Vox AC15 : fizzy sound

[ricothetroll]

Hi,

I recently transformed my Fender Hot Rod Deluxe into an Vox AC15. The amp is now working nicely but I've noticed that when I play single notes sometimes the amp sounds kind of fizzy. This is most noticeable with humbucker pickups, neck position, on the G string between like 5th and 12th fret.

Here are some sound samples, recorded directly in the circuit with an attenuator :

https://dl.dropbox.com/u/28610725/AC15/test2_Source.mp3 (original sound file recorded with a Jensen passive DI box, Amek preamps and RME Multiface soundcard)
https://dl.dropbox.com/u/28610725/AC15/test2-V1.mp3 (plate of the first gain stage, 1mag-10k attenuator)
https://dl.dropbox.com/u/28610725/AC15/test2-V2.mp3 (plate of the second gain stage, the one that's DC coupled to the cathode follower)
https://dl.dropbox.com/u/28610725/AC15/test2-V3.mp3 (cathode of the cathode follower)
https://dl.dropbox.com/u/28610725/AC15/test2-V4a.mp3 (plate of PI-a )
https://dl.dropbox.com/u/28610725/AC15/test2-V4b.mp3 (plate of PI-b)

Here is the schematic of my version of this amp :
https://dl.dropbox.com/u/28610725/AC15/AC15.pdf

Although those samples are not filtered by a 12" speaker, they are very representative of the "fzzzzzz" I hear on the notes decay when I play with the amp loaded by my Celestion Greenback Heritage G12M.

I remember having heard the same phenomenon when I once tried a Vox AC15C1 with a Fender Jazzmaster in a music shop : the amp sounded great when playing chords but sometimes fizzy on single notes.

I tried lots of things : altering the biasing of the preamp tubes, lowering/increasing the cathode resistor of the cathode follower, lowering/increasing the filter resistor to increase/lower the preamp supply voltage, changed some coupling caps to cut some low freqs...) ... without success : the "fzzzzzzzz" is still there ! The only thing that improves things if to crank the amp to get power amp distortion that covers the undesirable noise, but then the amp is VERY LOUD !!!

How can I reduce/suppress this phenomenon ? Is it typical of Vox AC15/30 amps ?

Any help/clue/explanation would be appreciated !

Thanx in advance.

Best regards.

Eric

[Seljer]

Hook up a scope to see if you have any oscillation going on. In my experiences fizzyness tends to be intermodulation distortion.
I opened up your audio files in audacity and the looked at the frequency spectrum, and theres a huge component at 15.8khz present at all times.

edit: or maybe one of your tubes is microphonic?

[ricothetroll]

Hi Seljer !

Thanx for answering me !

I checked my source file and the 15k6 whistle is actually in it, at about -122dB !

I also checked my circuity with a scope (an old 1960s tube scope, so thus not the most accurate). I found some oscillations at about 50kHz that I got rid of by using shielded cable between the input jack and the grid of the first tube. But... fizziness is still here !

I also found something else on the scope :



The frequency of the occurence of those bursts seems to be around 200Hz. I found those everywhere in the amp, from the plate of ther first preamp tube to the speaker output. Do you know what can cause such bursts ?

Best regards.

Eric

[PRR]

> around 200Hz

180Hz? (Are you in 60Hz land?) Get three spikes on screen, then disconnect and put your finger on probe to cach 60Hz hum from the air; do they sync 3:1?

Spikes like that are power rectifiers. "everywhere in the amp" suggests the PT CT goes to circuit ground then to fist power cap. PT CT must run to first cap, *then* to circuit ground.

Fizzy on simple note decay can be an almost-good connection on a filter cap.