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Thread: Class AB2: $5, .5 hr

  1. #36
    Senior Member Satamax's Avatar
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    Talking

    Quote Originally Posted by Arthur B. View Post
    Could you give the isodyne it's on floating supply and then use a variable voltage reference to bias it?



    The next question would be what's the driver?

    I like the way fender did it on the PS400. 6L6 and interstage transformer/spliter.
    There's another weird one, teh SMF tour series from sound city, i've never understood why an EL34 phase spliter with caps afterwards! Weird!

    Bye.

    Max. Ray, sorry to hijack!

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  2. #37
    Lifetime Member Ray Ivers's Avatar
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    Arthur,

    Could you give the isodyne it's own floating supply and then use a variable voltage reference to bias it?
    Yes, absolutely - the floating supply is pretty much what I meant to describe (in my long-winded way), and the VVR should make an excellent bias supply.

    The Isodyne's a really nice PI/driver setup - IMO it would also make a great (cathode-)driver for a P-P/P-P xfmr - and if the 4-gain-stage requirement is a problem the CF duties can always be performed by MOSFET's.

    Ray

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  3. #38
    Lifetime Member Ray Ivers's Avatar
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    Max,

    Hey, everybody hijacks from time to time...

    There's one thing i'm thinking tho, isn't an interstage transformer/spliter the easiest solution?
    Yes, it really is - one gain stage, a few associated components, a transformer, and you're done. As Arthur pointed out, though, it's gotta be the right type of gain stage for the job to work well, and there's also the power-supply thing to work out too.

    I like the way fender did it on the PS400. 6L6 and interstage transformer/spliter.
    ...and the 300PS uses a 6V6 and IT - still pretty beefy!

    Ray

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  4. #39
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    Quote Originally Posted by Satamax View Post
    I like the way fender did it on the PS400. 6L6 and interstage transformer/spliter.
    There's another weird one, teh SMF tour series from sound city, i've never understood why an EL34 phase spliter with caps afterwards! Weird!

    Bye.

    Max. Ray, sorry to hijack!
    http://www.schematicheaven.com/barga...y_smf_tour.pdf

    Seems like a self split EL34 amplifier for the PI.

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    Last edited by Arthur B.; 08-11-2006 at 04:23 AM. Reason: Corrected spelling errors.

  5. #40
    Lifetime Member Ray Ivers's Avatar
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    Arthur,

    The SMF PI/driver looks to me like like a self-biased LTP using two EL34's as the gain stages. IMO they must have been positively awash in EL34's and octal sockets to do it like this, as I really can't see any benefit the EL34's add; the PI component values and B+ would have worked fine with EL84's, or even a 12AT7! If only they had spent the transformer budget on a good driver unit rather than that balanced out... or that super-weird totem-pole distortion circuit... which might sound OK, for all I know...

    Ray

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  6. #41
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    Quote Originally Posted by Ray Ivers View Post
    Arthur,

    The SMF PI/driver looks to me like like a self-biased LTP using two EL34's as the gain stages. IMO they must have been positively awash in EL34's and octal sockets to do it like this, as I really can't see any benefit the EL34's add; the PI component values and B+ would have worked fine with EL84's, or even a 12AT7!
    I think they were trying to pry the grids positive through brute force.

    Quote Originally Posted by Ray Ivers View Post
    If only they had spent the transformer budget on a good driver unit rather than that balanced out... or that super-weird totem-pole distortion circuit... which might sound OK, for all I know...

    Ray
    I don't see a totem pole circuit anywhere. The distortion circuit seems to consist of a gain stage casccading into a concertina cascading into two more gain stages which drive the transformer primary.

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  7. #42
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    Smile

    Ray, I finally got to try the Zener mod at the second stage preamp of my Dumble clone. Looking at the scope wavefrom I determined that a good starting Vz was 6.3 volts, and this stage was the best candidate. I didn't have any 1N4001 - 1N4007s but a fast recovery 1N4936 would do the trick. After installing, the two diodes, the slightly gritty tones at higher volumes disappeared and more of a singing tone resulted. This clamped the higher F transients, although based on the waveforms alone, it could be further improved. Stores were closed on Sun so I'll try a lower Vz's 4.7, 3.9, etc. and report back. Thanks for posting this mod!

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  8. #43
    Lifetime Member Ray Ivers's Avatar
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    Bob,

    Most excellent! I was hoping it would work well on preamp tubes, too.

    Ray

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  9. #44
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    Smile

    Ray, It seems the 6.3 Vz was the best out of smaller 4.7, 3.9, and larger, 9, 12 zeners. This ckt was also tried at the the 3 previous stages w/o any tone improvement, so am satisfied with this mod at OD2 on my dumble clone. This is exactly the ckt I need to help remove the grit tone in the OD channel of my peavey (sorry) classic 30. thanks for your help.

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  10. #45
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    I should have mentioned, you won't hear the zenering effect until you have the amp cranked above normal bedroom volume. Using too low a value of zener in this circuit can cut too much off the top end, so either increase the value or add a series resistor to smooth the zener effect.

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  11. #46
    Lifetime Member Ray Ivers's Avatar
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    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

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  12. #47
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    Smile

    "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.

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  13. #48
    Senior Member chipaudette's Avatar
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    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

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