"Phasey Warble" in 5F2A when cranked

[tubeswell]

Having recently finished my 5F2A, and having played it for a couple of weeks, I noticed recently that there's what I can only describe a 'phasey kind of warble' on certain notes and when its cranked with the guitar vol wound back to give more headroom.

It sounds like its being put through a phaser effect unit, when its not.

What would the cause of this be?

I have 2 x 20uF Spargue Atom caps in parallel for the first filter, and a 10uF cap at the 2nd and at the third filter cap position

The voltages at idle are:

HT 290VDC either side, with a Toshiba 5Y3Gt rectifier (318VDC without load)

B+ = 368VDC

6v6GT Plate = 363-364VDC
6v6GT Screen = 302-303VDC
6v6GT Plate to cathode = 343-344VDC

12AX7 Plate = 164-165VDC

I am using a 11k 2W for the first supply resistor (2 x 22k 1W in parallel), and 22k 1W for the second supply resistor

[pdf64]

It could be a supply ripple getting through to the screen, causing intermodulation effects. Though the 11k resistor 10uF combo should be sufficient - Maybe the 10uF cap is dodgy. Try doubling that 2nd cap to 20uF. See
http://music-electronics-forum.com/s...ead.php?t=6144
Peter

[tubeswell]

Having another look and listen tonight, I tried running the amp with maximum volume, and with the guitar vol at various settings from nearly-cut to full-tilt, for about half an hour, so that it warmed up.

The distortion on full-tilt made it hard to hear any phasey sounds.

However, as I lowered the guitar vol, the sound got creamier and more phasey.

The voltages at idle were more or less as described above.

The voltages on the 6V6GT under load were more interesting. On full guitar and amp volume, the voltage readings seemed to vary (was I imaging it?, or can't I trust my meter that accurately?). When the guitar was 'quietly played' or played with the guitar volume nearly cut, the readings stayed pretty close to idle conditions, and I got that phasey sound, but maybe not as pronounced as I thought it was yesterday.

Under full guitar volume (and distortion) when I whacked out power chords, the plate did at times hand around idle voltages, although it varied a bit more, and actually had spikes that went into the 400s or 500s (was I imaging it? nah my meter had a fresh battery - but on the other hand its not a fancy meter).

The screen voltages also varied a bit around idle voltages, but tended to go down (as opposed to up) under these conditions. I was certain at one instant the screen voltage went down to around 230V. I suppose this is a normal situation for these playing conditions.

I have come to the conclusion that the phasey sound that I can only seem to hear when the amp is fully cranked but the guitar vol is cut (to give more headroom), is probably actually the pre-amp valve really singing and giving out maximum 'harmonic' (?) distortion, which I can only hear when all the other square-wave distortion is relatively absent. It actually sounds quite sweet.

As the idling and gently played voltages don't seem to be much different, I can't really see anything wrong with the power supply at this point. So I'm just going to hang in there and chill for a while, and enjoy playing and discovering its range of sounds.

[pdf64]

Re your voltage readings, maybe try aligator clips on your test probes, as it might be a dodgy connection causing the spikes. Could do with measuring the ac value of the screen supply, so revealing the 120Hz ripple.
It could also be swirl that you are hearing, though it's a slowly modulating change rather than a warble, in my experience. See
http://www.aikenamps.com/TechnicalQ&A.gif
Q: What is "swirl"?
A: "Swirl" is a dynamically-changing, slightly "phasey" sound as a note or chord decays, which is common to some tube amps. Typically, "swirl" is caused by a midrange "dip" or varying duty-cycle change in a clipped square wave that changes position as the note decays, giving a sort of mild phase shifter effect.

What happens is that first the phase inverter or output stage clips and produces a flat square wave. As the note decays, the signal level decreases, and the midrange frequencies start getting "unclipped" (either by the fact that their frequency band level is lower, or by phase cancellations due to the unequal phase shift with respect to frequency caused by tone controls and other RC phase shifts that occur in a gain stage) and show up as a "dip" in the top of the square wave, which will move back and forth along the top as the fundamental and other harmonics shift the operating point. Even if the clipping ratio isn't extreme enough to show the "dip" on the scope, the duty-cycle of the square wave will usually be dynamically changing as well.

Since the preamp stages are all AC-coupled to each other, the operating point shifts as the signal gets smaller, due to slight "blocking" distortion, where the gain stage clamps the top peak to a point slightly above it's cathode voltage, while allowing the wave to still increase in the negative direction. As the signal decays, it shifts upward and changes the duty-cycle of the clipping. It is this ever-changing shifting of the operating point that causes the "swirl" effect. The trick to good "swirl" is in the correct staging of the gain and frequency breakpoints of each gain stage in the amp, particularly in the phase inverter and output stage.

A similar effect can be caused by too much drive from the phase inverter to the output tubes. As the note decays, a riding "buzz" can be heard coming in and out. This is crossover distortion aggravated by too much signal swing to the output tube grids. Reducing the signal levels at the output of the phase inverter will cure this.

Another cause of a "swirly" sound is a parasitic oscillation that is riding on the output signal, causing intermodulation distortion.

[tubeswell]

Well the way you describe it pdf64, maybe it sounds like it is 'swirl' as opposed to 'phasey warble'.

The frequency of the 240V Mains AC in NZ is 50Hz. So I don't think I would get 120Hz (maybe 100?). The way the amp is set up, I can't hear much hum anyway.

The AC reading to ground on either side of the HT winding is 291 under load, and 318 without the other tubes plugged in. Someone said this was low for the B+ I was getting, but the guy who sold me the 5Y3GT, said that Toshiba made good rectifiers, so maybe that's got something to do with it?

I was using alligator clips to fasten the meter to the contacts. So maybe its my meter not being able to cope with the fluctuations?