late 70s AC30 master volume mod

[boroman]

My bandmate has Rose Morris-era AC30 (I believe it's w/o rectifier tube) like schematic below. As shows getting smaller and we need nicer sound on lower volumes, master vol. mod came to my mind. I've read that the best would be simple modding CUT by cutting cap across it and it then behaves like PPMIV. Okay. That is easy to try, but if we want to leave CUT pot like original, can I just wire 1M pot in seriers after CUT pot? So I'd have post-phase inverter, post-CUT master volume?

[Jon Snell]

Between the two arrows add a 250k linear potentiometer and a 4k7 in series. Similar to the treb cut but with 4k7 instead of capacitor.

[drewl]

I tried that and it does work fine.

What I think sounds better is to replace the two 470k grid resistors that go from the output tube inputs to ground with a dual 500k pot.

I installed it on both my AC30's and Marshall 18w clone I built.

[Jon Snell]

If you use a dual 500k pot, if there is an imbalance with one of the output valve grids, there could be DC on the pot and that will cause crackle as the pot is adjusted. DC pfset and pots, don't realy get on.

[pdf64]

If you use a dual 500k pot, if there is an imbalance with one of the output valve grids, there could be DC on the pot and that will cause crackle as the pot is adjusted. DC pfset and pots, don't realy get on.

It that a hypothesis of a potential issue, or one you’ve actually encountered? It’s just that unless a valve / coupling cap / pot was bad, I can’t envisage how it (non negligible control grid voltage) would occur?
By ‘bad valve’, I was thinking of one having excessive grid current (which then creates a voltage across the grid leak resistor). Which given the very high dissipation level in an AC30, is quite possible.
Safety back up resistors to maintain a grid leakage path in case the pot goes bad, as per the LarMar type 2 master volume variant, seem a good idea.

[Jon Snell]

It that a hypothesis of a potential issue, or one you’ve actually encountered? It’s just that unless a valve / coupling cap / pot was bad, I can’t envisage how it (non negligible control grid voltage) would occur?
By ‘bad valve’, I was thinking of one having excessive grid current (which then creates a voltage across the grid leak resistor). Which given the very high dissipation level in an AC30, is quite possible.
Safety back up resistors to maintain a grid leakage path in case the pot goes bad, as per the LarMar type 2 master volume variant, seem a good idea.

Good point. This could be hypothetical.
I have encountered up to 3volts DC floating on the control grid of the output stage with a +ve 16 volts across a very hot cathode resistor. Replacing the faulty valve normally cures this problem.
Being temporary, this is probably not an issue with the wiper of the pot to ground but I would prefer a coupling capacitor between each wiper and the grid with, as you suggested a grid leak resistor, if you use your aproach. An advantage with a coupling capacitor is, if a valve fails after being run on its limits for a while, there are less components to replace with the flashover damage.
Grid leak resistors are cheap, decent pots are not.
Using the pot between the grids, as opposed with grid to ground, ensures minimal damage if a valve fails catastrophically.
Seems more of a logical good design aproach to me. My logic could well be wrong though.

[pdf64]

It depends on the application really.
A type 3 (crossline) master can be fine for trimming down the amp’s on-stage sound level. But a basic one is useful for achieving domestic sound levels, can’t even manage a 20dB reduction usually, as they cause the LTP to flip into common mode and the balanced signal mutes. Your 4k7 series resistor should prevent that
Another option is a pair of series resistors between the LTP and the type 3, as per recent Vox models.
That can enable fine control down to whisper quiet levels.

[drewl]

I haven't encountered that problem.

FWIW I'm talking about the two grid resistors that go to ground, replace those with a dual pot.

Since the amp is cathode biased there's no bias voltage at that point.

I've installed several "Lar-Mar" type masters in other amps, installing the 'safety" resistors in case the dual pot fails.
actually on a few of my amps I have two of those masters installed that you can footswitch between for different volume levels.

[pdf64]

… Since the amp is cathode biased there's no bias voltage at that point.….

Bias is the voltage differential between control grid and cathode.
With cathode biased stages, if the grid loses its reference to 0V common, the DC level of the grid can float off, bias becomes undefined, redplating becomes likely.

[Helmholtz]

With cathode biased stages, if the grid loses its reference to 0V common..., redplating becomes likely.

Is this based on real experience?

Just curious because with a good non-gassy tube I would expect the floating grid to charge negative and lower plate current.
MIght be wrong, though.

[pdf64]

Is this based on real experience?

…

Yes, embarrassingly it happened on an early build of mine, then it crops up every so often on one forum or another, eg “my 5E3 clone is redplating, what gives?” type threads, where it eventually transpires that the 220k grid leaks haven’t had their node connected to 0V common.

Dunno why, or how it might work in the case of grid leak biasing, but with output stages, grid current generally seems to oppose bias voltage.

[Chuck H]

I'm thinking that reducing the "safety resistors" and making the dual pot of a higher value might mitigate differentials. Like a dual 500k pot and a pair of 390k grid leaks (safety resistors). A net grid load of 219k but much less differential in the event of tube set differences. The only rub would be how the the additional series resistance might affect tone at low master volume settings. But, of course, you're already compromising. Right?

[diydidi]

Use this LARMAR PPIMV.

[Chuck H]

Use this LARMAR PPIMV.

File is too small to see

[diydidi]

I believe here is the link.
http://el34world.com/Forum/index.php?topic=3496.0