Cathodyne PPIVMV

[willwaush]

Hello guys, some help needed here!
I have tried to implement the Post PIV Master Volume on this build but.. it's acting very strange.
I know that the 1M pot should be "mixing" the two out of phase signals to slowly cancel them more and more, but what's happening here in real life is by turning down this master volume I'm just making the amp more and more distorted (in a bad way, like if it was just odd distortion) and even loosing a lot of bandwidth.
From PPIV "Marshall" demos I've never seen this behavior, so I was wondering is this considered normal? Can I do something?
Because ideally I want to keep a Post PIV MV because in theory it should help the preamp compressing more when you hit the amp with a distortion pedal.. while Pre PIV MV seems to be cleaning the preamp level too much, so it leaves more headroom and you get a bigger jump in level when hitting a distortion pedal.

[sluckey]

What's this got to do with cathodyne? You have a LTP PI. That crossline MV is not liked by most people. I suggest using a LarMar type MV.

[WiderGates]

Hello sluckey,

V2b is the Cathodyne PI.
V3a & V3b is not a LTP PI.

[sluckey]

Ah yes. I should not post so early. Sorry.

[uki]

V3a & V3b is not a LTP PI.

What is it?

[pdf64]

A differential / balanced signal common cathode amplifier stage.

Type3 master volumes operate by loading, rather than by signal cancellation.
Their characteristics tend to leave a lot to be desired.
They just about work with the high anode impedance of 12AX7 LTP, but tend to sound awful with lower anode impedance valve types.
But try it on the other side of the grid stoppers, it may work a bit better.

[willwaush]

So would you guys suggest a LAR-MAR mod to behave better?

Since in this case the original resistors are 68K, which solution would fit better in my case?


Do you think something like this could work? I found it online, and while I am convinced that the 220K can be useful in case the potentiometer fails to still keep the bias tied to the output tubes grids to avoid anything blowing up, I'm not convinced that placing the 220K across middle and first lug of the pot would make the pot's value down to about 68K. I would agree one side of the pot would behave in that range, but what happens when the wiper and the bias potentiometer pin are tied together? Doesn't the pot become 100K once again?

[sluckey]

Most people use a 2.2M failsafe resistor rather than a 220K.

[SILVERGUN]

So would you guys suggest a LAR-MAR mod to behave better?
..... which solution would fit better in my case?

There isn't much gain to be harnessed in that PI and 12AU7 driver combination.
I wouldn't be too concerned with having to have the master where you're trying to squeeze it.
A simple prePI master might be the "better" choice. Just tack it in right after the treble wiper.
Wouldn't take very long to try it and hear it for yourself.

[dude]

Try both together, LaMar and just after the treble wiper. Now two knobs to play with, 

[willwaush]

So would you guys suggest a LAR-MAR mod to behave better?
..... which solution would fit better in my case?

There isn't much gain to be harnessed in that PI and 12AU7 driver combination.
I wouldn't be too concerned with having to have the master where you're trying to squeeze it.
A simple prePI master might be the "better" choice. Just tack it in right after the treble wiper.
Wouldn't take very long to try it and hear it for yourself.

The issue is a Pre Cathodyne volume (tied to treble) not only reduces volume, but grit and gain at the same time when being pulled down. I guess it could be useful to someone, but I'd like the MV to keep the original tone and just push it down in level.



About the 2.2M, yeah I know, the thing is here I need to approximate the original 68K resistors values in that spot.



I'd like to try both, but I'm still not sure about how to really perform correctly a LAR-MAR in this specific case.

[SILVERGUN]

You "could" do this

[dude]

You "could" do this

This how David Allen of Allen Amplification wires his PPIMV in his amps, works very well.

[kagliostro]

I'll give a try to what suggests SILVERGUN

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In my archive I've find this indication, may be, going on trying you can give a try to it (the MV for Concertina)




---


@ pdf64

A differential / balanced signal common cathode amplifier stage.

But which is the purpose for the use of that architecture between the Concertina and Power Tubes ?

is it someting to have the advantages of the chime of a Concertina plus something like a LTPI ???

Franco

[pdf64]

…
@ pdf64

A differential / balanced signal common cathode amplifier stage.

But which is the purpose for the use of that architecture between the Concertina and Power Tubes ?

is it someting to have the advantages of the chime of a Concertina plus something like a LTPI ???

Franco

The cathodyne can easily achieve excellent symmetry, its key limitations being its lack of gain and limited max signal output voltage.
A differential amp can have both very good gain and max signal output voltage (higher than an LTP as there’s no large tail resistor eating into the supply voltage).
So putting a differential stage after the cathodyne may achieve the best outcome, superior to an LTP.

The above is in context of a linear amp that’s intended to be operated within its linear range. For instrument amps, which may be overdriven a lot and need to recover rapidly from that, the higher max signal output voltage is probably a disadvantage, as it may increase bias excursion.

[kagliostro]

Many Thanks for the explanation pdf64

Franco

[jojokeo]

Lots of fretting over controlling output power on the most powerful tubes used in most of our geetar amps... Why have a rocket engine that you want to throttle down? Doesn't add up or make a lot of sense to me but oh well...


Crossline MVs get a lot of disrespect for no reason IMHO. I think they work at least as good as the PPIMV/LARMAR masters and a WHOLE lot easier to install too! In fact, putting that resistance in the signal path DOES in fact negatively affect tone when turned down. The PPMIV MV circuit is certainly "no angel" being the most transparent. No way no how. I have to install treble bleeds on them so that they don't suck tone. Another inconvenience is that it can also be noisy so shielded wiring is another thing required and more hassle to install... So what's the advantage again??? No, there is NONE. All the extra hassle, parts, wire, etc that if it were Leo? He wouldn't have done it I'm pretty sure? There is no free ride and the piper (or troll) usually always has his payment required as they say. ;)


*hint - there are several fancy "tricks" one can do when designing in a Crossline MV which removes it from circuit while not in use/activated.

[jojokeo]

"Hello guys, some help needed here!
I have tried to implement the Post PIV Master Volume on this build but.. it's acting very strange..."

Where's this schem from and/or how was it found? Just curious, thanks.

[pdf64]

… Crossline MVs get a lot of disrespect for no reason IMHO. I think they work at least as good as the PPIMV/LARMAR masters
…

A Trainwreck Pages Type 3 master vol (which uses a crosdline master vol) is specifically part of a 5F6A type 12AX7 LTP phase splitter.
That may be your experience of a crossline?
In that context, it does work, and seems to sound ok, but can’t achieve even a -20dB signal level reduction before the audio mutes *.
So it’s not much use for domestic friendly volume levels.

If transplanting just the crossline master vol aspect of the Type 3 for use with other phase splitter / balanced signal designs, my experience is that it either sounds bad, works very poorly, or both.
In the application noted in post 1, I envisage that it would indeed sound awful, as willwaush describes. Partly because the V3 circuit doesn’t overdrive nicely whatever, partly because the extreme loading effect of the crossline doesn’t work happily with many lower impedance circuits.

A couple of 10k resistors greatly helps, or totally resolves, the above issues. As per R31&32 of the AC30CC, see p6  https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf

…
*hint - there are several fancy "tricks" one can do when designing in a Crossline MV which removes it from circuit while not in use/activated.

Such as? eg no load pot.

* It’s worth noting that although the audio mutes, it’s because the LTP output becomes common mode, rather than balanced. ie some signal is still passed to the output valves, but gets cancelled out / muted by the OT ( due to being common mode, rather than differential).

[jojokeo]

I'm not here to debate you pdf... I just know what works and what doesn't. What I like and what I don't. What works better this than that, etc... YMMV as they say? I LIKE the crossline and it sounds and performs better than the LARMAR I feel without the extra bullsh!t. AND, as said, treble bleeds are necessary when in use because they DO suck out the high end with increased resistance. That is my experience without the technical BS. I'm a design, build, and repair guy and a player. Unlike most/many, they don't do both. I've been at this since the 80s and I don't just work with tube circuits. Thank you for your info, it is much appreciated and hopefully it helps others too? My philosophy has always been to read, educate, then implement, experiment, test/play, repeat...over and over if necessary. I have many decade boxes to assist with this aspect. Too much time is spent on these boards theorizing, speculating, wondering, etc...without just doing and finding out immediately what results are obtained and moving forward. But I digress...short of time as always w/more guitars and pedals to work on at the moment (heaven forbid it's not always a tube amplifier, lol). Have a good day pdf!