Tube Amps / Music Electronics
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|10/27/1999 1:53 PM|
||Dumble cathode values...|
I've been reading the threads on the Dumbles amps and I've spoken to many people about the 3.3K cathode resistor that H.A.D. used. Nearly everyone thinks that Mr. Dumble came up with this value as no one has ever seen it used before. As Steve pointed out in the "Dumble drawings removed" thread, H.A.D. "borrowed" his tone stack from a few other amps as he did with the 3.3K cathode values.
This value was used in amps like the Acoustics (G100T, AC-12), many Ampeg amps (B12X, B18X, B15S, B22, G12 Gemini 2, Reverberocket, etc), McINTOSH amps, etc but the point is that no one thinks of these amps now-n-days. These are not considered to be the "the sought after amps" but they were extreamly popluar in the 70's when H.A.D. began building his amps.
Lord knows I could get flamed for this ... but You can look at amp building the same as popular (or legendary) guitarists. They all had influences at some point or another. For example Albert King was to S.R.V. as Eric Clapton was to Eddie Van Halen.
I'm certainly NOT disrespecting H.A.D. in any way, shape or form. Like S.R.V. and Eddie Van Halen Mr. Dumble certainly stamped his name in the music industry forever.
Keep on experimenting!
|10/27/1999 3:31 PM|
Also Trace, remember that quite a few of these amps, Dumble amp included, did not use 100K plate load resistors.
Using an Rk of 3k3 and an Rp plate load of 220K would be nearly the exact same voltage ratio of 1K5 Rk and 100K Rp.
If you were using a power supply that fed a lot more B+ to the plate loads in the first place, this would still give you about the right ratio and the same plate voltage all at the same time.
Sorry Numble .. nothing new there either.
|10/27/1999 4:19 PM|
Ahh, same voltages on elements, and same voltage gains, but less plate current.
Less plate current means less gm and more distortion.
Not saying it's "new," but they aren't really eqivalent.
|10/27/1999 4:55 PM|
||They Belong to Randall Smith - Re: Dumble cathode values...|
Well Ken, sure on the " more distortion" note using 3.3K cathode resistors. But hey, look at the Groove Tubes Soul-O 75 amp... 15k on the cathode of the first dedicated overdrive stage (and 100K plate load).
For what it's worth guys, look at the schematic for the Mesa/Boogie Mark IIB... Do you guys see the 300K plate load resistors and 3.3K cathode resistor on the dedicated overdrive stage?
Everyone borrows from everyone. Boogie didn't go to the 2-stage cascated arrangement for overdrive until the Mark IIC, in 1983 or so. However, Dumble was already building that architecture since the 70s...
Then, Dumble was Fender all the way regarding the 100K/1.5K resistors, but Boogie was all over the place starting with the Mark II in 1978.
Conclusion? We should post every schematic known to man, then these people would litigate each other in trying to determine who's got the right to sue us for posting " their" work.
|10/28/1999 3:47 AM|
Some schematic I was looking at a few nights ago used a 420K plate load resistor! I forget what one that is now but it's in the 4.1 Groove Tubes book.
Good point and very true. I guess those were the "experimenting days" as one would say! (ha, ha)
Man, Lawyers would have a blast and also rake in the cash over that! :^)
|10/28/1999 12:16 AM|
||Re: Dumble cathode values...|
I have to respectfully disagree with you here on your assesment of distortion. I wish I could remember the exact numbers but I don't have my tube manual handy. A 12AX7 running a 220k plate load actually has a lower THD than 100k loaded stages. It's somewhere in range of 1.1% if memory serves me. Given this information, the 220k stage may actually produce a cleaner tone than a 100k stage. As I'm sure you're aware. it's the total package that makes the difference not just one particular stage. It's also going to affect the slew rate of the stage and this is going to make a huge difference in how the amp reacts.
Did that make sense? It's been another long day over here.
|10/28/1999 4:41 AM|
Carl, you raise a good point.
I was approaching it merely from the plate current POV, which of course doesn't tell the whole story at all.
I was also approaching it as if you were going to have 50VAC coming into the grid--like you'd normally see flying about the insides of a geetah amp.
But every triode I know of prefers a current sourced plate resistor--ie an infinite plate load--as far as distortion goes. Grab just about the ugliest set of plate curves you can find, and if you read it right across a steady Ip, the Vp will track in nearly a straight line. Barring a CCS, if you've got the B+ to spare with a bigassed Rl then you can get some very low distortions indeed. In doing so you will also get a huge output swing.
And in this case that I admittely overgeneralized, the plate current with the higher Rk would not be so drastically lowered as to "starve" the tube into pinch off.
Agree about the long day too.
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