VOX / Matchless circuits: crossline vs Lar Mar PPIMV?

[klobasa]

It seems Matchless, Bad Cat, VOX etc. are preferring crossline master volume over the Lar Mar PPIMV. Which do you prefer in british type Spitfire / Lightning / AC15 circuits and why?

I think that the 1M audio sweep is not a good choice in crossline, since it acts almost like a on/off switch.

Comments please...

 

[2L man]

"Crossline" will increase distortion to power tube drive signals for low MV settings because it "choke" PI outputs together. Increased distortion can somewhat compensate what is lost using lower volume?

LarMar is "transparent volume control" when NFB is not used. When NFB is used it has effect but I don't fully understand it.

I am about to begin Spitfire build and will use LarMar but also Reactive Load which most likely has only "slope simulation" which make it smaller and cheaper.

 

[gusfinley]

The "Crossline" Master Volume is simpler and cheaper since only one potentiometer is required, which may be why it is preferred in Vox Type circuits.

The two 100K plate load resistors in the phase inverter make this type of master volume possible. Marshalls use unmatched values (100k,82K) so there is an imbalance in the signal going to the power tubes. I tried a crossline master volume in a Marshall 20W circuit (100K, 150k plate load resistors) and it didn't work well at all. When I realized why I felt a little silly.

 

[FenderLover]

Crossline also does not work with a concertina... no one asked, but there you go.

My Chieftain has the crossline. Smooth response all the way up and down. I've also used it on my own amps.

 

[Peegoo]

I did a build a year ago and was going for the Matchless vibe, using an Eico integrated amp as the donor chassis. I used a crossline MV with a 1M pot. It is a screamer of an amp; it is loud and clean, or loud and really nasty, and everything in between. Very smooth. Build thread here:

Zippo 15 Amplificator: turning junk into a Matchless Lightning

This started out as a derelict 1959 Eico HF-12 mono integrated amplifier. I bought this carcass at a hamfest in the 80s for $6. The original Eico tube compliment was 3x 12AX7s (two preamp and one PI), 2x EL84s, and an EZ81 rectifier tube. This was an attractive donor for a Lightning...

www.tdpri.com

 

[klobasa]

I just installed a 330K parallel resistor to my Bad Cat 1MA crossline master to modify the terrible sweep (total resistance is now 250K). What a change! Now I can actually dial in bedroom sounds too. The pot has now very smooth taper and all of the "on/off" behaviour is gone.

I don't anymore miss the Lar Mar PPIMV at all...

 

[klobasa]

The "Crossline" Master Volume is simpler and cheaper since only one potentiometer is required, which may be why it is preferred in Vox Type circuits.

Could it be so that e.g. Matchless would use inferior MV to save a couple of bucks? They are overbuilt in everyway and cost a lot, so that would be weird if they would not use the "best" MV available...

My tech informed me that crossline is actually technically better than PPIMV, since the latter darkens the sound the more you use it and crossline doesn't. He was 100% right when he told me to try out the 250K pot in crossline. Very much transformed the amp for better.

 

[klobasa]

Just recently I bought an actual Matchless 15W amp. It also had the crossline master with the 1MA pot. The master also had a bright cap. The sweep was terrible for my use and at lower settings things got thin. It was basically almost like a on/off switch.

I need to use lower volumes too, so I replaced the master with a 250K audio pot (no bright cap) and voila, the sweep is great from 0,5 to 10. There is absolutely no way I would replace that crossline master with larmar.

I am beginning to think the reason why larmar gets the hype is that those double pots needed for larmar are 250K and not 1M. The pot value alone makes a huge difference.