Rockster Oscillation Woes

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dorrisant
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Rockster Oscillation Woes

Post by dorrisant »

I have an older Rockster build here that is giving me fits... This amp has been very good for quite a few years. About six months ago it started to drop volume as it warmed up. I checked the tubes and replaced a questionable preamp tube and thought no worries. It has gradually started dropping signal at the output in a shorter amount of time from powering on. I put it on the scope and noticed the oscillation right away. It starts out fine but snaps into oscillation with the gain set at about 2-3. It has a PPIMV (Lar Mar) and is turned all the way up. If I lower the MV to about half it will stop. Then if I turn the gain up just a bit, the oscillation comes back. If I turn the cut control to where it cuts the most highs then I can turn the gain and MV all the way up with just a little trashiness at the end of either pot's travel. If I leave the gain and MV maxed and turn down the cut control it breaks into oscillation immediately. I usually leave the tone controls at half way. To test the theory, i turned down the treble control to about 2-3. This allows me to turn up the gain a bit more before the oscillation. Still no joy. It doesn't seem to be in the preamp because it is non-existent if I remove the power tubes. With the power tubes (tried three different sets, known good) the oscillation shows up on every plate and grid in the preamp all the way out to the output jack. I swapped in another OT with the same results. All of the caps check fine as far as leakage. The electrolytics all test fine and the amp is only a few years old. I tried putting several different values of silver mica caps across the plates of the PI with the same results. I tried a snubber setup across the plates of the power tubes... same result. Screen grid resistors test fine as well as the signal grid resistors for the power tubes. I changed out the power tube cathode resistor on a whim... still no good.

I am at a loss. Does anyone have a suggestion?
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2tone
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Re: Rockster Oscillation Woes

Post by 2tone »

cold solder joint that maybe made contact for a long time and then started acting up after a year!?? I had one like this in an original Trainwreck..
2tone
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Re: Rockster Oscillation Woes

Post by 2tone »

Actually, I remember now it wasn't even soldered. Had forgot to solder it, but was in contact for a year..
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dorrisant
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Re: Rockster Oscillation Woes

Post by dorrisant »

Good suggestion 2tone... I reflowed all of the joints and still no good.
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romberg
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Re: Rockster Oscillation Woes

Post by romberg »

Since you have a scope and can see the oscillation, it might help to pull some preamp tubes and see if you can narrow down which stage it is occuring in. Is it there with just the power tubes? Power tubes and phase inverter? Etc.

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tubeswell
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Re: Rockster Oscillation Woes

Post by tubeswell »

Time for a filter cap change and/or output tube change.

Also check the tube socket pin clamps and pins are clean/free of oxidisation
He who dies with the most tubes... wins
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dorrisant
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Re: Rockster Oscillation Woes

Post by dorrisant »

I changed out the entire preamp circuit. When I built this one, I did some things completely backwards. In order to correct that and shorten all of the leads and other misc stuff, it seemed logical. No dice though.

I swapped in a another standby switch... No good. I replaced the rectifier tube with a SS substitute plug... No. I then temporarily removed the MV... Nope. Next, I subbed in a big ole resistor for the choke... Nopey. :?
Nopey.gif
The list of suspects is getting shorter... As tubeswell mentioned, it could be the filter caps. I built it 5 years ago with JJ can-type caps. I would be a bit surprised if it was one of them but I do recognized the possibility. I will change the power tube screens and grids next, maybe the sockets too. It still could be a dropping resistor. I wouldn't think it could be the PT but you never know. One thing at a time though... I really want to know what it is now more than ever.
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Stevem
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Re: Rockster Oscillation Woes

Post by Stevem »

Jump in new filters starting at the preamp section,in 5 minutes time you can atleast them rule them out.
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dpin
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Re: Rockster Oscillation Woes

Post by dpin »

When you temporarily removed the PPIMV what value did you have for grid reference Rg before the power tubes?
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