Add POST volume control to BF amp- Where?

[notchfox]

For all the amp guru's out there- Where's the best (or only) place to add a post volume control to a typical Fender blackface or 5F6A style amplifier, the amps with the so called long tail pair inverter/splitter? I'm assuming the pre volume control stays as it is .... Looks like it should be simple enough as to where I might think a post control belongs on a split load type ala Tweed Deluxe, but I admit to scratching my head on the later style.

Any thoughts out there? Thanks in advance for some input ....

 

[Wayne Alexander]

If what you're talking about is a post phase inverter master volume, it's just what the title says, it intercepts the wires between the phase inverter and the input grids on the power tubes. There are several variations on this, most involve a dual 500K or dual 1 meg pot , here's one explanation/schematic, applying to Marshalls but the phase inverter/power tube architecture is the same as on the 5F6a and the blackface amps. : (link removed)

 

[Cleeve]

Another crude but easier way is the way most all the matchless amps do it,
as the more correct way on the marshall forum involves a whole lotta wiring and making sure you connect things back to the bias circuit if needed..
The Matchless way could not be simpler- just a single pot and two wires.
http://www.schematicheaven.com/newamps/matchless_clubman.pdf

 

[JohnnyCrash]

That'd be a PPIMV.

It's installed at the phase inverter.

Use a 250k dual pot and two 2Meg resistors. Replace the 220k negative grid leaks at the PI's coupling caps with the pot/resistor combo. Also, the far lug on both of the 250k pots should not go to ground, but instead to the negative bias supply (where the 220k usually meet).

As far as the location for the control, perhaps on the back replacing an unused speaker jack. It'd be a shame to drill a new hole, but sometimes its worth it to get what you want/need out of an amp.

 

[notchfox]

Wow, thanks much for the specific help. I was thinking the post vol would placed ahead of the phase splitter, so I'm way off on that. And where I thought a post vol control on a split load inverter would be installed -before the splitting half- must be way off also.

Looks like a number of ways to get this done. None of these I hope relate to the mostly condemned Silverface amps with their Master Volumes? I think I'm fine with the Marshall page and the Clubman (simple for sure) page. JohhnyCrash- I am a bit confused at the moment where the 2.2M resistors are in the circuit- Are they in parallel with the 250K pots?

What made me think of all of this was some up close hearing/observing a Dr Z EZG 50 (into the matching 4 x 10 cab). I noted the post and pre controls. I don't know that the amp (in the ultimate end sound) was any better than my Blackface Tweed Bassman - with built in single knob reverb- but the whole idea of the pre-post controls seemed like a quite useful addition.

Thanks much again. I'm sure I'll be back with more questions on this one, bear with me as I've never gone this direction ....

 

[fauxsuper]

Blackface Tweed?

 

[JohnnyCrash]

This should give you a basic idea (it looks like it was drawn for a Marshall with EL34's, but the concept is exactly the same):



(link removed)



As you can see the normal 220k grid resistors are completely removed. "E" is the negative bias feed, "C" and "D" are the coupling cap feeds from the PI to the power tubes - this is why a dual pot is needed.

The reason 250k pots and 2Meg resistors are used is for when the pot(s) are turned all the way up the total resistance of 250k in parallel with 2M is 222.22k - pretty much the same as a "stock" 220k resistor (common manufacturing tolerances range from 5% to 10%) in that part of the amp's circuit.

"Pre" is pretty much the old volume which now controls the amount of drive/clean/overdrive and "Post" is now the actual "Volume" which is strangled or fed through the Post control to the power tubes.

I believe the MV Fenders had a 1Meg Master right after the channel summing resistors and just before the PI tube.

Marshall JCM800's and most early MV amps also had a Master before the PI. There's nothing wrong with either style of MV, it mainly depends on the amp/circuit and what you're aiming for... for instance I have a JCM800 build (and a few other builds) with a pre PI MV and a plexi build (and a Vox AC30 and a tweed Bandmaster/Super) with a PPIMV. I also have some amps where I built a VVR into them to adjust how much voltage the tubes' plates get, as well as attenuators and overdrive pedals - they all work well, if paired with the right amp.

 

[Wayne Alexander]

My first post above was more an orientation to the area. The type of PPIMV that Johnny Crash has given you the specifics on - that's the one I'd recommend from personal experience for Marshall, Blackface & Brownface Fenders, and tweed Fenders that have a long tailed phase inverter. I'd recommend the crossline-type that Cleeve recommended (just a pot between the two outputs of the phase inverter) for Vox and Matchless and similar cathode biased EL84 amps.

 

[notchfox]

You guys are great, I mean it. Didn't expect to find this much good info!

I do now understand the 250K with the 2.2M resistors. Earlier this evening I did jumper in the Cleeve/Matchless 1meg pot, it certainly works. I did note the 220K grid values (mine measure around 235K) had dropped to around 190K territory- I didn't measure the (1Meg) pot though. I had thought to step those up to 270K but will hold off until I try the other methods. Super that you've all provided the info to make this pretty painless to experiment with. I hope with the post pot wide open I can still have what it is that hear now.

About the SF style MV- if it was ahead of the inverter this would very much affect the high input impedance of this stage now that I'm thinking about it . Maybe this is why it wasn't so popular ......

Aside to fauxsuper- The amp was a 99 Bassman RI. I rebuilt to a PTP stock 5F6A amp firstly. Then I bought a 64 SR (still had 2 of the original C10R's) and immediately fell in love as it just had such a presence about it. So I modded the Bassman to a blackface circuit and bought a reverb pedal. It was great. But in the end I wanted the reverb in the circuit so I completely disassembled it again, made a new eyelet board, added an additional preamp tube socket and reverb transformer etc etc and ended up with a killer amp. I sold the SR after I felt it was that close. Yea I know ..... But I get pleasure out of building the dam# stuff.

Back to a maze of jumper wires .... Thanks again.

 

[blue56]

1 meg pot across grids of power tubes easiest -- mount it in the old ground switch hole

 

[muchxs]

1 meg pot across grids of power tubes easiest -- mount it in the old ground switch hole

That's a phase cancellation PPIMV. Simple but twitchy. It works different at low preamp volume compared to high preamp volume.