Blocking distortion

[yowza]

I have an old SF Vibrolux reverb that has been black faced and I recently replaced a lot of the old original carbon comp resistors to get rid of constant crackling, sputtering sounds and the amp sounds really good now except when I crank the volume knob up past 5 there's a nasty bit of distortion in the sound and I'm beginning to think this is blocking distortion. At first I thought it might be the speaker distorting so I tested with another 150 W speaker and the same distortion is present when the amp is cranked so I don't think it's the speaker.

What does blocking distortion sound like anyway?


I've been reading Aiken's site on blocking distortion and he says Fender amps can be prone to BD because of the large value of coupling caps on the power tube grids and can even get worse if the amp is black faced because the grid bias feed resistors get changed from 100K to 220K which exacerbates the problem. The Vibrolux schematics I've seen all have 100K grid bias resistors and PI plate resistors of 47K each as opposed to the BF PI plate values of 100K and 82K so I'm wondering if this is a good place to start.

He has a list of things that can fix this but I'm wondering what the order of priority is and what I should try first. I don't really want to take the amp away from the way it sounds now other than get rid of the distortion.

I was going to quote from Aiken's site but wasn't sure if it was ok so I didn't.

http://www.aikenamps.com/BlockingDistortion.html

So what should I try first? My thinking is to try upping the value of the 1.5 K grid stopper on each power tube and then maybe take the grid bias resistors back to 100K and the PI plate resistors back to 47K before I start messing with coupling cap values. I really don't want to make a bunch of wholesale changes or take the amp far away from the way it sounds now.

I'd appreciate any insight on this and thanks in advance.

Ed

[martin manning]

Sounds like a good plan. Increase the grid stoppers to 5k1 or 5k6. If you hear some improvement then you know you are on the right track. You can go bigger too, like 8k2 or 10k. And you're right, one thing at a time.

[Firestorm]

Blocking distortion sounds like the amp is cutting in and out. I don't think it could be mistaken for speaker distortion. But it wouldn't hurt to try .047s or even .022s as couplers to see if it makes a difference. I'd try that first, before messing with the PI values.

[Structo]

Blocking distortion occurs when an AC coupled tube is overdriven.

The tube is forced into cutoff.

It can be caused by a faulty coupling cap or grid stopper.

This can happen in the preamp as well as in the power amp.

If it were me I would first try the larger value grid stoppers on the power tubes.
Try 3K3 grid stoppers.

[pdf64]

Is this really blocking distortion - need to identify that first before going much further along this route.
eg does the unpleasant distortion get more or less evident as you overdrive the amp harder?
My thinking is that it's not something that would be noticeable with the volume as low as '5' - mild overdrive is unlikely to bring on blocking distortion.
More likely to be a regular distortion caused by a bad component, dry joint or bad connection.
Have you tried new tubes, power tubes especially?
Re-seated all tubes?
Have you replaced the screen grid resistors? If not use a metal film/oxide flame retardent type, 1 watt 470 ohm, rated for 500V.
Also more likely than blocking distortion is oscillation, caused by removing the suppression components as part of 'blackfacing'.
Have all electrolytics been replaced? Failing electrolytics with high ESR can result in positive feedback paths being created.
The regular BF AB763 etc design should sound great without any mods; problems generally arise because of how that design is implemented.
Could a soundclip demonstrating the problem be made available?
Pete.

[C Moore]

Just to varify.....
Does your amp presently correspond to the AB763 schem.?
Sometimes the term "Black Face" gets tossed around pretty liberally.
Best

[yowza]

Thanks to all.

I'm not sure if it's blocking distortion because I don't know what that sounds like, that's what I'm trying to get to bottom of. Can someone describe what typical blocking distortion sounds like or point to an audio clip? I couldn't find one. Firestorm said it sounds like the amp is cutting out, I wouldn't say mine sounds like that, it's just this funky distortion that happens on both the normal and reverb channels when I turn the amp up to 6 or above and it sounds about the same on either channel. It sounds to me like the signal is clipping and up until the amp distorts it sounds really good, the best it's sounded since I've owned it.

All filter caps have been replaced in the last year or so with Sprague's and all power and screen resistors were replaced around the same time with metal oxide, All of the old carbon comps have been replaced with carbon film, there are very few original components left. The bypass caps are now TADs and I used Mojo Dijon tone and coupling caps with a few orange drops in the reverb and vibrato circuits. The last Vibrolux schematic I can find is AB568 and as far as I can tell the amp is up to the AB763 BF specs.

Some of the tube sockets were very loose and I went through the amp and tightened them all but I'm going to go back through and check them again.

Unfortunately I don't have a scope to trouble shoot this further. I can try to get a sound clip recorded but it might take a few days.

I'm going to keep at it.

Thanks!

[yowza]

Forgot to mention the power tubes are Winged C 6L6s that have maybe ten hours on them.

[Firestorm]

What is your plate voltage and where do you have the bias set?

What year was the amp made? Some of the Silverfaces have lead dress from hell.

[yowza]

What is your plate voltage and where do you have the bias set?

What year was the amp made? Some of the Silverfaces have lead dress from hell.

Just checked and the plate voltage of each tube is 438v but for some reason my bias was set really low around 22 so I raised it so it's 38 on one tube and 40 on the other. I recently set the bias and I don't know how it ended up being set so low, feel kinda stupid on that one. With the hotter bias obviously the amp's tone has changed and it's a lot louder now, however that " fizzy, buzz" of distortion is still there with volume over 6 especially when I really hit the guitar hard, it's like a spike. It's a little late here now so I can't crank the amp up and wail but the sound of the distortion seems less nasty than it was with lower bias, but it still doesn't sound good.

In the next few days I'll try to get the amp back on the bench and take some gut shots so you guys can look at the lead dress. In the meantime I'm going to check the tube sockets again and tighten if necessary. I'm also going to take my time and roll some tubes and see if I've got a bad one before I start changing any components.

Thanks again, I really appreciate the help!!

[Roe]

try a 68k at the reverb driver (v3) grid

[crbowman]

I just got done working on a Princeton Reverb that was doing exactly what you are describing. Turned out to be a leaky coupling cap.

[Structo]

Blocking distortion can vary in sound but frequently sounds like a blatty, sputtery sound that can if severe enough, cut out completely.
If you have ever heard a vintage germanium fuzz pedal that is under biased, that is real close to that sound.

This can happen to preamp as well as power tubes.

With power tubes, it is usually the grid stoppers or coupling caps to them.

But it could be an interstage coupling cap as mentioned.

[yowza]

I'm at work now and won't be able to look at this again until later but I really appreciate everyone's suggestions. The fact that the distortion is on both channels leads me to think the issue might be later in the circuit than in the preamps unless both channels are coincidentally having the same issue.

Like I said before I didn't know what blocking distortion sounded like and right now I'm inclined to think that it's not BD after all. All of the tube sockets in this amp are relatively loose and noisy when lightly touching the tubes so I'm going to focus on that as it's an easy thing to fix to eliminate as a possible cause. My V1 was so loose a few weeks ago I was convinced that was the problem. I removed the original self tapping screws and tightened it down with nuts and bolts but that didn't help. There is so much low end on this amp it might shaking the cabinet and causing something to vibrate but I'll deal with the low end later if I can fix the distortion.

I hope it's not a leaky coupling cap as they were all replaced about a month ago if it is I'll be chasing down a new component that's gone bad. Do leaky coupling caps respond to a chopstick? I've already chopsticked the amp a few times but I may try that again too. Any troubleshooting tips for leaky caps? I know they have to be measured out of the circuit but what else am I looking for?

Sometimes you need outside eyes to give you perspective.

Thanks again!

Ed

[Tone Lover]

Ed I had one leaky cap right out of the gate an orange drop drove me crazy. Then when I found out what it was kind of pissed me off, not supposed to be bad right out of the box.
But like anything even if only 1 in 1000 is bad somebody has to get the bad one.
Not saying thats the problem more than likely the other things that were pointed out should be addressed first.
Just wanted you to know it happened to me once.
Thanks Bill