OD trim voltage value

[talbany]

This is kind of how I understand this..Not really being an engineer more of a player so if someone has a different take on it would love to revise...Thanks


Plate resistors setup in a typical D-Style/Fender/Marshall.. act as a voltage divider(Tubes plate/plate resistor)..Preamp tubes amplify voltage, output tubes amplify current..The voltage swing across the plate resistor sets the amount of voltage peak to peak..The higher the resistor the larger the swing..Say your typical 100k plate sets the peak to peak voltage at say 100 volts.. 220K doubles this ... More voltage = more gain but..(To me this is why some people don't like high plate amps on clean channel..Stiffer with less breakup) If your thinking in terms of transistors.. This sets the rails......The cathode resistor sets the bias..more gain =lower resistor.... With the tube bias set for more gain... the more input voltage (grid voltage) you need to hit the rails....Thats why when you up the plate resistor you need to up the cathode resistor since you still want the tube to clip especially in an OD amp...Plot this on a graph= Transconductance curve.. If you set the operating point closer to the point which the tube starts to clip ie 100K/1.5 RP RK.. (places you closer to the outer edges of the curve or operating point).. when enough voltage, is applied to the grid (less voltage needed in this case with the 100k/1.5's) it hit's the rails tube distorts...appealing anomalies emerge... Compression, touch sensitivity..Playing around with this imaginary boundary makes for a more expressive voice (light touch clean..heavy attack more voltage distortion appears)...For me I don't think in terms of gain anymore as much as where I am in the curve..
Generally speaking 100K/1.5 generates 2nd order harmonics.. 180/220k 2.7/3.3 generate 3rd order harmonics..I think and this is my theory 100k plate amps throughout don't sound has harmonically rich as low and high plates combined.. Both 2nd and 3rd order harmonics produced..This is just my theory.. Anyone?

Oh also with respect to tone...Raising the value of the cathode resistor decreases gain...Less gain=less bandwidth.. darker on the top..This is why I think 1.5k amps are inherently brighter!!

Preamp tubes for Dummies..LOL!!

Tony VVT

[groovtubin]

This is kind of how I understand this..Not really being an engineer more of a player so if someone has a different take on it would love to revise...Thanks


Plate resistors setup in a typical D-Style/Fender/Marshall.. act as a voltage divider(Tubes plate/plate resistor)..Preamp tubes amplify voltage, output tubes amplify current..The voltage swing across the plate resistor sets the amount of voltage peak to peak..The higher the resistor the larger the swing..Say your typical 100k plate sets the peak to peak voltage at say 100 volts.. 220K doubles this ... More voltage = more gain but most importantly more headroom..(To me this is why some people don't like high plate amps on clean channel..Stiffer with less breakup) If your thinking in terms of transistors.. This sets the rails......The cathode resistor sets the bias..more gain =lower resistor.... With the tube bias set for more gain... the more input voltage (grid voltage) you need to hit the rails....Thats why when you up the plate resistor you need to up the cathode resistor since you still want the tube to clip especially in an OD amp...Plot this on a graph= Transconductance curve.. If you set the operating point closer to the point which the tube starts to clip ie 100K/1.5 RP RK.. (generally at the outer edges of the curve or operating point).. when enough signal is applied to the grid hit the rails tube distorts... Anomalies associated with this are same as output tubes...Basically..Altering the tone and feel.. Obviously..
For me I don't think in terms of gain anymore as much as where I am in the curve..
Generally speaking 100K/1.5 generates 2nd order harmonics.. 180/220k 2.7/3.3 generate 3rd order harmonics..I think and this is my theory 100k plate amps throughout don't sound has harmonically rich as low and high plates combined.. Both 2nd and 3rd order harmonics produced..This is just my theory.. Anyone?

Preamp tubes for Dummies..LOL!!

Tony VVT

hey man, i`l agree with you! jim@ O$&%^^^ps

[bluesfendermanblues]

larger the swing..Say your typical 100k plate sets the peak to peak voltage at say 100 volts.. 220K doubles this ... More voltage = more gain but most importantly more headroom..Tony VVT

Not true. 220K puts out less output

Generally speaking 100K/1.5 generates 2nd order harmonics.. 180/220k 2.7/3.3 generate 3rd order harmonics..I think and this is my theory 100k plate amps throughout don't sound has harmonically rich as low and high plates combined.. Both 2nd and 3rd order harmonics produced..This is just my theory.. Anyone?
Tony VVT

Don't know about the amount of 2nd and 3rd order harmonics with 220K, but that must be a matter of different phase swing with 220K/150K vs straight 100K

[groovtubin]

larger the swing..Say your typical 100k plate sets the peak to peak voltage at say 100 volts.. 220K doubles this ... More voltage = more gain but most importantly more headroom..Tony VVT

Not true. 220K puts out less output

Generally speaking 100K/1.5 generates 2nd order harmonics.. 180/220k 2.7/3.3 generate 3rd order harmonics..I think and this is my theory 100k plate amps throughout don't sound has harmonically rich as low and high plates combined.. Both 2nd and 3rd order harmonics produced..This is just my theory.. Anyone?
Tony VVT

Don't know about the amount of 2nd and 3rd order harmonics with 220K, but that must be a matter of different phase swing with 220K/150K vs straight 100K

output doesn`t matter when you got the TRICKS to make up for it! Great discussion!

jp

[bluesfendermanblues]

larger the swing..Say your typical 100k plate sets the peak to peak voltage at say 100 volts.. 220K doubles this ... More voltage = more gain but most importantly more headroom..Tony VVT

Not true. 220K puts out less output

Generally speaking 100K/1.5 generates 2nd order harmonics.. 180/220k 2.7/3.3 generate 3rd order harmonics..I think and this is my theory 100k plate amps throughout don't sound has harmonically rich as low and high plates combined.. Both 2nd and 3rd order harmonics produced..This is just my theory.. Anyone?
Tony VVT

Don't know about the amount of 2nd and 3rd order harmonics with 220K, but that must be a matter of different phase swing with 220K/150K vs straight 100K

output doesn`t matter when you got the TRICKS to make up for it! Great discussion!

jp

Which tricks are you referring to??

[talbany]

Not true. 220K puts out less output

By output you mean what?

Don't know about the amount of 2nd and 3rd order harmonics with 220K, but that must be a matter of different phase swing with 220K/150K vs straight 100K

2nd and 3rd order with respect to tubes bias.. (cathode resistor)

The lower value cathode resistor will bias the tube hotter so more current flows and the tube will be able to take hotter input signals before clipping. When it distorts it will have slightly more 2nd order harmonics. A "rounder" tone.
Higher value cathode resistors will lower current flow and the tube will not take as hot of an input before clipping, and will have more 3rd ordered harmonics when distorting. A "sharper" tone that you can distort easier. The higher you go the less gain out of the tube untill you reach a point of unity gain- 1 in 1 out.


Tony VVT

[bluesfendermanblues]

Not true. 220K puts out less output

By output you mean what?

Don't know about the amount of 2nd and 3rd order harmonics with 220K, but that must be a matter of different phase swing with 220K/150K vs straight 100K

2nd and 3rd order with respect to tubes bias.. (cathode resistor)

The lower value cathode resistor will bias the tube hotter so more current flows and the tube will be able to take hotter input signals before clipping. When it distorts it will have slightly more 2nd order harmonics. A "rounder" tone.
Higher value cathode resistors will lower current flow and the tube will not take as hot of an input before clipping, and will have more 3rd ordered harmonics when distorting. A "sharper" tone that you can distort easier. The higher you go the less gain out of the tube untill you reach a point of unity gain- 1 in 1 out.
Tony VVT

So you're saying the more distortion, the more 2nd and 3rd components ?

[heisthl]

removed

[dogears]

Henry, not sure I agree with this. Wouldn't the 220K drop less voltage as it is in series but second in the audio path? According to all formulas, the higher plates have more output.

Layman's explanation - For a voltage amp, it helps if you think of the tube as just another resistor that can be ANY value from infinity to short. The plate resistor is always going to be in series with whatever R value the tube presents. The junction of the tube plate and the plate resistor is where we take the signal. So its just a voltage divider circuit. A 220K is always going to drop more voltage than a 100k so the voltage swing at the output will always be smaller with the 220k resistor.

[butwhatif]

Perhaps an easier way to understand it is by plugging ohm's law into the current produced by the tube 'valve' action, a voltage is 'created' across the plate resistor which interacts with the b+ . This voltage swing (ac component) relative to ground is translated into the output signal by the coupling cap. Easy to see what the plate resistor size does in the equation.

[odourboy]

Layman's explanation - For a voltage amp, it helps if you think of the tube as just another resistor that can be ANY value from infinity to short. The plate resistor is always going to be in series with whatever R value the tube presents. The junction of the tube plate and the plate resistor is where we take the signal. So its just a voltage divider circuit. A 220K is always going to drop more voltage than a 100k so the voltage swing at the output will always be smaller with the 220k resistor.

H2 - I think you're only half right. Your layman's explanation is correct for DC characteristics - the higher plate resistor will drop more voltage, resulting in a lower DC plate voltage. However, the gain factor is higher with the a higher plate resistor (as Dogears has correctly reminded us), so the AC operation is for a larger P-P swing voltage of the output signal with the higher plate value.

[Tonegeek]

Layman's explanation - For a voltage amp, it helps if you think of the tube as just another resistor that can be ANY value from infinity to short. The plate resistor is always going to be in series with whatever R value the tube presents. The junction of the tube plate and the plate resistor is where we take the signal. So its just a voltage divider circuit. A 220K is always going to drop more voltage than a 100k so the voltage swing at the output will always be smaller with the 220k resistor.

H2 - I think you're only half right. Your layman's explanation is correct for DC characteristics - the higher plate resistor will drop more voltage, resulting in a lower DC plate voltage. However, the gain factor is higher with the a higher plate resistor (as Dogears has correctly reminded us), so the AC operation is for a larger P-P swing voltage of the output signal with the higher plate value.

odourboy is on the right track. Its AC we care about. The 220k will produce higher AC output at less current. It is the CHANGE IN VOLTAGE as that grid V swings that is the key here. look at the attached image (thanks to Merlin the Valve Wizard) and you can work out that if the grid V goes from -2 to -1 that there is a larger output swing with the 220k plate R than the 100k plate R. With the 100k there is more DC current used over the same change, but less AC voltage output. If someone is getting higher db down the line with 100k than with 220k then I have to wonder if there is some other explanation besides the plate resistors. It could be that the cathode resistors are shifting the bias point such that the tube is clipping on only on one side of the waveform causing anomalous readings. I would assume though that given each resistor value, that the tube is biased to clip symmetrically, or close to it.

[odourboy]

Yep - what Tonegeek said!

[heisthl]

Layman's explanation (Revised) - For the AC output of a preamp stage, it helps if you think of the tube as just another resistor that can be ANY value from infinity to short. The plate resistor is always going to be in series with whatever R value the tube presents. The AC signal is developed by the current flow through the plate resistor. So the larger the plate the higher the AC voltage (E = I x R). A 220K is always going to drop more voltage than a 100k at the same current flow so the AC signal voltage swing at the output will always be larger with the 220k resistor.

[butwhatif]

That's correct, except I always think of the signal (ac) as a voltage that is created, not 'dropped' . Sometimes it's a little hard to understand initially- but if you think of a resistor with a current going thru it, a voltage is created across it.
This is the effect of the tube w/ bias, with the residual 'noise' ac signal superimposed. As the input causes the control grid to encourage or inhibit electron flow, this voltage increases or lessens, in accordance with the amplification factor of the tube. This is the output that's created, relative to ground by the output blocking cap, 180 deg out of phase. Effect of the the plate resistor size is easy to understand.