Overdriven Fizz Sound

[Theashe]

I've been trying to sort this one out for days now. The amp I'm working on sounds fizzy as soon as it starts to overdrive. The fizz gets louder and quieter until the guitar signal dies away.

I experimented with overdriving different parts of the circuit. If I overdrive V1b but turn down the next volume to make sure the rest of the amp passes that signal clean, you can hear the fizz I'm talking about. However, if I turn down the first volume to make V1b clean, and turn up the volume later in the amp, you can hear the SAME fizz sound. The amp doesn't make that sound unless tubes are starting to clip. I saw a thread in the Trainwreck forum that sounded similar to my problem.

Sound clip: https://soundcloud.com/steeledriver/amp-fizz

I've tried:
- Rolling four different brand 12AX7s through the amp as well as 12AT7s and 12AY7s. If I can get tubes to overdrive, that sound will happen, no matter what tubes and where.
- Bypass caps on the power supply, PT primary, and PT secondary.

Could it be too much gain causing grid conduction? Or oscillation from lead dress/wires running too close? Maybe someone has heard this type of fizz before, knows what causes it, and can help me get to an answer.

Schematic attached. I've been playing with LEDs for biasing, but the amp had this problem before the LEDs.

[pdf64]

Have you scoped the waveforms along the signal path, eg to check for oscillation?

I've not tried diode biasing, but Merlin advises the diodes are bypassed.

Your fx loop arrangement seems a bit dodgy, eg fx send from a diode biased cathode, very different dry and 'wet but bypassed' signals; has it been proved elsewhere?

[Littlewyan]

That is one crazy ass amp!

http://www.ax84.com/bbs/dm.php?thread=445256

Reading through this it sounds like you're better off just sticking to resistors and capacitors for biasing the stages. Or maybe just try bypassing the diodes with capacitors as I believe thats what Mesa Boogie did. Seems a bit pointless though as you may as well use a resistor instead of a diode.

[Theashe]

PDF64 - I scoped the FX send jack this morning, and I get the same signal whether I use an LED to bias the tube or a 2k2 resistor. The FX return goes to a common anode mixer almost identical to the one in Merlin's book, and right now I have the mix set to all dry signal.

Littlewyan - I noticed this problem with fully bypassed Rk/Cks on each cathode before I went to LED biasing, but the phenomenon the article described seemed similar to my problem so I gave it a shot again. After replacing overdriven stages' LEDs with resistors alone, the problem was still there once overdrive was hit. I tried the LEDs with large bypass resistors, nothing, and bypass capacitors, again, nothing. The same fizzy distortion on top.

I'm noticing that when my post-tone stack volume pot is all the way down, a little bit of signal is still making it through. I tried connecting each of the pot terminals to ground with a jumper cable to see if it was a high resistance solder joint, but no change. I'll try to trace that one today.

[JazzGuitarGimp]

I'm noticing that when my post-tone stack volume pot is all the way down, a little bit of signal is still making it through. I tried connecting each of the pot terminals to ground with a jumper cable to see if it was a high resistance solder joint, but no change. I'll try to trace that one today.

Try moving the volume pot's ground wire to the bottom of the cathode resistor (or LED) ground of the stage the pot's wiper feeds.

[Theashe]

Found the source of the signal feedthrough problem - the V1b plate wire running to the tonestack was too close to the tonestack return wire that goes to the grid of V2a. The wires are in phase with each other. I'm moving the components around and shielding the wires now.

[Theashe]

I think that the signal feedthrough problem is causing the fuzz, so this may just be a lead dress issue. If I hit a big chord while the pre-tonestack volume is maxed and the post-tonestack volume is at 0, and max the pre-PI master, I can hear signal coming in and out and fuzzing just like the overtones I'm hearing on top of the overdrive.

Three pieces of shielded wire, physical separation, and a ground plane later, the first feedthrough has been minimized. I can't get it to completely go away. Would a run of wire DC and AC be more likely to couple than a wire with just AC? If so, I might try adding a large cap before one of my wire runs.

I found another signal feedthrough problem when the pre-tonestack volume is 0 and the post-tonestack volume is maxed, which I think may be from part of my tonestack coupling with V1a. It's gonna be a long day to sort these out.

[pdf64]

PDF64 - I scoped the FX send jack this morning, and I get the same signal whether I use an LED to bias the tube or a 2k2 resistor

When diode biased, I don't understand how any clean signal can be present at the fx send cathode?
Even with regular resistor, the signal level may be limited, though perhaps sufficient.
The signal level on the dry path, being taken from the stage's plate, will be a lot larger than that of the wet signal path, at its cathode?

Something that I noticed when tinkering with a high gain pre-amp a while ago, was that unbypassed cathode stages seem to have a lot less of that fizzy breakthrough when overdriven.

[Theashe]

So you've heard noise just like what was in my sound clip before in a high gain preamp? Can you say for certain whether the effect was because of removing the capacitor, or just because the unbypassed cathode has a lot less gain and doesn't hit the grid of the next stage as hard?

The FX Send signal level is indeed limited. At the overdriven tone I like, it's 200mV, but it can go up to 700mV at full crank. Any effects connected would have to have their own gain or a boost to match the dry volume.

[Malcolm Irving]

...
The FX Send signal level is indeed limited. At the overdriven tone I like, it's 200mV, but it can go up to 700mV at full crank. Any effects connected would have to have their own gain or a boost to match the dry volume.

When V2a is not overdriven, the LED which biases V2a should hold the cathode voltage (1.7V) very close to constant - which would mean almost zero signal for the FX send.
If V2a is overdriven, I think the LED might work as a kind of half-wave diode clipper - giving a very distorted FX send.
Could there be a mistake in your schematic and you actually have a resistor in there as well as (or instead of) the LED?

[Theashe]

Right now, there's an LED in there because I was playing with LED biasing some of the amp. I put a 2k2 resistor in instead of the LED earlier today, but my scope shows exactly the same signal on the FX out jack as it did with the LED.

[JMFahey]

You do NOT repeat NOT have a malfunctioning amplifier.

Your soundcloud video shows no fizz but plain distortion/clipping when stage is overdriven, what else do you expect?

The amp I'm working on sounds fizzy as soon as it starts to overdrive. The fizz gets louder and quieter until the guitar signal dies away.

I experimented with overdriving different parts of the circuit. If I overdrive V1b but turn down the next volume to make sure the rest of the amp passes that signal clean, you can hear the fizz I'm talking about.

Of course, once overdriven/clipped passing it through later clean stages will not clean it, they faithfully reproduce what's fed them.

However, if I turn down the first volume to make V1b clean, and turn up the volume later in the amp, you can hear the SAME fizz sound.

Of course, now you clip a later stage or the power amp.

The amp doesn't make that sound unless tubes are starting to clip.

Proof of what I'm saying.

That's the way amplifiers are supposed to sound: clean until overdriven.

Unless broken, which is not the case, you have nothing to "fix" there, except getting a louder cleaner amp.

Steel guitar players use 200 to 400W SS amplifiers, with limiters carefully set to avoid clipping at all cost, and driving very efficient speakers, such as JBL/EV/Black Widow.

[Theashe]

It doesn't *quite* sound like plain distortion to my ear. At 0:45 in the clip, when I hit a big chord, there's a transient fizziness on the top end of the overdriven sound that increases and decreases in volume, comes and goes, and finally fades away, leaving the overdriven undertone. It doesn't sound like a lot of the overdrives I've heard on other amps and in recordings, where if they hit a big chord, there's no transient fizziness on top. I'm trying to get rid of that fizziness, but no amount of tube rolling, center biasing, and plate bypass capping has helped yet.

Yesterday I managed to minimize some of the cross-talk between wires in the amp by moving them around, using shielded wires, and putting a ground plane between them, but that doesn't appear to have been the cause.

[Littlewyan]

I know what you mean, its not pleasant distortion. It might be a simple matter of EQ. How have you got the tone controls set?

Also a bit of crosstalk with the volumes down can be normal. Both my Marshall and my Express have this. Even my old Marshall 1987xl with shielded wire on all inputs and volume controls had it.

How does the amp sound with the gain turned up? On the edge of overdrive can sound a bit pants at times.

[Theashe]

That's a relief to hear. I was getting pretty frustrated that I couldn't completely get rid of the cross talk no matter how much shielding I put between two wires. If the first volume in the amp is turned to 0, the second one is maxed out, and the master is maxed out, you can just faintly hear the guitar signal making it through.

When testing I keep all the tone controls around a 6, but I've tried every combination of cut and boost to try to clean the overdrive sound, to no effect.

The problem gets worse the higher the gain goes. When the amp is balls-to-the-walls distorted, it's very easy to make that fizziness happen. At light overdrive, I have to really dig into a note or chord.