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Mcintosh MAC 1500 Bad tube? Bias Adjust?

SudoSpencer

SudoSpencer

Member
I came across a Mac 1500 the other day. After firing it up on a dim bulb tester and seeing no issues, I then proceeded to plug it in normally and try to set the bias. It's supposed to be .7v for all 4 of the output tubes. I tried this and 3 of them got there, the 4th tube won't go past about .55v. If I swap the location of the tube it still won't go above .55v. Is this indicative of a bad tube?

Should I try and just replace the single tube with a new one if so, or should I just go with 4 brand new tubes.

Thanks
 

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Also, could I just lower the voltage on all 4 of the tubes to say .5v? I'll just loose some output volume right but it would be safe to run?
 
Replace the tube as a minimum, though the best thing to do is replace all 4. When an output can not be set to its bias level it also can not make rated power and distortion goes up.
For longer tube life the voltage at the test point can be lower slightly to .6 V but this will give a sacrifice in performance, small but probably noticeable.
 
If you cleaned all the crud off I bet the top of that tube would be milky white, means air leaked in, it's toast. There is no sign of any getter left, the silver color inside of the other tubes. It's job is to combine with any left over gas from manufacturing and any gas that might migrate in.

BillWojo
 
BillWojo said:
If you cleaned all the crud off I bet the top of that tube would be milky white, means air leaked in, it's toast. There is no sign of any getter left, the silver color inside of the other tubes. It's job is to combine with any left over gas from manufacturing and any gas that might migrate in.

BillWojo
Click to expand...
Good eye Bill,
 
I think you both were right. I ended up bringing the tube to a local tube guru and he tested it and it was basically completely dead. Threw in a newer RCA (not the coin-base style) and biased everything again without issue this time.

I did notice a nice hum happening, and when swapping through the FM tuner section the hum became VERY loud. So I pulled all the tubes in the FM section and the hum went away. Looking closely at the tubes it appears that 3 of them are clear at the top, so air appears to have leaked into those also as @BillWojo says.


Would you all recommend replacing all electrolytic caps in this? From my understanding of the MAC 1500 this doesn't have a rectifier tube, so if I have leaky caps and say they short it won't destroy a rectifier tube but it could destroy a power transformer or something correct?
 
A clear top does not mean that a tube has "gone to air". RCA made 6CG7/6FQ7s and 12AU7s, possibly along with other miniature tube types, which didn't have the usual top getter. Even if a tube has a faded or toasted getter, as with the power tube that you had to replace, it doesn't mean that air has leaked in. Only a milky white getter indicates that.
-Adam
 
AdamAnt316 said:
A clear top does not mean that a tube has "gone to air". RCA made 6CG7/6FQ7s and 12AU7s, possibly along with other miniature tube types, which didn't have the usual top getter. Even if a tube has a faded or toasted getter, as with the power tube that you had to replace, it doesn't mean that air has leaked in. Only a milky white getter indicates that.
-Adam
Click to expand...
Thanks for the info. Its definitely helpful as I learn. I did replace the three I tubes that I assumed were bad and now the FM tuner is working spectacularly!

The entire MAC 1500 now functions well and sounds excellent. I'm just debating taking the time and effort to swap all electrolytic capacitors or if I just use it as it is now and hope for no failure.
 
SudoSpencer said:
I feel like I'm talking to myself mostly, but this could help someone.

I searched everywhere for those knobs, finally found this: http://www.tubebooks.org/file_downloads/McIntosh/knob_info.pdf
Which lists the Mcintosh Part numbers for knobs.
Then I searched that part number on google and found that Audio Classics had them:
http://www.audioclassics.com/cat.php?cat=Parts&p=16
They are on order and should arrive in a week!
Click to expand...

Good to know. Keep posting any updates because they are of interest so some.

Thanks,
 
I ordered all the parts to replace the electrolytic capacitors including the cap cans. Plan to work with someone more knowledgeable to replace the parts, I'll report back once it's done.
 
Hooray!

Spent about 4 hours last night replacing capacitors and then can capacitors. I originally planned to re-stuff them but decided against it and got new can caps from CE manufacturing:
40-40-40 replaced with this: https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-404040uf
50-50-50-50 replaced with this: https://www.tubesandmore.com/products/capacitor-ce-mfg-350v-50505050uf
60-40 replaced with this: https://www.tubesandmore.com/products/capacitor-ce-mfg-60-f525v-40525-10450-2550 (and ignored the 10 and 25 sections)

All the other parts I got at mouser.

I did all the work on this myself, but had the guidance of the owner of Rain City Audio in Seattle WA. I would highly recommend him for any work you have. I did the work at his workshop with his tools.

First shot of the underside before touching the capacitors
737093-c6b9272111748543d7427218ae7cb5ac.jpg


The first capacitors to go. (See end photo for replacement)
737096-42619e6615bb6f048b4502defb2c3473.jpg


Next in line
737097-32886bf937de491ec770998a45bf11c3.jpg


The new capacitors are so much smaller!
737098-edb642bc29924ae665bef107ac8481d6.jpg


This one was somewhat hard to access, it's in the FM section
737099-59de090a38539f420d0c0c44a4ed7ace.jpg


2 each on each of these parallel boards that needed replacement.
737100-49f234e1a1148f0c36708a476803060f.jpg


The new ones installed
737102-498417bd03d5fff7a49d914d4250d675.jpg


Looking similar which I am rather proud of!
737103-8e4a767ec90ffe1ef8b6589186d17136.jpg


The first Can Capacitor to go, the 60-40. I took this photo for reference and then also wrote down that the single red wire went to the 60uf and the orange wires went to the 40uF, the Black wires are ground, obviously.
737106-fc63c254ecf90398cb292317e013db60.jpg


New Can capacitor installed. I oriented it slightly different, but it is correctly wired.
737107-def2467ab94a60f2416c9f442d948e7c.jpg


Photos of the 40-40-40 uf Can Capacitor before replacement.
737108-a3dce499df608b28de469e233be83741.jpg


Photos of the 50-50-50-50uf Can Capacitor before replacement
737112-deed33f0007f9244b9b2ab2fe4ba7ff8.jpg


An After shot of the replacement capacitors. You should be able to make out 6 on this if you look close.
737114-529b969f35b0a31acf91786395f5c8fa.jpg


Old vs New Can Capacitor. If you look at the old one you see a bit of a bulge near the etching. Not sure if this is due to failure or if this was normal due to the etching?
737110-359576e9b8be4be6d9d292a90d457e85.jpg


All the parts I replaced (missing one black capacitor which fell of the table and I found later). Notice the bulges in the cans? I assume this is due to them leaking but I'd be curious if anyone has knowledge of if this is normal due to the etching process.
737113-4f527f214157403df384f8bd63f2b7e7.jpg


Fired it all up and re-biased the tubes and it works well and is sounding good. Now to go give it a long listening test.
 
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Congratulations on your first rebuild... and gateway drug to the world of McIntosh and Tube HiFi. Nothing can stop you now from buying gear and refurbishing it for use. It's a dangerous game, a very dangerous game ...

Very cool of him to allow you use of the gear nonetheless.:thumbsup:
 
SudoSpencer said:
Thanks for the info. Its definitely helpful as I learn. I did replace the three I tubes that I assumed were bad and now the FM tuner is working spectacularly!

The entire MAC 1500 now functions well and sounds excellent. I'm just debating taking the time and effort to swap all electrolytic capacitors or if I just use it as it is now and hope for no failure.
Click to expand...

I have a 1500...just bought it. FM section super noisy with hum. I can tune a station, but the hum overpowers the audio.

Phono and aux are dead quiet. Which tubes did you replace to quiet the noise on fm?

Any other user adjustments I can make?

Thanks- Jeff
 
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