Master Volume

[W5FH]

I realize there are volumes of information regarding the different master volume circuits, including the PPIMV ones. I have been studying all of this until my head is just about swimming in it.
     My question is this: There is some tone interaction when changing master volume settings. Attached is a crude drawing of a master volume circuit I used in a recent build. The 100K series resistor from MV pot wiper has not been included yet, I just now read about using it. The tone stack and master volume circuit is from the JCA2112RC (correct version) schematic. I am asking if it is best that I just add the 100K resistor which should reduce interaction some and live with remaining interaction? I have studied the various PPIMV circuits, with great interest in the Lar-Mar version, but am afraid there will be too much affect on presence control and possibly tone.
Thanks,
Byron

[sluckey]

What kind of tone interaction are you experiencing? Be specific. I don't see how the 100K is gonna change interaction.

[pdf64]

As I see it, the concept behind the 100k resistor might be a bodge. Parasitic capacitance effectively adds a bright cap to high impedance circuits.
By quartering the impedance, a 220-250k master volume pot will reduce the effect of such capacitance by 4.
Which should be significant.
So I suggest to try getting rid of the 330k resistor and change the master pot to 220k.

[W5FH]

Thank you both for the responses -
    I have attached a schematic of the preamp section of the amp I cloned, the JCA2112RC. This is the CORRECT schematic, I searched a LONG time to find it. It is a copy of the Soldano Astroverb. All of the Astroverb schematics are grossly in error and most of the JCA2112 copies are in serious error too. I built my clone using only the preamp as shown here. I used the old Fender parallel 12AT7 reverb tank driver with upgraded transformer (OT3SE USA 25K to 8 ohm) to replace this one shown on schematic. A fixed bias Hiwatt style phase inverter and traditional P-P 5881 output stage with slightly oversized output transformer (APD8025BH) finished it out.
    Sluckey asked about tone interaction of master volume - when MV reduced to low settings the treble would shift requiring treble pot adjustment. That is what I want to reduce. Also Sluckey when reviewing this schematic would the master volume wiper be a good place to insert to Trem-O-Nator intensity pot? I plan to use a 250K pot and leave the one end ungrounded.
    Thank you for your suggestions pdf64, I am open to your suggestions esp after viewing this schematic.
Thanks!

[pdf64]

It won’t hurt anything to try the 100k.
But my finding is that they don’t do anything I can hear.
If the tone gets brighter at low master settings, it may be more of a layout issue.
Photos of your build?

[sluckey]

when MV reduced to low settings the treble would shift requiring treble pot adjustment.

If this means you have to increase treble at low MV settings then put a bright cap on the MV. Connect between mV hot lug and wiper. Try a 120pF.

would the master volume wiper be a good place to insert to Trem-O-Nator intensity pot?

yes

[dude]

I've had issues with a PPIMV with a 6V6 PLexi, loosing highs when turned down.  Sluckey's suggestion of a bright cap on the MV solved my issues, in my case a gang pot, needed two bright caps, yours is a single pot . I used 120pf 

[PRR]

...I have attached a schematic of the preamp section of the amp I cloned, the JCA2112RC. This is the CORRECT schematic,..

That's the first here which makes sense. The reverb returns to a plate-mixer V2b V3a both on R44. My gut says a safe first bet for R43 is much lower, like 400 ohms; but Soldano is a known tuner and the "too high" R43 may add just the right spice.

The 'mf' caps would be 'μF' (aka uF) in modern notation. (In the past, milliFarad caps were absurd, but today they show a lot in welders and transistor amps.)

With four stages of gain in the main path (and side-gain in reverb path), the layout may be critical.

Would be nice to know tube-types.

[W5FH]

Thanks everyone!
   The JCA2112RC schematic I posted has all 12AX7 tubes. I copied this preamp into my build, except I used a different reverb driver (parallel 12AT7 old standard circuit). I posted pictures of it here about a month back, I believe I called it an Astroverb clone. I used the Hiwatt fixed bias 12AX7 phase inverter as shown in the tutorial on the Tube Amp Books site. I followed that with a push-pull pair of USA Tungsol 5881's much the same as in the Fender Hot Rod Deville ML212 clone I built last year. Tried to get as clean as possible P/I and output stage.
   This amp has worked out very well. It is very quiet. It does just what the guitarist wanted, he is in love with the tone. He owned an Astroverb in the past but did not like the EL84's. This one was built on a Deluxe Reverb chassis made about 1/2" deeper front-back. Housed in DR combo cabinet with Celestion Neo Creamback speaker. I tried out a Revisit reverb tank in this one, same part number as in Astroverb, a 9" 3 spring medium decay.
    Used polyester film caps for 1 uFD cathode bypasses but in next build will try polypropylene film.
Thanks all suggestions on reducing treble/MV interaction. Based on all this what is your final suggestion on this?
Thank you!
   

[pdf64]

The 1M control value will cause the frequency response to change over the range of the pot rotation. Doubly so when interacting with the Miller capacitance if the following grid.
Hence I think 1M volume pots are pretty unusual in valve hifi.

[W5FH]

An update on this project:

1. Removed the 220K/330K voltage divider between wiper of treble control (preamp output) and master volume top end. Used a 22K resistor in its place, connecting the treble wiper to master volume top end. Placed a 56 pF bright cap across MV pot. These changes resulted in much more volume and pretty much elinated the loss of treble as MV reduced.

2. Improved the Sluckey Tem-O-Nator by using a 250K RA intensity pot as a variable resistor between MV wiper and VTL5-C1 (resistance cell has one end grounded). Therefore no loss of signal to ground through intensity pot. Pretty deep, sounds good.

3. Using film caps for the 1 uFd cathode bypasses (3 places) helps tone in my opinion. I used polyester.

4. A very noticeable overall improvement in tone/character of amp was noticed after these further changes -
       a) 1 ufd cathode bypasses changed to 1.25 ufd CDE WMF polyester 10%. It is believed this change very slightly improved bass without muddying up crunch tone.
       b) 100 ufd active mixer cathode bypass was originally a high grade electrolytic. Replaced it with 3 film capacitors, giving in sum 30 ufd of polypropylene, 10 ufd of polyester and 10 ufd of polycarbonate. These were what I had on hand, as an experiment. Made a very noticeable change. In fact, I did two of the Astroverb clone builds back to back, they were identical with identical components throughout. We would make changes on one, then compare to the other, then make same changes on other one bringing it up to very similar tone. It was very noticeable playing the one with film caps (replacing active mixer cathode bypass) compared to playing the one with 100 ufd electrolytic. When we replaced 100 ufd electrolytic with similar film caps in second one it improved tone very close to first one.

5. I used the JB JFX metallized polypropylene film caps throughout both builds. As an experiment we replaced all preamp tone caps (not past preamp) with SoZo Mustard caps (polyester film). A slightly more desirable tone resulted in opinion of guitarist.

6. The HiWatt phase inverter (12AX7 circuit) presence control compliments this preamp. It is a high boost/high cut circuit. This is the fixed bias P/I described at Amp Books site.

7. Having the 5881 output section following this P/I adds further to the amp. The preamp has a very clean tone if volume is kept below 3.5 setting. The amp is fairly loud up to this point and has more clean headroom than original Astroverb.