Fender - (preamp hiss/cured) Now have buzz/sputter

[Janglin_Jack]

I finished up my 40w Fender build, (Blackface AB763) and everything is working. The Normal channel is silent, but the Vibrato has some noise, like buzz/hiss that gets louder as the volume is increased. I pulled V4 to eliminate the Vibrato/Reverb circuit and the Normal channel is quiet as can be (vol 1-10). Once I plug in V4, the amp is dead quiet with volume all the way down. Turn up the volume on the Vibrato channel and the noise get louder. Based on that I have elimited the power amp section and focused from the inputs to V4.

First I just cleaned up some wires by shortening, etc. I tried chopstick and nothing chaged. I put a shielded wire from V2 pin 7 to the treble pot, no change. I removed the wire from the input jack to V2 pin 2 to elimiate the Vibrato input jacks from the circuit, and no change. I have tried various tubes in different positions, etc. I tried remove the foot switch, I tried removing the reverb pan.

I used mostly carbon film/metal film resistors. I have one or two carbon comp in there as that was all I had for those values. In particular the 3.3M which I believe is the Reverb Mix Resistor.

I am out of ideas. Is there something I can test? My suspicion is a bad componenet, but without just shotgun and replace stuff, I am not sure what else to do with it.

Prior to first power up, I tested all the voltages with my meter and every looked good.

Mike

[Janglin_Jack]

Well. I spent a long time poking around and cleaning up wiring, but nothing worked. I pulled V5 and amp was noisy. Pulled V3 and amp was noisy. Pulled V4 and amp was quiet. I knew it had to be in the circuitry for V2, and decided I might try a new tube socket and just wire all those conections again, including board components.

As I sat there deciding to get started, and shaking my head that all looked and tested OK. I just grabbed some tubes and decided to keep trying. The first tube I grabbed was a Groove Tubes GT12AX7M, (sort of a "Mullard") tube. Popped it in...amp quiet!!!! I had already taken a quiet tube from V1 and put it in V2????

Anyhow, decided, if the tube was bad, it should make noise in the Normal Channel V1. Yes...it does. Problem solved. But not sure how I assumed V2 was a quiet tube. Maybe during my substitutions I got mixed up on what was a known "good" tube.

Amp fixed...I guess I chased my tail a bit, but I do think the extra work making the amp tidy was not wasted effort.

Mike

[gldtp99]

I'm glad you found the problem----- preamp tubes can work fine in some positions and can be problematic in others---- but the first thing to rule out in troubleshooting is tube problems.
I'm close to finishing up a single channel AB763 build w/reverb---- 50 watts in a 1x12 combo cab for my friend's band----- also has a Raw control and a Lar/Mar PPIMV.
Hopefully this build will come out with a minimum of build problems/troubleshooting---- because another friend has dropped off three amps for service-----and there are other builds i'd like to get to in a reasonable amount of time..................gldtp99

[Janglin_Jack]

I always try to do the simple things first. Doesn't always work out.

What is the Raw control! Like a resonance control?

[Roe]

250k pot after mid pot (to ground)

[Janglin_Jack]

Did I read correct that it bypasses the tone stack? I think the insertion loss is big on the tone stack off the plate ((Fender) vs carhode follower like Marshall. If I am not mistaken something similar is done with a Dumble with the "rock/jazz switch? Is that overkill in a 40 watt? Cool mod. Tell me more......

[Roe]

yes bypasses

[Janglin_Jack]

New details on new amp build. I got rid of the hiss/ noise with the preamp tube change. I played it at practice at loud stage volume and it performed well. I was at home messing with 12AU7 vs 12AT7 reverb driver and when I was at home noticed sort of a buzz/sputter on the reverb/trem channel with the volume on 7 or higher. I started to reason it was crossover distortion and raised the bias from 55% of max plate dissipation to 60%. I also changed the screen resistors on the power tubes from 470ohm to 1K wire wound. Pushed it at practice tonight and the issue was still present. I plugged into the normal channel and cranked it to 10 and got some really nice overdrive, but no noises.

I found that high volume on reverb/ trem channel AND treble control at 7 or higher the noise was there. It also got worse as the treble was maxed even going to cutoff. So I am guessing oscillation vs anything in the power section. What should I check?

Mike

Update - as mentioned above, I have changed screen resistors on 6L6 to 1K wire wound. Rechecked and adjusted bias to 60% of max plate dissipation on 6L6GC 30w*.6/450v = .04 (40mA). I changed V2 pin 3 cathode cap to 5.6uf/1.5K for a little bass reduction. I added 33K grid resistor to V2 Pin 7 lead from volume pot.

I still get the same buzz starting with volume on about 7. If I go higher, it gets worse and more sputtery and thin and cuts out a bit, too. I tried going with 12AU7 in V2 position and it is better, but still can make it sputter at max volume.

The weird part is the Normal Channel on 10 overdrives nicely and none of these issues are present.

Any tips on what to do/check next??Mike

[Janglin_Jack]

Found it. I had the wires coming from the doghouse swapped. I had the wire to the power tube screens on the preamp plates and vice versa. I also rewired the input jacks. Took off the carbon comp resistors and added metal film. Also the 1M resistor body was touching the guitar cable shield so I moved it for more space. Now. I can crank the amp up to 8-9 and I just get a hint of a buzz. Not sure what to look at to get the volume on 10 with no noise but where it is I am happy as it sounds awesome. Any tips on what to check to get that last bit of output without the buzz. The bass is strong in the amp. Not sure if I know how to describe the buzz but it rides over the top of the note at high volume. Blocking distortion? Any hints?

[Janglin_Jack]

Buzz is still there with the volume up over 7-8. The amp is loud. I have done a few things to try and fix this. Not sure if I am dealing with Blocking Distortion for Crossover Distortion. Notes are loud and clear, clipping at high volume, but there is a buzz riding over the notes. I changed V2a cathode bypass cap to 5.6uf. I changed the phase inverter caps from .1 to .047uf. I added a cap across the PI plates 47pf. I changed the 1.5K power tube grid resistors to 4.7K. I changed the screen grid resitors on the power tubes from 470ohm to 1K. I read this on the Aiken site. I don't hear a swirl, but the buzz reference at the bottom in BOLD is what I think I have. Would changing the PI "Tail" resisotor help here??

I am new to using an oscilloscope, but what I have seen is the wave starts to clip at 6 or so and then as the volume is increased, it starts getting more to squarewave. Then it appears there is some hint of crossover distortion and some "noise" - like small lines on the top of the square wave that start.

Any ideas??

Q: What is "swirl"?
A: "Swirl" is a dynamically-changing, slightly "phasey" sound as a note or chord decays, which is common to some tube amps. Typically, "swirl" is caused by a midrange "dip" or varying duty-cycle change in a clipped square wave that changes position as the note decays, giving a sort of mild phase shifter effect.

What happens is that first the phase inverter or output stage clips and produces a flat square wave. As the note decays, the signal level decreases, and the midrange frequencies start getting "unclipped" (either by the fact that their frequency band level is lower, or by phase cancellations due to the unequal phase shift with respect to frequency caused by tone controls and other RC phase shifts that occur in a gain stage) and show up as a "dip" in the top of the square wave, which will move back and forth along the top as the fundamental and other harmonics shift the operating point. Even if the clipping ratio isn't extreme enough to show the "dip" on the scope, the duty-cycle of the square wave will usually be dynamically changing as well.

Since the preamp stages are all AC-coupled to each other, the operating point shifts as the signal gets smaller, due to slight "blocking" distortion, where the gain stage clamps the top peak to a point slightly above it's cathode voltage, while allowing the wave to still increase in the negative direction. As the signal decays, it shifts upward and changes the duty-cycle of the clipping. It is this ever-changing shifting of the operating point that causes the "swirl" effect. The trick to good "swirl" is in the correct staging of the gain and frequency breakpoints of each gain stage in the amp, particularly in the phase inverter and output stage.

A similar effect can be caused by too much drive from the phase inverter to the output tubes. As the note decays, a riding "buzz" can be heard coming in and out. This is crossover distortion aggravated by too much signal swing to the output tube grids. Reducing the signal levels at the output of the phase inverter will cure this.Another cause of a "swirly" sound is a parasitic oscillation that is riding on the output signal, causing intermodulation distortion.

[Janglin_Jack]

Interesting update. I am using a JAN Phillips 12AT7 in the PI. I popped in some cheapie, (Ruby 12AT7), just to see if there was any change, and there was no difference.

I thought about a lower gain tube earlier in the preamp so in V2 I put in a JAN 5751. The problem was worse!?! Without the amp pushing as hard it started crackling, buzz and splatting at like 4 on the volume control vs 7-8 as before.

Could this mean there is a physical issue, (maybe at the tube socket??). Why would it get worse with lower gain tube??

Mike

[Janglin_Jack]

While my kids were at summer camp, I got into the amp with my scope. I looked at the normal channel after the cap on V1B. Nice clean wave all the way through the range on the volume pot from 1-10. Then I moved over to the vibrato/reverb channel and did the same. The wave looked good up to about 5 on the volume pot, then it got all distorted. The bottom of the wave got flat like a squarewave and then got a weird spike. Something was going on here. I tired the low input on the same channel and got different weirdness. The wave was all over the place in a different way. Went back to the high input and got even more drastic movement and distortion to the wave. At one point, the lower half of the wave dropped off the bottom of scope screen while the top part of the wave stayed put.

The issue is right at the input and V2. I started testing all the resistors and they all looked fine. I tested the bass and treble pots and those showed 250K resistance and smooth variable resistance through their sweep. When I got to the volume pot, it tested normall for the first half of so of the sweep, then the resistance sort of dropped off the curve and showed continuity in the upper range.

I think I have a faulty volume pot, which would explain the weirdness at volumes above 5, since that is where the amp started to exhibit noise and scoped showed distortion and noise.

Later tonight, going to pull the pot and replace with another. Hope this is it!!!

[Janglin_Jack]

Chaged the volume pot, changed the treble pot. Did a few other items and it wasn't really inproving. Then all of a sudden the wave looked great and I could crank the amp and treble and all was well. As I was patting myself on the back, I noticed I didn't have the reverb pan plugged in. Once I did that, the wave went back to the same issues.

So whatever it is, causing this, it doesn't do it with the pan removed. And it does make noise when the pan is connected. Didn't matter if the footswitch is on or off. So the oscillation/buzz takes place with the pan connected. Any ideas what to check??

Mike

[Janglin_Jack]

Cleaned up some leads and wrapped the pan with a dish towel. Now with the scope I can crank the volume with treble on 7-8 or crank the treble with volume on 8 or so. But there is still some oscillations going on. Not sure it will be a problem as those setting will be unlikely in a band situation, but would like to figure out the oscillation. Is the a cap or resistor to add to suppress the oscillation to get that last bit of juice from the amp?

[Janglin_Jack]

Well. After all that, it seems it is the reverb driver tube. I made all the wiring neat and tidy, shortening leads and routing them better than before. Changed a preamp tube socket. Changed each of the tubes one by one and made several substitutions in each spot.

I was tried a 12AU7 in V3 and bam....no buzz. Still plenty of reverb even at 2, and the amp is loud and punchy. Not sure why my JAN Phillips 12AT7 reverb tubes are making this noise, so not sure if the lower gain is helping or if those tubes are just faulty.

For now, it seems like I finally got it.

Mike