Who fights the power that fights the power?

[jfutral]

I want to read up some on the power supply side of tube amps. I've read some people talk about higher voltages in pre-amp tubes having a different overdrive tone than lower voltages, etc. Interested in finding out more about this.

What got me thinking about this is my current build. I'm ignorantly happy with the voltages on the power tube side of things (two 6v6s) and PI (ax7), but the pre-amp side of things seem kind of low. This is a PPIMV build and I seem to be getting a bit of a raspy over-drive sound and I would like to smooth that out some. I was wondering about the voltage and the affect that has. I was wondering how to keep the voltage on the 6v6s about the same, or even lower, but boosting the voltages on the preamp side. Not that will change things, but I just don't know.

I am not exactly looking for advice on this specific build, though. This question has me wondering about the considerations in designing the B+ structure in general. Why do certain amps use higher value capacitors? Why do some amps use 10k or 22k resistors between stages? Why do some have different values? What is the effect of changing the value? Why don't amps jump all the tubes off the same point? Why separate stages? Why does one note follow another?

Is any of this, from a vintage amp stand point, mostly mathematics or tone folk lore?

Does TUT vol 1 cover any of this? Just curious, I recently ordered the book. Anywhere online talk about this?

Thanks,
Joe

[Jazz P Bass]

Most of the power supply data comes directly from the individual tube datasheets.
The preamp tubes are basically voltage amplifiers.
They do not require high plate voltages.
Now the output tubes are Power Tubes.
ie: Voltage times current.
Now you need high voltage.
If you look at a lot of different schematics you will find that most designs adhere to specific rules.
Again, these rules where developed from the tube datasheets.
(well grounded theory helps)

[jfutral]

So voltage levels on the pre-amp stages don't affect the tone?

The resistors separating the stages are strictly about affecting voltage levels to be within a certain range for each stage as per data sheets?

Joe

[riz]

Hopefully I can be some small help here... First, your post instantly made me think of one of the books I'm reading, Merlin Blencowe's new power supply book. Maybe not exactly what you're looking for but I'm learning a ton... Second, without a schematic, it will be hard for anyone here to give advice on what to do about a "raspy overdrive" sound. There's a lot of things besides voltage that can influence that. I'm learning from my tinkering, trying things I'm absorbing from threads here, and books I'm reading. Voltage definitely has a role to play in the tone, and I had a big discovery the other day working on another in a seemingly endless stream of Champs that have been coming my way. But I can't say that lowering the B+ voltage actually did more for the sound than the screen resistor I added... This Champ now sounds sweet, and I'm going back into my own Champ next to implement some of my new knowledge, to get rid of rasp, and I WON'T be lowering the preamp voltages specifically. Just sayin'... I'm focusing on the power supply and output section more than anything, including bias. I think a very broad generalization would be low voltages="brown", higher voltages=big clean. "Rasp" could very well have nothing to do with it.

[Chuck H]

Pretty much. But if you ask me there is a significant enough difference in tone that it's worth considering. It's tricky though because circuit EQ and loading can make for a good amp with both lowish and highish preamp volts. The difference I notice is that higher volts on the preamp tend to be more clear in the high and low end with a basically cleaner sound AND more gain, but... What happens now when you OD the next stage??? This can be a good or bad thing depending on many variables. High preamp volts can make for a nice, dynamic clean tone but tricky to voice for OD. I tend to use higher preamp volts. Most popular high gainers seem to be in the 150V to 170V range. IMHO I have noticed that preamp voltages below about 130Vp sound messy and grungy to my ears. Not only is there a predominant mid character (brown sound?) but it's not good smooth mids. Sort of choppy and sloppy.

Carefull adjustment of the HV rail can usually get what you want. Don't be afraid to raise or lower a resistor value. You won't know what works for you or a specific design until you try.

[d95err]

Does TUT vol 1 cover any of this? Just curious, I recently ordered the book. Anywhere online talk about this?

TUT1 has a chapter on power supplies. Hopefully it covers some of what you want to know. If you want to discuss the TUT series, you can always try Kevin O'Connor's own forum at Untitled Document. Kevin answers many posts there himself.

[jfutral]

The search to clear up the rasp is really only mentioned as the inspiration to learn more about power supply. I do understand there are all sorts of things that could be affecting that. This is analogue. Everything affects everything. My issue could easily be my John Cage approach to build with found parts as much as possible (until I get it in my head I want to achieve something specific, like my post-PI MV). And since this is a post-PI MV, I figure the voltages on the pre-amp and PI stages will have a greater impact than a non-MV cranked, that is assuming voltages can have an affect. But I am open to being wrong about that. I mean, I suppose gain is gain regardless if it is achieved with 150v or 190v.

So I started pouring over schematics in my Vintage Tube Amp book looking at how different amps powered each stage. Mostly I was wondering about raising the voltage on the preamp stage, so I wondered about dropping the resistor value from 10k to 5k if that would raise the voltage to that stage. But I also recall when biasing any tube and changing the voltages,there seems to be an affect across the board, everyone's voltages gets tweaked to some degree. So I was wondering if that was why the rail is broken up into stages, to minimize the affect elsewhere.

I also noticed that if the value of the resistors were different, they were always lower between the output and the PI stages. A 5k might be between the output and PI and the higher value was always after the PI. So I was curious if there was some principle at play there.

This is my 5th build. And in addition to my John Cage approach to parts, I also take a Frankenstein approach to structure. but in doing so I am accepting that the original designs worked at a component level (output/PI/preamp). But this VOX top-boost pre-amp in an otherwise Fender-ish build seems to be messing around with me. So I guess I am looking more at the fundamentals at play. What affect is there to a basic Fender power supply chain to a Vox stage and why? Which then makes me ask why Vox did things the way they did and Fender the way they did? Which leads me to What am I trying to understand? And what of the affect of Derrida?

Joe

[GregS]

The effect of reducing the value of dropping resistor(s) in the power supply to pre-amp stage(s) will not only increase the supply voltage at the pre-amp nodes of the power supply and the voltage at the pates of the pre-amp tubes, but will also shift the bias voltage making the grid more negative relative to the cathode. The bias shift will affect tone and the increased supply voltage will increase the headroom of the stage.

[jfutral]

In my purely intuitive examination (I'm half-asian, so my wife says that is in my nature regarding technology), I would imagine the series resistance also has an affect on sag, at least at the pre-amp stage. I also imagine this is the reason for the capacitors along the way, to help with or adjust for that. Or am I smoking oregano?

Joe

[Jazz P Bass]

I have not heard the term "sag" mentioned before, as concerns the preamp.
When you setup a tube stage, there are a number of considerations at play.
The tube has to be biased.
That is the role of the cathode resistor.(in concert with the plate resistor)
Together with the tube type, you get amplification (gain).
If you change the overall voltage (up or down) going to the preamp, all of these design values change.
The power supply resistors are not picked willy nilly.
The preamp itself is a design.
The capacitors are for smoothing out the pulsating preamp voltage, in stages.
They decouple one stage from the next.
Each tube , when it amplifies, is pulling current from the power supply in a sinusoidal manner.
Hense the need for the capacitors.

[Chuck H]

Since preamp tubes are used as voltage gain amplifiers and not current sources there isn't much sag. If you have two or three 12ax7's in there they pull, what?, about 7mA of current through the resistor (not much). And since the preamp stages are class A self biased that 7mA doesn't shift much. So what's to sag if there's no (or very little) current change? But since the power tubes do sag voltage considerably on the same rail there is a corresponding voltage drop in the preamp.

[riz]

And in the process of learning about the power supply, you may discover some connections you had never realized before, that's all I'm sayin'! How, in the amp system, all these voltages relate to each other, and to what you HEAR coming out of it. It's a never-ending process. Have fun, sounds like you are, and I think we're more or less on the same page! Let us have some fun, too, got a schematic?

[jfutral]

Great things I'm learning here. Thanks for the responses. The questions that come to mind at this point based on the comments thus far:

So the compression effect, even in a purely ax7 effect like a BK Butler Tube Driver, is purely inherent to the tube and not induced (though probably enhanced) by sag?

I am sure the resistor values are not chosen willy nilly (which is why I haven't willy nilly changed them... yet). The question I am asking is how are the values chosen? Purely on how much voltage they drop? In relation to the other resistors? In relation to the capacitor?

Why decouple the stages? Only to adjust voltages along the chain?

Capacitors are also, essentially and effectively, batteries. They store power. Does this not benefit the stages they supply as well as smoothing any pulses. Probably not just from the pre-amp, but also from the output tubes? I imagine this works in both directions.

Joe

[jfutral]

Application is how I learn best. I have a hard time just understanding formulas. 1+1=2 has little meaning to me until I learn 1 what, and why do I need two anyway? Learning what I've simply taken for granted, that all those parts meant something to someone at some point and that was enough, is not enough now. Crap! But it is fun. I don't have an updated schematic, but I can get it together probably later today, now that I am in-between shows.

It's all about the tone!

Joe

[jfutral]

Schematic

This is the latest iteration with the new PT installed. And it is much quieter than the last iteration (although I still want to rework the heater leads somewhat. Going from 12.6v to 6.3 is a bit different since I don't run the 12ax7s' heaters in series, now.)

Joe

edit to add: The 6v6s are EHs.