Variable voltage regulator in SE amplifier question

[jackplug1]

I'm currently building a single ended plexi similar to the SEPLEX project but with a VVR to control B+1 and B+2 on output tube 6V6 and also B+3 V2b 12ax7
however I would like an opinion on whether V2b should be left out of the VVR circuit and left to operate at the standard high voltage or is it advantagous to include it.
I noted that the 18watt LiteIIb with VVR (PA&PI) phase splitter is included in the VVR circuit.

https://chasingtone.com/the-seplex/

[jojokeo]

Greetings,


I suggest only the plate and screen on the 6V6. As you control the voltages downward, G1 becomes more and more sensitive. This is more than enough to achieve the desired result you're looking for. I speak from experience and the result is the same whether I have a 6V6, 6L6, or EL34 plugged in.

[jackplug1]

The reason I asked is that V2b cathode is connected to the NFB loop

[jojokeo]

I don't think it matters like the grounding area of the tail does because of the feedback loop. It's a very easy fix either way in any case, so no need to waste time thinking about it.

[jackplug1]

I don't think it matters like the grounding area of the tail does because of the feedback loop. It's a very easy fix either way in any case, so no need to waste time thinking about it.

Okay - thanks

[jackplug1]

Greetings,


I suggest only the plate and screen on the 6V6. As you control the voltages downward, G1 becomes more and more sensitive. This is more than enough to achieve the desired result you're looking for. I speak from experience and the result is the same whether I have a 6V6, 6L6, or EL34 plugged in.

By experimentation I can confirm that you are indeed correct - thanks

[kagliostro]

As far as I can know, on PP amps the preferred way is to lower only the Power Tubes B+ and, may be, to add a PPIMV, this way the PI can push hard the Power Tubes and be under control (via the PPIMV)

In an SE amp you have only preamp tube and Power Tube, so, if you want to push it hard to obtain some sonority at low power is better you don't low the preamp tube B+, only Power Tube B+ is to be lowered to achieve this result

One option to obtain the same result (if you have a low power Mosfet on the VVR) is to lower only G2 voltage and feed the Plate of the Power Tube at full voltage

Something similar to the SAG effect of the PP amps in an SE can be achieved putting a "high value" resistor in series with G2 (in addition to the standard G2 resistor of the project) and a switch in parallel with the "high value" resistor to bypass this added resistor when the resulting effect is not wanted

Franco

[jojokeo]

Franco,


How about getting SAG in a PP amp? I tried a number of things searching for SAG a while ago in a PP amp but had no luck and went a different direction. But I'd like to revisit this and try again at some point? Is screen voltage lowering the trick to acquiring SAG in a PP amp? (without weakening the power filter supply side)

[kagliostro]

The effect will be very close

Mesa use that resistor + switch

Franco

[HotBluePlates]

How about getting SAG in a PP amp? ... Is screen voltage lowering the trick to acquiring SAG in a PP amp? ...

High Resistance is the key for sag.  You don't want "lower screen voltage" but want "voltage to lower A LOT when current increases with signal."

You could do it at the screen, but you could also do it for the entire amp (output tube plate current variation will be a lot more than screen current variation).

The methods that use a MOSFET greatly increase the FET's resistance so that current variation cause a lot of voltage-variation.

[kagliostro]

High Resistance is the key for sag.  You don't want "lower screen voltage" but want "voltage to lower A LOT when current increases with signal."

This is what I was talking about

The methods that use a MOSFET greatly increase the FET's resistance so that current variation cause a lot of voltage-variation.

About this I was thinking to the MB Mark V but seems I was remembering badly

the circuit purpose on the Mark V seems to be different
(some time ago I know it but now I don't remember)



Franco

[jojokeo]

I know I tried adding resistance on a switch prior  to both plate and screens separately but neither worked barely if at all so I abandoned ship on it. It’s been about 1.5 years ago so not sure of I recording things on it? Usually I do, crossing my fingers i can find design notes?


I was using an EF86 and made the changes in gain to it instead.

[jojokeo]

I know I tried adding resistance on a switch prior  to both plate and screens separately but neither worked barely if at all so I abandoned ship on it. It’s been about 1.5 years ago so not sure of I recorded things on it? Usually I do, crossing my fingers i can find design notes?


I was using an EF86 and made the changes in gain to it instead.

[kagliostro]

Try only with G2, standard resistor plus resistor paralleled with switch, try with differenti (added) resistor values (Mesa used 2.7K resistors)

If the resistor value isn't enough high, no effect

Franco

[HotBluePlates]

I know I tried adding resistance on a switch prior  to both plate and screens separately but neither worked barely if at all ... I was using an EF86 ...

We're describing output tube hacks, not preamp tube.

Look at Page 4 of the 6L6GC data sheet.  Screen Current (dashed line) spikes up at the left end of the graph, where we would have a loadline touching peak plate current.  So when played LOUD the screen has a moment when its current gets very-large.

Now we stick a 3-4kΩ resistor on that screen.  Voltage stays mostly-steady except the moment when the 6L6 is trying to touch maximum output power.

    - Screen current spikes up
    - Voltage-drop across the 4kΩ is large at this instant
    - Screen voltage drops A LOT, which pulls back plate current.

We get compression, but only when playing the amp very loud.  "How Loud" and "How much compression" depend on your choice of screen resistance.


For a preamp tube you'd do something very different for compression.

    - Half-wave rectify the audio signal, use that to charge a cap to a negative voltage.
    - Apply that negative voltage so it adds to some preamp stage's bias, turning that tube more-Off.
    - R and C in the circuit determine Attack & Release times (really, "Cap Charge" and "Cap Discharge" time).

[kagliostro]

In my archive I've find this



Franco