Preamp Sag Dilemma

[zendragon63]

Hey gang. I have been thinking about this for a while, have searched for it and burned up a few MOSFETS trying different ideas but honestly do not have the solid state know how to figure it out. Maybe one of you have already done this. I will try to be as concise as possible.


To control the overall volume, I like the use of a 'VVR' and typically use it only on the power and PI tubes--preceding it with a master volume--because it allow the preamp to operate close to full design voltage; it just sounds the best to me out of several configurations. However because the preamp drop precedes the PA supply, one downside is that as you turn the voltage/current load down on the PA section, the voltage predictably goes up in the preamp section. 


One of the more desirable traits of the traditional tube amp is that the preamp will sag with the rest of the amp--since it is at the end of the power supply string. Adds a certain character to the amplifier responsiveness. So the question is whether or not you can sag the preamp voltage drop by means of a MOSFET(?) regulator based on the change in power tube screen current BUT independent of the PA operating voltage set by the 'VVR'. So, in other words, regulate voltage on one string only as a function of a change in current on another. Hope that make some sense.


Attached is my typical 'VVR' scheme. Where to start, thoughts and critiques welcome--I could be overthinking this one. Thanks to all in advance. Regards.


dennis

[jjasilli]

With 29 views but no Replies, I submit that your concept is unclear.  Do you want the option to drop voltage to the Preamp only, while keeping voltage the same at in the PA?

[92Volts]

You could try bleeding voltage directly into the poweramp, for example if you feed the preamp PS through a 5k resistor from the unregulated HV line, put a 5k resistor after that to the poweramp supply, so preamp voltage is drawn down towards that level (values can obviously be tweaked to your needs).

I don't know if trickling voltage into the "downstream" side of the poweramp regulator could damage it. The current should be nothing compared to what the powertubes draw, but before the tubes warm up or if this amp is run without powertubes? Could be a problem.

There should still be increasing ripple and decreasing average voltage (once ripple is filtered out) at the pre-regulated part of the supply, in response to more current draw by the poweramp. If you shrink the main reservoir capacitor you should see this location and the preamp supply sag, for pretty much the same reasons it would sag in a normal amp. The downside is this limits the available voltage and steadiness when you turn the VVR to its maximum voltage.

[zendragon63]

Yep, maybe this isn't as easy to describe as I would like. Thanks for your thoughts 92volts; I need to think about what you wrote--I usually have to 'empty my teacup' to understand what it is that you guys suggest so as not to mix ideas.

On a traditional power supply string, I will see up to a 50VDC drop on the power supply drop to the PI--and to subsequent drops to the preamp--under load due in largest part to the influence of the PA screens. The goal is to be able to vary the PA/PI voltage via the VVR and still have the influence of varying/sagging the preamp voltage supply because of the change of current on the PA screen.

I have gotten as far as the idea that the PA screen resistor taking the quiescent voltage drop over one of the PA screen resistor to bias a MOSFET(remember that I am very not solid state savvy) to vary a gate of another MOSFET that then would vary the voltage to the preamp section. Tweaking that bias notwithstanding.  See the attached idea.

Heck, there may be another application for such a function if it can be done. Anyway thanks for your thoughts and criques. Regards

dennis

[kagliostro]

May be you are interested to read this

http://el34world.com/Forum/index.php?topic=13340.msg124825#msg124825

may be it will be of some help to you

About your question, if I understand correctly what you mean, you must remember that a VVR have not a stabilized output ant it will SAG

so you can consider something like in Sluckey's proposal, only you are working in a preamp and a PA on the same chassis

Franco

[sluckey]

So you want the preamp B+ to sag when the PA load is heavy, but you don't want the preamp B+ to rise when the PA load is light?

Think about that a minute.

[zendragon63]

Revised a bit....

Almost sluckey...I would like the preamp supply to sag along with the PA, as if it were connected in a traditional string--while still being able to drop the PA/PI voltage via the VVR. That the preamp voltage does not track the same when the load increases is predictable with the preamp drop being in front of the PA/PI.

I have done the VVR for the entire amp but once you drop the voltage/volume to a tolerable practice level, it just gets to be little too muddy IMHO.


The sag in the preamp is not a big thing--in fact it is a small enough difference--but you feel it when you do have it. Slight compression I think. Just was thinking that there would be a way to track it to the sag of the PA screen though it doesn't look like an easy fix. And I just don't understand solid state design parameters as well as I could to make any appreciable headway.

Hey, I really appreciate the comments and questions. kagliostro--thanks for the reference--I will chip away at it. Regards

dennis

[pompeiisneaks]

It makes me think you don't want a VVR in that stage, just a VR.  Meaning that you prefer a specific input voltage to the preamp stage to be pretty much consistent so that even if you're pulling less voltage in the PA, you don't want that excess unused voltage at the PA side to hit the preamp.  This sounds like just a plain old voltage regulator.  I think the problem with that is that a voltage regulator for 2-300 VDC is pretty expensive/rare, maybe a tube could do the job? 

Does something like this fit the bill?

http://diyaudioprojects.com/mirror/members.aol.com/sbench/reg1.html

~PHil

[VMS]

Londonpower has got these sustain kits. Maybe you are looking something like that. Although it does say on the page that it is a subtle effect.


http://www.londonpower.com/sustain-sag-kit-selection

[zendragon63]

Thanks pompeiisneaks and VMS. The way London Power describes it (much better than I) is about the best. Indeed a subtle thing that you just sort of feel. Thanks gang! Regards

dennis