Super Reverb Distortion

[martin manning]

Thanks, Martin. I rechecked the voltage on V2, pin 6, and it's definitely 280 volts. Pin 6 is a plate, correct? Isn't 280 volts in the ballpark?

Typo on my part- I meant pin 3. Looks like a 12AT7 grid voltage. Try the Excel file again... I was messing with the format and replaced it.

[RightLurker]

First, Martin, I went back and tried again and your spreadsheet downloaded. I don't know what was wrong.

Second, Firestorm thanks for the pointers. I'll check the cap. I have a substitution box and a bunch of spare caps lying around.

[martin manning]

Another possibility is blocking distortion at the power tube grids. Reducing the value of the coupling caps out of the PI (try 0.05 or 0.02uF, say, instead of the 0.1's), or increasing the value of the grid stopper resistors on the power tubes to 5k6. See Aiken's site for some discussion: http://www.aikenamps.com/BlockingDistortion.html

[RightLurker]

Firestorm, it does sound like what I understand to be crossover distortion (as described by Gerald Weber as sort of buzz-like), and to my novice brain, what you're saying makes sense. You mention the junction of the 100, 820 and 27K resistors. The 27K is actually a 22K, correct?

Martin, I used your Excel spreadsheet. I checked everything again except the vibrato tube. My meter (a $75 Sears Craftsman - I know, Fluke Fluke Fluke) came up with a good solid 1 on pin 8 of the 6L6s - I assume that's a fault of the meter. It wouldn't give me .03X, no matter what the range.

I'll attempt to attach the spreadsheet to this email.

Now it's time for an adult beverage, which means I'm done with this project for the day. Mixing alcohol with high voltages seems like a poor idea!

Thanks you guys for your help. A productive day.

[martin manning]

Sounds good! Enjoy a beverage while reading Aiken's article. Note that blocking distortion creates crossover distortion by upsetting power tube bias. The PI looks well balanced and correctly biased at this point, so I really don't think there is anything wrong there. My first move would be to try smaller coupling caps out of the PI and see what that does.

[Firestorm]

Firestorm, it does sound like what I understand to be crossover distortion (as described by Gerald Weber as sort of buzz-like), and to my novice brain, what you're saying makes sense. You mention the junction of the 100, 820 and 27K resistors. The 27K is actually a 22K, correct?

Yes, sorry. It was 27K in the AA763 model. Was just wondering about that cap because it's the one place in the PI where DC readings won't tell you anything. The grid needs to go to AC ground, so if the cap is open or the solder joint is funky, half the PI won't work and you'll get something like I think you're describing. The fuzzy sound is pronounced, right? Not a fainter insect-like sound riding the note? That would be something else.

I concur with Mr. Manning's and Mr. Aikens' suggestion to try dropping the couplers to .047 on general principles. SRs are rather too boomy to begin with. But I have mostly encountered blocking distortion at much higher volumes and it seems to take the form of the well-known "farting out" syndrome.

[martin manning]

...was just wondering about that cap because it's the one place in the PI where DC readings won't tell you anything. The grid needs to go to AC ground, so if the cap is open or the solder joint is funky, half the PI won't work and you'll get something like I think you're describing.

You make a good point here, Firestorm. I think the power amp comes out of class A pretty quickly, and therefore the problem would show itself at pretty low volume, but it would be easy to check this cap by paralleling another one across it. From the DC readings we know it can't be shorted, but it could be open.

[Firestorm]

It's rare for a cap to fail open (unless, like the UL death caps, they're designed to) but I have seen broken leads and especially leads pulled out of the eyelet by board warpage. This is the section of the Fender circuit board that seems to warp the most as the amp ages. I saw one warped so badly that it pulled the PI ground connection out of the solder on the grounding bus. This makes the PI ground THROUGH THE SPEAKER via the feedback resistor. The speaker did not like that.