Making Assumptions

[52 Bill]

Had a couple of Magnatones in the shop this month. Haven't seen one of these for a few years, so having two in at the same time is odd. One M10 and one 460. The older 460 had one dead channel and the vibrato channel was weak and distorted. The M10 was DOA.

The M10 took a half an hour, a new fuse, 4 new rectifier diodes and some DeoxIt and it was sounding like a dream.

The 460 drove me nuts. Replacing a 12AX7 with one dead triode fixed the dead normal channel. The vibrato channel worked, but was just enough out of spec that nothing seemed right. The voltages around the vibrato tubes were wrong, but not so far off that they made me want to rebuild the entire circuit. I knew that I didn't have any 6CG7s in the shop, so I drag out the tube tester and set it up to test the two vibrato tubes.

As I am setting up to test the tubes, I find that someone had replaced the original 6CG7s with 12AU7s. The amazing thing is that it actually worked with the wrong tubes in it. Turns out the filaments are the only difference in the pinouts. So the tube had 6 volts ac applied to pins 4 and 5, NC on pin 9, so the filaments were getting 1/2 normal voltage. No wonder why the channel was weak and distorted. Once a set of 6CG7s were installed everything was back to normal and sounding great.

Who would think that there would be two wrong tubes installed, and that the two wrong tubes would actually work. I made the assumption that the amp once worked as it was brought in. Obviously it never worked right as it was, but I assumed that it had.

[Chuck H]

It could have been a new owner of the amp that wanted it snuffed up and not the person that installed the wrong tubes. I know this wasn't the issue, just thinking out loud because reading your post made me think of how customers will occasionally misinform a tech out of embarrassment. "The amp just stopped working." "No, I didn't do anything to it". Then you pull a power tube that YOU installed and find the alignment pin busted off. You then have to take any info the customer gave you and just toss it. Treat the amp like you found it by the freakin' road side with a sign taped to it that just said "DUH!?!" I never get snarky about such things either. I just return the amp in good working condition and tell the customer what was wrong. Just the facts are usually enough to let them know that you're onto them and letting them skate. Which, BTW, is way better than whining at them about it

[52 Bill]

...just thinking out loud because reading your post made me think of how customers will occasionally misinform a tech out of embarrassment. "The amp just stopped working." "No, I didn't do anything to it". Then you pull a power tube that YOU installed and find the alignment pin busted off. You then have to take any info the customer gave you and just toss it. Treat the amp like you found it by the freakin' road side with a sign taped to it that just said "DUH!?!" I never get snarky about such things either. I just return the amp in good working condition and tell the customer what was wrong. Just the facts are usually enough to let them know that you're onto them and letting them skate. Which, BTW, is way better than whining at them about it

When the customer picked up the amp, he told me that he ordered a complete set of NOS tubes and must have ordered the wrong ones for the vibrato section. I only wish that he had told me that when he brought the amp in.

And I've had the same problem with the mysterious, "It just stopped working." failures. Or the even more popular "I lent it to my buddy, and he said it just stopped working." problem.

[Leo_Gnardo]

As I am setting up to test the tubes, I find that someone had replaced the original 6CG7s with 12AU7s. The amazing thing is that it actually worked with the wrong tubes in it. Turns out the filaments are the only difference in the pinouts. So the tube had 6 volts ac applied to pins 4 and 5, NC on pin 9, so the filaments were getting 1/2 normal voltage. No wonder why the channel was weak and distorted. Once a set of 6CG7s were installed everything was back to normal and sounding great.

Perhaps the crustomer read somewhere that 6CG7 is a bigger tougher version of 12AU7. Which is - sorta - true. And making that sub has been recommended by those who rebuild McIntosh tube amps, BUT with the additional info that the filament is wired differently and that must be done to make it work successfully. Crustomer or his informants got half the message, at least nothing was damaged & the solution simple. Glad it worked out, those old Magnatones are wonderful amps.

About 30 years back there was a rash of guitarists around here who wanted to use 6DJ8 "because the hi fi guys are nuts about them." Same thing, you needed to rewire the filament connection. Only one guitarist brought his amp in to try the 'DJ, I ran the filament correctly, then he backed out quickly after playing it a couple days with a handful of them to try. No fantastic improvement in tone, the experiment over.