Farty bass on '66 Fender Pro

[rfgordon]

A guy brought his well-worn 66 Fender Pro to me for some work. It has really nasty farty distortion on the bass notes.

So far I've rulled out:
- Power tube issues
- PI caps
- OT issues (put a new heyboer OT in)
- preamp tube issues

At some point in the amp's history, the entire board was re-done with 1/2 watt cfs and PS OD caps.

The owner says the problem developed over about a year of heavy gigging.

Do y'all think it could be a plate resistor or two? I don't have very much time "under the hood" of old Fenders, so any advice is most welcome.

[jaysg]

How did you rule out the post PI caps? Did you rule out the speaker?

[rfgordon]

When he first brought it over, he played it to let me hear the farty distortion. I first thought it might be a bias issue, because it got worse with a new set of tubes. We did, however, test the amp through another speaker/cab--same behavior.

BTW, the voltages throughout the amp are spot-on. All the preamp tubes have been swapped out to eliminate them as suspects.

He wanted it converted to cathode bias, which I did, but that didn't help the situation.

The OT (and the rest of the amp) is way rusty. I spliced in a toroidal OT I use for testing, and the problem seemed to go away. So I put in a new Heyboer OT--drop in replacement for the Pro. The farts came back.

I then swapped out the OD PI caps for a pair of fresh Mallory 150s (what I had on hand). Both new and old are .1 mF No change

I tried several values at the PI entrance--no change.

I may try smaller caps at the PI next.

Ideas? Seems like it should be a cap--voicing issue, but is it?

[jaysg]

You need to progressively shrink the size of the 0.1uF's that follow the PI. .082, .068, .047, .022, etc.

OT swapping can change the ultimate value quite a bit. I can't remember whether the Pro had the small or large OT, but I'd guess the Heyboer you put in is the size of the SR OT, and that extra low end range just makes it worse.

[rfgordon]

I'm goin' crazy with this thing!

I dropped the two caps on the PI output to .022, and I've tried clipping in various values of PI entry cap down to as low as .0047mF. No substantial change.

The behavior exists for both normal and vibe channels, so I figure it must be a PI issue.

When a low note is struck hard, like low E, it starts out good, but as the note decays there's this farty, sizzly sound. It sounds very "electronic" if you know what I mean.

I'm gonna change the plate resistors on the PI tube.

Also, the previous tech put grid stoppers on the power tubes, which is fine, but they are 1k5 3 watt wirewound power resistors. Could having these types of resistors last in the signal chain make a difference? I usually put 2 watt cfs on that spot. Thoughts?

[jaysg]

hmm...power resistors as grid stoppers? Seems like overkill.

If you have a scope, pull the 6L6s and look at the PI output as you hammer it with low E. I've got a function generator which is helpful for isolating sections. I would pull everything but the PI tube and drive its input to see what's going on. A synth is a decent substitute. hth

[rfgordon]

Well, I have no scope.

I'm going to focus on the parts the two channels have in common, from the mixing resistors thru the PI.

[mlp-mx6]

Have you ruled out the power supply caps? How old are they? Also - measure the "in-between" voltage on the plate supply totem pole. See if one cap on the stacked caps is carrying too much of the load.

[jaysg]

I think there's a shared 25uF/25V on the cathodes of both channels.

[rfgordon]

I checked the filters, they're all OK. Even clipped in a new one just to make sure.

Replaced 82k & 100k plate resistors on the PI. no change.

I'll try the shared cathode resistor/cap combo. I've read that icky cathode resistors/caps can cause all sorts of mysterious problems. I hadn't thought of that as a shared component. It does "set the tone' to a certain extent. Worth a try.

[rfgordon]

Tried the cathode resistor/cap swap for V1/V2. No change.

Could it be that I have to take those post-PI coupling caps a lot smaller? I've got them at .022mF now. Maybe I do have to strangle this thing to keep it from farting out....I am REALLY frustrated right now.

What's left to try:
-change the grid stoppers on the 6L6s
-change out all the resistors from the 220k mixing resistors throughout the PI
-try even smaller values of caps on the post-PI caps
-??????

[paddy]

I had a similar problem with a dumble clone-turned out to be
the resistor values in the NFB loop.You could check/change these
values and see what happens.I'd start by reducing the value of
the series resistor.
Just my 2 cents.

cheers
paddy

[mlp-mx6]

Could it be that I have to take those post-PI coupling caps a lot smaller? I've got them at .022mF now.

Absolutely not. Something else is causing it. Fix the problem, not the symptom.

What's left to try:
-change the grid stoppers on the 6L6s
-change out all the resistors from the 220k mixing resistors throughout the PI
-try even smaller values of caps on the post-PI caps
-??????

Are you intentionally ignoring the power supply? Honestly, you SHOULD make sure it's all good there before you change anything else in the PI.

[jelle]

I also suspect the power supply...I recommend you put the 100n PI output caps back.
Q: is the rectifier tube OK? I've had a batch of Sovtek GZ34's that sounded nasty...I have used them for slide since....not as tubes

Are the resistors in the PS rail OK? Sometimes these carbon composition Allen Bradley resistors go bad here...I know you wrote that the voltages are OK but you never know. How about the choke under load? Does the problem go away when you replace the choke wih a 500 ohms 10W resistor?

I hope you find it soon

Jelle

[Bob-I]

Since you mention that the note starts out ok and becomes farty, I too suspect power supply. You could have a PT, filters or rail resistors causing the issue.