1964 Ampeg Jet Questions

[olddawg]

I have a 1960 Rocket which is about the same thing. I wouldn't change the speaker (unless you want to keep from blowing it at full volume). I would recommend a Weber Minimass attenuator. You could use a 25 watt but I would get the 50. Your hum may simply be a noisy tube. Even new ones can cause him, noise, and microphonics. Swap some spares in and out.

[Tom Phillips]

...The two silver things I referred to earlier are marked 12/15V. I also just noticed way up under the hood, attached to the volume and tone pots are two red "tiny chief" capacitors ... maybe I should just replace all 4 of these and see what happens to the hum. Any thoughts?

It is unlikely that any of those components are causing excess buzz.

It's a buzz and it really isn't that bad...I thought it was normal for vintage tube amps until I had my other vintage amp with the same thing recapped and now it is totally silent. It gets louder as I turn the volume up on the amp with nothing plugged in the input. It isn't too bad until around halfway, from there it is definitely louder than I think it should be....

The Ampeg Jet used a non-shorting jack for one of the inputs. Those amps did have more buzz under the conditions you describe. (Volume turned up with nothing plugged into the input)

How does the background noise of your amp compare to other amps with your guitar plugged in and played at your usual volume levels? Usually the noise picked up by the guitar swaps out the internally generated hum & buzz.

Another possibility is a problem with the tube heater circuit grounding / balance.

...I have a friend who can replace caps but probably can't diagnose the hum that well...

Your need right now is exactly for someone who can diagnose and troubleshoot not just replace parts by guessing the need for replacement. The photos show that the previous work on your amp was done by a skilled person. The MEF members could help but you, or a helper, need some basic test equipment and the ability to trace circuitry.

The first goal, however, is to determine if there is really anything wrong with your amp as I discussed above.

[Leo_Gnardo]

It's a buzz >snip< gets louder as I turn the volume up on the amp with nothing plugged in the input. It isn't too bad until around halfway, from there it is definitely louder than I think it should be.

As Tom mentioned, there's no shorting bar on these Ampeg inputs, as opposed to Fenders & many other amps which do have that feature to minimize hum when no instrument is plugged in. So . . . with volume control up buzz is normal, no worries, don't go swapping out parts to try & "fix" it. Accept it as a harmless quirk in an amp that's otherwise competent.

If you really feel the need to do something, find some replacement preamp tubes. 6KD11 are just about unobtainium and someday you may need a good working one. Consider this: I have an Jet identical to yours but I'm stuck because I have only one of those tubes and it needs two. Can't use it, can't sell it. Don't let that happen to you.

[Jazz P Bass]

Isn't the compactron a 6BK11?

I thought 6C10 was a good enough sub.

Here are the gain figures:
" The only difference is an unused, internal connection on pin 8 and the gains are 6BK11: 70/100/100 and 6C10: 100/100/100."

[Tom Phillips]

...find some replacement preamp tubes. 6KD11 are just about unobtainium ...

I think you mean 6BK11. Maybe not a problem in this case though because the first two versions of the Jet used 6LS7 preamp tubes. Maybe Triocd has the 6LS7 version?

[g1]

Leo, do you mean 6BK11 ?
Can you use something similar in one of the spots to get it somewhat useable?
I believe Justin Thomas mentioned using 6K11 in one spot in one of his amps?

[Justin Thomas]

Triocd has octal pre tubes - lucky SOB! I think I got the last 6C10s on Earth... I won't tell you the price, you'll stone me!

Justin

Simul post... g1, I had a 6K11 & a 6AV11 in mine, both of which are quite a bit lower in gain. But I scored 2 6C10s. It didn't fix the lack of volume, but it did bring the trem to life. I still have that little amp, and ONE DAY I will fix it!

[Triocd]

Dope....I should've picked up on this earlier, but I just discovered that input 1 is totally fine. I was recently plugged into input 2 (randomly). Input 2 seems to have a problem because I just plugged a patch cord into input 1 and put the volume on 10. The amp was practically silent. Then I moved the patch cord to input 2 and put the volume up and there is a loud hum/buzz that definitely indicates a problem.

This would lead me to believe tubes and caps are all fine, and that there is something wrong with something in that 2nd jack/circuit. Should I just leave it alone? Any reason to fix it?

[Tom Phillips]

The stock Jet uses a shorting jack on one input and an open jack on the other.

The behavior you are describing seems backwards to me. Are the jacks actually labeled #1 and #2?
You should plug into whichever input works best and then play your guitar. I think the amp is fine and is acting per the design. At lease with respect to the buzz issue.

Edit: If you can post some photos of the outside and inside input jack area we can maybe verify your amp's wiring configuration

[Leo_Gnardo]

Isn't the compactron a 6BK11?

I thought 6C10 was a good enough sub.

Here are the gain figures:
" The only difference is an unused, internal connection on pin 8 and the gains are 6BK11: 70/100/100 and 6C10: 100/100/100."

Whups sorry folks, yes 6BK11 as on the schemo and 6C10 a working sub. Some of the other triple triodes, 6U10 for instance have low-mu sections, if you swap one in you'll be lacking gain and/or vibrato. But that's for Ampegs that have compactrons; the Jet in question predates Ampeg's triple triode travesty. Note to self: don't type answers early mornings until you've had a coffee or two. Put dunce cap wearing, bleary eyed smiley thing here.

[Triocd]

They aren't numbered. I'll get some photos and possibly a video up this evening/tonight. What's weird is I slightly like the tone better in the input that gets really noisy at high volumes. I just normally play it at bedroom volumes so the buzz isn't really noticeable. At high volumes that jack would be unplayable due to the noise.

[Justin Thomas]

They aren't numbered.

It may have worn off, but I wouldn't be surprised if one wasn't labelled "Accordion" at some point!


Justin

[Triocd]

Here's a quick video. I couldn't really play because the baby was sleeping upstairs. The hum from the "bad" input is a low frequency hum that wasn't really picked up by the iphone mic. It rattles the floor somewhat.

https://youtu.be/jMybYYHVsoo

Here are a few internal pics of the wiring of the inputs. The jack in question is the switching jack.

[Tom Phillips]

Triocd,
Attached is a clear schematic for the J-12B Jet which, I believe, is the version you have.

The input jack you are calling number 1 is the non-switching jack and is the upper one shown in the top left corner of the schematic. When you plug into the #1 jack the input signal is attenuated a little by a resistive divider circuit and the fact that the signal is loaded down by a medium input impedance of ~141k Ohms. When you plug into the #2 jack with nothing plugged into the #1 jack the attenuator is switched out of the circuit and the input impedance increases to a very high value. These different conditions combine to produce the different buzz levels and the different tone response you get from each input. The #2 input is a little higher gain than the #1 input and I think that the amp is operating normally for what it is.
Regards,
Tom

[Triocd]

Thanks for the input! The label on the inside of the cabinet says Jet 12-A but it can't be very different from the 12-B, right? That would make sense that the hotter input has more hum...I just mentally had the two inputs backwards. I thought the jack with more hum was actually the one with less gain, because on my other vintage tube amp, a Kalamazoo Model 1, the input farthest from the control knobs is the hotter input.

I don't have much experience with single coils as I'm currently borrowing a strat from a friend, but the single coil buzz when the amp is at 6-10 is really loud compared to the humbuckers. Like not even in the same ballpark. Does that sound right? I knew there would be hum with single coils but it's freakin loud. Could be his guitar isn't shielded well or something.