Distortion.....Blocking distortion.?

[C Moore]

I rebuilt a Bassman head that you guys helped me find a power supply problem with. This is an amp that I gutted, re-using only the transformers, circuit board, and tube sockets.
It is basically the AB165 layout (non of those "suppressor" type caps) with the AA864 NFB.
The amp seems to function fine with the exception of the notes distorting on both channels. If it is blocking distortion, I was thinking that with both channels involved, it is probably a problem with blocking distortion in the power tubes.
I removed the NFB, that makes the amp louder with the same distortion problem.
I have gutted and rebuilt 10-15 BF-SF Fender amps. I have never had this problem before. This is the first time I have deviated from the Fender ground scheme. I installed a ground buss for the preamp filtering. I used 14 AWG solid wire, grounded at the input jack end of the chassis.
Is it possible/likely for the ground scheme that I used to be causing this.? I have no problem with taking it out and going with the "normal" Fender system. I was just curious if I have done something that the amp does not "like".
The distortion seems to come on as the notes peak, and then start to decay. Not right as you pic a string.
Thank You

[martin manning]

I doubt the ground scheme is the cause. What values do you have for the PI input and output caps, and the power tube grid resistors?

[C Moore]

Input from both channels to the PI goes through a .1uf.
Between PI and power tubes are .022uf.
Grid resistors are 1.5k (guess I could increase those.?)
Thanks

[martin manning]

You can try 5k6 for the grid stoppers, and/or reduce the 0.1 PI input to 0.05 and see what happens. The 0.1 PI input seems pretty big to me, maybe try that first.

[Cliff Schecht]

I'll bet your phase inverter is farting out. You really have to be careful about shoving too much bass into even the later sections in your amp or it will fart out distortion wise. Fender uses large coupling caps later on in some of their amps and a lot of them fart out when pushed too hard, especially if you don't roll back the bass in the EQ a good bit.

Grid stoppers are typical for a phase inverter so your only real option is to reduce those coupling caps. I think 0.022uF is much more common here. I also agree that some grid stoppers on the output tubes might help.

[C Moore]

I'll bet your phase inverter is farting out. You really have to be careful about shoving too much bass into even the later sections in your amp or it will fart out distortion wise. Fender uses large coupling caps later on in some of their amps and a lot of them fart out when pushed too hard, especially if you don't roll back the bass in the EQ a good bit.

Grid stoppers are typical for a phase inverter so your only real option is to reduce those coupling caps. I think 0.022uF is much more common here. I also agree that some grid stoppers on the output tubes might help.

Hey Cliff -
I was already under way with Martin's suggestions before I saw your post.
I changed the input cap to .033.
I changed the grid resistors to 4.7k.
It still sounds the same.
I can go lower with that input cap.......
The distortion seems to be most prevalent as the note just peeks and then decays. Kind of the frying bacon type of sound. The only other time I have ever really heard this was with a Twin that had a speaker with a bad voice coil. As the note started to fade the sound would distort real bad.
I have changed cables and speaker cabs with no difference.
I realize there could be A Lot of things that "might" be causing this. But I cannot stop thinking about that ground buss I used. If it were causing some type of problem, maybe "starving" the B+ somehow, might this be the cause of what I am hearing.
I will try whatever you guys think is the logical chain of repair.
Thank You

[Cliff Schecht]

Could also be a leaky coupling cap.

[C Moore]

All the components are new, but that does not mean one of them could not have died.
I could replace them.....are you talking about between the PI and power tubes.?
Is there something in the NFB that can cause this.?
Thanks

[C Moore]

This amp is about 40 years old. I re-used the fiber, eyelet board. I am not really familiar with "tweed disease".....or what happens when these old boards become conductive.
Is there something specific you check for.? Was the problem mostly a case of B+ leaking over to an eyelet it should not be at, or is there more to it than that.?
Thanks

[martin manning]

So far what you've tried rules out the blocking distortion theory, which seemed likely given the large PI input cap and small-ish power tube grid stoppers. You should have seen some change in the distortion even if it didn't clear up. I don't know about a leaking coupling cap... I'd think that would tend to let the bias drift and affect the whole note instead of just the peak. Is it possible that a HF parasitic oscillation is comming in?

[C Moore]

Hey Martin -
Not to be dense.....but I do not know how to check for high frequency, parasitic, oscillation.?
Thanks

[Cliff Schecht]

Martin got me thinking, you definitely need to be scoping this out. There are types of distortion you want and types you don't want. Every type of distortion (good and bad), and even parasitic oscillations, can be quickly viewed on an oscilloscope and if you are serious about amp debugging you will know how to use one. I've traced out every problem imaginable with a scope and a signal generator..

[martin manning]

hh, usually an oscilloscope is the easiest way to find oscillations above the audio range as Cliff says. Being that this is a transient phenomenon may make it a little tougher. You'd have to get it to exhibit the distortion with a sine or square wave input, and then look for its source with a scope.

To review, you said that a note or chord cuts out somewhere between the initial attack and decaying out, with an accompanying crackling sound? Does it cut out and then return? If so, how long does the cut-out last? And, you said that disconnecting the NFB doesn't make any difference?

Since the problem is on both channels, the PI and power stage is the likely place to look. Can you measure the voltages on the PI and power tube pins, and the supply voltages, one more time? For the PI, also measure the voltage at the bottom of the 470-Ohm bias resistor (at its junction with the two 1-Meg's and the tail resistor). You can't get a good reading at the grids because your meter will change the operating point. Are you sure that your power supply is wired correctly now and the grounds are all good?

['67_Plexi]

This amp is about 40 years old. I re-used the fiber, eyelet board. I am not really familiar with "tweed disease".....or what happens when these old boards become conductive.
Is there something specific you check for.? Was the problem mostly a case of B+ leaking over to an eyelet it should not be at, or is there more to it than that.?
Thanks

'Tweed disease' affects not just Tweeds, but any amps that use the pressed fibre boards. Over time they soak up moisture like a sponge.
Unfortunately the only real way to 'dry' them out is by removing all the components and putting them in an oven on a low setting for a few hours. I've done this many times. You can normally spot a board that has sucummed to this disease at some point because the board will have become distorted and 'wavey' and won't sit horizontally flat.

That said, your symptoms are not typical of that issue, you would normally expect to hear odd pops and crackles and other weird noises.

Clearly I can't make any 'technical' diagnosis without at least a) a recorded sample or b) a scope shot. I find this just costs people time before they have checked the simple stuff first. Back right out of the minutia a moment and take an overview.

To me it actually sounds like you potentially rewired something incorectly when you put the amp back together, an incorrect value resistor (misreading a color code by a factor of 10 or 100 etc), wire on a wrong connection, missing or incorrect ground point or a rogue wire strand etc etc.
After building 100's of amps I still can make simple mistakes like mis reading orange for yellow on a resistor.
Don't overlook the obvious and never assume new means working on new components, especially tubes.

If you intend to build/restore more amps, you really need a signal generator and a scope and learn diagnostic methodologies so you can logically work back though the amp from the power amp backwards. Much easier to isolate a problem or to find if it's systemic in the power supply etc.

One last thing.......lead dress, bad/incorrect lead dress can cause all sort of weird signal anomolies and noises. I have seen many situations where induced out of phase signals can stir up some very strange distortion effects that are signal amplitude dependant. Basically farty noises that cut in and out like someone is flicking a switch when the signal gets to a specific amplitude. These issues are often found in high impedance circuits in the early stages of the preamp and higher gain overdrive circuits are definately much more prone to this, but it's worth being aware. Often people incorrectly diagnose these sort of issues as being in the feedback/presence circuit because moving wires in that circuit sometimes changes the noises. The feedback circuit itself is rarely the cause unless it's become somehow disconnected or the feedback resistor from the speaker has got shorted/bypassed.

Sorry I can't be more help.

Alan Phillips.

[Firestorm]

I'll bet it's parasitic and I'll bet it's lead dress related. Do you have current gut shots?