Q: Voicing

[10thTx]

There are a number of us on the Hoffman amp forum that use this "enhance cap" across the plate going into the LTPI to smooth out the highs.

This was done on 4 different amps I've had with similar results.

It seems to smooth the highs withOUT eliminating the highs. I can not hear any loss of high frequencies using this.

It also seems to allow more "blooming" on Dumblish inspired amps such as the Tweed Overdrive Special and D'Mars OD special. My unscientific impression is that is removes some of the "hash" (harshness) of the high frequencies which allows the notes to sustain and bloom more smoothly.

IF you try this carefully use insulated alligator clipped wires and values between 82p to 150p across the plate resistor. This is a very quick and easy mod to try.

IF it works on your amp, please post the results so others can know about it.

With respect, 10thtx

[Darkbluemurder]

What you are doing with this enhance cap is stabilize the amp with respect to its negative feedback loop. This is explained in great detail in chapter 9 (feedback theory) in ValveWizard's preamp design book. Definitely a sensible enhancement.

Cheers Stephan

[JazzGuitarGimp]

What you are doing with this enhance cap is stabilize the amp with respect to its negative feedback loop. This is explained in great detail in chapter 9 (feedback theory) in ValveWizard's preamp design book. Definitely a sensible enhancement.

Cheers Stephan

Is that to say there is nothing to be gained from this mod if the amp doesn't have NFB?

[surfsup]

Interesting, I just saw this. I actually tried this a couple weeks ago on one of my amps screwing around tweaking and learning wondering what an anode bypass would do on the PI. When I did it, the amp volume dropped, became very fizzy and the EQ was like a telephone. I put it on the other side though, not the primary signal plate resistor.

I wonder if this is signaling a problem with that amp? Without it, it sounds great. So not sure why this happened and I've been meaning to look into that more.

[Colossal]

Surfsup,

Hmm, that's odd. I've done that to a hot rodded Marshall with 4 gain stages and it was a tweak that really took it from great to wow, that's really freakin' great. It improved the separation even more; icing on the cake so to speak. I have a resonance and presence circuit in the feedback loop.

[M Fowler]

deleted

[10thTx]

Any chance that you're thinking of Randall Smith of Mesa Boogie rather than Dan Torres? I found out that Mesa Boogie does this on a couple of their amps.

With respect, 10thtx

[M Fowler]

never mind

[JoeCon]

Newbie question:

Why the cap on one side only?

[10thTx]

Why the cap on one side only?

Fair question .........

It worked. That was all that my 4 amps needed.

I prefered the tone of that over having both LTPI sides having caps across the plate resistors (which I also tried). I thought the tone was more articulate and expressive with just the one side.

with respect, 10thtx

[Colossal]

I thought the tone was more articulate and expressive with just the one side.

This is exactly my observation.

[JoeCon]

Thanks guys.

Can't beat first hand experience no matter what the MATH says!

[rmroza]

Thanks for everyone's support. I keep getting sidetracked with other new builds, but still in monitoring this!

A couple notes...the amp is still currently stock as designed, I find the G12H30-70th Anniversary very flat and not a bright speaker at all, I am and have been using a 40'cable over the last year.

As soon as I can, I'd like to get in and Tweak the TW Express to my personal liking. You guys gave me some things to check out and think about. Amp design is funny....you complete a baseline design and then really needs to be tweaked to suit.

Thanks for everyone's help. Back to my new 2-channel design!!

[gingertube]

By putting the cap across the first anode load of the PI (input side) you are putting a high frequency roll off on the whole PI - why? becasue the other side of the PI is driven from its cathode by a signal generated by that input side - so the cathode driven side will follow the input side response. For 100K + 100pF the roll off is at 15.9 kHz. At this frequency you are only knocking off some higher order harmonics which tend to be nasty sounding anyway and a bit of high frequency noise. In addition the PI can be subject to supersonic parasitic oscillation, particularly if there is a treble boost just ahead of that stage. This mod will tend to stop parasitic oscillation BUT I still recommend the use of grid stop resistors on both sides of the PI. Might be over the top but I generally use 10K for these.

Putting a cap across the anode load on the other side of the PI (cathode driven side) will be sus (as experienced above) as this will unbalance the PI at high frequencies whether or not you have global NFB.

Cheers,
Ian

[Colossal]

^^^This!!

Great post as always Ian.