Q: Voicing

[Blackburn]

Kinda curious about giving this a try. Do you guys recommend 120pf? I think I might have one in the neighborhood.

[martin manning]

Yes, good explanation. I'll pick one nit: Supersonic refers to velocity with respect to the speed of sound. It's Ultrasonic that refers to the range of human hearing.

Maybe try as much as 220pF? Experiment!

[Reeltarded]

^^^This!!

Great post as always Ian.

Always, huh?!

Thanks for pointing out the stoppers on the PI.

[iknowjohnny]

Hmmmm, what would the gurus say about they way i did it, a cap from plate to plate on the PI? Seems to work well, but is there anything i should be aware of? I've tried all ways. Cap across input anode only, cap across the other anode, both at same time. Cap from plate to plate seems best to my ear. But just curious what you all would say about that.

[Reeltarded]

That cap is called for in most every modern amp. Marshall and Ken say 47p. I start with 100p.

[iknowjohnny]

mine is 50pf. 100 just seems to dull the top every so slightly but enough to change it too much to my ear.

[Reeltarded]

You think.

I use even bigger on super nutso gainy amps. The only place where smoothing doesn't mean dulling.

[martin manning]

That cap is called for in most every modern amp. Marshall and Ken say 47p. I start with 100p.

Leo said it first...

[Reeltarded]

Well yeah, if Marshall said it he must have heard it somewhere.

[gingertube]

I must try this too.

On my Junkbox Trainwreck I put a 300pF across the 100K anode load of the final gain stage, that seemed to knock off a slight agressive edge.

https://tubeamparchive.com/viewtopic.ph ... k&start=15

Also on that amp I did have parasitic ULTRAsonic oscillation which took me ages to track down. On the CRO it showed up as high frequency burrst on just the positive going peak of the input signal. There was a hint of where to look in that it only happened passed a certain volume control level but as is often the case with ultrasonic problems, when it took off, then you could see it everywhere throughout the amp. It was definitely the phase splitter which was the culprit and grid stop on both sides fixed it.

While scoping around looking for this problem I also noted power supply diode switching spikes on the output. Replacing the 1N5408 diodes with Ultrafast Soft Recovery Diodes fixed that.

By the way, for those of you who like to try weird things, may I recommend the parallel input stage I added to what is otherwise basically the Trainwreck A0 schematic. This was intended to add some harmonic emphasis right at the front, besides we had to do something with what otherwise would be a spare triode, right?

Cheers,
(Not always right) Ian

[LeftyStrat]

... besides we had to do something with what otherwise would be a spare triode, right?

Of course!

[gktamps]

My 6V6 KF60-inspired build runs 82k and 100k PI plate resistors. Is it important which plate resistor this enhance cap is connected to? The 82k resistor is connected to pin 1 on the PI...

Gingertube wrote that 100k + 100pf gives a 15.9kHz rolloff; may I ask for the equation to calculate rolloff for other values or R and C?

Last, any problem/benefit with combining this with a 47pf cap across the PI plates?

Thanks!

Greg

[hitchcaster]

when someone says voicing, it seems like such a vague description. for me it's mostly parts selection and then tolerances... some mods too... others its just roll a couple tubes. but the express is designed right IMHO. the bright switches and where u add and subtract briteness is interesting. dumbles are the same way where u can add and subtract at different points. effects loop, cables, switches etc. I never really got the express until I plugged it into a vintage 4x12 with the right speakers and the brite switch on at 500pf.... it's just a different thing. I did a small mod with a three way brite so I can use a 250pf with different type cabs... if your getting tricky and using Teflon, and/ or silver platd wire that's going to cause briteness problems depending on the synergy you have going on with everything else... in the end u just gotta mess with it

[gingertube]

Calculating the roll off.
Corner frequency (-3dB point) fc = 1/(2 pi R C)

so 100K + 100 pF = 15.915 kHz.

Cheers,
Ian

[gktamps]

Thanks, Ian!
Greg