The Plate Resistor.......more = compression, less = "Fendery"

[Mook]

Ok.....this may be well known amoungst you guys, but I'm come to determine that the plate resistor value(s) is what determines the "compression" level of the amp.    When I plug into an amp, and just play a few notes, you can tell what the plate resistor values are set at.

A Fendery sound = 100K values are used.

A more compressed sound = values ABOVE 100K (150, 180, 220, +++), and this compressed sound is what contributes to the "Dumble" sound.     

Case in point, I build the 97 Dumble schematic but used the usual 220K (or 180K in some places) and I did give that Dumble "tone".   But, I changed them to 100Ks to see the difference.    It still had gain, but the "compression" was gone.     

Yes, the higher the plate resistor value, the more "gain", but also, the more compression.


Discuss?


Mook

[Fresh_Start]

Plate resistor doesn't function in a vacuum (pun intended).  Plate voltage and cathode resistor - or resistor & cap in parallel - also are key variables in basing a tube.

220k plate over 2.7k cathode resistor isn't typical Fender, but it's not Dumble either.

Cheers,
Chip

[HotBluePlates]

... I'm come to determine that the plate resistor value(s) is what determines the "compression" level of the amp.   ...

...  Plate voltage and cathode resistor - or resistor & cap in parallel - also are key variables in basing a tube. ...

To help you see what Chip is talking about, please tell us the plate resistor/cathode resistor combinations you listened to, and the (at least approximate) supply voltage.

Plotting those out on a set of curves may prove instructional.

[Mook]

Yes, I understand the cathode resistor helps set the bias and overall "range" of the tube.     I'm just saying, if I plug into an amp with 100K/1.5K values, it sounds Fendery.    If the Plate is 220K and the cathode is, let's say, 3.3K, it sounds more compressed -- every note has a distinct cut off, whereas with the 100K plate resistor, the tone has a decay, not a cut off.

I'm saying I can hear a different in 100K vs. 220K plate resistors.


Mook

[jazbo8]

It's more likely that you are hearing the the cathode resistor, 1.5k is the typical mid-bias and the 3.3k would be considered cold-bias by most, i.e., it cuts off sooner.

[jjasilli]

+1 to Freshstart & Hotblue. 


I have used 220K plate resistors with 2.7K and 3.3K cathode resistors in a Princeton & VibroChamp.  Sounds totally Fendery to me.


The Dumble OD channel has cascading gain stages in the preamp, with lots of oomph to also overdrive the PI, then the power tubes. 

[MFowler]

Review this gain chart.

[terminalgs]

I'm saying I can hear a different in 100K vs. 220K plate resistors.

what is the rest of the circuit?


On some circuits, Fender used a DC coupled CF to drive the tone stack.  The  Bassman 5F6A used a 100K plate, and a 100K cathode under the CF. The Bassman 6G6 used a 220K plate and 100K cathode under the CF.  Matching those two resistors, like the 5F6A is the most common approach, but using a smaller cathode R under the CF will result in more compression of the audio signal.  See Vox Tone-boost circuit too.


If it isn't a DC coupled CF, then plate resistor alone, I don't think will compress any part of the signal.  What is the load impedance that the AC signal sees after the triode (call it "next")?   Changing the plate R, changes the out Z of the triode.  If "next" is a volume pot to feed another stage, it's input Z is fixed.  If "next" is a tone-stack, it's input impedance will be frequency dependent.   maybe that's the subtly you hear?  the ability of the triode to drive the input Z of "next"?

[tubeswell]

... the plate resistor value(s) is what determines the "compression" level of the amp.    When I plug into an amp, and just play a few notes, you can tell what the plate resistor values are set at.

A Fendery sound = 100K values are used.

A more compressed sound = values ABOVE 100K (150, 180, 220, +++), and this compressed sound is what contributes to the "Dumble" sound.     
...
Yes, the higher the plate resistor value, the more "gain", but also, the more compression.

Discuss?

Compression isn't simply a matter of the plate load. It also depends on whereabouts the bias point is along the load line, the supply voltage, the tube's transconductance and the strength of the input signal. A centre-biased tube with a big input signal and a bigger plate load will compress more at both peaks of the signal than a warm or cold biased tube. A centre-biased tube with a high supply voltage and a high load that is only being fed a small input signal at the start of the signal path, may not compress as much as a tube with a lower plate load that is being fed a larger signal further along the signal path. And so on.

[2deaf]

He may be talking about the effect of increased cathode-grid current with larger plate resistors.  The rounding-off of the signal in early stages can sound like compression.

[Mook]

He may be talking about the effect of increased cathode-grid current with larger plate resistors.  The rounding-off of the signal in early stages can sound like compression.

Yes....this is what I'm trying to get across.     Bingo.    THere is a distinct "compression" with higher values of plate resistor.