Class AB2: $5, .5 hr

[Ray Ivers]

Bob,

Thanks (again!) for the info! I had a few questions:

1) Does the stage the Zener is being used on have a cathode bypass cap, and if so, what are the Rk/Ck values (if you don't mind sharing them) ? This question relates to the treble loss mentioned above; if you had (for example) a 1uF Ck across a 1.8K Rk, the Zener would have a different effect on the mid/treble frequencies than on LF.

2) I think the reason the Zener only really does anything on the final OD stage is that the previous stages all have pretty heavy attenuation, either via tone stack or resistive voltage divider. I'm going by the '97 Rev. 1.2 schematic I have, which has 1M 'Volume', 100K 'OD Trim', and 250K 'Drive' pots. If the existing circuit is designed to minimize the effects of grid blocking, it stands to reason that any grid-blocking 'fix' will have little or nothing to work with (just thinking out loud).

Ray

[BobW]

"1) Does the stage the Zener is being used on have a cathode bypass cap, and if so, what are the Rk/Ck values (if you don't mind sharing them) ? This question relates to the treble loss mentioned above; if you had (for example) a 1uF Ck across a 1.8K Rk, the Zener would have a different effect on the mid/treble frequencies than on LF."

I agree, Rk = 2.2k, Ck = 2uF. (1+1). Am using the hybrid revA w/additional tweaks.

"2) I think the reason the Zener only really does anything on the final OD stage is that the previous stages all have pretty heavy attenuation, either via tone stack or resistive voltage divider. I'm going by the '97 Rev. 1.2 schematic I have, which has 1M 'Volume', 100K 'OD Trim', and 250K 'Drive' pots. If the existing circuit is designed to minimize the effects of grid blocking, it stands to reason that any grid-blocking 'fix' will have little or nothing to work with (just thinking out loud)."

You're right, the stage gains are more attenuated = no grid blocking action in the previous stages, hence the zener ckt doesn't work there, but since I had the parts ... I gave em a try anyway.

I'm ready to close the amp up now, well... at least till next time. The curse of the Dumble clone, never ending tweaks.

[chipaudette]

Hi Ray (and all),

I came across this long-past thread by searching for "mosquito". That's one of the bad sounds that my power amp makes when being overdriven. Another series of words I could use is swirly (in a bad way) and mushy. I found this thread and was really intriqued.

I have a Fender 65 DRRI. I've owned lots of amps, but most were pre-amp distortion focused, not traditional amps where power-tubes were the center of a good overdrive sound. After discovering that my power amp made this mosquito/swirly/mushy sound when driven hard (turn to to 10!), I spent months groping around in a completely uneducated fashion trying to figure out what was "wrong" with my amp. I then just acceptabed that that's how class AB amps must sound in real life (as opposed to tweaked to heck on famous records). This thread gives me hope.

So, my Fender (6V6s, of course) has a bias level around -37V. It's got 0.1uF coupling caps after the PI. Following your guidance an Zener's with 2x the bias voltage, I got the closest I could -- 62 V Zeners. The only regular diodes that I could get quickly are some silicon ones (forgot the number) which are wicked fast but only have a 40 V reverse breakdown voltage. So, I got 2 and put them in series.

After wiring them up, I certainly got rid of the swirly/mushy sound. That's fantastic. I also got rid of the particular mosquito sound that I had. That's good too.

But, I feel that I can clearly hear a diode-clipping type sound whenever I hit the amp hard. I feel like I can hear the sharp corners of the square-wave being induced by the Zeners. I'd like to soften those edges up a bit.

Any recommendations? Bigger zeners? Resistor in series with the zeners (what value?).

Thanks,

Chip