Subjective Mods, Preferences and the likes..

[cboysen]

Hey!

Besides dealing with good amp etiquette; decent layout, grounding schemes, lead dress etc, it seems that a lot of tube amp building and design is down to personal preference. After all sound is pretty subjective, so it wouldn't come as a surprise if people favor certain 'styles' above others.

While I'm still new to building amps, I have already found methods and mods that work well for me, and I was wondering if you guys had any tips and tricks of the craft and "trade secrets" to share.

I'm sure many of you know these, but here are a few mod-tips from others, that I've found to work well in some amps.

Snubber Caps & Tubenit's "Enhance Cap".
Small capacitors (100pF to 500pF) from cathode to plate, parallel to the plate resistor, or from grid to plate, seems to yield a way to shelve off very high frequencies and somewhat stabilize the circuit from potential oscillation. Tubenit's enhance cap places a 150-250pF cap across the plate resistor of the phase inverters inverted output signal, which seems to alter the frequency response of the upper mid/highs. I have found that these caps and to some degree Tubenit's mod work well in designs with large preamp signals and master volumes where the preamp tends to distort before the power amp. This often leaves one preamp triode to distort before the rest, giving off a rather harsh distortion. Knocking off some highs can help reduce the harshness, but may leave the overall amp too dark, if used with a lot of power amp saturation. It also explains why these mods are often seen on high gain amps, and Dumble overdrive channels.

Merlin's Grid-stopper Resistor&cap
Listed on the valvewizard pages, and discussed extensively on the ax84 forum. Using a small-ish grid stopper (10k) in conjuction with a 150-500pf cap to ground, reduces the noise of the grid-stopper resistor, which otherwise would be amplified through the circuit. This is especially helpful on high gain amps. Furthermore, I have found this mod to increase the relationship between the guitar and the amp, making it easier to dial down the gain with the guitar volume knob, without sacrificing much volume. Not using the capacitor yields a bassier/darker tone, where my personal preference is 150pF-220pF, though going higher will start to roll off highs.

Bright Cap on Pre-amp Master Volume
Using a traditional pre-amp master volume between the output of the tone stack and the phase inverter, seems to cause the same problem as any other volume knob. Turning down the volume, shelves off highs. This is easily resolved with a 47pF cap between the input and the wiper of the pot. I know a lot of people do this, but many tend to use 150-500pf caps, which in my opinion is way too much. 47pF is just enough to retain some highs at lower master volume settings, without altering the response of the amp. The rest of the treble increase, I believe is due to the Fletcher–Munson effect.

Parallel Triodes & Sluckey's November Build
I recently built an amp for my friend, which included a parallel triode with separate volume knobs, that would blend together as bright and dark mixing channels. This is a straight up rip-off from Sluckey's November Build, but boy does this work well! First of all, using a parallel triode at the first stage, makes you able to effectively dial in bass and treble. Furthermore it makes this intermodulation effect that thickens the tone in a very cool way. Maybe this is one of the reasons, why so many enjoy the Plexi and Bassman amps with the 4 inputs? For this particular amp I tigthened up the bass response, using a 2.2uF cathode bypass-cap in conjuction with a 1.5k cathode resistor on the dark channel, and lessened the treble response, with a 100pF bright cap and 390pF cap parallel to the 470k treble peak resistor on the bright channel. Blending these two channels, creates an extremely versatile amp, that works great with any guitar, at any volume. So much so, I consider replacing the treble/bass pots with resistors on future builds.

That's it for now. If you're interested, I have a few more. Also I'll appreciate any comments on my findings :)

Kind regards
Christian

[Lectroid]

So, yeah, it's nearly four years later but I wanted to tip my hat to cboysen for Tip #3

Bright Cap on Pre-amp Master Volume
Using a traditional pre-amp master volume between the output of the tone stack and the phase inverter, seems to cause the same problem as any other volume knob. Turning down the volume, shelves off highs. This is easily resolved with a 47pF cap between the input and the wiper of the pot. I know a lot of people do this, but many tend to use 150-500pf caps, which in my opinion is way too much. 47pF is just enough to retain some highs at lower master volume settings, without altering the response of the amp.

I built a single-channel Deluxe Reverb clone using Hammond organ hardware, specifically the PT and OT from an AO-43 chassis.  I also added a Master Volume and Mid pots.  As built, the amp has plenty of power and the Volume/Master combination gives me plenty of control over the clean/dirt spectrum as well as good level control.  Its only problem was a harsh, almost buzzing, and very treble overall tone.  By dialing down the Treble knob to zero I could get a decent tone but IMO the amp should sound clean with all tone pots at 5, right?

After searching on 'harsh' and 'buzz' and trying several suggestions, I stumbled on this post.  I tried #3 and it went a long way to clearing up the amp's sound. 

I clipped in 120pf, 250pf, and 500pf silver mica caps in turn, and quickly chose the 120pf cap.  The tone cleaned up immediately and eliminated the harshness and buzzing.  The treble knob became useful again over its whole range, and the versatility of the Vol/MV combination stayed the same.  Best of all, it added in an airy, mid-range clarity.

All thanks to Christian, this is a mod worth trying if your amp has symptoms like mine.

Rich