Class AB2: $5, .5 hr

[Ray Ivers]

Watch out, my scanner's up again... ;)

Here's the simplified version of the circuit from the February issue of audioXpress magazine:

http://music-electronics-forum.com/attachment.php?attachmentid=74&stc=1&d=1152223483

This circuit or a similar one may already be in use over at the 18W Marshall page, I don't know - at any rate, here it is.

According to the article, the component values can be adjusted to allow positive grid shift during heavy drive conditions. This would allow the output stage to be idle-biased a bit cooler for a crisp and dynamic clean sound with the guitar turned down, with an automatic shift to a hotter bias point kicking in during overdrive for a more compressed/"woodier" dirty sound if desired; might sound pretty good!

I've got the complete amp schematic here too; I'll scan & post it if anyone's interested.

Ray

[Chris333]

I'd like to see the complete schematic. Probably won't understand why it does what it does, but I'd like to see it anyway...

[Satamax]

Hi vrybody!

Well, Ray, that's the zenner bias you were talking about? If yes, please explain how it works, cos i'm bit daft you see. There's neg grid bias, but how is it dropped by the back to back diode and zenner?????

Bye.

Max.

[Ray Ivers]

OK, here's the full circuit, resized to fit the site restrictions - it's really small, sorry:

http://music-electronics-forum.com/attachment.php?attachmentid=76&stc=1&d=1152265535

and here's the first circuit again:

http://music-electronics-forum.com/attachment.php?attachmentid=77&stc=1&d=1152265716

Operation is pretty simple, really - the Zener conducts when the negative signal swing exceeds the Zener voltage, dumping excess grid electrons to ground instead of allowing them to build up on the downstream cap plate and causing negative bias shift during grid conduction. The regular diode isolates the Zener from the grid during positive signal swings, and the resistor eases the transition into Zener conduction to prevent sharp transients within the NFB loop.

Ray

[BobW]

Ray, How does it sound? Have you performed any A/B tests to determine the ideal zener Vz?

[Ray Ivers]

Bob,

I'm gonna try it either tonight or tomorrow, and I'll post back with the results. From the article, it appears that a Zener voltage of about twice the idle bias voltage maintains a 'neutral' bias situation during hard drive, while reducing the Zener voltage from this point results in positive OD grid shift and increasing the Zener value allows increasing amounts of negative shift.

Ray

[R.G.]

The idea is that a grid usually sits at one bias voltage negative. To get grid conduction and bias shift, the signal has to be more than one bias voltage big on the positive excursion. If the zener is two bias voltages to breakover, then the next half cycle will break over the zener about the same amount as the previous one caused grid conduction, draining off about the same amount of charge, and keeping the bias on average steady.

[Matt T.]

OK, silly question: Could you use this (or something very similar to it) on a preamp triode? I'm thinking specifically about the input triode of a Long tail pair phase inverter. Sometimes, for example, a Marshall circuit will sound amazing and other times the exact same circuit, with the very similar components, at similar voltages will sound all fuzzy/fizzy and bad and it seems that whether the phase inverter is distorting 'well' or not is the issue.

[Ray Ivers]

Matt,

I still have yet to build & try this thing, but it should also work on preamp triodes provided enough current is available to avalanche the Zener. Unfortunately, one place where it may not work so well is at the input to an LTP PI, where very little grid conduction is going on due to the high tail resistance and input bootstrapping - but little or no grid conduction means little or no bias shift, so there may have been no real improvement to be had. This is just supposition as I haven't tried it, but if you do, be sure and take into account the fairly high grid voltage present at an LTP's input when setting up the Zener voltage.


PI idle point, tube type, component values, feedback situation if any, output tube loading - FWIW these are the things I feel affect the sound & performance of the PI stage the most. Different examples of the same tube type plugged into the same LTP PI circuit can often give widely varying idle bias points, with correspondingly wide variances in sound.

Ray

[R.G.]

Ray's right - there are a ton of variables there.

As to generalizing the technique for triodes:
- you could do this with any tube setup that is driven into grid conduction. What it really does is to compensate for grid conduction with driven-really-really-into cutoff conduction in the zener.
- how well it works depends on what it's fed. If the waveform is notably assymmetric, there may not be the balance of grid and zener conduction at all levels. There may be a certain critical level where one side of the waveform is clipped by the preceeding stage. If that clipped side is the positive going side, the bias will be driven off in the overconduction direction by zener conduction at signal levels where the zener coducts and the grid doesn't. If the clipped side is the negative going side, then you can have transient movement into the cutoff region as in normal grid blocking.
In a PI, you could put the zener network between grid and source instead of grid and ground. As Ray notes, the value of the zener will change for every different type of tube and bias point.

[Ray Ivers]

R.G.,

I actually managed to get the test amp ('73 Marshall Major) onto the bench tonight, and perhaps tomorrow sometime I'll actually manage to do something with it. I figured a high-power UL amp would be as good a test platform as any to try this thing out with - I'll be using only two output tubes, though, just in case things go horribly wrong. :D

It will be interesting to see how it handles asymmetric guitar signals you mentioned, as the article just used pure sine & (briefly) square waves for testing. I'm hoping it doesn't have a lot of idle-point 'bouncing', and closely mimics direct coupling instead. It would also be cool to get some real AB2 Ig and Ik flowing, but we'll see - this PA and PS will surely supply both if the drive is there.

I always wanted to try driving the PA with a signal consisting mainly of just positive PI swings; I guess this one achieves a similar effect, only on the far side of the coupling caps.

I wonder what a typical minimum Zener-breakdown current level is? I'll try throwing one across a preamp-tube grid and see what happens. In my 'symmetrical clipping circuit' testing (which I'll post about after I try it again using my own amp :(), I saw a 12AX7 grid source over 2mA of current (!) which seems promising.

Ray

[Ray Ivers]

I've been screwing around with this thing the last couple of days, and while it works pretty well in some ways, it's not the drop-in replacement I thought it would be - and why I thought that, I do not know. ;)

The Major's bias voltage is @ -76V, so I've got 100V and 51V Zeners in series with a 1N4007 on each driver output, as per the print in my original post. The stock 5.6K grid stoppers are still in place (mistake), and the stock 56K plate resistors are still on the 12AU7 driver stage (bigger mistake). Tubes are one new, unmatched pair of original Svetlana - now SED - KT88's. Bias point is 20 watts, or roughly 50% P.D.

As far as bias shift goes, the circuit works exactly as claimed - I can detect absolutely NO bias shift on the 'scope, at any drive level. The Zener-ed drive signal clips symmetrically around the bias voltage - the top half from grid conduction, the bottom half from Zener conduction. The NFB loop has been disconnected, and while the stock circuit shows ample evidence of bias shift/crossover distortion at full unclipped power, switching in the Zeners removes almost all of it. FWIW, I like what the Zeners do to the PA during heavy clipping - removing the chewing-tinfoil crossover stuff results in more of a smooth grinding distortion.

The trouble I have is with AB2 operation - so far I've seen little evidence of it, although as I mentioned above I can't believe I thought it would happen by just popping some diodes into a high-impedance AB1 driver circuit. I ran an 8 ohm load on the 16 ohm tap, and got pretty much the same power output with and without the Zeners switched in, although the Zeners did allow the grids to be driven a few volts more positive than the stock circuit before grid-circuit clipping.

The circuit in the article uses a 6BX7 driver - IMO the "heavy hitter" of small octal dual-triodes - along with 22K plate loads, gigantic 2.2uF coupling caps, and 1K grid stoppers; the Major uses none of these things. I think the 12AU7 can stay for now, but the "$5/.5 hr" thing is looking pretty unlikely at this point. :( I'm hoping to get it working well with minimal changes to most guitar amps, but it looks like a low(er)-Z 12AT7 driver circuit, bigger coupling caps, and smaller stoppers will be minimum requirements, in addition to the Zeners and associated components.

I'll keep at it, and see what I can come up with. I'm using an isolation box/12" Eminence Omega Pro/SM57/mic pre/headphones for listening tests - it's like putting the sound under a microscope, but if it sounds at all good through this setup, it always sounds good through a normal speaker cab.

Ray

[Ray Ivers]

First off, it's just come to my attention that this circuit was initially created by Paul Ruby, of the AX84/18 watt/surely many other BBS's. Thanks, Paul - it really works quite well!

OK, I've finished up testing of this circuit, and FWIW I'm quite happy with the results overall. I didn't get the AB2 operation I had (foolishly) hoped for, but what I got, I really liked. It should drop right into practically any SE or push-pull output stage w/no other circuit changes needed, and although I didn't try it on a preamp stage or self-split design, it should also work on those with a bit of tweaking. The test circuit was also fixed-bias; I won't cover cathode-bias here as this post will be WAY long enough as-is.

http://music-electronics-forum.com/attachment.php?attachmentid=117&stc=1&d=1153927785



As an overview - this circuit consists of:

a) a conventional silicon diode (I used a 1N4007) w/cathode (stripe) to the 'upstream' side of the output tube grid stopper, if any - in series with

b) a half-watt or higher Zener diode, cathode to the output stage ground point; I used a 1N4757A (51V) and 1N4764A (100V) in series, as I had an unusually large -76V bias to contend with.

A small-value resistor (< 470 ohms) can be inserted between the Zener and ground to soften the Zener-conduction transition, but to my ears this just diluted the circuit's effect so I didn't use it.

The Zener value should ideally be about twice the idle bias voltage, at least for a starting point. Go higher than double the bias and the circuit has less and less effect; go lower and the bias voltage will actually shift POSITIVE during output-stage overdrive/grid conduction; way cool IMO. You can't go too far in this direction if you want to retain a clean sound, though, as the duty-cycle shift from Zener conduction on the negative signal peaks adds distortion 'shoulders' to the waveform slopes.

The test circuit was a '73 Marshall Major, 600V B+ in UL, using two National KT88-USA's (cool tubes, made in the U.S. w/the original GEC machines!). The driver is a 12AU7 differential amp, 56K plate loads, .47uF coupling caps, 68K output grid bias feed resistors. NFB was disconnected for testing, and levels were set such that only the driver and output stages distorted.

I first tried what I consider an extremely hot AB1 bias point - 50mA, or 70% plate dissipation. I didn't hear too much of what I call the 'springy mosquito' sound of crossover distortion with the Zeners switched out, but with them in it vanished completely. Chords sounded noticeably cleaner with the Zeners engaged, and with them out things got pretty muddy, as you'd expect using huge coupling-cap values like these, along with plenty of 'envelope effect' (swell/decay). I thought the clean sound wasn't too bad, but the night was young...

Next was what I considered the 'acid test' - I biased to 5mA, or 7% (that's SEVEN percent) plate dissipation, which even I consider ridiculously ice-cold. With Zeners out during overdrive, the mosquitos were a-springin' in swarms :>) - it was hard to keep from wincing on palm-muted 'chug' notes and chords - although no real muddiness was apparent. The clean sound was kind of thin, extremely percussive, and edgy/bright; cut-through-the-mix qualities, for sure. In comparison, the 70% clean sound was mostly mids and bottom end, and actually sounded like a compressor with high threshold and medium ratio was being used.

Zeners in: what a difference! The mosquitos vanished almost without trace, and apparent gain increased - it was more of an 'expansion' effect than compression, as the harder I picked, the more gain and overdrive I got. I had one 'scope channel on the grid, and sure enough, the entire signal shifted slightly positive during notes, with the bottom half clipped at the Zener voltage (obviously you won't hear this, as it occurs well below the tube cutoff bias point), and the top half smoothly round-topped as it went above the 0V grid-conduction threshold,up to @ +5V. Chords stayed just as defined as before - if not more so - and while the clean parts of notes stayed 'spanky' and crisp, the overdriven/sustained parts took on more of the higher-current characteristics (darker, more mids, compression, etc.) instead of becoming harsh and metallic as they did without the Zeners. I dug it!

I did more testing - ending up liking 25% P.D. for this setup the best - but this is more than enough out of me for now; I just wanted to get out a 'review' on this circuit before too much time passed, along with two thumbs up.

Ray

[Shea]

Sounds cool, Ray, thanks for the report.

I've been thinking that if you could generate a negative voltage rail that can supply at least double the expected bias voltage, then you could set up a bias circuit consisting of a pot, a 1:1 voltage divider, and maybe a couple of source followers, so that one bias pot would set both the actual bias voltage and another voltage of twice that amount. Then instead of using a zener, you'd just need one silicon diode per side to chop off the signal when it goes more negative than the higher (i.e., most negative) voltage. That way you wouldn't need to change zeners when you change tubes and rebias.

Shea

[Ray Ivers]

Shea,

Just when I thought everyone had lost interest in this thing... :D It was great to get back to the bench again, even if only for a few days.

Great idea - "Zener scaling"! Maybe you could even get away with just using a MOSFET "gain stage" set to a gain of 2, with the adjustable bias voltage fed to both the MOSFET gate and the grid resistors, the source thru an Rs to ground, the drain thru an Rd to the existing raw bias voltage (if it's not high enough, circuits tweaks or a doubler could be used), the threshold diode to the drain, and AC bypass caps? Maybe I'm missing something...

Ray

[Shea]

Great idea - "Zener scaling"! Maybe you could even get away with just using a MOSFET "gain stage" set to a gain of 2, with the adjustable bias voltage fed to both the MOSFET gate and the grid resistors, the source thru an Rs to ground, the drain thru an Rd to the existing raw bias voltage (if it's not high enough, circuits tweaks or a doubler could be used), the threshold diode to the drain, and AC bypass caps? Maybe I'm missing something...

Ray

That sounds simpler, but, I'm embarrassed to say, I'm not sure I know how to create a MOSFET gain stage with a gain of 2. :( Do you just make Rd twice the value of Rs?

Shea

[Ray Ivers]

Shea,

Yes, I think it actually is just that simple with most MOSFET's, unless super-low resistor values are used. An Rs of 3.3K and an Rd of 6.8K should pull maybe 6mA or so from a -70V raw bias supply (hopefully this current is available, IME bias supplies are notoriously non-beefy) and should stay pretty solid during electron dumps when the diode conducts.

Ray

[Slobrain]

Ray,
That mod looks great, Man, you have sparked my interest in amp mods just reading this post.
Thanks...
SLO....

[Ray Ivers]

SB,

Thanks - I'm psyched about it too! I started out thinking it was only a non-master-amp kinda thing, but not any more - it should make just about any amp sound better when pushed, and at lower levels it just sleeps.

Ray

[Shane]

Nice preliminary review Ray! sounds like a good Mod well worth further research.
What kind of changes, if any would be needed for cathode biased amps or self splits?

looking at Paul Rubys original article he seemed to set the zener voltage at a bit more than the cathode bias voltage.
Cheers
Shane

[Ray Ivers]

Shane,

Thanks! I just re-read Paul's article, to try and get a little better handle on the cathode-bias thing.

If the cathode resistor is unbypassed, I would think the most precise way to figure out a ballpark Zener value would be to use a use a dual-trace 'scope, with one probe at the cathode, and one probe at the grid. Turn up the input signal until grid conduction occurs, then subtract the Vk idle voltage from the Vg-Vk peak voltage (possibly using the 'scope's A-B feature if available); the resulting value times 2 would be your Zener voltage, at least to start. I realize nobody's going to go to all this trouble :) - it's just for example's sake.

With a large-value Ck, I would say double the idle Vk would be a good starting point, and with a small-value Ck I guess you'd have to try different Zener values out to see what sounded best to you.

A typical self-split stage should be pretty similar to a standard cathode-bias situation as far as grid drive goes, I'd think.

Ray

[BobW]

Ray,

thanks for the feedback, I'm tdy w/work and was unable to try out the zener mods on the preamp section. I bought a few zener values to try out, so I'll report back once I get back to try it out.
Also was wondering if anyone has had success using a digital cam to take a snapshot of an analog display Oscope? Using your suggestion to measure grid conduction and then making a pdf would help understand, to me at least, at what point should the zenering take effect vs. tone? Wish I was back in my shop! :(

[BobW]

You'd have to make sure you don't use an N channel MOSFET with the built in freewheeling diode, (anode to source, cathode to drain) otherwise it defeats the purpose of the ckt.

[Ray Ivers]

BobW,

Here's a link to Paul Ruby's page describing his experiences with the original circuit:

www.paulamps.com/18watterbuzz.html (http://www.paulamps.com/18watterbuzz.html)

About halfway down the page there's a section called 'Cross-Over Distortion Root Cause', and below the title there's two 'scope photos showing the downward average bias shift, with the waveform tops clipped around 0Vg and the bottoms free to descend as far as they can go, with no Zeners to limit them. You're looking to get this same waveform on top, but with a matching clipped bottom - i.e., a symmetrically-clipped waveform with its center at the idle bias voltage. This is for starters - after you've played with the circuit a while you can obviously go up and down with the Zener voltage, but I would recommend starting with the double-bias-voltage Zener value as sort of a 'home base' you can always return to and have it sound acceptable.

I really haven't taken any 'scope photos with my digital camera, although I took some a long time ago with my Nikon that came out OK.

As a side note - most discussions I've seen on the Web regarding 'scopes seem to mainly focus on bandwidth (IMO the least important feature of an audio oscilloscope since about 1955 ;)), and tend to ignore the many cool features some of today's digital 'scopes have - one of which is taking waveform snapshots and outputting them on disk, USB, etc. And IMO it's hard to beat that rock-solid 60Hz display!

Ray

[BobW]

Ray,
Thanks for the Paul Ruby article, it looks like he's already captured the particular waveforms so no sense in me 're-inventing' them. Although his waveforms are not crystal clear, his discussion makes sense, can't wait to try it. cheers :cool:

[Arthur B.]

Sounds cool, Ray, thanks for the report.

I've been thinking that if you could generate a negative voltage rail that can supply at least double the expected bias voltage, then you could set up a bias circuit consisting of a pot, a 1:1 voltage divider, and maybe a couple of source followers, so that one bias pot would set both the actual bias voltage and another voltage of twice that amount. Then instead of using a zener, you'd just need one silicon diode per side to chop off the signal when it goes more negative than the higher (i.e., most negative) voltage. That way you wouldn't need to change zeners when you change tubes and rebias.

Shea

If you had a decent bias supply (a.k.a separate from the main PS) you wouldn't need the diodes in the first place.

Shea,

Just when I thought everyone had lost interest in this thing... :D It was great to get back to the bench again, even if only for a few days.

Great idea - "Zener scaling"! Maybe you could even get away with just using a MOSFET "gain stage" set to a gain of 2, with the adjustable bias voltage fed to both the MOSFET gate and the grid resistors, the source thru an Rs to ground, the drain thru an Rd to the existing raw bias voltage (if it's not high enough, circuits tweaks or a doubler could be used), the threshold diode to the drain, and AC bypass caps? Maybe I'm missing something...

Ray

What you're describing is called a rubber diode.

[Ray Ivers]

Arthur,

If you had a decent bias supply (a.k.a separate from the main PS) you wouldn't need the diodes in the first place.

What you're describing is called a rubber diode.

Can you elaborate on these statements?

Ray

[Arthur B.]

Arthur,



Can you elaborate on these statements?

Ray

Oops, :o I think some explaination is in order.


If you had a decent bias supply (a.k.a separate from the main PS) you wouldn't need the diodes in the first place.


From the first post of this thread:


According to the article, the component values can be adjusted to allow positive grid shift during heavy drive conditions. This would allow the output stage to be idle-biased a bit cooler for a crisp and dynamic clean sound with the guitar turned down, with an automatic shift to a hotter bias point kicking in during overdrive for a more compressed/"woodier" dirty sound if desired; might sound pretty good!


If you had a robust enough bias supply, it could deliver the current that the grids need when they conduct. It will also cause the positive bias shift because the cathode capacitor will charge on the positive half of the waveform.


What you're describing is called a rubber diode.


Here (http://en.wikipedia.org/wiki/Voltage_source) is a Wikipedia article that details the operation of a rubber diode. Look for the heading "VBE multiplier voltage source"

[Ray Ivers]

Arthur,

If you had a robust enough bias supply, it could deliver the current that the grids need when they conduct. It will also cause the positive bias shift because the cathode capacitor will charge on the positive half of the waveform.

When the control grids conduct, the electron flow is out of the grids into the 'downstream' cap plate; it is then the job of the device driving the 'upstream' plate to pull that plate positive, pulling that current thru the bias supply and into the positive driver power supply. Merely beefing up the bias supply isn't sufficient; it must be accompanied by an increase in driver-stage power capabilities, or the current will never flow in the first place, and electrons will merely accumulate on the downstream cap plate and cause bias shift. It is important to have a low-impedance bias supply in an AB2 driver to maintain a constant bias voltage in the face of drastic changes in grid impedance during grid conduction, but not to sink grid current per se.

Here (http://en.wikipedia.org/wiki/Voltage_source) is a Wikipedia article that details the operation of a rubber diode. Look for the heading "VBE multiplier voltage source"

Thanks for that link - I've seen this circuit before, in SS power amp drive stages. I've never heard it called a 'rubber diode' or 'rubber Zener', though. ;)

Ray

[Arthur B.]

Arthur,



When the control grids conduct, the electron flow is out of the grids into the 'downstream' cap plate; it is then the job of the device driving the 'upstream' plate to pull that plate positive, pulling that current thru the bias supply and into the positive driver power supply. Merely beefing up the bias supply isn't sufficient; it must be accompanied by an increase in driver-stage power capabilities, or the current will never flow in the first place, and electrons will merely accumulate on the downstream cap plate and cause bias shift. It is important to have a low-impedance bias supply in an AB2 driver to maintain a constant bias voltage in the face of drastic changes in grid impedance during grid conduction, but not to sink grid current per se.


Ok, so a stiff bias supply is important, but not the determining factor. Would an isodyne work as a good driver in this situation?


Thanks for that link - I've seen this circuit before, in SS power amp drive stages. I've never heard it called a 'rubber diode' or 'rubber Zener', though. ;)

Ray

Yeah, that's what it's called.

[Ray Ivers]

Arthur,

I have to admit, I completely mis-read the Isodyne schematic the first time around - I concentrated on the two left-most triodes, seeing the two right-side ones as push-pull outputs, for some reason - maybe because of the bias control, or more likely just losing it, I guess. ;) This is kinda-sorta a "buffered Schmitt w/local NFB".

The Isodyne could work great for power-grid drive IMO, but it would have to be modified for direct-coupling to the output tube grids. The existing positive bias supply would have to be changed to a low-Z negative one and run to the PI and CF Rk's as well as the bias control, and the 300V PI and 400V CF B+ reduced - the ultimate goal being a correct negative bias voltage at the CF cathodes/output grids, along with low-impedance positive-swing drive provided by suitable (i.e., non-12AX7) relatively hot-biased CF stages. If one was going to all this trouble, the CF's could be eliminated completely and a low-rp PI tube (w/suitable Rp value - ideally less than 5K) used to drive the output grids directly, in a straight-ahead Schmitt or PI/diff amp configuration.

Ray

[Analog Assassin]

Could this be a cure for the loose bottom end on blackface fenders? I find it daunting often to get a tight sound out of my super reverb without thinning down the bass too much.

[Ray Ivers]

Could this be a cure for the loose bottom end on blackface fenders? I find it daunting often to get a tight sound out of my super reverb without thinning down the bass too much.


Yes - I assume you mean the Zener mod (the AB2 circuit would also work, but the Zener thing's so much easier to do). In my testing I used .47uF coupling caps from the PI to the output grids, and I'm sure I could have gone a lot bigger than that. In AB1 these big caps seem like ridiculous overkill - 12Hz F3, who needs to go that low? - but when the grids conduct you're looking at more like 180Hz for F3, inside the NFB loop (where a reduction in bass is a reduction of bass NFB and apparent damping factor, with corresponding reduced control of speaker-cone motion).

Bigger coupling caps + Zener mod = more/tighter bass when the power amp's working hard.

Ray

[Satamax]

Huh, guys! I've tried to follow, but with my limited knowlege, i haven't understood much.

There's one thing i'm thinking tho, isn't an interstage transformer/spliter the easiest solution? :D

Bye.

Max.

[Arthur B.]

The Isodyne could work great for power-grid drive IMO, but it would have to be modified for direct-coupling to the output tube grids. The existing positive bias supply would have to be changed to a low-Z negative one and run to the PI and CF Rk's as well as the bias control, and the 300V PI and 400V CF B+ reduced - the ultimate goal being a correct negative bias voltage at the CF cathodes/output grids, along with low-impedance positive-swing drive provided by suitable (i.e., non-12AX7) relatively hot-biased CF stages. If one was going to all this trouble, the CF's could be eliminated completely and a low-rp PI tube (w/suitable Rp value - ideally less than 5K) used to drive the output grids directly, in a straight-ahead Schmitt or PI/diff amp configuration.

Ray

Could you give the isodyne it's own floating supply and then use a variable voltage reference to bias it?

Huh, guys! I've tried to follow, but with my limited knowlege, i haven't understood much.

There's one thing i'm thinking tho, isn't an interstage transformer/spliter the easiest solution? :D

Bye.

Max.

The next question would be "what's the driver?"

[Satamax]

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. :D 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! :D

[Ray Ivers]

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

[Ray Ivers]

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. :D 6L6 and interstage transformer/spliter.

...and the 300PS uses a 6V6 and IT - still pretty beefy!

Ray

[Arthur B.]

I like the way fender did it on the PS400. :D 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! :D

http://www.schematicheaven.com/bargainbin/soundcity_smf_tour.pdf

Seems like a self split EL34 amplifier for the PI.

[Ray Ivers]

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... :D which might sound OK, for all I know... ;)

Ray

[Arthur B.]

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.


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... :D 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.

[BobW]

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!

[Ray Ivers]

Bob,

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

Ray

[BobW]

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.

[BobW]

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.

[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