I've got a DIY, KT-88, push pull, mono bock that has, apparently, developed a short and I could use some help in tracking it down. When the tube heaters get to operating temp, the main power supply drops from from 135 vdc to around 120, B+ from 450 to 350, and ac output from 20vrms at eight ohms to 10. I plugged it into my dim bulb tester and get a small glow. With the output section disconnected, all voltages (ac and dc) in the power supply and driver board are correct. All connections from the driver board to the power tubes to the OPT seem fine. There is continuity among the leads on each side of the transformer, but not between the primary and secondary. The binding post assembly is detachable from the transformer and the problem persists when that assembly is detached.
I've run out of stuff to test; ideas/advice would be appreciated.
I've run out of stuff to test; ideas/advice would be appreciated.
How about running the KT-88s in one channel but not the other, then do the same for the other channel.
Keep in mind that you could have an issue inside the amp, or you could have a tube issue.
If you do that test on the first side, be sure to use that same pair of tubes on the second side. If you get the same results for both channels (good or bad), then swap to the other pair of tubes and recheck.
Keep in mind that you could have an issue inside the amp, or you could have a tube issue.
If you do that test on the first side, be sure to use that same pair of tubes on the second side. If you get the same results for both channels (good or bad), then swap to the other pair of tubes and recheck.
What do you mean by this? That beside B+ there is another DC voltage of (only) 135Vdc to feed part of the amp? Or is it a negative voltage to bias the KT88's?
If the KT88's are OK, than I would suspect one or more capacitors in the power supply (maybe one or more got leaky).
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Check the plate current for each tube (perhaps over the cathode resistor). Something seems to cause a high current draw and it could be a tube.
Regards, Gerrit
Regards, Gerrit
It'a a mono block amp, so I cannot run one channel at a time. However, I did swap tubes with the other amp and no luck.
Yes, the main power transformer has three sets of secondary leads: a HV center tap for B+, a 6.3vac for tube heater, and a third dc center tap that is not used. Instead, a second, power amp is employed to make 135 vac + and - to run the 6cg7s and the rest of the driver section. The amps have been running fine for over a year, so component deterioration is possible, either in the power supply or the driver board. As noted, with the output section not connected, the power rails deliver 135 vdc + and -, so I think the ps is probably ok. My suspicion is that that any component failure is more likely something on the driver board. However, all checkable dc readings are ok. Clearly, once the system comes under load, something becomes power hungry. Maybe a power tube, a bad wire or connection, or a component: like a transistor or voltage regulator. Pretty tough to find a bad component when the available check points read ok.
What I could do would be to swap the driver boards between the amps and see. Fair amount of work, but doable if necessary.
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Do you have a way to measure the current of each KT-88? Typically on a fixed bias amp, you would use a 1 or 10 ohm resistor between cathode and ground, then measure the voltage across that.
Bias is adjustable. My understanding if that the range for KT-88 runs from around 40mA to around 65. I usually run at 50-55 mA. More and more I'm thinking I'm going to have to swap driver boards. That way I could identify or eliminate the driver board as the culprit. If that is the problem, I'll probably have to start replacing parts.
Didn't think until now, I should probably swap the 6cg7s around, just to be sure; but, I'm not hopeful.
Didn't think until now, I should probably swap the 6cg7s around, just to be sure; but, I'm not hopeful.
450-350=100, 100/450=22% usually only 10% drop. Sound like the short is in OT primary winding, you can compare DCR with the other amp and is OT hot?
This big drop of DC voltage shouldn't happen unless something is drawing excessive current or you have a leaking cap, which should show up as a DC voltage in the circuit that is incorrect, or possibly you have an oscillation. Are you able to scope the output?
I've been reluctant to drive the amp for any extended period, so I don't know about OPT overheating, but I'll check that right away. The ac output wave is smooth, but, just half the voltage it should be. I've not dealt with DCR, but there's no time like the present to learn. Thanks for the suggestions.
FWIW, I was away for several hours this afternoon and left the amp on at idle. There was no noticeable heat in the OPT.
OT will heat up only when it driven by signal. There are many types of short like aching, inter-winding short near end or far end, all result in abnormal current drawn and power consumption.
not unheard of, drops like that happens....the question is how stiff is your power supply, what are the series resistances along the b+ line....
tube psu is pretty simple to figure out if you know tube loading and the psu impedances along the b+ line...
you did ot give us enough information to be able to help you....a schematic for starters and a picture of your amp goes a long way...
btw, you gave me that impression that your power traffo is normal by itself....but we need more from you...
Is there a way to test the OPT? (Perhaps it relates to DCR which I'm not finding much about on line.) Removing the trans from the chassis would be cumbersome; but secondary leads are connected to the binding posts via a four wire molex so I've got clean access to them. Naturally, the primary leads are soldered in place, but I would be willing to disconnect all of them if I could identify that the trans was (or was not) the culprit.
Along the same lines, I could swap both the the driver boards and the power supply boards between the two amps. Obviously these steps would take a little time, but it seems to me that eliminating major sections of the amp would be preferable to digging into schematics, discussing amp design, etc.
Thanks
how to check a transformer - YouTube
Watch this video, it shows how to measure DCR and others. There is no need to disconnect the transformer wire lead except where there is resistor parallel with the winding. All needed is to unplug the output tubes, the schematic of output stage such as one attached or at least the pin-out of the tubes and measure across the pin out and output posts with DMM in resistance scale. Beware of the high voltages even after switch off you need to discharge the capacitors on the HT and use only one hand.
Watch this video, it shows how to measure DCR and others. There is no need to disconnect the transformer wire lead except where there is resistor parallel with the winding. All needed is to unplug the output tubes, the schematic of output stage such as one attached or at least the pin-out of the tubes and measure across the pin out and output posts with DMM in resistance scale. Beware of the high voltages even after switch off you need to discharge the capacitors on the HT and use only one hand.
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Thanks for the information. Testing was very straightforward and only took a few minutes. All of the resistance specs listed on the datasheet checked out well within the 20% tolerances listed. So, I assume I can remove the output transformer from the suspect list. I'm going to proceed to swap the input section boards and see what happens.