Master Volume for Paraphase Inverter

[passaloutre]

I'm certain I'm not the first person to come up with this, but i wanted to report a discovery I made recently.

We all like to crank up a tube amp. I mean, that's why we're here. I'm lucky enough to have a garage shop where I can play as loud as I want, but I find the stages I play on need to be quieter and quieter (probably why a lot of people are switching to modelers, etc). Even the 10-15 watt amps I play are frequently too loud these days. I also have attenuators, but sometimes it's nice to have a "loud" knob on the amp, so I find myself installing a master volume in most of my projects, especially where space permits.

In most cases this means some sort of post-phase-inverter master volume, as most of the designs I'm interested are low-gain enough that there's no point in attenuating twice before the phase inverter. In higher-gain designs, as outlined by Merlin and others, it is critical to put attenuating voltage dividers between preamp gain stages, but I'm finding this thinking can be extended to controlling the tone of "low-gain" designs also.

Anyways, I don’t need to extol the virtues of a master volume around here. You get it

My latest project, with a paraphase inverter, I decided to try an idea that was floating around in my head. The donor amp is a Silvertone 7350--a late 40s PA only a handful of parts away from early Fender Deluxe designs--left me with an unused control knob on the faceplate begging for a purpose. Instead of spending big bucks on a dual gang pot that would allow me to attenuate the signal going to BOTH power tubes, it occurred to me that since the second triode in the phase inverter only has unity gain (or more accurately, -1 gain), I could attenuate both phases by placing a single-gang volume knob after the first triode of the phase inverter but before the second triode. This would directly reduce the signal going the grid of the first power tube, and indirectly reduce the signal going to the other power tube--with only a single pot.

Off to the Spice I go...and find that, particularly if I use the "self-balancing" paraphase design, this single pot provides a very smooth volume sweep with very closely-balanced signals going to each power tube. I've attached the schematic and simulation below showing the signal level on each power tube grid throughout the range of the master volume.

In practice, it has proven to be a very useful control for me and extends the range of these "primitive" designs beyond their typical characteristics. I'd definitely recommend it for any designs utilizing the paraphase inverter. Obviously it's best to use a high-value pot (1M) to minimize the additional load on the first triode, but this amp has a 500k and it's working great. In my simulations, it takes a lot of tweaking to balance a standard paraphase inverter, but the "self-balancing" design with the local feedback does exactly that (this one taken straight from the Fender 5D3). Note it's also important that these two triodes have their cathodes separated, or else the second triode will get a signal from the first through its cathode and the volume control will not work.

[passaloutre]

Oh and here's a photo of the amp! It's a cool design where the speaker cabinet (with 10" speaker) acts as a little suitcase with a space to carry the amp head inside. There was another version where the amp head had a turntable built into the top of it...ready for spinning the hits and calling the numbers at your local bingo hall!

[Carlsoti]

I did something very similar to an old Knight PA I tweaked for a buddy. I really liked the sound of the amp, though it did slightly oscillate when everything was dimed. Someone here convinced me that, if it's never gonna be played with everything dimed, a little oscillation on top isn't the end of the world. So far, it's survived everything he's thrown at it, including a "great purge" when he got married.

[passaloutre]

Was it this Knight? I converted that to a Matchless Spitfire. I guess my Knight is actually for home hifi, not a PA. I got each of these for under $100 on reverb, with tubes! And I just got a tiny Heathkit 3-watt with EF86 and a single EL84 for $75.

[Carlsoti]

The one I worked was older, I think. Cool, weird shaped, goofy blue/green deal from the late 40's or early 50's. I really love those kinds of chassis, but I've got a few boxy ones, too. You got some smoking deals on those other gadgets. I had to curtail my buying a few years ago, due to the growing pile of weird old tube things, most still in need of major rework. Some Eico bits, a couple Bogens, an AIMS 4 channel PA, vintage no-name hifi kits, etc. I tend to try to make weird things out of them. Who needs yet another 5E3 with 6L6s when that base is already covered? On the Knight, I was attempting to make an amp that "sounded broken in the best way possible", so I leaned into the assymetry of the paraphase inverter when implementing the master volume. I did convert one of the inputs to be switchable from grid-leak bias to a more traditional input. Aside from that, it's got a ginormous dropping resistor between the plates and screens. Most of the way up the dial, it plays normally. If you really rip into it, it's got all the sag and bloom one could want. Pushed hard with a pedal, and the input dimed and master rolled off a bit, it transitions from sag into "totally falling on it's face." Super compressed, with an un-natural "bloom/volume recovery" that sounds like the leading edge of feedback as the note dies away. Most of that range is available from the pedal stomp switch, volume on the guitar, and picking dynamics.

[passaloutre]

the growing pile of weird old tube things, most still in need of major rework. Some Eico bits, a couple Bogens, an AIMS 4 channel PA, vintage no-name hifi kits, etc.

I know the feeling and I’m attracted to the same weird stuff. I have a couple shop shelves full of console amps, tube reel to reels, PAs, and film projector amps. Most of them have pretty alien (to me) circuits from the pre rock’n’roll years, and I’ve learned way more dissecting these (and simulating them!) than I ever did changing tone caps in a Twin Reverb. Having started this hobby by modding 70s Fenders and trying to perfect that tone, I’m much more interested these days in seeing how these odd designs respond to being pushed to their limits.

This Silvertone is a fast favorite, and the master volume helps me get those broken Crazy Horse tones at a more reasonable volume. That Knight Spitfire is no slouch either, chimey and rich. It helps that I upgraded both with nice alnico speakers, Jupiter and Celestion respectively.

[roarshock]

You're always up to something two steps ahead of me. I still havent made that stereo harmonic tremolo, but I have a few things to clear out first. Nice work using LT Spice to figure things out before building. I made a similar move with a Newcomb TR-16c conversion.

[passaloutre]

That’s funny because I always feel two steps behind around here! I’m just out here exploring all the unconventional (i.e, not Fender/Marshall/Vox) designs I can find, though this one came out to be quite close to a 5D3.

Nice work on the Newcomb. I like having a pentode in that position. And 560k on the 12AX7 plate! I admire your bravery lol

[sluckey]

I've never seen a MV pot used in that position of a paraphase inverter but I have seen several amps that put a switch in that position. The switch provided a standby/mute function.

[passaloutre]

I can confirm it does mute the signal when the MV is all the way down!

A few changes and I suppose it could be used to attenuate/mute just one power tube, for a pseudo single-ended sound.