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First I would like to thamk ENZO for answering my PM, and am moving this to a new thread as he suggested. Here is the original PM and reply:
Enzo
Senior Member
Join Date: May 2006
Location: Lansing, Michigan, USA
Posts: 1,030 Re: Alamo hot chassis
Well, I don't mind a PM, but why not just start a new thread and let us all discuss it? Then you get insights from the rest of the group too, instead of just me.
Do not confuse hot chassis with two-wire power cords in general. A two wire power cord is just that. A two wire can power a hot chassis or not. Hot chassis means ther is NO POWER TRANSFORMER. The 120v mains are directly rectified. Typically the tube heaters will be high voltage types and wired in series, their heater voltages adding up to 120v, then the whole string is wired across the 120VAC. Usually it requires a resistor to get the voltages right. That is why there are tubes like the 50L6 and 35Z5. Add three 12v tubes to that, and voila - 120v (OK OK 121)
SInce the mains are directly rectified, one side or other of the mains will wind up on your chassis. 50/50 chance it will be the neutral instead of hot.
If you have a power transformer for all the working voltages it is unlikely you have a hot chassis. In case it is confusing, you can get a shock from a non-hot chassis just from leakage currents. Just that hot ones are far more dangerous.
The amp hums without tubes, yes? Does this amp have a field coil speaker? That is instead of a permanent magnet there is a large wound coil on the back of the speaker. Or in other words, there are four wires to the speaker instead of just two.
Field coil speakers make their magnetic field with an electromagnet - current through a coil of wire. This replaces the permanent magnet we see in modern speakers. The field coil gets its power from the same B+ supply as the tubes. In some it is wired in series like a power supply choke. In fact it acts like a choke. Others have it wired from B+ to ground. Either way, if your filter caps are bad, or there is some large leakage to ground from B+ causing current through the coil in excess. It will make the speaker hum. Is this the case?
OH wait, hums through your test speaker? Sorry, I just wasted some time, I hope it was interesting though
The output transformer feeds the speaker, if it drives a hum signal into the speaker, then some current must be flowing in the primary or is being forced into the secondary directly. With power removed, check for shorts or leakage from the primary winding to the frame, and to the speaker winding. Disconnect stuff from chassis. Ther should be infinite resistanxe between primary and secondary
Power transformer? It makes 6v for the heaters and high voltage for the B+. I don't know what readings you are expecting, but I couldn't tell you what any particular transformer ought to measure. The 6v winding will have almost no resistance, less than an ohm usually. The HV will have more resistance, but it all depends upon the gauge of wire and how many turns of it are in the transformer winding.
I look for open windings: bad. windings shorted to one another or to frame: bad. windings intact showing continuity: good.
Then I apply 120VAC to the primary and see what comes out the secondaries. Hopefully about 6v and maybe a couple hundred on the other. I also monitor AC current through the primary during this test. It ought to be negligible. If the transformer draws more than just a tiny bit of current sitting there with the secondary wires not connected to the circuit, then it has issues.
What has confused you from the debug page?
Quote:
Originally Posted by gtrplr2 Hello,
I hope you do not mind the PM. You answered a post I had about a month ago about an Alamo Paragon Special I am working on. I read your post about the other person that has an Alamo, and the hot chassis came up, and I did not want to hi-jack his thread. The amp I have came to me with no power cord and some missing parts. I wired the power cord like I have many of times when replacing the 2 prong type cords with 3 prong. I can pull all the tubes out of the amp and switch on the power switch, and immediately I get a hum out of my test speaker. Is it possible that I have the power cord wired wrong for this type of chassis? I am also trying to determine if the power transformer is good, do you have a method for testing the resistance of the leads when they are unplugged from the sockets? I followed the directions from the amp de-bugging page, but I am sort of confused as my reading are way different than those directions specify. Thank you.
gtrplr2
Enzo
Senior Member
Join Date: May 2006
Location: Lansing, Michigan, USA
Posts: 1,030 Re: Alamo hot chassis
Well, I don't mind a PM, but why not just start a new thread and let us all discuss it? Then you get insights from the rest of the group too, instead of just me.
Do not confuse hot chassis with two-wire power cords in general. A two wire power cord is just that. A two wire can power a hot chassis or not. Hot chassis means ther is NO POWER TRANSFORMER. The 120v mains are directly rectified. Typically the tube heaters will be high voltage types and wired in series, their heater voltages adding up to 120v, then the whole string is wired across the 120VAC. Usually it requires a resistor to get the voltages right. That is why there are tubes like the 50L6 and 35Z5. Add three 12v tubes to that, and voila - 120v (OK OK 121)
SInce the mains are directly rectified, one side or other of the mains will wind up on your chassis. 50/50 chance it will be the neutral instead of hot.
If you have a power transformer for all the working voltages it is unlikely you have a hot chassis. In case it is confusing, you can get a shock from a non-hot chassis just from leakage currents. Just that hot ones are far more dangerous.
The amp hums without tubes, yes? Does this amp have a field coil speaker? That is instead of a permanent magnet there is a large wound coil on the back of the speaker. Or in other words, there are four wires to the speaker instead of just two.
Field coil speakers make their magnetic field with an electromagnet - current through a coil of wire. This replaces the permanent magnet we see in modern speakers. The field coil gets its power from the same B+ supply as the tubes. In some it is wired in series like a power supply choke. In fact it acts like a choke. Others have it wired from B+ to ground. Either way, if your filter caps are bad, or there is some large leakage to ground from B+ causing current through the coil in excess. It will make the speaker hum. Is this the case?
OH wait, hums through your test speaker? Sorry, I just wasted some time, I hope it was interesting though
The output transformer feeds the speaker, if it drives a hum signal into the speaker, then some current must be flowing in the primary or is being forced into the secondary directly. With power removed, check for shorts or leakage from the primary winding to the frame, and to the speaker winding. Disconnect stuff from chassis. Ther should be infinite resistanxe between primary and secondary
Power transformer? It makes 6v for the heaters and high voltage for the B+. I don't know what readings you are expecting, but I couldn't tell you what any particular transformer ought to measure. The 6v winding will have almost no resistance, less than an ohm usually. The HV will have more resistance, but it all depends upon the gauge of wire and how many turns of it are in the transformer winding.
I look for open windings: bad. windings shorted to one another or to frame: bad. windings intact showing continuity: good.
Then I apply 120VAC to the primary and see what comes out the secondaries. Hopefully about 6v and maybe a couple hundred on the other. I also monitor AC current through the primary during this test. It ought to be negligible. If the transformer draws more than just a tiny bit of current sitting there with the secondary wires not connected to the circuit, then it has issues.
What has confused you from the debug page?
Quote:
Originally Posted by gtrplr2 Hello,
I hope you do not mind the PM. You answered a post I had about a month ago about an Alamo Paragon Special I am working on. I read your post about the other person that has an Alamo, and the hot chassis came up, and I did not want to hi-jack his thread. The amp I have came to me with no power cord and some missing parts. I wired the power cord like I have many of times when replacing the 2 prong type cords with 3 prong. I can pull all the tubes out of the amp and switch on the power switch, and immediately I get a hum out of my test speaker. Is it possible that I have the power cord wired wrong for this type of chassis? I am also trying to determine if the power transformer is good, do you have a method for testing the resistance of the leads when they are unplugged from the sockets? I followed the directions from the amp de-bugging page, but I am sort of confused as my reading are way different than those directions specify. Thank you.
gtrplr2
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