Fuzz/Phase shifting effect (muskito flying by) in the mids

[Ricky Lee]

What would you recommend for tail resistor value?

[novosibir]

Were those grid stoppers supposed to reduce possible PO, or lower the p-p voltage?

To avoid possible PO.

The grid stopper doesn't do anything against a possible clamping effect of the bias voltage or blocking distortion.

A bigger tail resistor is smoothing the PI's operating, so that - especially when driven into saturation - both systems don't amplify so much different anymore.

Larry

[Froumy]

Thanks for clearing that up Larry. One possible cause of PO has been addressed, but not the root of a possible crossover issue.

No clue on the tail resistor value. New to me, too. I think it might depend on your p-p voltage, and what you're trying to get it down to, no? Any one measure it yet? Until you can try the diode thing(certain solution?), and assuming you have to get the p-p voltage down to double the bias voltage, it might be useful to lower the plate voltage on the finals so you can up the bias voltage. For testing purposes, anyway. Not sure about this, just trying to draw some conclusions from what we know at this point. Please correct me if I'm going the wrong way. Good luck.

[raiken]

Were those grid stoppers supposed to reduce possible PO, or lower the p-p voltage? If I understand correctly, the goal is to reduce the p-p voltage to less than double the negative bias voltage. Something the diode trick should accomplish. Have you measured your p-p voltage? I'm guessing attenuating more with the Tail resistor might accomplish this. If you're almost there, I'm guessing bumping up the bias voltage just a little, might get you closer. Hope the tail resistor or diodes work.

They were only to correct a possible parasitic oscillation. They will not reduce the drive to the output tubes. If parasitic oscillations were not the cause of your noise, they will not do anything.

Randall Aiken

[Froumy]

Thanks for your continuing help on this. Any ballpark range on the tail resistor? Is using some type of current mirror an acceptable solution? Or is the diode trick acting like a modified version of one?

If time allows, I'll open my amp after rehearsal tonight, to get some readings and see what it looks like on the scope. I haven't noticed this issue yet, but I'd like to see to what degree I have crossover distortion. If I do, I might be able to try some things before making assumptions.

[raiken]

Thanks for your continuing help on this. Any ballpark range on the tail resistor? Is using some type of current mirror an acceptable solution? Or is the diode trick acting like a modified version of one?

If time allows, I'll open my amp after rehearsal tonight, to get some readings and see what it looks like on the scope. I haven't noticed this issue yet, but I'd like to see to what degree I have crossover distortion. If I do, I might be able to try some things before making assumptions.

Look at the output of the PI on the scope, both phases, right at the grid of the power tubes. Increase the tail resistor (the 10K, not the 470/820 bias resistor) until you see the peak-to-peak signal limited enough to not drastically overdrive the output stage, while still reaching zero volts on the top peaks with a 1kHz sinewave.

You'll probably have to go up to around 100K-150K or even more (put in a 500K trimpot and adjust to suit). This typically works best on amps without global negative feedback, but you can try it in this application. It will change the way the output stage is driven, so it may change the way the amp reacts, for better or worse. The zener clipper is probably your best bet, if this is what is causing your noise, as it doesn't change the operating conditions as much, except for possibly reducing the amount of duty-cycle modulation of the output waveform, which can make the amp sound a bit less "organic".

Randall Aiken

[Ricky Lee]

Just curious to why Marshall never did all these type modifications? If we're building amps that are pretty spot on with the vintage Marshall's and yet having these issues, shouldn't we be looking into the differences between then and now for possible solution/s, especially in the tranny department since everyone seems to claim their hardware is closest to the vintage stuff. Build two amps identical and one does it and the other doesn't.

[Froumy]

Just curious to why Marshall never did all these type modifications? If we're building amps that are pretty spot on with the vintage Marshall's and yet having these issues, shouldn't we be looking into the differences between then and now for possible solution/s, especially in the tranny department since everyone seems to claim their hardware is closest to the vintage stuff. Build two amps identical and one does it and the other doesn't.

$$$$$. Same reason for low filtering, undersized transformers, etc.... Within reasonable operating specs. We're not exactly being reasonable when we jump and dime them....Why fix it if your public is begging for more? Some pleasant underdesigned mistakes in there. The old ones do it, too, to varying degrees. Mr. Aiken pointed out that a different tranny/speakers tubes could make it more obvious. Class B and AB are susceptible to varying amounts of crossover distortion. We need to prove if this is the problem before moving along.

FWIW, I don't hear it my build. I probably used some of the same parts as you. That's why I'll try scoping it. I'm guessing it will show up on the screen, even though it's not apparent. 5150(or anyone else for that matter), you should give it a go, too. My experience is mostly with circuit design software scopes. Still working on the learning curve with the real one.

Ricky Lee, I totally hear what you're saying,though. We should compare the ones that are audible to the ones that aren't, and see if any components help out. Seems like Billy Batz has already done a lion's share in that department.

[Billy Batz]

I dont think there was much base to draw from for example. At the time the idea of cranking an amp to distort on purpose was a novel thing. I wonder if they even heard a lot of these things. Ken and Jim werent guitar players.

[Froumy]

OUCH. Painfully long post.

Finally got a chance to scope it out. It think I do have crossover distortion. My notch is starting much higher than the reference on Aiken's site, though(10V both side of zero), and at low volumes. At high volumes, I witnessed the shift of the wave downward, but was unable to determine a "notch". Another thing of concern, was that one output tube had a significantly lower peak - peak voltage. Wonder if that RCA blackplate is severely unmatched, or if I have something else going on. I'll revisit ASAP.

Now for the good stuff. Seems like 18 watt guys have been implementing the diode thing for a while. I found a post on another site for a guy who did it on a Marshall Major. His results were, bye bye mosquitos, hello richer darker mids, and organic palm mutes, with perhaps more gain. I can't link directly to the post, so I'm going to have to quote it. I think the trick to the diode circuit is to let it crossover a little, but stop it right before it pisses you off. Please remember, we can still pay homage to the classic circuit, with a little "improvement" when we need it. If you do it right, those diodes should only effect the mosquitos, not the tone.

Here's for the Marshall implemented crossover eliminating-diode review. Posted by Ray Ivers on ax84:



Quote

WARNING - WAR & PEACE POST-LENGTH ALERT

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.

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
End quote.



Fantastic review. Not sure if I'd go so high on the Zener's, but that'll be a matter of taste. Don't be afraid to try it If you have the problem, and report back. Better tone should be the only result. Also, here's another link to help understand the problem.
http://www.paulamps.com/18watterbuzz.html#ResistiveLoad

I also found more detailed information on eliminating blocking distortion, in case that's the issue. I'll PM if you need it. Good luck gents.

-Bob

[Froumy]

Ohh yeah. Randall, seems like you have/had a book in the works. Is it done? And where/when can I buy it?

[5150loveeddie]

Bob from what planet are you from LOL, that is a hell of a cool review, wowy, I gonna try this next. 25% PD sounds good, if it does this will same some tube up big time in the process...!? Anyway this must be looked at seriously, thx dude and don't stop!!!

So he says to use a zener twice your sellected bias voltage per tube?

Where can we find those Zener diodes??

[Ricky Lee]

This place seems to have the most popular value Zeners.

http://www.partsexpress.com/webpage.cfm ... &sm=1&so=1

[Ricky Lee]

Any problem using the Zeners when the 220k bias resistors have been removed for the PPIMV? Also, with the dual voltage tranny, where plates defer 400v/500v and naturally bias -35/-43 respectfully, would using a Zener value for the low voltage side (400v) be the most appropriate?

[Froumy]

Not sure what planet I'm from, but it's filled with people that get a little obsessive.. Fun using the scope and learning a little more about the PI section. Just passing along whatever I could dig up.

Seems like diode recommendations are varying. Double the voltage to slightly over. 18 watter tried less than the voltage, and it still showed a marked improvement in tone. The fact that the same basic wave form may or may not have audible results, makes me think the higher the better. Leave enough of it in there to retain the classic tone, and cut it off right at the point where it sounds like crap. Think that's why Randall gave the surest spot to start(just a few volts over), and encouraged experimentation from there...

I'll try it in mine after I figure out why my signal is so unbalanced.. Good luck.

Bob