Will PPIMV wiring cause oscillation?

dynaman · Post by **dynaman** » Tue Mar 22, 2005 12:03 pm

Fired up my JTM45 clone yesterday and it oscillates when cranked up. I know this has been covered a million times and the usual recommendation seems to be to use shielded cable from the input jacks to the first preamp tube, which I have already done. Here are some details and questions:

1) I don't have the usual four inputs. I have installed a single input jack that is wire to a switch (on-on-on) that allows me to switch between the clean and lead channels or a combination of both. The switch is wired to the preamp socket with shielded cable. Soldered directly on pins 2 and 7 are 33K resistors.

2) I didn't use shielded cable for the MV. I used regular teflon coated hookup wire and heat shrink tubing. The run of wires is about 8". Could this be the problem?

3) Should I scrap the wiring for the tone controls and just use shielded stuff? What are the most sensitive wires?

4) No, I don't want to lose the bright caps. It's a little dark as it is.

5) I remember reading a post here somewhere that claimed some oscillation could be cured by placing a 5 pF cap somewhere on the preamp tube socketes. What's that about?

6) Thanks for any help!

Flames1950 · Post by **Flames1950** » Tue Mar 22, 2005 12:37 pm

That's a nice clean wiring job, the source of oscillation isn't obvious from just looking. But I don't deal in PPIMV's to help there.
It was common in Fender CBS amps to see small caps across the plate load resistors in the preamp to help stop oscillations......kills some high end sparkle though. Same with using about a 2000pF cap off the power tube input grids to ground -- yucky.

Billy Batz · Post by **Billy Batz** » Tue Mar 22, 2005 3:49 pm

In my experience a PO is almost always directly caused by something specific. A missing ground. A mixed up connections. A cap that puts the signal over the top of stability. etc... And its found about 90% of the time by cranking it to where its really oscillating (use ear plugs if you have to- sometimes there so high pitched you cant hear it as much as feel it splitting your brain) and move some wires around with a pencil or something wooden or plastic. The only problem with twisting and running everything together is that if two things are together that shouldnt be then its hard to take em apart and see. As long as you dont have anything questionable twisted together then try proding the stuff with a pencil, etc.. When proding a specific thing changes the PO, even if it only changes the pitch/speed/etc.. or even makes it worse then its a good possibility its either causing it or connected to whats causing it. Try prodding socket pins and terminals as well. See what you get.

Billy Batz · Post by **Billy Batz** » Tue Mar 22, 2005 3:53 pm

BTW - 2 possible cures are to put a cap value of, say, 250p-390p jumping the cathode and plate and also jumping the plate and grid with something like 5p-25p. That only works if the PO is isnt caused by some specific problem or mistake. Also sheilded wire is only really used on the grids. So when people use sheilded wire its almost always going in long runs to pins 2 or 7. Like when the output of a volume pot goes accross the chassis, under the board to the grid.

Guest · Post by **Guest** » Fri Mar 25, 2005 6:45 pm

it looks like the output tranny wires go right past the 1st preamp tube.
that would be where i'd start looking. you should keep them away from the input side of the amp.

dynaman · Post by **dynaman** » Sat Mar 26, 2005 4:17 pm

You can't see in the picture, but the OT wires don't run across the preamp tubes' wires. When they enter the chassis they are within a half inch. I'll try moving these around and see what happens.

jbrew73 · Post by **jbrew73** » Sat Mar 26, 2005 5:00 pm

that was my post above(i forgot to log in).
i think in general you want the ouput tranny wire away from the input side of the amp so maybe you will find something by moving those wires around. something else i thought about is the primary leads on the output tranny may need to be reversed at the power tubes. if they are backwards the amp will have positive feedback.

Flames1950 · Post by **Flames1950** » Sat Mar 26, 2005 5:34 pm

If those output tranny leads were switched you wouldn't get a chance to crank it up. As soon as you hit the standby you'd be presented with the world's loudest most irritating hum ever. Sounds like all your filter caps took a crap in a big way......but changing caps won't help he he he he........

Billy Batz · Post by **Billy Batz** » Sat Mar 26, 2005 7:57 pm

Flames1950 wrote:If those output tranny leads were switched you wouldn't get a chance to crank it up. As soon as you hit the standby you'd be presented with the world's loudest most irritating hum ever. Sounds like all your filter caps took a crap in a big way......but changing caps won't help he he he he........

I dont know about that Flames. At least in all cases. When I put together my TWreck clone a few months back I had the primaries mixed up and I didnt get an obnoxious hum or squeal until I started to up the volume at which point the squeal was horrible. I wouldnt assume it in this case but if all else fails it only takes a second to switch them.

Flames1950 · Post by **Flames1950** » Sat Mar 26, 2005 8:15 pm

Billy Batz wrote:
Flames1950 wrote:If those output tranny leads were switched you wouldn't get a chance to crank it up. As soon as you hit the standby you'd be presented with the world's loudest most irritating hum ever. Sounds like all your filter caps took a crap in a big way......but changing caps won't help he he he he........
I dont know about that Flames. At least in all cases. When I put together my TWreck clone a few months back I had the primaries mixed up and I didnt get an obnoxious hum or squeal until I started to up the volume at which point the squeal was horrible. I wouldnt assume it in this case but if all else fails it only takes a second to switch them.

Whoopsie, I didn't read Jbrew73's original post close enough -- I thought we were talking about switching the secondary leads around. Did that on one of my '68 Bassman's incarnations and man oh man was that an ugly noise......

Then your negative feedback is emphasizing everything bad about your signal that you want to stabilize and cancel out.
Never have swapped the primaries around yet, and I don't think I'll deliberately try it just for the sake of it......