why the higher plate loads on Dumbles?

[pula58]

Why the higher (than 100K) plate load R's in most Dumble schematics (in the first ax7 tube. i.e. pre and post Tone-stack) I have seen. For more gain? What does this do to the tone?

Thanks.

P.

[David Root]

Historically, HAD was going with the flow. The '70s tended to favor relatively crunchy overdrive (exception, Duane Allman, who used Dumble modded Twins and/or Supers I believe).

The '80s and '90s looked for higher gain, more "violin" style overdrive. So HAD went to higher plates. What he did that no-one else did was use the "rule of thirds" which resolves to having the cathode resistor 1/66 x the plate resistor. This keeps the DC operating point in the middle of the load line. This also alters the distortion characteristics compared to Marshall, Mesa-Boogie, Soldano and other luminaries. All a matter of taste.

Max, if you're reading this, I know the above is an over-simplification and ignores HAD's Marshall type moves with the Bluesmaster PI, for example. I have built a '70s ODS, a #124 and Bluesmaster, and I love them all!

[dave g]

exception, Duane Allman, who used Dumble modded Twins and/or Supers I believe

Curious, where did you hear this?

[David Root]

Just my recollection, I could be wrong, I was there in the '60s and '70s but that's no guarantee of historic accuracy, is it?!

[Drumslinger]

Interesting. Perhaps Max can chime in and elaborate a little more.

[guitardude57]

Duane used an original twin in the pre Allman studio stuff, and until the Brothers got Phil to spit out some money to upgrade the Van to a Winnebago and amps etc.

He then played a late 60's JMP 50 watt bass (1986 circuit) head........same as Paul K. (Free)

[pula58]

Thanks folks...I am more interested in the tonal and/or gain effect of the higher load resistors than the history..although the history is interesting too.

Truthfully, I am also wondering why the reduced (Compared to Blackface/silverface fender) cathode bypass caps in the front end of the Dumble amp schematics I have seen.

[martin manning]

Thanks folks...I am more interested in the tonal and/or gain effect of the higher load resistors than the history..although the history is interesting too. Truthfully, I am also wondering why the reduced (Compared to Blackface/silverface fender) cathode bypass caps in the front end of the Dumble amp schematics I have seen.

The short answer is treble boost. May I suggest that you do a little reading? Here is a very good source that contains the answers to your questions: http://www.freewebs.com/valvewizard1/gainstage.html

[thyx]

Higher gain and greater touch-sensitivity are what you get. You also get less distortion (prior to clipping), which may also be a factor Dumble considered. I use high plate-loads sparingly as, although they exhibit greater gain and less distortion, what distortion they do retain is largely 3rd order harmonics...which don't sound as pleasing as the 2nd order harmonics you get with a 100k plate-load.

[Structo]

An interesting observation is that just about every plate/ cathode resistor pair you will see has a ratio of 66:1

100,000/ 1,500

150,000/ 2,200

180,000/ 2,700

220,000/ 3,300

All have the same ratio.

So this ratio is what sets the operating center point?

[greiswig]

Tom, the meds are affecting your vision. Look at David's post a few clicks up this page for a concise explanation.

An interesting observation is that just about every plate/ cathode resistor pair you will see has a ratio of 66:1

100,000/ 1,500

150,000/ 2,200

180,000/ 2,700

220,000/ 3,300

All have the same ratio.

So this ratio is what sets the operating center point?

[dobbhill]

Martin:
I respectfully disagree with your "treble boost" comment. I believe the more accurate answer would be bass reduction. The lower cathode bypass cap value changes the bandwidth of the circuit by raising the bass rolloff frequency.
D

[Structo]

Tom, the meds are affecting your vision. Look at David's post a few clicks up this page for a concise explanation.

An interesting observation is that just about every plate/ cathode resistor pair you will see has a ratio of 66:1

100,000/ 1,500

150,000/ 2,200

180,000/ 2,700

220,000/ 3,300

All have the same ratio.

So this ratio is what sets the operating center point?

I realize that.
I was giving examples for people to visualize.
My "meds" had nothing to do with it.

Nice

[talbany]

In a nutshell

Things that concern us is TONE!!.. The larger plate resistors (which sets the rails) generating a greater voltage drop across the resistor giving you a wider voltage swing in effect offering greater headroom.. (whether you can actually hear the increase in headroom in the output section is the question) though it might respond differently in the feel..IMO.. What it does do is effect how and when the tube clips (harder or softer) generating different order harmonics as this effects tone and breakup character..The whole purpose.. The cathode resistor value sets the tubes bias point so the 66:1 is maintained to retain the tubes transfer curve..In a nutshell
Low plate resistor values have been known to yield a slightly brighter/crunchier breakup.. High plates produce a low end/low mid girth with a slightly smoother OD tone..

That's my take..

Tony

[martin manning]

I respectfully disagree with your "treble boost" comment. I believe the more accurate answer would be bass reduction. The lower cathode bypass cap value changes the bandwidth of the circuit by raising the bass rolloff frequency.
D

You are correct, of course. But in my own defense, it's all relative ;^)