PPI Master Volume or Power Scaling?

[big_teee]

Here is the info From the Metro Page:
Also I doubt you are as old as some of the rest of us!
T

The LarMar Type-II Post Phase Invertor Master Volume Modification
How Does it Work?
This modification is an ‘improvement’ on the established Ken Fisher Type-II ‘Rich Mod’ Master volume. The changes were identified and tested by Mark and Larry from the Metroamp Forum boards (RockStah and Novisbir). Although the LarMar PPIMV was designed independently, a similar PPIMV can be found in Kevin O'Conner’s “Ultimate Tone Book” and Randall Aiken uses the same type in his Aiken amps. The changes succeed in:

- Lowering the sensitivity of the Master Volume to interference and noise. - Maintaining a Bias-Feed resistance of 220K, seen from the PI coupling caps - Keeping the resistance between the PI’s coupling caps and the output tube grids as small as possible (preventing loss of high frequencies when master is set to low volume) - Preventing the output tubes failing if the pot suffers a wiper failure.

Many who have installed this Master Volume report that it is the best Master Volume modification they have ever used, achieving that ‘dimed plexi’ sound at ‘non-pain’ volume levels.

As mentioned, the mod is the same as the Ken fisher type II mod, but with a 250K audio pot (log taper) instead of a 100k linear and with the addition of 2.2M resistors soldered between the wipers and the outputs (bias voltage supply).

The benefit of the 250K PPIMV as opposed to a 500K PPIMV is that, when you've backed off the pot, you don't lose as much of the high frequencies. This is because the input capacitance of the output tubes is creating a low-pass/hi-cut filter with the entire resistance from the previous coupling cap to the grid.

This mod involves removing the two 220K bias-feed resistors on the board. The 2M2 resistors must be installed between the centre lugs (to grids) and the left lugs (from bias supply). The resistors make the pot = 220k when dimed - which means this PPIMV should sound the same when the amp is dimed as if you didn’t have a master at all!

Where do I put the Pot? Recommended practice, to avoid excessive interference from the input stages of the amp, is to mount it in a speaker jack hole leaving yourself with a single speaker jack (you only ever use one of them anyway right?). In this way, you don't need shielded wires to the tube's grid resistors, (but you have to twist them).

Because the effect of a PPI Master Volume renders the Presence control almost redundant at low volumes, some people like to place the Master Volume pot in-place of the presence pot, and re-route the presence pot to one of the speaker jacks. Special care must be taken routing the wires to avoid interference. Shielded wires to the output tube's grid resistors are recommended when using this method.

[imaradiostar]

I love that this "larmar" thing is a mod. I've been doing that for years on amps because it's logical to do so- nothing special so how is it a mod? It seems like it makes sense to use a pot value close to that of the bias grid leak resistors and put a resistor across it in case a pot wiper goes open circuit. That said, 2.2meg probably won't save you from much if the pot is truly open and the tubes are working hard. Maybe it'd give you a chance to hear that something is wrong and fix it before one of the outputs melts down.

15 years ago I was on the ampage forum with the user name "plexi jr" building plexi circuits with EL84 output sections. What isn't the November amp or any of a number of low powered bassman/plexi clones named after me? Because it's not a new or particularly innovative idea! It's just a logical extension of building amps. I'm sure lots of people built similar things 20 and 30 and maybe even 40 years ago and we don't really name the amps after them either.

The other thing- the input capacitance of an output pentode is very very low. I'm seeing 15pf for an EL34 and 10pf for a 6L6. That means you'd need upwards of 500k series resistance to get a rolloff in the audible range, much less one that can be heard in a guitar speaker. 1meg ohm grid stopper would rolloff around 10k with a single El34 connected. You could use a 250k master volume pot and 100k grid stoppers to avoid grid blocking and still reproduce frequencies only the family dog would be able to hear.

OK, sorry, had to rant a bit.

jamie

[Chuck H]

One change to the typical PPIMV that I thought always made sense is to swap the power tube grids on the wipers to the PI outputs on the wipers. This does two things. It makes it so amps with bias vary trem can use the circuit and it makes it so the bias voltage isn't dependent on pot wiper contact. Usually when a pot fails it's the wiper. Having the power tube bias on the wiper is a bad idea IMHO.

[Han die Blume]

Hi, its called LarMar because the idea is from Larry Grohmann (LARRY AMPS) and Mark testet it in severell Amps.

Han

[Steve Conner]

Although the LarMar PPIMV was designed independently, a similar PPIMV can be found in Kevin O'Conner’s “Ultimate Tone Book” and Randall Aiken uses the same type in his Aiken amps.

And presumably, nobody can be bothered calling it by its full name, the Larry-Mark-O'Connor-Aiken-Radiostar PPIMV.

One change to the typical PPIMV that I thought always made sense is to swap the power tube grids on the wipers to the PI outputs on the wipers.

This is a good idea indeed. I guess as a second small bonus, it keeps power tube grid current away from the wipers, stopping them from going scratchy. The wipers are connected to the PI output coupling caps, so there is no DC path.

Having said that, the signal level is so high here that scratchy pots may be a non-issue.

[J M Fahey]

You tubeheads are so funny

[Wombaticus]

And presumably, nobody can be bothered calling it by its full name, the Larry-Mark-O'Connor-Aiken-Radiostar PPIMV.



This is a good idea indeed. I guess as a second small bonus, it keeps power tube grid current away from the wipers, stopping them from going scratchy. The wipers are connected to the PI output coupling caps, so there is no DC path.

Having said that, the signal level is so high here that scratchy pots may be a non-issue.

The only problem with connecting the PPIMV wipers to the PI outputs is that you will then be variably loading the PI itself, and it will experience very heavy AC loads at low volumes. Seems likely to cause some oddities in the taper of the response as you dial down -- probably the all-or-nothing effect with sudden changes in volume occurring over small changes in the pot position, as is seen with tweed style gain knobs.

[Chuck H]

Yes. We've covered this "problem" before. So far though no one has reported on how much of a problem it is. There is a thread around here somewhere in which the OP used the PI outputs to wiper config for an amp with bias vary trem. He said it worked fine. I will try it myself at some point when I have an amp on the bench and time to play around.

[kg]

it would be a problem if it was a hifi amp.

[Chuck H]

Right. Thinking about it... Since the most audible affect of loading the PI outputs would probably be top end roll off that might actually sound good. Especially since most players complain that attenuators make their amp sound thin. If you wanted to preserve the top end to some degree you could always add a capacitor (or a cap in series with a resistor to adjust the effect) to each pot gange from the wiper to the lugs on the grids.