. Verify that you can adjust this current with the bias pot. This would show that the output stage is working from a DC point of view. I would check the wiring of your IT and OPT for errors.
The capacitor from the BOTTOM of the IT to ground is OK (required for A2) and provides a signal bypass to ground across the output of the bias supply. The DC path is provided by the bias supply. If the cap was accidentally miswired to be from the TOP of the transformer to ground, it would cause no sound.
This observation shows that your input tube and driver are working. You have current flowing through the 845. Verify that you can adjust this current with the bias pot. This would show that the output stage is working from a DC point of view. I would check the wiring of your IT and OPT for errors.
The capacitor from the BOTTOM of the IT to ground is OK (required for A2) and provides a signal bypass to ground across the output of the bias supply. The DC path is provided by the bias supply. If the cap was accidentally miswired to be from the TOP of the transformer to ground, it would cause no sound.
Hey Guys,
Your line of reasoning seems logical, and consequently what I have done.
The 845 has plate voltage, negative bias, and current which is adjustable with the bias pot. Yet, no sound. I will double check the wiring around the secondaries, as it is possible that I am shorting the AC path with a miswired cap off the TOP of the IT....
Heck, maybe my speaker is busted and I just don't know it... I'll check that as well.
With respect to signal on the output, what should I observe here if I probe the output terminals. Obviously some AC fluctuating... But at what voltages...
BK
Your line of reasoning seems logical, and consequently what I have done.
The 845 has plate voltage, negative bias, and current which is adjustable with the bias pot. Yet, no sound. I will double check the wiring around the secondaries, as it is possible that I am shorting the AC path with a miswired cap off the TOP of the IT....
Heck, maybe my speaker is busted and I just don't know it... I'll check that as well.
With respect to signal on the output, what should I observe here if I probe the output terminals. Obviously some AC fluctuating... But at what voltages...
BK
If the plate current of the output tube is swinging with the signal, then it is obviously getting signal. This implies that the problem must be between the tube and the speaker. There aren't to many things in that path (the OPT and the wiring). Check to make sure that your speaker is connected, and there is no accidental short across it. I know it is unlikely but check to be sure that you don't accidentally have the OPT connected backwards (primary and secondary switched). I have actually done this using the Handwound transformer, because of the non - standard color code.
With the power off and everything discharged, measure the resistance of the secondary of your OPT. It should be under 1 ohm. The primary must be OK or you wouldn't have any tube current but it should be 50 to 200 ohms. Then check the continuity of the path to the speaker.
If you put a voltmater (set on AC volts) across the speaker leads you will get random numbers (or a bouncing needle) with music. If you have a test CD play a continuous tone near 1KHz and you should read an AC voltage that increases as you turn up the volume. 30 watts will be about 15 volts with an 8 ohm load. If there is the possibility that there is no load on the amplifier, do not turn it up loud. The OPT could be damaged.
The tube current will usually stay pretty constant until you start to leave class A1 then it will increase on peaks. A digital meter will read the signal current and fluctuate around. A mechanical analog meter should be pretty constant until you get loud. If it is moving around a lot with a relatively small signal, I would suspect clipping caused by an open or shorted load.
With the power off and everything discharged, measure the resistance of the secondary of your OPT. It should be under 1 ohm. The primary must be OK or you wouldn't have any tube current but it should be 50 to 200 ohms. Then check the continuity of the path to the speaker.
If you put a voltmater (set on AC volts) across the speaker leads you will get random numbers (or a bouncing needle) with music. If you have a test CD play a continuous tone near 1KHz and you should read an AC voltage that increases as you turn up the volume. 30 watts will be about 15 volts with an 8 ohm load. If there is the possibility that there is no load on the amplifier, do not turn it up loud. The OPT could be damaged.
The tube current will usually stay pretty constant until you start to leave class A1 then it will increase on peaks. A digital meter will read the signal current and fluctuate around. A mechanical analog meter should be pretty constant until you get loud. If it is moving around a lot with a relatively small signal, I would suspect clipping caused by an open or shorted load.
Tubelab,
I took the resistance measurements last night. The primary of the OPT is 320ohm, and the secondary was about 0.5ohm. I compared these readings to the other OPT and they are the same.
It is probably just a SIMPLE wiring issue with the terminals and then the connections to the test speaker. The fairly large degree of current swing and loading sounds like this could be the issue.
Hopefully I can confirm this tonight with relative ease.
BK
I took the resistance measurements last night. The primary of the OPT is 320ohm, and the secondary was about 0.5ohm. I compared these readings to the other OPT and they are the same.
It is probably just a SIMPLE wiring issue with the terminals and then the connections to the test speaker. The fairly large degree of current swing and loading sounds like this could be the issue.
Hopefully I can confirm this tonight with relative ease.
BK
Some input.
I've just checked the OPT wiring with the terminals and the test speaker. All seems fine. There are no shorts from the positive terminal to ground. There is also no connection referrencing the negative speaker terminal to ground, which is correct, right?
One thing I am noticing is that voltage is swinging on the grid of the 845. When I start to bring up the 845 B+ with the variac I immediately see the ammeter bouncing, enev when there is relatively low voltage on the plate of the 845.
Still no resistor between the top and bottom leg of the IT secondary... The cap (100uF electro) is present with the positive terminal to ground and the negative terminal to the bias which also conncets to the lower leg of the IT secondary as in the schematic....
Hmmm....
A final thought/question. What happens when one wires the plat to the grid, and the grid to the plate???
I've just checked the OPT wiring with the terminals and the test speaker. All seems fine. There are no shorts from the positive terminal to ground. There is also no connection referrencing the negative speaker terminal to ground, which is correct, right?
One thing I am noticing is that voltage is swinging on the grid of the 845. When I start to bring up the 845 B+ with the variac I immediately see the ammeter bouncing, enev when there is relatively low voltage on the plate of the 845.
Still no resistor between the top and bottom leg of the IT secondary... The cap (100uF electro) is present with the positive terminal to ground and the negative terminal to the bias which also conncets to the lower leg of the IT secondary as in the schematic....
Hmmm....
A final thought/question. What happens when one wires the plat to the grid, and the grid to the plate???
Your IT wiring should be fine. Swapping the plate and grid wires would usually bring on a short unhappy life for the 845. If you can set your tube current by adjusting the bias, this is NOT the problem.
I am assuming that the voltage swinging on the grid is due to signal input. This is normal. The voltage on the grid, and the plate current should be steady with no signal. The plate current will change with signal in normal operation, most analog meters won't move fast enough to show it though.
The resistor on the IT is not needed to make sound.
The negative speaker connection should be grounded for safety reasons, but it is not necessary for amplifier operation. If your transformer ever developed a short from primary to secondary it would put 1000 volts on your speaker leads. The amplifier will keep on working normally until an unlucky person touches the speaker terminal. This is an unlikely event, but I have seen it happen! Grounding the negative lead will blow the power supply fuse.
It still seems to me that the amplifier (up to the OPT) is working. Try measuring the resistance between the speaker terminals (speaker disconnected) you should still get 1/2 ohm. Just to make sure measure the resistance of your speaker (including the wire) right at the point where you would plug it into the amp.
Are you sure that the sound is coming from the IT and not the OPT? Don't fry yourself by puting your ear too close to the HV.
I am assuming that the voltage swinging on the grid is due to signal input. This is normal. The voltage on the grid, and the plate current should be steady with no signal. The plate current will change with signal in normal operation, most analog meters won't move fast enough to show it though.
The resistor on the IT is not needed to make sound.
The negative speaker connection should be grounded for safety reasons, but it is not necessary for amplifier operation. If your transformer ever developed a short from primary to secondary it would put 1000 volts on your speaker leads. The amplifier will keep on working normally until an unlucky person touches the speaker terminal. This is an unlikely event, but I have seen it happen! Grounding the negative lead will blow the power supply fuse.
It still seems to me that the amplifier (up to the OPT) is working. Try measuring the resistance between the speaker terminals (speaker disconnected) you should still get 1/2 ohm. Just to make sure measure the resistance of your speaker (including the wire) right at the point where you would plug it into the amp.
Are you sure that the sound is coming from the IT and not the OPT? Don't fry yourself by puting your ear too close to the HV.
It's a Tamura transformer right?
Make sure you don't have the "E" or "G" connection confused with the "0" connection on the secondary. The speaker needs to be attached between the "0" connection and the "8" or whichever tap you're using. The "E" or "G" connection is for grounding the transformer case.
I made this mistake with my Tamura trans and had exactly the symptoms you describe.
Make sure you don't have the "E" or "G" connection confused with the "0" connection on the secondary. The speaker needs to be attached between the "0" connection and the "8" or whichever tap you're using. The "E" or "G" connection is for grounding the transformer case.
I made this mistake with my Tamura trans and had exactly the symptoms you describe.
I hope you get this figured out. I'm sorry that my advice appeared to be stupid because it was based on a schematic that I was referred to (post 226) that is missing circuit paths and a bias supply.
As shown, the capacitor is interupting the DC path from the 845 grid and not only will class A2 not be possible, nor will A1. In fact, that circuit as presented has no class at all! My advice based on what I was shown, to nix the cap was not wrong.
Please try to support questions in the future with better information.
As shown, the capacitor is interupting the DC path from the 845 grid and not only will class A2 not be possible, nor will A1. In fact, that circuit as presented has no class at all! My advice based on what I was shown, to nix the cap was not wrong.
Please try to support questions in the future with better information.
RCA... No woriies what so ever. I appreciate all advice. We are all trying to help each other, right? Your suggestion about posting clear concise questions is well taken. I will try to be more clear.
Tubelab and/or others...
Resistance at the speaker terminals (speakers disconnected) = 0.2ohm
Resistance across speaker leads (including speaker wire) = 4.3ohm
I am sure the audible sound (music) I hear is coming from the IT, as I still hear the sound with NO B+ to the 845.
Jeff Mai, It is a Tamura. I have 0, 4, 8 and 16 taps on one side of the txfmr and P and B on the other side. I had the negative terminal attached to the "0" terminal and the positive terminal attached to the "8" terminal (although it now appears my test speaker is a 4 ohmer???. The "P" connection goes to the plate of the 845, while the "B" connection attaches to the B+ at the last cap in the CLCLC filter.
I may try it with the cap removed on the secondary of the IT, as A1 would be nice to hear over nothing at all...
Thoughts?
BK
Tubelab and/or others...
Resistance at the speaker terminals (speakers disconnected) = 0.2ohm
Resistance across speaker leads (including speaker wire) = 4.3ohm
I am sure the audible sound (music) I hear is coming from the IT, as I still hear the sound with NO B+ to the 845.
Jeff Mai, It is a Tamura. I have 0, 4, 8 and 16 taps on one side of the txfmr and P and B on the other side. I had the negative terminal attached to the "0" terminal and the positive terminal attached to the "8" terminal (although it now appears my test speaker is a 4 ohmer???. The "P" connection goes to the plate of the 845, while the "B" connection attaches to the B+ at the last cap in the CLCLC filter.
I may try it with the cap removed on the secondary of the IT, as A1 would be nice to hear over nothing at all...
Thoughts?
BK
With the amp unplugged from the mains, Discharge the supply caps. Pull the 845 and set it aside. Get a 9 volt smoke alarm battery and with clip leads hook it temporarily between the plate and B+ terminals on the output xfmer. When you make and break the contact from the battery you should hear a faint clicking in the speaker. if you hear nothing you have a dud output xfmer or a wiring error associated with it.
I think you should be seeing more than you are for DC resuistance on the speaker terminals of the output xfmer. I read 1.2 ohms on the 4 ohm output of my SET amp currently available to measure on the living room floor with the speaker disconnected. It sounds to me like you are measuring a short circuit. As a test if you still have no sound, hook the speaker to the 4 ohm tap and the 8 ohm or 16 ohm taps, forgetting about the ground tap entirely. If this gets you clicking sounds with th battery then procede to try the amp. It won't hurt anything to operate the output xfmer hooked up to the speaker this new way until you figure out what is up.
Don't bypass the capacitor on the bottom of the IT secondary if you have a bias supply hooked up across it. You will short out the bias supply.
I think you should be seeing more than you are for DC resuistance on the speaker terminals of the output xfmer. I read 1.2 ohms on the 4 ohm output of my SET amp currently available to measure on the living room floor with the speaker disconnected. It sounds to me like you are measuring a short circuit. As a test if you still have no sound, hook the speaker to the 4 ohm tap and the 8 ohm or 16 ohm taps, forgetting about the ground tap entirely. If this gets you clicking sounds with th battery then procede to try the amp. It won't hurt anything to operate the output xfmer hooked up to the speaker this new way until you figure out what is up.
Don't bypass the capacitor on the bottom of the IT secondary if you have a bias supply hooked up across it. You will short out the bias supply.
Can you post the complete schematic....
I don't have anything showing how the bias circuits....
Also the 845 heater supply is critical on how you bypass that, since that is passing the AC signal current, ie..audio signal... The hot cathode needs to be at AC Ground potential...
I would also bypass the 100uF with a smaller cap for better high frequency response...such as a .1uF or .01uF ....
Chris
I don't have anything showing how the bias circuits....
Also the 845 heater supply is critical on how you bypass that, since that is passing the AC signal current, ie..audio signal... The hot cathode needs to be at AC Ground potential...
I would also bypass the 100uF with a smaller cap for better high frequency response...such as a .1uF or .01uF ....
Chris
Here is the 845 heater circuit. The 0 VDC is referrenced to earth at the fillament pin.
I dont have a proper schematic for the bias. It is a simple CRC supply feeding a voltage divider of 5K - 10K pot - 10K. I take the bias from the wiper on the pot directly to the grid of the 845.
I should mention that I am now hearing some faint distorted sounds from the test speaker...
The 9 volt battery test gives me no clicking at the speaker.
I dont have a proper schematic for the bias. It is a simple CRC supply feeding a voltage divider of 5K - 10K pot - 10K. I take the bias from the wiper on the pot directly to the grid of the 845.
I should mention that I am now hearing some faint distorted sounds from the test speaker...
The 9 volt battery test gives me no clicking at the speaker.
Attachments
Bryan....
I don't see how the CM filter will do much where you have it located.... The Audio is CM at the Cathode.... In fact you may get poor high frequency response with it located there...Remove it from that location...For effective CM rejection filtering you need to put the CM choke in the AC lines after the transformer secondary but before the rectifier bridge....then you need to put in Y-caps to ground..... Now you will have effective CM filtering, which is used to remove noise that is generated in the Hot and Nuetral...
To test the transformer out..make sure it is completely disconnected...apply AC signal across the primary..such as your 120V mains voltage at 60HZ, 60 Hz will be fine for the test...then measure the AC voltage you get across your unloaded secondary side, such as 0 to 8 ohms.... Then take the AC voltage you measure across the primary and divide by seconday AC voltage..this is turns ratio...then sqaure that and multiply by 8 and you should roughly get 10K .....
ALso...make sure your jumbo 4-pin socket is wired correctly to the proper pin #'s ......
Chris
I don't see how the CM filter will do much where you have it located.... The Audio is CM at the Cathode.... In fact you may get poor high frequency response with it located there...Remove it from that location...For effective CM rejection filtering you need to put the CM choke in the AC lines after the transformer secondary but before the rectifier bridge....then you need to put in Y-caps to ground..... Now you will have effective CM filtering, which is used to remove noise that is generated in the Hot and Nuetral...
To test the transformer out..make sure it is completely disconnected...apply AC signal across the primary..such as your 120V mains voltage at 60HZ, 60 Hz will be fine for the test...then measure the AC voltage you get across your unloaded secondary side, such as 0 to 8 ohms.... Then take the AC voltage you measure across the primary and divide by seconday AC voltage..this is turns ratio...then sqaure that and multiply by 8 and you should roughly get 10K .....
ALso...make sure your jumbo 4-pin socket is wired correctly to the proper pin #'s ......
Chris
Bryan said:
No, no. The wiper output on the bias adjustment pot shud go to the bottom point on the IT secondary where the capacitor is connected and the other end of the cap is grounded. If the pot wiper is tied directly to the grid it is shunting the audio feed to the 845, therefore no audio.
The 845 grid shud only go directly to the top of the IT secondary winding. A resistor across the secondary of the IT is optional and will only improve the sound, once you get it working.
I fear that you have a short on the secondary of your output xfmer. Look for a short or solder blob as another suggested, Did you try hooking the speaker between ohm taps as I suggested? Try lifting ground off the "0" terminal just for test purposes.
Ok Guys, Time for me to take a break....
RCA. You are correct. The bias feeds the bottom leg of the IT secondary. My mistake in wording.
I did however notice a strange phenom. When I turn up the input signal, I am seeing a bright focused light in the 845 near the center where a metal post passes through the graphite part. Maybe I have a bad tube?
I'll try a different tube and see what happens. I also changed to the 4 ohm tap and I do hear a very faing clicking with the 9 volt battery, but very faint.
BK
RCA. You are correct. The bias feeds the bottom leg of the IT secondary. My mistake in wording.
I did however notice a strange phenom. When I turn up the input signal, I am seeing a bright focused light in the 845 near the center where a metal post passes through the graphite part. Maybe I have a bad tube?
I'll try a different tube and see what happens. I also changed to the 4 ohm tap and I do hear a very faing clicking with the 9 volt battery, but very faint.
BK
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