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Old 15th April 2012, 03:13 AM   #31
nazaroo is offline nazaroo  Canada
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Thanks for the Soldano example.

However, I think what we see here is what I talked about earlier in the thread:
Doing the tone shaping BEFORE the power stage.

All he's got on the grids of the Power tubes is 2.2k stoppers.
Nothing really like 'tone control by choice' there.

I certainly didn't deny that distortion/overdrive/compression is carefully crafted in guitar preamps, and apparently Soldano spent a lot of time manipulating grid stoppers in his.

But was talking about a method of selection of OUTPUT tube grid-stopper based on Tube choice (aiming for a flat audio response with low distortion).

At this point, we seem to have gotten sidetracked into a discussion about the merits of capacitor-coupling a driver stage prior to the Output, due to issues with grid-current in positive swings.
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Old 15th April 2012, 03:38 AM   #32
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Yes he is doing the shaping in the preamp stage, it was more for an exercise.

That particular schematic produces bucketloads of high frequency harmonic content, so there is no shortage of treble despite the aggressive measures.

The Peavy Classic 30 has 47K gridstoppers.

I doubt you will hear much of a difference in gridstopper values until you reach the point where it starts to cut into the audible treble range. These are relatively low values, series connected.

Have a look here for some handy math.
The Valve Wizard

The preamp stages have different Miller capacitance therefor need different values. At the end of the day it is the same result, similar levels of RF attenuation despite being different values for preamp vs power tubes.

So just use one high enough to attenuated RF at a desired level.
Go higher if your really going to push the amp dirty or wire like spaghetti.
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Old 17th April 2012, 01:28 AM   #33
nazaroo is offline nazaroo  Canada
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Just as an aside, what do you guys think of adding a rectifier-tube,
not for rectification (I would leave the bridge in), but as a 'pass-tube'
just to drop some voltage and add a bit of sag to the powersupply rail?

I know that sounds counter-productive but if there were some sonic advantages
(could it help in an overdrive situation? and with this PS transformer,
I have too many volts to risk driving it into class B2/cutoff with say 5881s...)

Also, maybe choking out the HV supply with a rectifier tube
would lower risk of excessive screen-grid currents, without resorting to bigger SG resistors,
that might harden the sound and remove the sought-after 'bounce'.

Last edited by nazaroo; 17th April 2012 at 01:32 AM.
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Old 17th April 2012, 01:38 AM   #34
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What, like a passive tube? Not sure what you mean, have the tube rectifier serial connected with a SS bridge?

At the end of the day the ultimate fix for over voltage, is too instal a Variac before the power transformer, and use a separate heater transformer.

Then you can dial it into whatever voltage you want, whenever you want. Even step it up 20% or so, depending on the model.
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Old 17th April 2012, 02:25 AM   #35
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See the 57 Deluxe RI schematic. Not sure how using resistors to drop the voltage will harden the sound though. The tube resistance causes a voltage drop. The resistor's resistance causes a voltage drop.
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Old 17th April 2012, 06:53 AM   #36
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Alvis View Post
What, like a passive tube? Not sure what you mean, have the tube rectifier serial connected with a SS bridge?
Yes. A "pass-tube" meaning, not 'passive', but passing the current to the amp.

Quote:
At the end of the day the ultimate fix for over voltage, is too instal a Variac before the power transformer, and use a separate heater transformer.

Then you can dial it into whatever voltage you want, whenever you want. Even step it up 20% or so, depending on the model.
Well, that works great for your test-bench.
But I have never seen a schematic with a variac built in.
Kinda cute idea though!

But if you're gonna do that presumably for line-adjust,
why not automate it, so the user can't make a booboo?
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Old 17th April 2012, 07:00 AM   #37
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Printer2 View Post
See the 57 Deluxe RI schematic.
Ok I'll have a look.
Do you have a link?

Quote:

Not sure how using resistors to drop the voltage will harden the sound though. The tube resistance causes a voltage drop. The resistor's resistance causes a voltage drop.
In regard to screen resistors,
I've read a lot of internet discussions on it,
and some of them actually had some 'meat', i.e., somebody actually tried different values in a commercial amp or two.

What I read left me with this impression:

(1) Although higher resistors limited screen current, something happened to the guitar 'sound'. SR values higher than 600 ohms made the output stage "too stiff", harsh and non-musical in the judgment of the players.

(2) This was thought at first to be some kind of distortion, but turned out to be the opposite! Specifically, the 'attack' was too sharp, possibly reproducing the dismal performance of the pickups or previous stages. No clear results could be given in regard to the 'decay' of a note, but since that was less important it was less noticable.

(3) The cure was to go back to lower screen resistors, to preserve a 'softer' attack, but then make some other adjustment (presumably fixed bias adjustments or lower B+) to protect or enhance tube life.

I was really interested to find out that this approach (increasing screen resistors) was effective from an engineering standpoint but was disastrous musically.

Another case of solving a problem without regard to the overview and/or preserving some desirable flaw.
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Old 17th April 2012, 12:15 PM   #38
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Quote:
Originally Posted by nazaroo View Post

Well, that works great for your test-bench.
But I have never seen a schematic with a variac built in.
Kinda cute idea though!

But if you're gonna do that presumably for line-adjust,
why not automate it, so the user can't make a booboo?
The main reason is because it is cost prohibitive. The name escapes me at the moment, but there is at least one modern day manufacturer of a boutique guitar amp with a built in Variac.

Also once it's used to dial in the sound for a particular genre, the power transformer can be custom wound to simulate the voltage.

Some nice sound can be had by cutting the line voltage to ~90v from ~120v
Eddie Van Halen - Wikipedia, the free encyclopedia

I suppose a Variac could be modified with mechanical stops or possible by removing windings to limit a users adjustment.
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Old 18th April 2012, 03:07 AM   #39
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Quote:
Originally Posted by nazaroo View Post
Ok I'll have a look.
Do you have a link?
I normally Google when I want it.

Quote:
In regard to screen resistors,
I've read a lot of internet discussions on it,
and some of them actually had some 'meat', i.e., somebody actually tried different values in a commercial amp or two.

What I read left me with this impression:

(1) Although higher resistors limited screen current, something happened to the guitar 'sound'. SR values higher than 600 ohms made the output stage "too stiff", harsh and non-musical in the judgment of the players.

(2) This was thought at first to be some kind of distortion, but turned out to be the opposite! Specifically, the 'attack' was too sharp, possibly reproducing the dismal performance of the pickups or previous stages. No clear results could be given in regard to the 'decay' of a note, but since that was less important it was less noticable.

(3) The cure was to go back to lower screen resistors, to preserve a 'softer' attack, but then make some other adjustment (presumably fixed bias adjustments or lower B+) to protect or enhance tube life.

I was really interested to find out that this approach (increasing screen resistors) was effective from an engineering standpoint but was disastrous musically.

Another case of solving a problem without regard to the overview and/or preserving some desirable flaw.
The screen resistors limit the current to the screens and more importantly reduce the voltage on them so when the plates drag down the voltage on the OT the screens do not become higher than the plates. Since you are running ultralinear the voltage on the screens drop more less in relation to what the voltage to the plates are doing. At least from my understanding, I do not know enough about ultralinear operation to design an output stage.

On regular guitar amps screen resistors are a compromise. They protect the output tubes from the likes of tube eating Marshalls.


Oh, and not that I know much about it also, but larger screen resistors would cause the voltage on the screens to drop more than lower resistors. And the gain of the tube is less when the screens are run at a lower voltage which would cause me to believe that the larger screen resistors would cause a compression effect.

Last edited by Printer2; 18th April 2012 at 03:11 AM.
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Old 20th April 2012, 07:57 AM   #40
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Alvis View Post
...

Some nice sound can be had by cutting the line voltage to ~90v from ~120v
Eddie Van Halen - Wikipedia, the free encyclopedia
I've been told repeatedly that this was one of Van Halen's (off the cuff interview) pranks.

That is, he really didn't use a variac,
unless perhaps experimentally in studio once, and its not a part of his 'sound'.

Those running out to try this ought to take serious heed of the obvious warnings, such as:

(1) A variac on most amps would also drop the heater voltage,
and this directly contributes to tube death.


So much so that RCA did a long comprehensive study
and warned designers not to vary the heater voltage
from recommended values more than 4%:
LOWER VOLTAGES especially KILL TUBES QUICKLY.

Click the image to open in full size.

With 85% of the rated voltage on a tube, it goes from a 5,000-hour+ tube to a 3-hour tube!.
Letting your tech experiment with a variac on your amp for a couple of hours will cost you a whole set of tubes within weeks of installation!

Don't let the heater voltage on your tube drop below 96% for any extended length of time:

12.6 v heater (signaltubes): greater than 12.1 volts or else! (Never use 12 volt regulators (7812) without resistor adjusts)

6.3 v heater (powertubes): greater than 6.05 volts or else! (Never use 6 volt regulators (7806) without resistor adjusts)


Thus rather than a Variac, for most amps, we really want an ANTI-Variac (surge/voltage regulator) that provides correct heater voltage regardless of MAINS or LINE INPUT.
To do the opposite is to destroy tubes, drastically shorten tube life, and is only sensible if you are an idiot millionaire.

"cutting the line voltage to ~90v from ~120v" may give you some 'nice sound',
but it would cut your heater voltages to about 75%,
and for any length of time, cut your tube-life expectancy to only a few hours!

You can get the same results without destroying your tubes several other ways (which we'll explore later)

Last edited by nazaroo; 20th April 2012 at 08:13 AM.
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