Output tube impedance

[Fretts]

... it has already been said that "Kenny" was not a heavy duty electrical engineer. You'd expect a proper, disciplined engineer to correct the problem of spurious coupling like that and keep those wires well apart. Being free from electronic dogma probably is what allowed him to think, 'wait a minute, that sounds great' and tinker with it rather than instinctively engineer the "problem" out.
I never knew him outside some postcards and a few phone calls, but those who did know him well and have been interviewed, paint pretty much that kind of picture. He would just move things around and see what they did, and then obsessively refine it to the nth degree, conventional wisdom be damned.
Besides the actual sound that came out of his amp designs, that's why I admire him so much.

[Leo_Gnardo]

... it has already been said that "Kenny" was not a heavy duty electrical engineer.

Perhaps not "heavy duty" as an MIT Electrical Engineer, but he was trained by US Army Signal Corps leading to practical knowledge that served him well. In his Ampeg years he distinguished himself by getting off the production line and becoming "the guy who could fix anything."

There are certain basic truths in electronics, they are inescapable, call it "dogma" if you will but that's what the universe hands us and we all have to work with it. Maxwell's equations, Ohms law, power law, etc. Thevenin and Kirchoff gave us techniques to engineer circuitry so that we could reasonably expect plans modeled on paper to work the way we expect when we build them. Ken Fisher cannot have been ignorant of them, and nobody can escape them even if they think they're such a genius they can color outside the lines. You can complain about conventional wisdom but you can't run away from it. Stand on the shoulders of giants and get used to it.

[Enzo]

Seems like the wire type wasn't at fault so much as that it could move. I bet if the sproingy wire had been solidly anchored to something, it wouldn't have happened.


And the super high gain folded up circuit? Sounds a lot like a regenerative radio set. An amplifier about to break into oscillation goes through an area of high gain before instability. In the old days of regen sets, you turned up the RF gain until it almost oscillated, then the radio was super sensitive, and you could receive weak signals. The regenerative feedback gave you much more gain than just the stage gain.

And Leo is right. Whether we understand what is going on or not, it is all explainable by the laws of physics, the laws of electronics.

[Fretts]

Ken's bio does show his military electronics training, sure. Not discounting the value of the tried and true either.
He had a track record of being kind of an iconoclast. The only point I wanted to make is that I doubt he went into the design of the Express with thoughts of exploiting spurious coupling or stray capacitance as a game plan. Only that he was of a mindset that wouldn't immediately try to engineer it out if/when he encountered it, without tinkering with it first and evaluating its... value.

[Leo_Gnardo]

Seems like the wire type wasn't at fault so much as that it could move. I bet if the sproingy wire had been solidly anchored to something, it wouldn't have happened.

I think you're right about that Enzo, or I could have placed a layer or 2 of heat shrink tubing over it to damp its movement.

I've run across a similar problem in lots of classic 60-70's Fender reverb amps. The wire that connects the reverb/dry mix node to the next pre tube's grid has a tendency to "talk" a lot. Tap it with your handy chopstick and you'll hear it clack. Not as much of a problem as the Ceria but in some amps it really is annoying to have this electro-mechanical noise. I tried tacking the wire down with RTV, no dice. Change of wire, mmm, not that either. In those amps where it's really buggin' me, I attach the mix resistors directly to the tube socket, then there's NO wire to cause a racket. One fly lead each to the eyelets on the board that source dry, reverb, and ground, no more problems. Mind you I don't do this to every amp, just the ones that exhibit an objectionable problem. Why some do, and others not so much, there remains a mystery.

[loudthud]

One unique feature Juan didn't mention was the .1uf coupling cap to the PI. Since the PI is doing most of the duty cycle shift (swirl and bloom) this cap value is important to the effect because it charges and discharges slowly (time constant) by comparison the the typically lower values used here. I've experimented with this and watched it on a scope and the results are very interesting. I did try it out on a model I build that's sort of like a Trainwreck design. I didn't like it (for that amp). It compressed the attack in a way that felt strange to me. But in the Express design I think it's a major part of the amps tonal character.

The thing you didn't mention is the effect of the PI being driven by the Cold Clipper stage. Bias on the Cold Clipper means it doesn't overdrive the PI too hard on positive peaks so bias shift is limited. It wouldn't react the same way if the PI was driven by a normally biased stage.

On the clone I built, the magic seems to be in the power supply. There is a sweet spot with B+ around 400V. With an 80uF reservoir cap, B+ is stiff, hardly any ripple or signal on the main B+ node but the 1K "choke" resistor gives plenty of sag to the preamp. Despite this, measured compression is low. The plate Voltage on the EL34's can swing very low due to the high OT impedance and that causes high screen current.

[Mike Sulzer]

Remember regenerative radio receivers? They were invented in about 1912 and obsolete for most purposes in 1918 when superhet was invented. They used a tickler coil to get positive feedback, reasonably controlled, but used a gain control just for the purpose of adjusting the gain for current conditions. A regenerative guitar amp, sitting on the edge of oscillation under all conditions at a useful audio frequency with the positive feedback feedback resulting from stray capacitive coupling would be an unusual design accomplishment.

[Chuck H]

They used a tickler coil to get positive feedback,..

I like that for a component name. Tickler coil. Says it all technically, but still maintains a mad scientist feel Who wouldn't want to try one of those into a guitar amp

[Mike Sulzer]

I like that for a component name. Tickler coil. Says it all technically, but still maintains a mad scientist feel Who wouldn't want to try one of those into a guitar amp

But first you need the main coil of a resonant circuit to tickle, or other inductor. Otherwise, you are stuck with doing it capacitively.

One inductive approach would be to use an interstage transformer with a special extra winding. Positive feedback is usually rejected as a good approach in audio circuit design because it increases distortion. Oh, but wait; that is exactly what you want for a guitar amp.

I suppose you can dream about a design in which you have a push pull output stage and one high gain pentode driving it through an interstage transformer that would provide the phase splitting as well as a tickler winding, allowing positive feedback around just the output stage so it would not be too hard to control. Well, maybe.

[tubeswell]

Hmmm - this has got me thinking about Tickler Coils

Dave's Homemade Radios Tube Radio Schematic Selector

[Fretts]

...One unique feature Juan didn't mention was the .1uf coupling cap to the PI. Since the PI is doing most of the duty cycle shift (swirl and bloom) this cap value is important to the effect because it charges and discharges slowly (time constant) by comparison the the typically lower values used here. I've experimented with this and watched it on a scope and the results are very interesting. I did try it out on a model I build that's sort of like a Trainwreck design. I didn't like it (for that amp). It compressed the attack in a way that felt strange to me. But in the Express design I think it's a major part of the amps tonal character.

On the Komet 60, there is a fast/slow switch on the back of the amp which changes something in the PI. I didn't have access to the opened chassis long enough to dig into details, but it sounds like that may be what he was doing in there -- changing the 'tightness' of the response. The little cylindrical switch body was wrapped in wire. I was in there long enough to determine that it was a grounded wire, wrapped around the switch body as a shield. Shield?, gimmick capacitor? antenna?... with Ken Fischer I'd believe almost anything. The Komet is a tremendous sounding amp - that I can vouch for.

[Jazz P Bass]

A little info that I found on "The Gear Page":

"The Fast gradual switch is just a plain old Split plate load switch. It takes the standard 100K plate load resistor and splits it into 82K and 22K in this case. Fast mode is the full 100K load resistor. Gradual mode takes it's signal from the junction of the 82K and 22K to soften the tone by reducing gain. "

[Fretts]

Ah, great! Thank you for that. The handmade spiral wire shield looked tricky though.

[mikepukmel]

because there is no "dead match" for the tubes. All this happens in a spectrum. There is a wide range of values that are perfectly fine for the circuit. The output impedance at the load is only a nominal value, if you look at any speaker impedance curve you see a huge range of values with respect to frequency. All those impedances are reflected back to the tubes in equally varying amount.


The designer also has to consider how it sounds. We are not making hifi amps, here, we are making the opposite: amps that do indeed color the sound and distort. If a "wrong" impedance pushes the tube operation down its curves a bit and we like the result, well, it isn't wrong then.

Evidence for the "magic" 1 in 1000 amps.

[Dmask]

Reading all of this thread is great. Now, my next amp will go to Eleven.