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Dirty-South Blues Harp forum: wail on! > Wiring tremolo pot on ampeg jet: Help please
Wiring tremolo pot on ampeg jet: Help please
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SuperBee
4003 posts
Aug 17, 2016
5:40 AM
a while back I posted photos of an ampeg Jet I picked up in trade.
It was kind of a jet. Started out as a J 12 B but had no original circuitry.
Only original parts were cab, handle, knobs, chassis, transformers. Maybe one pot and valve sockets.
PI had been rewired for more gain than original circuit and power tubes were 5881 rather than original 6V6.

I left the power amp as it was but I rebuilt the preamp, tremolo and phase inverter circuits.
Having done that I've a much clearer idea of how I'd lay it out differently if I did it again, but it all works and I'm quite happy with it. I think. I intend to play it a while in garage jams and band practice and see how it goes.

Just one thing bothers me and I haven't been able to get my head around the issue. Maybe someone here can steer me right.

When I switch the tremolo on, it starts up immediately and it's strong and deep. But it starts out fast and gets slower as I rotate the knob clockwise. I've no idea if that is how it's supposed to work but I'd expected it to start out slow and get faster as I turned the knob.

I'm not clear on how to rewire it to make it operate that wAy.
I haven't been able to find any info which clues me into what goes on with this, and just can't seem to get my brain around the principle of what is going on with it. I need a nudge
Wendell
51 posts
Aug 17, 2016
6:44 AM
Swap the connection to ground to the other terminal on the pot.
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No Harm In Harmonica !
1847
3618 posts
Aug 17, 2016
6:50 AM
JET
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.600_439660165
SuperBee
4004 posts
Aug 17, 2016
6:54 AM
Ok, that's interesting. That is what I tried but the tremolo doesn't get going when I do it that way
JustFuya
944 posts
Aug 17, 2016
8:23 AM
I would rule out the pot with an ohm meter and/or try a couple of different resistors in its place (one at a time) to see if the circuit is behaving as it should.

Have a grain of salt on hand because my eperience is limited.

Edit to edit add: WHAT HAPPENED TO THE MBH SPELL CHECKER?

Last Edited by JustFuya on Aug 20, 2016 7:44 AM
SleepingCat
20 posts
Aug 18, 2016
8:15 PM
SuperBee, if you haven't got it sorted and you are OK posting the schematics, I would happy to help (in real life I am a EE)
SuperBee
4009 posts
Aug 19, 2016
4:16 AM
Thank you SleepingCat. That would be cool.
The link 1847 posted is pretty much it.
I am running sovtek 5881WXTs instead of 6V6s and I have a couple of screen resistors which aren't shown on the schematic but the tremolo is as shown.

Of interest and possibly pointing to the sense of JustFuya's idea; tonight I took the amp to practice and just for the heck of it I turned on the tremolo. It came on but it stayed at the same quite fast rate. It did not respond to turning the knob, other than on/off.
In other news, the old thing held up quite well through a couple hours of rehearsal. Better than the Princeton Reverb I think, perhaps mainly thanks to the 12" speaker.
SleepingCat
21 posts
Aug 19, 2016
7:40 AM
Hi SuperBee. Well, as Wendel said, swapping the ground to the other terminal on the pot (but not swapping both connections) should have done it for you. Which points to JustFuya's point, as you said.

I marked up the schematic to show what I believe *should* give you the correct behavior. Note that this isn't really a change to the circuit, just illustrating how to switch the connections to the pot to get the sense that you prefer.

Last Edited by SleepingCat on Aug 19, 2016 8:42 AM
SuperBee
4014 posts
Aug 19, 2016
6:00 PM
Cheers SC. yes that was my first thought but when it didn't work I thought maybe there was something about the oscillator circuit which I wasn't understanding. I'm actually sure of that, but I mean regarding it starting up. The fact it now seems to have stopped responding to the movement of the shaft does seem to point to a problem with the pot. I'll check that out
Barley Nectar
1249 posts
Aug 19, 2016
7:15 PM
Always wanted to try one of those. Good job Supe!
SuperBee
4016 posts
Aug 21, 2016
7:11 AM
Funny thing. That pot has to be dodgy. Switched it on today to demonstrate and the speed control was working again.
Anyway
I'll get it sorted.
I really like this amp. Jammed with it in the garage today. 3 guitars plus bass but no drums. Another player there with a Classic 30. No problem. Just as loud and sounds better IMHO. Sounds great with my crystal as well.
now I need to build something with an octal preamp and enough grunt to drive 4 speakers


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