Phaez gives away EL84 design

[Chuck H]

There ARE ways to make EL84's behave. The biggest reason I know of for EL84's buzzy/fizzy reputation is that they grid load very easily causing crossover distortion. Second to that is the fact that most of these amps are cathode biased. So as the amp pushes harder the bias cools. If things aren't set up just right this causes even more crossover distortion. Combine this with high gain and the need to crank them for volume and it's a mess really. The other option is to NOT push the power tubes that hard and generate most of your distortion in the preamp. Either way you get thin, buzzy/fizzy tone.

Bias on the hot side. EL84's don't seem to mind.

Use the "Paul Ruby" mod to stop grid loading.

Don't run plate volts excessively high.

Use a zener across the cathode resistor to "fix" the bias at the onset of clipping.

If needed you can use a small conjuctive filter to pull down excessive overshoot spikes on OD. With an 8k priZ a 10k resistor and a 1500pf cap only effects the spike and won't dull your tone.

Use an OT with some meat on it.

Since you can now smash the PI and power tubes without fear of crossover distortion or excessive overshoot spikes there is no need for excessive preamp overdrive.

A good efficient speaker can add more volume than trying to push the power tubes into more watts.

Ta Daaaa... Now your "pair of EL84's" amp won't offend Bruce

Uh oh... Did I give something "secret" away like Randy Fay?

Chuck

[clyde1]

I belong to a blues society and we had Nick Moss and the Flip Tops to the club Saturday night. He had a Category 5 amplifier, not sure what model, but he had earlier told me it had a Plexi channel and a JTM 45 channel so I'm assuming it was a pair of EL34's. Long story short, he was way too loud. The soundman didn't even have him in the mix through the fronts and on the dance floor his tone was super-shrill with bottom end flub. I guess it's the Jensen Neo in there, loud but not my favourite for guitar. With the system we have, like a lot of clubs, 20 watts should be more than enough to get that sweet tube sound without messing up the mix. Need more volume?-just turn up the monitors.

[Bruce / Mission Amps]

...Ta Daaaa... Now your "pair of EL84's" amp won't offend Bruce

Uh oh... Did I give something "secret" away like Randy Fay?

Chuck

Ha ha... I don't think there are any secrets anymore anyhow...
Cool... I use the zener diodes on the "modded" PA, old school, cathode biased harp amps... well a couple other little tiny tweaks too.
The conjunctive filter works really well on just about every amp I've tried it on with the largest caps being 2n2 to 4n7 in series with big honkin' 10K higher wattage resistances.... just like Orange did, in the day, with their bad dog 200 watt PA heads.
I guess that secret got out too...

[Chuck H]

Hasserl,

No one mentioned your amp yet. Sounds great. It's hard to get all the nuances in that vid because it's REALLY compressed. But it sounds very JTM with a hint of Vox top end jangle. Not shrill. If you can't play with that amp, you can't play. Nice.

I like it better than the Daisy Cutter clip so far.

Chuck

[J M Fahey]

Not to agree with Bruce , but I started googling a little and found 2 things
1) In a page called non other that EHN's AX84 Page , where a fine chap who built all AX84 projects called Nyberg posts some tasty licks, many recorded live, on stage (the best kind ), says the following about the Conjunctive Filter (oh my God, who invented that name):

The filter definitely smooths out some of the high-end harshness in the EL84 output section,

Ahem !!
2) And what would that wonderful cure-it-all 7º wonder be?
Nothing more than a d_mn low-pass filter: a capacitor wired in parallel with the load, somewhat damped by a series resistor which dissipates harmlessly all that unnecessary high frequency energy (who needs highs anyway?).
Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
Anybody for a "subtle" effect?
They might just use a car subwoofer speaker, it will roll off at about that frequency.
3) Does it mean I hate AX48s? No, on the contrary, I love them.
They sound about 37.455.348 times better than the typical 15W Msh/Fnd/Lny/Pvy/Crt SS "practice" amp, with their typical muffled-and-buzzy sound. (don't know how they achieve both opposite sounds at the same time)
So: keep on rolling, AX84!!

[Hardtailed]

Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
Anybody for a "subtle" effect?
They might just use a car subwoofer speaker, it will roll off at about that frequency.

Actually, I believe the math is a bit more complex because you have to take into account the load that is in parallel with it.

Still, I am of the belief that if you need a conjunctive filter, you have a problem somewhere else.
But then, I like a clear and transparent circuit with minimal filtering.

That said, the only EL84 amp I've built so far (AC30 clone) didn't sound harsh at all, at least not to me. It sounded glorious once cranked, chiming yet smooth as a baby's butt. Of course it does have sort of a conjunctive filter before the power amp (the Cut control), but I was leaving it off all the time.

But I have yet to hear a high-gainer using EL84 that I like. It is my experience that, the more gain you try to extract from the preamp, the more power you need to keep it clear and solid.

[yunger]

regarding fizz, fuzz, bumble bees, etc.... i've noticed much of that coming from the pi when hit hard. from there, the pa compresses and muds-up. speaking from limited experience however.

[Bruce / Mission Amps]

Not to agree with Bruce , but I started googling a little and found 2 things
1) In a page called non other that EHN's AX84 Page , where a fine chap who built all AX84 projects called Nyberg posts some tasty licks, many recorded live, on stage (the best kind ), says the following about the Conjunctive Filter (oh my God, who invented that name):

Ahem !!
2) And what would that wonderful cure-it-all 7º wonder be?
Nothing more than a d_mn low-pass filter: a capacitor wired in parallel with the load, somewhat damped by a series resistor which dissipates harmlessly all that unnecessary high frequency energy (who needs highs anyway?).
Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
Anybody for a "subtle" effect?
They might just use a car subwoofer speaker, it will roll off at about that frequency.
...

Well in his defense, he does mention that he prefers the 5k6 .015uF combo now... about 1900Hz.
I don't, I'll stick to the 2n2 to 4n7 and around 10K resistance. No sense in snuffing the life out of the amp.

[Chuck H]

And what would that wonderful cure-it-all 7º wonder be?
Nothing more than a d_mn low-pass filter: a capacitor wired in parallel with the load, somewhat damped by a series resistor which dissipates harmlessly all that unnecessary high frequency energy (who needs highs anyway?).
Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
Anybody for a "subtle" effect?

I avoided trying a (to use the common associated term) conjunctive filter for the same reason. Every clip I heard of amps before and after the conjunctive filter sounded like someone threw a couple of blankets over the speaker cabinet. But then while building my last design (with EL84's) there was a modest overshoot spike that lent a cracking striking unpleasantness that played inconsistently on the top end. I chose my resistor value based on a couple of articles I read on line and I used the smallest cap value that quenched the spike, 1500pf. A far cry from .047uf.

IMHO this is a viable solution to the problem. Most EL84 amps I've heard that really push the power tubes exhibit some of this behavior to my ears. Not mine, not anymore. There was no notable loss of any useful top end but the refinement of the clipping improved a lot. The cap values Bruce mentioned are also much smaller than the typical values used.

I didn't take any scope pics but the sketch below is somewhat representivive of my before and after. Who wouldn't want to make this kind of improvement? Higher cap values did round off the tailing edge of the + swing and had the undesireable effect of killing the livelyness and dynamics of the amps top end. I wouldn't suggest using this type of circuit for tone shaping because of it's negative effect on an amps dynamics when too large a cap value is used. But as a corrective circuit to maximize performance it served me well.

[J M Fahey]

Now speaking a little more seriously, when I saw that whatever-filter I instantly recognized it for what it actually is: our old friend the snubber circuit.
Very acceptable to dampen unwanted (some would call them "parasite") resonance peaks, usually above the actual frequency band where we are working (or in its top octave).
It can allow a little bit of extra NFB without instability and so, in an indirect way, let us polish our sound a little more; good!!!
But to rely on them as an equalization trick scratches me the wrong way.
There are so many other points where I can work !!
I prefer my snubbers in my SMPSprojects, where they belong, or at most as their cousin, the Zobel Network, which really "absorbs" nothing to speak of but enhances stability.

PS: I hate any blanket over speakers; if you need them, then something is very wrong somewhere along the path.

[GregS]

I recently built a copy of the Marshall 2061 Lead and Bass 20W amp with EL84's in cathode bias. The pre-amp is low gain (60's design) so the only distortion it produces is power amp. There is no buzziness or unpleasantness to this at all. It is a good classic rock sound. There is also no conjunctive filters, diodes or anything like that - just a simple straightforward power amp design. So I don't think EL84 amps have to be buzzy.

Also, the amp is seriously loud. It has too much headroom to get into any significant distortion at small club or bar gigs. Maybe a tiny bit of grind at most. My brother uses this as his main amp in the band. For distortion he needs to use a pedal in these types of venues. The cabinet is a 2x12 with Celestions

Greg

[Bruce / Mission Amps]

I recently built a copy of the Marshall 2061 Lead and Bass 20W amp with EL84's in cathode bias. The pre-amp is low gain (60's design) so the only distortion it produces is power amp. There is no buzziness or unpleasantness to this at all. It is a good classic rock sound. There is also no conjunctive filters, diodes or anything like that - just a simple straightforward power amp design. So I don't think EL84 amps have to be buzzy.

Also, the amp is seriously loud. It has too much headroom to get into any significant distortion at small club or bar gigs. Maybe a tiny bit of grind at most. My brother uses this as his main amp in the band. For distortion he needs to use a pedal in these types of venues. The cabinet is a 2x12 with Celestions

Greg

Of course they don't have to be buzzy... there are tons and tons of old EL84/6BQ5 Hi-Fi mono and stereo amps out there from many years gone by that sound glorious and still can make 12- 18 watts.
As there a many fabulous sounding VOX EL84 amps, Matchless amps, all kinds of EL84 amps that don't sound like trashed out buzz bombs.
That was part of my point....
What I said was:
"Ho hum... just what we need, another wanky, over driven El84 amp..."
I never said there were no good sounding EL84 amps.

[Steve Conner]

So what you're actually saying is, you hate high-gain amps?

[GregS]

Of course they don't have to be buzzy... there are tons and tons of old EL84/6BQ5 Hi-Fi mono and stereo amps out there from many years gone by that sound glorious and still can make 12- 18 watts.
As there a many fabulous sounding VOX EL84 amps, Matchless amps, all kinds of EL84 amps that don't sound like trashed out buzz bombs.
That was part of my point....
What I said was:
"Ho hum... just what we need, another wanky, over driven El84 amp..."
I never said there were no good sounding EL84 amps.

Just to clear up any misunderstanding I wasn't arguing with you about what you said. It was the notion presented in some of the other posts that you had to add conjunctive filters, diodes etc in order to make an EL84 power section sound good. That's what I was responding to. My point being that Marshall did it with a simple design without any of that stuff.

I also hate high gain amps.

Greg

[Chuck H]

It was the notion presented in some of the other posts that you had to add conjunctive filters, diodes etc in order to make an EL84 power section sound good...

My point being that Marshall did it with a simple design without any of that stuff.

You do need to add all that stuff to a high gainer to get EL84's to behave. IME you can't push them into a full square clip like you can with the big bottles.

Often the right OT and speaker can mask the uglies well enough. I didn't want to search for any magic parts so I just designed the flaws out with brute force instead. I would prefere a simpler more eloquent circuit.

And FWIW the Marshall 18 watter designs are hit or miss. Even from the factory. Some sound OK and some buzz like bees. There are threads on AX84 that discuss using the right OT and speaker to avoid this. Ok, so now everyone has the same PT, OT and speakers. The same tone I just wanted options since I don't think the "18 watt" design is all that.

It has intrigued me that rare EL84 amps seem to defy this. In many cases building a second identical amp fails to produce the same desireable result. Fine, so it CAN happen. But if I build two amps I want them to perform the same. So I designed mine that way.

JM2C

Chuck