fizzy distortion at nearly any level-- AB763

[The Dude]

The "fizz" is likely crossover distortion. Since the tubes are biased so low and closer to cutoff than they should be, neither tube is conducting when the wave crosses over the zero reference point. So, the waveform becomes distorted.

[Chuck H]

You mentioned that the waveform becomes "very jagged in a fractal-looking way". It would be great if we could see this. Do you have a camera that can interface with your poot? Without seeing it, it's hard to say. considering the low current I'm guessing crossover distortion.

And FWIW... The 470k v. the correct value of 470R for screen grids has been seen here a dozen times. I blame it on the color code difference between standard and high spec resistors. Not builders. It's easy to either take something for granted or even miss a decimal point (or a "K") when your brain is already aligned for a certain result.

[DC Bluez]

Hey,

Check the plate resistors on V1 or change them and also the coupling caps on that stage. Also check solder joints. Also check the input jack. It can be corroded over all of those years and not making good contact and probably needs changing.

On those old Fenders the plate resistors are probably carbon comp and 1/2 watt. Big trouble. I use 1 watt metal film for the 100K plate resistors and no noise at all. If your hearing hiss then its probably the plate resistors.

Also recheck your voltage on the 6V6 cathode bias section. Filter caps change is good idea also. cant go wrong there. Doug has some Vintage style coupling caps on his site now. I use Mallorys without an issue on many amp builds.

[Enzo]

Hi DC, welcome to the forum.

He established this is a reissue model, not the old original. Probably should not have referred to it as an AB763.

His super low power tube conduction seems to be causing severe crossover distortion, and he narrowed the cause of that down to 470k screen resistors instead of 470 ohm.

To the OP:

Sure, order the right resistors, but anything you have in the 100 ohm to 1000 ohm range will work well enough to see if that cures the problem. I bet it does. Tack them in for service. Then replace with correct type when they come.

[electron obvious]

And FWIW... The 470k v. the correct value of 470R for screen grids has been seen here a dozen times. I blame it on the color code difference between standard and high spec resistors. Not builders. It's easy to either take something for granted or even miss a decimal point (or a "K") when your brain is already aligned for a certain result.

hey thanks,
but of the many many components I ordered, measured to sort, then measured before soldering, that was a pretty glaring slip. 1000x.

[electron obvious]

thanks to everyone, and g1 for tipping me off first.

I couldn't wake the fam with a ringing 1kHz before I left this morn ( I got no dummy load 'cept in my head, apparently). I can post a pic from my phone later today. The funky wave form has to be crossover d although it looks a bit more wicked than a cursory search of google images. Probably because it is stupid extreme.

Since this was a problem before my bungle I can only hope that I had crossover d before in a lesser degree that I can now (with scope and this forum) identify and totally eradicate. The B+ is high though only 10% above schematic listing.

While I wait for a package from Doug (the fastest tracking # in the east) I will likely follow Enzo's advice if I can find a value to fit (of at least 1W).

Thanks again all

[Chuck H]

I couldn't wake the fam with a ringing 1kHz before I left this morn ( I got no dummy load 'cept in my head, apparently). I can post a pic from my phone later today. The funky wave form has to be crossover d although it looks a bit more wicked than a cursory search of google images. Probably because it is stupid extreme.

I'm certain that it also looks different because you're not using a dummy load. Actual speaker impedance changes with frequency and is all over the map compared to a purely resistive dummy load. Which is likely how all those Ooogle images were done. The difference between speakers and resistors for the load is usually visible as peaks and dips in the wave form. So you'll be seeing some of this too.

[electron obvious]

I'm certain that it also looks different because you're not using a dummy load.

thanks again,

that's prolly the fractal part then.

what is a good option for an econo- dummy load?

[Chuck H]

what is a good option for an econo- dummy load?

Usually a quad of big, honkin' 8 ohm resistors. All in parallel for 2R, Two in parallel for 4R, All in series/parallel for 8R and two in series for 16R.

There are a number of threads here on the matter that can be searched and read. Switching arrangements and alternative loads like water heater elements in cooling vats are covered.

For this one project a simple 100W/8R or 10R resistor would do it.

[g1]

The B+ is high though only 10% above schematic listing.

It should come down some when the power tubes start conducting.