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wordsandsigns

112 Sixty Five w/PI tube: Distorted preamp + buzzy chassis!?

Hello All! Thanks for all the help I've had so far on this great forum. This has been a great resource for my amp for the last decade!

My friend recently got a 112 65 with the PI tube for cheap, but it was distorting badly at low volumes (sounded ugly, maybe crossover distortion). I replaced the tubes, same story. While in there, I noticed that the electrolytic caps were all old, except for one which was way out of spec. I replaced all the electrolytics, and the distortion improved dramatically.

But the amp is still crunchier than what I'd expect. I have a 112 65 model without the PI tube, and I'd expect these two amps to sound pretty similar, since their preamp circuits are identical. But my friend's amp seems to break up more easily, and also has a lower output volume at the same settings. I ordered replacement LM307Hs, and I'm going to try to replace those in case one got fried from the bad caps I replaced.

Any other thoughts on why this amp would be so crunchy? It sounds like it's in the preamp, because it increases when I turn up the channel volume, but the master volume seems to provide clean gain.

There's one other thing I noticed while working on this amp. When it's powered on (with standby on and with it off) the chassis buzzes. Not the speaker, but the chassis. It kind of sounds like it's coming from the output driver board. In addition, when I grab the power switch on the front when it's on, it vibrates a little. This seems really bad! I'm gonna hold off on working on this until I get some input, because it seems kind of dangerous! I've been really safe working on these amps, discharging all the caps, checking them, one hand deep in my pocket whenever there's power, but I'm too young to die!

Thanks in advance for the help!

inertian

Hmmm...

Hmmm...

As far as volume and distortion issues:

Have you checked the volume output/tone clarity through a known good speaker?
Have you tried a known good speaker cable (they do go bad)?
Have you installed known good tubes?
Checked the bias?
Checked for correct voltages?
Swapped in known good LM307s (from your amp)?

Regarding the buzz and vibration:
If you are getting a buzz (mechanical) vs a hum (electrical), you may have a loose component in the chassis. Check that everything that should be bolted/screwed down tight to the chassis is so, especially the transformers. Check the bolts holding the sides of the transformers together too. If you can localize where the buzz is coming from, apply pressure (using a wooden or plastic probe) in the area to see if you can make the sound change. You might get lucky and find a bad solder joint, lifted board trace, or busted component.

It is possible you have loose laminations in your power transformer, which would explain both the sound and the vibration. Here is a video showing how to diagnose and repair. I would try that before purchasing and replacing transformers.

https://www.youtube.com/watch?v=dtOwewfqucI&feature=youtu.be

Good luck and let us know what you find.

wordsandsigns

Thank you for the suggestions

Thank you for the suggestions!

I checked my friend's amp through my amp's speaker and speaker cable, which are known good. Same issue.

Have new tubes and double checked the bias, both good.

Checked for voltages, they all look good.

I didn't swap in new LM307s from my amp, but I did swap in new ones that I got from ebay. The distortion issue remains, though I know that the new ones from ebay might be bad so I'll try swapping out the ones from my amp in the next couple weeks.

As far as the buzz and vibration, I think it's just a noisy transformer. I localized the sound and vibration to the smallest of the three transformers, a transformer that my amp doesn't have (I guess they're only in the PI 112 65s). It's the 3 Henry transformer that runs to point G on the schematic and then to the (screen grid of the?) power tubes. I'm not super worried about it knowing that it's transformer hum, I was just worried that the chassis was live and I was going to get shocked. Seeing that all the voltages test fine makes me pretty confident that's not the case!

Any suggestions on what else to check to try to fix this distortion issue? I'm uploading a sound sample so you can hear what I'm talking about. The audio is a little low quality, but you can hear the amp breaking up when I play more than a couple notes at once. I was playing a strat with the amp on normal settings: Channel volume 6, EQ all at 5, Reverb and Trem controls all at 0, master volume at 2.5.

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mgriffin155

HV Power Supply

I suggest that the Hi Voltage power supply be thoroughly checked out. The 3H inductor (choke in the old days) is there for suppression of AC ripple on the HV voltages for the 12AX7. If it is buzzing, that is a sign that it is seeing far more AC ripple than the designers expected. It would be good to check to if one of the diodes in the Half Wave Voltage Doubler circuit (D9-D10) has shorted. Also, if the 3H inductor buzz became evident after the Cap replacement you mentioned, go back and double check the work for reversed polarity cap and stray solder blobs. -mgriffin

wordsandsigns

I'll check those diodes next

I'll check those diodes next time I'm inside the amp! I don't remember if the buzz was there before I replaced the caps - it may have been. I definitely put the caps in the correct polarity (triple checked) and I'm pretty sure there's no stray solder blobs (I checked for continuity at each end of each cap, and they're all connected to the right places).

One solder pad did come off the PCB when I was replacing the caps, but I connected that cap lead really well to the trace. Is there anything else you think I should do to make this as clean a fix as possible?

inertian

I know there was some

I know there was some variation as to where on the Volume dial the earlier 12AX7 PI tube Sixty-Fives started to breakup. Best I recall the first one I had years ago was clean until about 8. I've since played some that broke at 5 or 6. So your amp might be behaving normally.

That said...how loud can you crank the Volume with the Master at 10 before breakup? Any difference in behavior on Hi vs Lo power setting? Taking the Master Volume control out of the equation may (or may not) reveal a problem.

That little 3 Henry transformer is the choke, part of the DC power supply noise filter for the output tube grids. Works with the the 2 x 20uF filter caps to filter out noise. It went away with the 12AX7 tube PI. I think there is a way to jumper around it, would be interesting to see if the vibration and distortion then disappear. Hopefully someone here has done that and can advise.

wordsandsigns

Thanks for the reply! I

Thanks for the reply! I think you're right and the amp may be behaving normally! I was expecting it to sound very close to my 112 65 (without PI tube) at the same settings, but it makes sense that they'd be different. I usually set my amp with a channel volume of 5 or 6, and then a master volume of 1.5. For this amp, it sounds like I may need to do the opposite to get a clean tone - master volume at 5 or 6, with the channel volume down to 3 or so.

I will mess around with the channel volume while the master is at 10 next time I'm at my friend's house. I remember the same thing happening in the Hi and Lo power settings, but I'll double check that too.

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