A good first step for him is to just measure resistance from B+ bus to ground, that’ll at least reveal if its a dead short or a few ohms. Then start testing the electrolytics . Alternately, disconnect the B+ bus lines one at a time. Great video, thanks!
that choke is interesting but not realy surprising. those output transformers are equaly intricate. and the first schematic i downloaded has the same shorted resistor and 150k resistor. thanks for sharing. your videos are always interesting.
I am glad I did not change the circuit in the amplifier and "fix" it... I should know not to make decisions bases on a schematic, especially as well as the amplifier performed. You guys help solve a lot of problems for me. 🙂
Greetings. This comment is more relevant to your ham radio builds - I work professionally doing electrical field repairs and testing (utility transformers, switchgear, distribution etc) and we recently got to work at the Jim Creek VLF transmitter station. They were kind enough to show us around, and being an amateur AF amp nerd / hobbyist I was on the moon checking out the equipment. Thought often about what you would think. If you ever would like to come up to Washington state, I’d be happy to see if a tour can be arranged.
That certainly sounds interesting. I fluctuate in my interests between AF and RF but ultimately lean towards the RF world. Small uH and pF values in the RF world and huge soft-iron-core inductors in the AF world. One is as complicated as the other. Unfortunately, El Paso is a long way from everywhere - I certainly appreciate the invitation and if I get up you way I will do my best to look you up. Invitation is the same for you if you come to El Paso. Around here we have the VLA, public.nrao.edu/visit/very-large-array/
Good stuff thanks. I just picked up 2 MC40's. One works fine, the other blows the fuse. I'm going to start with replacing the caps. Although my basic tests look good. Digging in a bit more and learning. SR1 and SR2 test good. Maybe SR3, I need to find it and test next. I went to college and have an AA in electronic and one in computer science. I went the computer science route for work and never really did electronics past college and fiddling with stuff myself. Lots to learn.
Great find - congratulations - I would pull all the tubes in the one that blows fuses and eliminate them first. Since you have two and one is good, comparison measurements between them can make things easier. I would measure a point in the power supply right out of the rectifier using an analog meter - switching the + and - leads around and comparing to the good one. That will tell if you have a "short" to the chassis. Best of luck with your repair. You have some real jewels.
@@ElPasoTubeAmps I have switched all of the tubes from one to the other, with no change. So the tubes must not be the issue. I'll test for the short like you suggested. Thanks
12:30 There is a schematic of the MC40 amplifier labeled 154-298 where R26 is shorted and R27 is 150k. Both of these changes are probably explained in the service manual in Notes 2 and 5.
Thank you so much for that information - I had to hold myself back and not go in reckless and change things. We all know schematics and reality may be different. I don't think the amplifier can perform any better. I am surprised the performance was exactly the same with the switch in 117 VAC and 125 VAC. I suspect the clipping level might be slightly lower at the 125 VAC position. Stay safe and thanks again...
Just to add, the last page in the service manual has a replacement parts list. T2 is different below and above serial# 511LO. No mention of T2A or B. Still confusing. Maybe T2B became incorporated inside T2A... Hoping there is a McIntosh historian out there to explain it.
It’s been my understanding, largely from TomTekTest’s presentation of original literature published by the engineer responsible for them, that McIntosh amps perform so well due to a rather unique output transformer design. I’d have to go back and look up the articles from the 1960s (1950s maybe?), but it’d be neat to see a detailed overview of how their output transformers are designed and what makes them different.
No doubt their transformers are a huge part of the performance of McIntosh amplifiers. Also, the way they couple the output tube to the transformer it provides a load to the tube of about 1K instead of the common value of 5K. There is some good information to be found on the interesting design of Mac amps.
@@ElPasoTubeAmps I know they were among the first, along with Williamson’s original designs, to incorporate feedback circuits to reduce noise. Tom’s short series on the origins of hifi from 4-5 years ago are excellent resources into that. I’ve also dived into the 100 Tube Amplifiers online “book” you mentioned a while back. I even reached out to the author (in Denmark or Holland I think?) and we discussed putting it together in a more professionally packaged form to release as a proper ebook and website (I am a professional graphic artist and web developer). He was receptive to it, but we both got busy with other projects, and it’s been back-burnered. I need to reconnect with him, I think it’s a worthy project to dedicate some time to.
@Bill0656 Not sure. He was planning a new series of videos earlier last year, and asking for suggestions while demoing a series of function generators for a class he was scheduled to teach. When things went quiet, I assumed he was busy with teaching, as that wasn't unusual, but it's been a long time now. Several of us have posted well-wishes on his last video, but haven't received any response. I hope all is well, between this channel and Tom's, I have learned an immense amount about EE and vintage audio engineering--it's really incredible. I'm glad to hear I'm not the only person keeping Tom in their thoughts, hope everyone is healthy and safe!
Very interesting and enjoyable video! That amp is beautiful both inside and outside. Never seen that choke integrated in PT construction. However in many 50's tube radios the extra OT winding of SE output stage supposedly acts as a choke when screen and preamp and radio stages get their voltage filtered through it. BTW, when you drive that amp near clipping using for example 1kHz, can you hear the OT make clear audible noise at that same frequency or is it very silent?
I know you're measuring THD+N but I always check SNR separately. You can do that with the AA501: select 'Level' and 'dB ratio', press the 'dB Zero' button below it a few times until it zeroes, then turn the signal off and you have SNR in negative dB. I was wondering if it might improve if you un-short that resistor, but reading below I don't see any reason to even try it if it was a factory mod. Interesting that they already had the 10K in place. Probably put there by a piece worker and stockpiled, then the mod came in and they did the rework. I wonder if the choke is wound into the transformer or just mounted inside the same can. Keep well.
It is amazing what I sometimes learn about my own equipment when I post videos. I guess it is because I am taking a really close and serious look at it. We have always heard that if we really want to learn something - try to teach it - certainly seems to be truth in that saying. My HP 8903A does SNR and I thought about but just didn't do it. I have learned that even the connecting cables are critical in measuring a S/N ratio down to -90 dB and anything that I measure better than about -75 dB, I can't hear it on my efficient Klipschorns anyway. I was truly puzzled when I couldn't find the choke but luckily the schematic labeled the wires "orange" and that helped me follow it out. I am sure you are right about it just being part of an assembly line and the trouble to take a 10K, 2W resistor out was not worth the effort. I seem to remember this amplifier did not perform as well as it should (but did in this video) at the upper end of 20KHz but... maybe it was just the output tubes. I can't imagine the choke being a magnetic part of the OPT and assume it is just mounted inside the case to save space and work under the chassis. We can certainly drive ourselves crazy trying to understand the flow of every electron so I think I will just let this one rest... for now...
I have been following you for years and love your videos on these vintage amps. I have an MC40 that I refurbished about 6 years ago, and T2B is present. Most pictures I saw for this amp did not show it, though, and had me confused for some time. Can't remember all the details but based on the service manual (starting with serial# 100L1) that I downloaded there appears to be changes to the power supply design above and below serial# 428L7. My amp is serial#446L1 with T2B present - I would need to open it up to verify the other changes. Do you have the service manual - I am sure it will help clear this up. You will find it quickly with a google search. Hope this helps.
I am so glad you mentioned serial numbers. Mine is 240L8 so that might explain the choke being in the power transformer. I was like... what?? I can't explain why I thought it did not perform well. It's perfect. Later, after this video, I put on a Heathkit with push-pull 807 and tested at 16 watts (24 max output below clipping) and it is just as good as the Mac. I may post a video on it tonight. Thanks for your comments.
I have a Heathkit 807 amplifier that I have made a video of but it doesn't use an 83 MV rectifier. Maybe I can "clip" an 83 in and test it if that will help. Here is the link. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-GYdQ144Ie_8.html
@@ElPasoTubeAmps yes I love that video it would be cool to play some audio over it and maybe do a update video I love that amplifier I wish they made a kit that people could make but with the war in Russia I know tube prices have been crazy I’m so glad I got so many cases during Covid
@@jimcatanzaro7808 I was looking at the price of Hg rectifiers recently on Ebay and they are outrageous. I know this is an esoteric component for a hobby but prices just don't make sense anymore.
I have a debating question I don’t run my tube amplifiers out of my wall socket as it is 122vac +_ .5vac my amplifiers say 117vac and test the correct voltage on this output from my variac if I go to the wall socket it’s over 60volts at 475 this exceeds all my cap ratings and 50 over my 7189a tubes Do you agree how I run my amplifiers
Hi Jim, I agree that you don't want to exceed the ratings of the electrolytic capacitors. Lots of things to think about. An example is Dynaco amplifiers. They have always been (allegedly) known to have their electrolytic capacitors run very close to the operating voltage and can easily have warmup (no-load) voltage exceed this rating for a short time until the output tubes warm up and draw down the PS voltage. The 5AR4/GZ34 they use as a rectifiers has a long warm up time so this helps prevent/reduce over-voltage time giving the output tubes time to warm up. I have read articles where they clearly state that the most important factor in the life of electrolytic capacitors is the voltage they are run at in relation to their rated voltage. If it is possible to raise the voltage of those PS capacitors by putting two in series, for example, you could solve the problem of over-voltage on them. Just a thought. I run as many as 12, 470uF, 450 volt capacitors in series (each shunted with a 470K, 2W resistor). That gives me 5400 volts that I run at 4KV. Never had a problem. Lots of bad ideas on the Internet about replacing vacuum tube rectifiers with solid state diodes. A grand example, as a ham radio operator, is replacing the 5U4 and 5R4 in the PS of the S-Line radio equipment. This equipment was made in the '60's when line voltage was more in line with 110 or 115 volts. As you state, it is now 120 or better and then to raise it another 50 volts, or so, with SS diodes is Asking... for trouble. There will be a bit more power but the heat is not going to be a good thing. The PS capacitors are very conservatively rated but, why push it ? I've got some more videos to make concerning rectifiers that I hope make sense. I have been doing a bit of research on old rectifiers like the 80, 82 and 83 and the 5V4, 5AR4, and of course the 5U4. The 80 and 82 (the 82 and 83 being Hg) have a plate current rating around 125 mA for the 82. Too small for push-pull 6L6 level amplifier. The 83 and 5AR4 and even the 5U4 are rated at double that at 250 mA and are perfectly sized for a 30 watt output amplifier needing 200 mA at maximum output. I have made a statement before about line voltage to my friends that if my line voltage was 140 volts - I would trip the main breaker into my house - but what if my blood pressure is 160/100 .... (I am laughing...) It's not that high - it's actually OK - but for the example, we should go to a doctor because we will be running at "high-voltage" in our body - but would we go to the Dr. ? Voltage is synonymous with pressure so it is a good comparison. So, I do agree with you operating your equipment at a proper voltage - but what about the fridge and everything else? I have seen my line voltage as high as 122 but right now it is 116 in my backhouse. Thanks for keeping in touch. Stay safe.
I live in a apartment building in a city so I made a input box with led gauges on it measuring the voltage and current and it’s always the same it dips up and down more in the summer obviously the type 83 I’m using is running great I have had the amplifier running for 4 days now and everything is running good and med temperature I’m a little concerned because the 83 uses 3amp at 5v for the heaters and the 5AR4 uses 1.9 amp at 5v I figured the transformer would get hot but it actually runs cooler with the 83 I’m running the amp at 114vac that measures to factory specs
@@jimcatanzaro7808 Sounds good to me. I just made a video with a 5V4, 5U4, 83 and a SS version of a 5R4 for a vintage Heathkit amp running push-pull 807 in a Williamson design. Not sure I have the energy to post tonight. The 83 I have is the one I removed from my TV-7 tube tester some time back and replaced with 1N4007 from a sacrificed four pin tube. Makes the tube tester run a bit cooler and calibration is exactly the same with filament voltages spec dead on so, that is how I run my tube tester. I leave the 5Y3 in for screen and bias voltages.
