Hi all!
I've been going through some of my family's old stereo gear, and one of the systems is a Luxman CL35mk3 preamp with a MQ68C 30W amplifier. Although I have a distant background in electical and electronic circuits (a degree in EE long ago), I know nothing about tube circuitry.
Both seem to work fine at first, but then I noticed that the volume on one speaker was louder than the other. Upon opening the MQ68C, I noticed that some of the old trim potentiometers had broken so the wiper was no longer contacting the resistive strip. Though replacements were at first hard to find, I did find some at Digikey and went about removing the broken originals and replacing them.
I downloaded the service manual and, using the steady-state voltages, I began to check DC voltages at the cathode resistors. At first everything seemed to be operating as expected; with a little adjustment of the appropriate trim pot, voltages would be in line with the service manual.
Then, on the last tube I checked, there was almost zero volts (0.01V) across the resistor coming from the tube's cathode. Not knowing if the problem was with the tube or with the circuitry, I switched the position of the tube, and the same reading was found in the new position, so whatever is happening is definitely linked to the tube.
Visually the tube looks okay; It glows orangey-red (and not purple). I don't see any visible cracks or discoloration, so I thought I'd post here and see if there are any other possibilities I am missing. I would really hate to have to find a replacement set, especially if all the other tubes are in good condition!
Thanks!
I've been going through some of my family's old stereo gear, and one of the systems is a Luxman CL35mk3 preamp with a MQ68C 30W amplifier. Although I have a distant background in electical and electronic circuits (a degree in EE long ago), I know nothing about tube circuitry.
Both seem to work fine at first, but then I noticed that the volume on one speaker was louder than the other. Upon opening the MQ68C, I noticed that some of the old trim potentiometers had broken so the wiper was no longer contacting the resistive strip. Though replacements were at first hard to find, I did find some at Digikey and went about removing the broken originals and replacing them.
I downloaded the service manual and, using the steady-state voltages, I began to check DC voltages at the cathode resistors. At first everything seemed to be operating as expected; with a little adjustment of the appropriate trim pot, voltages would be in line with the service manual.
Then, on the last tube I checked, there was almost zero volts (0.01V) across the resistor coming from the tube's cathode. Not knowing if the problem was with the tube or with the circuitry, I switched the position of the tube, and the same reading was found in the new position, so whatever is happening is definitely linked to the tube.
Visually the tube looks okay; It glows orangey-red (and not purple). I don't see any visible cracks or discoloration, so I thought I'd post here and see if there are any other possibilities I am missing. I would really hate to have to find a replacement set, especially if all the other tubes are in good condition!
Thanks!
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Since the lack of cathode current follows the tube, you'll need to replace the tube. Those are very, very expensive tubes!
It's a bad tube, so start looking on ebay, etc. Maybe someone has a single good tube for sale.
Otherwise, you can sell the amp on ebay as-is and get a different one.
Make sure the resistors for measuring current are still good, they can be damaged by a bad tube.
This equipment is from the 70s-80s, so they got their money's worth of use out of them.
Maybe you can sell both pieces together as a package, but check the typical resale prices first.
If possible, replace (only) the bad tube before selling them, since then their value would be much higher.
Look for the original boxes and manuals as well.
Otherwise, you can sell the amp on ebay as-is and get a different one.
Make sure the resistors for measuring current are still good, they can be damaged by a bad tube.
This equipment is from the 70s-80s, so they got their money's worth of use out of them.
Maybe you can sell both pieces together as a package, but check the typical resale prices first.
If possible, replace (only) the bad tube before selling them, since then their value would be much higher.
Look for the original boxes and manuals as well.
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Thanks so much for the quick replies! I guess it's just as I had expected.
I'll look further into replacements, etc.
I'll look further into replacements, etc.
If you buy new or used tube(s), only do so with a right of refund if it (they) do not work properly.
Use a credit card or Paypal.
Use a credit card or Paypal.
The safest long term course to take is modification, by replacing the defunct 50C-A10 tubes with readily available 6CA7/EL34 types. Octal sockets will be needed as well as a suitable heater transformer. The latter item can be fitted in the position of the C-1744 choke, which should be removed as part of the exercise along with changes to the B+ filter components. I believe the OY15-6 output transformer has Ultra Linear taps and if this is correct then this connection option would be worth trying.
That would destroy the value of the amplifier. He cannot do such redesign work anyway,
even if this suggestion has actually ever been done successfully, which is not the case.
even if this suggestion has actually ever been done successfully, which is not the case.
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China is manufacturing That tube again if you can't find good NOS. The amps value and quality is probably worth recovering for the $170 cost of a new pair, especially now that you've fixed the issues.
Shuguang 50CA10 new Hi-Fi tubes
Shuguang 50CA10 new Hi-Fi tubes
I am looking for replacements for the 50C-A10 in my Luxman 38FD. The tubes in it are good right now. May have to find a good reputation tube seller and get a quad of these to try out. Shuguang 50CA10 new Hi-Fi tubes
I have nine 50CA10 and some of them are bought from seller at AliEXpress. Even there they are getting more and more rare and will set you back a hundred dollars.
NOS tubes from Japan will probably require a kidney and a pair cornea and a major deposit at a Maltese bank.
NOS tubes from Japan will probably require a kidney and a pair cornea and a major deposit at a Maltese bank.