Voltage divider ettiquete

[ECV02]

Hi all,
Finally got my p-p 6BM8 build in working order, so naturally, it is time for me to complain.

Upon startup, I played a lot with the clean channel. Worked just fine, got a bit crunchy when cranked, just the way I like it. However, I mostly play all the hair-band hard rock, which was the main purpose of this amp (I was really going for a Scorpions style dirt; thick and pronounced bite). With the gain knob cranked it almost sounds like I'm playing through a Big Muff. Did some probing, asked around, and a friend brought me to the idea of a voltage divider between stages 2 and 3. I've learned the hard way that grid circuity is sensitive, so sticking a 500k grid stopper followed by a 500k grid leak seems like it would come with it's fair share of problems.

Schem of the dirty channel's gain is down below. Each tube has a 1k grid stopper as well; I simply omitted them for clarity.
Thanks,
E

[ac427v]

Sounds like you are coming to the thought that all that gain is killing your tone. Many amps with three gain stages include a tone stack which reduces excess gain. Yours must come later in the circuit. So, rather than using a voltage divider, you could reduce excess gain by  cooling the bias of the preamp stages. Use a 3.3k cathode resister rather than the 1k. Try it gradually starting with the last stage first. If you like the result and want more gain reduction (my ears would) do the same swap on the second stage--or lift one leg of the 10u cathode cap on the second stage. Or both.

[jjasilli]

1.  Posting an erroneous or incomplete schematic is the opposite of clarity.  It's in your interest to always post the entire, true schematic.


2.  You say grid stopper.  a) A grid stopper is on the grid side of the grid leak resistor of the input stage, not on the plate side (output) side of the prior gain stage (or input jack).  Hence, a grid stopper does not perform a voltage divider function.  b) 500k is pretty small to yield a grid stopper function.


3.  Presumably you're using 12ax7 tubes, but it's not clear.  If you swap in 12at7's or 12ay7's you can test what less gain would be like.

4.  If you use a listening amp or a 'scope, you can pinpoint the origin(s) of the overdrive.

5.  If you simply install a voltage divider somewhere, you'll get more rotation of the gain knob before overdrive kicks-in; but not necessarily more actual headroom.  IOW, the amp may still break up at the same signal voltage and audio volume.

6.  Another approach is to plagiarize the cascading gain topology from a known amp that you like; e.g., a mesa boogie or a marshal.

[st]

Another voltage divider between stage 2 and 3 would be like a second (fixed) volume/gain  control... It might work, it might not (tone sick). Clip in a real pot (1meg) to eind out which resistance values would work for you. Take jjasili's advice to heart though, esp. points 1 to 4.

Edit:
I was just now looking at the schematic of Mesa Maverick (http://tubefreak.com/fschema.htm). Heed advice no. 6 as well and learn from known working designs. When you're done with the Mesa back catalog, you'll know all about voltage dividers and  other ways to tame gain.

[PRR]

What are you asking? You want to do this (below)?? Cost you 12 more cents and 5 minutes with a hot iron. Been done many times before. See what "troubles" you run into.

[ECV02]

Thanks for the help guys. I apologize for not posting the whole schematic, my main PC crapped out the other day and Falstad will crash with more than 3 triodes on my old laptop (though I am at fault for not including the tube types, they are 12AX7). 

I have had major trouble in the past with grid stoppers, especially values over 5k. Awhile back I learned the trick of soldering them directly to the tab on the socket, and this helped somewhat. I've still never dared to go above 10k, so slapping 500k in series with the grid seems a bit foreign to me.

My tone stack is after these 3 stages, I have 2 more for the clean channel, all fed into a DC coupled CF driving a tone stack. In future builds I'll experiment with putting it between gain stages.

Thanks again guys, I'll let you know of any other problems I run across pertaining to this and (hopefully) post a full schem.

[tubeswell]

1k grid stoppers won't do much to help. You want at least 10k, preferably 33k with your garden variety 12AX7 inverting gain stage. Even higher still for some stages. Soldered right at the grid.

[jjasilli]

Ok, but remember were talking apples & oranges.  Grid stoppers do not divide voltage.  PRR's circuit is a voltage divider.  As st alludes to, sometimes this kills hi's or mids.  This can be fixed by bypassing the series R with a cap; usually a good value for the cap is a pF number the same value as the R.  IOW 470pF or 500pF for a 500K series R.


Missing from this discussion is the design goal.  What's the point of cascading gain stages?  Not gain! A 12ax7 can effectively boost voltage 60X.  200mV into a 12ax7 stage will produce 12V output -- way more than the next stage can accept. 


The purpose of cascading gain stages is to sculpt overdrive tone in the preamp.  In a gain stage only the bottom of the waveform is clipped.  To get symmetrical clipping the next gain stage must also clip what was the top of the waveform in the prior stage.  This happens because each gains stage is 180 degrees out of phase.  Alternatively, a chosen amount of asymmetric clipping can be designed for. 

There is no point in stringing gain stages together, and randomly putting a voltage divider somewhere.  It is also important where the tone stack is located in the string of cascading stages.  See, Mike Funk's Tube Amp Workbook.  These things have been worked-out by proven amp designs: mesa boogie, marshal, soldano, etc.

[st]

Your point about cascading gain stages is pertinent, as is the advice to take inspiration from known good designs.
In this case, however, the thing has already been built. If you're curious about what a voltage divider in a certain position would do, I feel you should just tack/clip it in.