Low end buzz issue

[jordan86]

Hey guys...Wondering if any more experienced ears could help identify what might be causing this. Sound sample attached. Worst at like 30-35 seconds on the neck pickup.
https://drive.google.com/file/d/1yo9vIKLMKqxuMayNPdcK2tPqHSWoD8wJ/view?usp=sharing

I have an AA1164 PR build, that's recently been doing something strange in the low end.  Buzzy, flappy thing. It may have done it before, but I can't confirm. Seems to be worse now, and happening with low volume and bass settings too. When I just recently noticed it, I thought I had blown my Celestion Blue!!! Attached a sound sample. Things confirmed while troubleshooting....

1) The PR does this across all my different speaker cabs.
2) My other 18 watt amps do not do this.
3) Not my action. My action is low, but there is clear difference in the fret buzz from the hard attack, and a clean note causing the fuzzy buzzing in the speakers.
4) Tried different bias levels on the 6V6's. No change.
5) Tried different setting on bass control. No change.
6) There seems to be a marked improvement when I put an EQ in front of the amp, shelving everything at 75hz off by 7-10db. Amps just sounds real lean and bright at that point.
7) Removed 12AT7 reverb driver. No change.

I know the PR has a reputation for being farty/flabby. But I tried to address all those during the build. Seemed to have worked before. My build has the following mods....

1) 5uf bypass caps at first two stages. Later stages left stock (25uf)
2) 40uf first filter cap and GZ34 rec
3) 470k grid stopper coming into the phase inverter
4) Grid stoppers and screen resistors on 6V6s
5) Stokes mod. Higher supply voltage on PI (260v measure at PI plate)
6) Allen TO20B output transformer

My typical settings are:
Volume:5-6
Treble: 8
Bass: 8

Is it still just putting out way too much low end, or could I be clipping something internally at one of the gain stages?

[shooter]

what caps size is feeding the PI?

[jordan86]

Standard value = .02

And to clarify....this is with 5 different 12" speakers (not 10's)
Alnico Blue
G12M Creamback
G12H Creamback
G12H30 anniversary
Vintage 30

The speaker in the recording is a G12H30 Anni

[HotBluePlates]

My typical settings are:
Volume:5-6
...
Bass: 8

Hey guys...Wondering if any more experienced ears could help identify what might be causing this. Sound sample attached. Worst at like 30-35 seconds on the neck pickup. ...

The entire clip before ~30 seconds sounds like a "Fender guitar" to me.  There might be momentary distortion on the transient peaks, but for the most part this section of the clip "sounds normal, like every other amp."

Don't know if the guitar/amp was turned up at the 30 second mark, but that sounds like "onset of distortion" (and grid-blocking, per Shooter's question).  I would like the hear how the amp sounds strummed at that same volume.

Trying turning the Bass down a lot.  For reference, I usually play a Tele on both-pickups, with the bass on my blackface amps down around 1-2, but I tolerate (and want) a lot more midrange where your settings imply a lot of scoop.

          -  If you want to reduce grid blocking, make the coupling caps running into the 6V6s ~0.01µF, and consider dropping the 220kΩ bias-feed resistors to 100kΩ.  This raises the -3dB frequency from ~7Hz (0.1µF coupling caps, 220kΩ) or 36Hz (0.02µF, 220kΩ) to 72 (0.01µF, 220kΩ) or 160Hz (0.01µF, 100kΩ), speeding the recovery from grid-current induced bias-shift.

          -  Realistically, the amp has plenty of bass elsewhere, so it's fine to cut at the output tube coupling caps.

          -  Normally, large caps are used to put phase shifts well outside the pass-band to keep the feedback loop stable, but Fender isn't using a lot of feedback, and overload performance is of more concern to you (since that's why you're asking).  If stability became a real problem, you'd just disconnect the feedback, and accept smoother transition to distortion for less control over the bass resonant frequency & no-oscillation (and even obtain a resonant bass-boost offsetting the bass-shave you had to imposed to fight the blocking distortion).


My first good amp was a 1967 Princeton Reverb, which I got in maybe 1991 or 92.  I even foolishly tried to gig with that amp in Nashville in the late-90s.  Stock, they sound like garbage when they're dimed (or at least, they "sound trashy").  Like many Fender amps of the 60s-70s, they don't do much distortion gracefully, for many reasons.

[jordan86]

Thanks, HBP....At 30 seconds...I just switch to the neck pickup on my Tele. Fralin Blues Specials.

I'll look more into those mods. I run the treble at 8 because its a very warm sounding amp to me. Much warmer than I expected. Maybe the Allen OT is the cause of that? I intend to due a switched bright cap to help in that.

I like the bass at 8, not so much for a need for more low end, but for the scoop/shift in the midst that finally hits the spot for me at 8 on the dial.


Strumming some wide open chords, the amp sounds good. It the attack on the low E that pushes things into buzzing. If I push the amp with a pedal it does seem to be fuzzing out. Noticed that for the first time yesterday as well. Changed 6v6's also, but no change.

I'll try to make some more recordings. Definitely sounds like something is wrong in person.

[Cree]

Let me just add my 2 cents. My first build made this same sound. I finally disconnected all by pass caps after V1. One at the time till I found the caps at my output tube was the cause. That was it. I know the PR does not have by pass caps on the output tubes. But I would start there at all my by pass caps. I mean the bias resister by pass caps. 

[shooter]

got a spectrum analyzer ?


I asked the most nit-picky guitarist I know play an amp;
His only complaint, "playing B on the X fret isn't right"
so I found B's frequency for "that" fret (can't recall which)
set my sig-gen up for 1k to baseline my scope (FFT mode)
set it to "B" freq and sure as sheet's, intermittent harmonic kept popping up, swapped some caps in a "fixed TS" section and it's gone


I didn't hear it before or after 
was one of the weirder tube amp problems

[HotBluePlates]

Thanks, HBP....At 30 seconds...I just switch to the neck pickup on my Tele. Fralin Blues Specials.

Does that mean the first 30 seconds are the bridge pickup?  Or bridge & neck?

The track below is a blackface Pro Reverb, Tele on both pickups (Fralin Tele pickups that Callaham once sold; maybe called "half & half"?  Alnico 5 for wound strings & Alnico 2 for the plain strings).  The Bass was on 1-2, Treble around 4.5-5.  That was intentionally done to raise the mids, and also trim the low end.  IMO, it still sounds pretty full, certainly good enough for a mix (and maybe too full for an actual mix).

https://soundcloud.com/hotblueplates/next-time-im-in-town-2

There's a reason all the Marshall amps since the 70s (and essentially every other amp focused on its distorted sound) strips out tons of bass from the distortion channel.  And there's also a reason why Fender (with a big mid scoop & lots of treble/bass, but also a bunch of low end passed stage-to-stage) never had a passable "distortion amp" until fairly recently. 

... My first build made this same sound. I finally disconnected ... by pass caps ... at my output tube ...

The cathode bypass cap at the output tube(s) also leads to bias-shift unless it is either gone or many-many µF's.

[jordan86]

Thanks all. I’ve found backing the bass off to 6 does help.

Any preference on Trimming via bypass cap vs coupling?

I do have a premium 2uf Bypass cap I could put at stage 1. And move that 5uf down to stage 3.

Or could do a .01uf or .0047 on the coupling cap going into the PI. Those eyelets don’t share any other components, so easy swap.

Or I could do some .033 blue sozos feeding the power tubes. I’m a little hesitant to go down to .02 there.

[jordan86]

got a spectrum analyzer ?

No scopes or analyzers :(

[HotBluePlates]

... Any preference on Trimming via bypass cap vs coupling? ...

Your amp is fixed bias, and the output tubes are the first thing that will be overdriven.  The right move is to adjust the coupling caps to Pin 5 of the 6V6s (and possibly the 220kΩ bias feed resistors), since this is the exact place the bias shift & blocking distortion are occurring.

Cree was right to mess with the cathode bypass cap in his amp, but it was an output tube bypass cap, as his amp was cathode-biased (and so, also the exact place the bias-shift & blocking were occurring).

[jordan86]

Quick update. Found an old stash of parts. Had two orange drop .047’s and two Mallory M150 .022 that I decided to test. The M150s cures the blocking distortion completely, the top end sparkle was gone though. Distortion is still there with the orange drops, maybe only a slight improvement than stock .1

As for the tone...sadly, I don’t love either. Maybe it’s just psychological but both make the amp feel smaller to me. There’s still low end. Just different.

I did find using the .047 and a 5v4 rectifier clears up the issue as well.

Debating now whether I live with the issue with .1 stock caps and augment my playing. Just have an always on EQ, or try trimming somewhere else. I may try some .047 or .033 blue sozos just out of curiosity.

Thanks everyone for your help. I’m
headed in the right direction.  The quest continues.

[bmccowan]

My first thought on "low end buzz" was that it reminded me of the Commander Cody song: Down to Seeds and Stems Again. My first project ever was bringing back a non-working PR. When I go it working stock, it had exactly that issue. It was a Silverface, with the BF circuit, but with those little blue turd drop caps. It only sounded good with the bass control on 0-1. I changed all the caps to 716s - helped some. And then changed the .1 to a .022 716. That helped more, but the amp never did sound good with the bass turned up as high as 8. Not much help I know, but thought I'd share my experience.

[jordan86]

Yeah my problem is that I LIKE the sound with bass on 8. Doesn’t sound too muddy or flubby. But when I hit things hard on the low E, there is a real problem that needs to be addressed or avoided by adapting my playing

[HotBluePlates]

... The [.022] cures the blocking distortion completely ...

Yeah my problem is that I LIKE the sound with bass on 8. ... But when I hit things hard on the low E, there is a real problem that needs to be addressed or avoided by adapting my playing

The above translates to: Use a 0.022µF cap OR turn your volume down until the blocking stops.

Those are your only 2 options aside from "learn to live with the blocking distortion."

... Had two orange drop .047’s and two Mallory M150 .022 that I decided to test. ...

There are many kinds of "orange drop" caps.  715P/716P (polypropylene)?  223P (polyester)?  Something else?

I ask because:

... The M150s cures the blocking distortion completely, the top end sparkle was gone though. Distortion is still there with the orange drops, maybe only a slight improvement than stock .1 ...

If you were using polypropylene orange drops, then some folks feel they have a "bit more top end" than the polyester M150s.  So your solution would be to try a 0.022µF 715P orange drop.

But "distortion" adds harmonics and "top end sparkle."  So one has to wonder if that plays a role.

... Maybe it’s just psychological ...

That's also a possible explanation. It's why I built a cap substitution box to directly test 12 different types of caps (of the same value), as well as 3 different options for cap-value.

I have a whole separate commercial cap decade box to try different values in-circuit to hear which performs best.

[jordan86]

Update and another question. Strangely enough, I am still experiencing the issue now with .022 coupling caps going into the power tubes and the bass pot set on 5. Mind you, I do have to whack the low E pretty good. But I wouldn't call it an "abnormally" hard attack. So an outboard EQ coming into the amp is the only "cure" at this point?

Is it possible that the issue is coming before the PI? Could a scope show me that if I were to read the signal after each gain stage? I do have a friend with a scope.

Or could a failed solder joint or failed resistor cause this? I did measure every resistor religious before installing FWIW.

[bmccowan]

Taking wild shots here. You mentioned that switching from a GZ34 to a 5v4 helped and you mentioned the stokes mod that upped the voltage to the PI. Any chance that this is a subtle voltage thing happening? I just looked that Stokes mod up, and according to Rob Rob it shifts distortion toward the power tubes, and many people run 6L6s or 5881s in their PR with that mod.
And - you said that the problem has been developing over time, and now is continuing to get worse. To me that sounds like a failing tube or other component.
Like I said - just wild pot shots

[jordan86]

Good thought. I tried the 5v4 again last night and it was back to the same buzz. Swapped all tubes as well. Still have the issue. I’m thinking I may have a failed component somewhere. Gonna try the scope if my local friend here has time to help.

[bmccowan]

If the 5V4 helped before, but not now, then it certainly seems like a failing component. If you have somebody who could hit that low E for you while you probe, you could try a listening amp and start at one end of the signal chain and see if you could narrow it down. There are a lot of caps, resistors and pots in that amp to use the replace and hope approach! I recall one time a somewhat similar problem and using a listening amp finally tracing it to one resistor. When I removed it, I was able to easily break it apart with my fingers.

[shooter]

If you have somebody who could hit that low E for you while you probe

It's a frequency, look it up and use as "test" signal, won't come with the string harmonics, but should be close enough for a fun day 

[bmccowan]

It's a frequency, look it up and use as "test" signal, won't come with the string harmonics, but should be close enough for a fun day

I wondering that as I usually use a tone generator app on an iPad. But then I was thinking "that could be pretty annoying to have that tone blasting steady." But maybe no more annoying than hitting the E string hard repeatedly, eh?

[shooter]

that could be pretty annoying

look behind the 2 white tube holders, they make 0 sound while testing 

[bmccowan]

Love it - scope, signal generator, fluke, and 4 - 2x4s. Looks like my shop. Anyways, I was referring to hearing the buzz, not seeing it on a scope. It seems he hopes to borrow a scope.

[sluckey]

Love it - scope, signal generator, fluke, and 4 - 2x4s. Looks like my shop. Anyways, I was referring to hearing the buzz, not seeing it on a scope. It seems he hopes to borrow a scope.

You're not seeing the whole picture. Look "behind" the two white tubes.

[HotBluePlates]

... I am still experiencing the issue now with .022 coupling caps ... Mind you, I do have to whack the low E pretty good. But I wouldn't call it an "abnormally" hard attack. ...

You guys are ignoring the realities of the voltage waveform coming the pickups.  Don't you think maybe those initial transients are a factor?

Prove it to yourself by clipping your meter to the output tube grid (Pin 5) set to display "peak A.C. volts."
     -  Play until you hear the bad artifact.
     -  Compare your reading to your tube's bias voltage.

[shooter]

 

won't come with the string harmonics

I just buried it in a sentence

[bmccowan]

You're not seeing the whole picture. Look "behind" the two white tubes.

The dummy load? I get it. I should make up one of those.