Lar-Mar Master Volume | How It Works, Pros & Cons 

Great idea! I claim “Circuit Collapse” for a post-punk project.

@@ColeWheeler4Lyfe That is exceptional !

@@ColeWheeler4Lyfe I vote for “ Common Mode Rejects” band name! LOL

I tried the variable feedback pot as well as the presence control with switchable 1,5, 10 uF caps in a ‘69 champ amp and it gets pretty “Raw” as the NFB goes away like a tweed. Very nice!

@@tjminasi1442 The trick is more NFB when you turn down volume to keep NFB to similar levels when volume is up. Of course you can also use it to give you less NFB at higher volumes to give a rawer tone.

See the ‘67 Bassman video yesterday where I used the same MV with the Fender Bias Balance circuit. So you can fudge tube matching that way, though I always prefer matched tubes myself.

In a modern amp you can use teflon coated 22AWG and end up with a very neat small diameter twisted run. But in these old Bassmen I used the cloth covered 22AWG purely for cosmetics/vibe.

Well, the AB763 phase inverter/output tubes usually stay pretty clean, so a simple pre-PI MV ala late ‘70s Fenders can also be a good choice for those (without the dumb bright cap). Depends on how much you turn up the preamp volume. For “regular Fender use” (volume no higher than noon but you just want less output) the pre-PI works well. If you turn the volume to 8 a lot ala Bloomfield then the slightly more complex Lar-Mar is your friend.

Thanks Pete. The frequency change is more apparent in amps that have a Presence circuit. Trying to compensate is tricky as you don’t know how the player is using the Presence circuit. Easy to overdo it and have the amp too bright for many (but still not enough for others). I advise clients that any MV involves some compromise, but this circuit is close to transparent over 1:00 and not objectionable between 10:00-1:00. If they find themselves using the amp with the Master at 8:30 it’s too much amp for that stage. Better to get a second smaller amp or look into the various reactive load attenuator devices (Suhr, Fryette, UA, etc). There are ways to compensate within the circuit (high pass filter increases as MV is turned down, very minor treble bleed circuit incorporated, etc) but like I said, that is pretty much guesswork as to what *most* players will find good. Others will hate it.

@@PsionicAudio Thank you for the quick and thoughtful response.Much appreciated.

You want to steer signal away from high voltage/current AC as much as possible. Certainly away from the mains.

Thanks man. That’s a tail not really worth chasing, for reasons I don’t have enough coffee to get into. Let’s just say if it was doable and there was a real reason to do it, designers would have been doing it a long time. Simple is good.

Thanks for the well deserved scolding. @@PsionicAudio

Given that no attenuator and no MV are “perfect” the best solution for “transparency” would be to use both at once, each just doing a little volume reduction. Say MV at 1:00 and attenuator doing -6dB. So neither is really changing the sound. But you get a very noticeable level reduction. But this kind of MV is much more “transparent” than people think. The gear forums are full of hyperbole. Nothing ever has some weakness - IT SUX. Nothing ever has some advantages - IT RULZ.

Thanks. Type 3 (crossline) doesn’t have a very good taper and sounds a bit fizzy in comparison. I’m not a fan, but for minor volume reduction it can be OK. PS no disrespect to Ken, but the Type 2 version in my schematic is better to what he showed as I (like others) include a fallback resistor.

@@PsionicAudio Sir you are right. The taper with an 1MA pot was horrible, tried 250KA and 100KA pots. Kept the 100KA one. Your videos and the AB763 Mods page from rob robinette helped me a lot working on my amp. Thank you very much again! I'm looking forward for your next livestream. Maybe you could talk a little about the SLO cirucuit and your opinion. Here in Europe there is a nice source for a SLO Kit and I'm intrigued in building a 30 Watt one as a high gain amp since my clean tones are well covered with the DRRI. Cheers from Germany

It doesn’t really matter if the difference is 10% or less. That’s something I’ve tested with matched and unmatched dual gangs. The high pass frequency difference is negligible, and the effect on bias isn’t enough to cause hum/noise. So long as each gang’s taper is the same (and they usually are as that’s consistent in production) the MV works the same. I spent a lot of time down that rabbit hole.

Brilliant. Thanks a lot for that. Much appreciated.

It shouldn't be a circuit thing as mine can get really cavernous when you turn it up. Maybe check your V3 tube, which is the reverb driver and should be a 12AT7. V4 is the reverb recovery tube so that could possibly have something to do with it too.

I’d want to see the schematic before answering.

The Lar-Mar is out of circuit when at full volume. The type 3/crossline usually always attenuates slightly.

KiCad

@@PsionicAudio thank you

Not at all the same circuit but IMO comparable sound with fewer parts and no heat.

Thanks. Linear can sometimes be better in cathode biased amps. Audio works better in fixed bias in my experience. The value will depend on the value of the coupling cap before it, but in general use the closest match to the existing bias/grid leaks.

I share an awful lot. I don’t, however, share everything. ;)