New MC30 owner with red hot outlet tubeHello AK Mac Heads. First time poster, long time listener. First, thanks to you all for helping me take the plunge into vintage tube gear. I have found AKers to be great resources and the service this board provides is excellent. Now on to my issue.
I recently purchased a pair of excellent condition MC30s (serial #s 19023 and 23297) that were awaken from a 10-15 year hibernation. They were serviced with new caps, new can, selenium diode replacement, NOS tubes and thorough cleaning. After about 10 hours of use, ser# 23297 developed a red hot outlet tube (I think V5 on pin 3, but don't quote me on that) and Xfmr M152B was very hot to the touch. I have attached a few pictures and a marked up schematic. Note the first picture showing the glowing red filament in the one outlet tube, all other tubes appeared normal. I switched tubes around and determined it was the socket and not the tube that was the problem. The amps sounded wonderful and I did not detect any difference in sound quality. I have shutdown the amp and plan to have it checked out, but I am curious about what the Mac experts and enthusiasts think has gone wrong here? Perhaps you can instruct me in a little DIY.
Feel free to comment on what you see in the pictures.
You have lost bias on that output tube which if run for a minute or more, while bright red, is now damaged and will be weak. Have your tech remove the output tubes and check all tube socket pin voltages and compare to the schematic.
Repair or replace any out of spec parts and do not run the amps until the voltages are correct.
Thanks Ron. The amp is on it's way to the tech's shop and he believes it is something very simple. He agrees with your diagnosis and will check the resistors against spec. I will report back once it is fixed and back singing in my home office.
I am studying these beauties and after I get some time with them I will gradually get to DIY status. Very neat being able to restore something as magical and industrial as these things. The learnings I get from this forum is terrific, I hope I can give back a little in the future.
Nice looking pair of '30's.
I hope those are not the Chinese (Sequa) PIO capacitors in the last stage, I have had 2 out of 4 fail under high voltage operating conditions, they became leaky when warm but checked good when cold.
Murray - thanks, their cosmetics are great and I haven't had much of a chance to judge their sound yet, but I will. I have also just sourced a near mint MX110 in completely original condition. I am considering a DIY restore on the MX110, little bits at a time to judge the differences as some AKers have suggested. I will report back if I take that route.
Terry - yes, those are Sequas. What is the best way to test them and what do you recommend as replacements if /when needed? My DMM doesn't measure capacitance as best I can tell.
Problem I had was they check good on the Sencore 'Z Meter' at room temperature, correct capacitance, and no leakage. After an hour in the amplifier I had a red plate (ate a 6BQ5), a quick check with the cap warmed up from the heat under the chassis showed it had leakage, Simpson read about 1Meg ohm, enough to kill the bias to the output tube.
Order up the orange drops please!
Yum, Ron.....got any cherry drops?
For our OP, mjmCVX, unless you're well-versed in electonic restorations, I'd suggest you start with the MC-30s for a restoration project. They're easier to work on. Its much more cramped in the MX-110.
Thanks for all the tips Fellas. Update on the red hot outlet tube. Terry and Ron were spot on, leaky C10 and a marginal resistor. The amp is being burned in this weekend and hopefully back to me Monday or Tuesday. I am getting tired of listening to stereo thru one channel (nice feature of the MX110)......
The other MC30 has been run for about 20-25 hours and all appears well, but I plan to change the Sequa PIOs to other better quality PIO or orange drops.
Dave - understood and good guidance. I have found a local tube tech to help me with the MX110, but right now it has been running for 20-25 hours with no signs of electrical problem. I will be doing the cap upgrades for reliability and sound improvement as described in this most excellent AK post
The tube tech is a good hand, but not up on all this AK Mac intelligence. I plan to do the MX110 little bits at a time (power supply first, then the phono stage, etc) so I will chronicle each step to give feedback on each improvement.