changing the cathode follower into a gain stage

Brentsp · Post by **Brentsp** » Mon Apr 25, 2005 12:13 pm

Billy Batz wrote:I sent you a layout of how to do it. The only problem would be fitting it on a standard board. If you are going to try it let us know how it sounds. Id be curcious to know as well.

Whenever you get that layout let me know because I haven't received anything unless you mean the email you sent. I talked with Doug a little bit last night and he gave me some more info on how to drive the tone control without using the CF.

Billy Batz · Post by **Billy Batz** » Mon Apr 25, 2005 12:54 pm

Check that first email again. There was an attachment. If you didnt get it Ill send ti again but make sure your email settings are cool.

Brentsp · Post by **Brentsp** » Mon Apr 25, 2005 12:56 pm

Billy Batz wrote:Check that first email again. There was an attachment. If you didnt get it Ill send ti again but make sure your email settings are cool.

Really?.....I don't remember getting anything but I'll have to check when I get home from work to be sure. I'll let you know either way. Thanks bro.

rjgtr · Post by **rjgtr** » Mon Apr 25, 2005 5:03 pm

You can convert the CF. The tone controls work almost the same, but aren't exact due to the impedence change. Blackface Fenders don't use CFs and the tone circuits work fine. Don't let this aspect of the design bother you. There won't be that much difference in the tone controls in practice. On most Marshalls, the only controls that do a lot are the treble and mid controls anyway.

So how to do it? I would suggest looking at an amp like the DSL401 or Rivera R-100. They have 4 gain stages between two tubes. The bigger DSLs add a cathode follower and put the tone stack after it, but I don't think it is necessary for good tone.

What I typically do with three tubes is use V3 for the PI. I will call each triode of V1 and V2 a and b. V1a takes the input from the jack and gets it to a usable level, like normal. V1b is similar to V1a, but is set up with a grid resistor and grid bleed to adjust it's gain and tone properties. V2a is used to add another gain stage and is biased with a higher k resistor. THis then feeds the tone stack. If you want it more like a cathode follower you can always feed the tone controls from the cathode, but I don't think it is necessary. On some amps I use a 250p treble cap. The tone controls then feed another normal looking gain stage in V2b. This goes to the PI as normal.

The problem with most 80's designs is that they slam each new gain stage with too much signal. The trick to a good sounding multi-stage preamp is controlling the gain. Sometimes you can do this by increasing the cathode resistor, but too much of this makes the amp sound cold and sterile. Sometimes you can do it by using voltage dividers between stages (Soldano anyone). I find too much interstage gain can be a little buzzy.

So that is the dance - gaining each stage properly, by the means that gets you a convincing cranked tone. And also the art of amp design.

No I won't post a schematic - there are several high gain designs from Rivera, Marshall, Soldano or Mesa you can look at and start with. Start at http://www.schematicheaven.com or other site. Copy the one closest to the sound you are after. Then it is just a matter of adjusting the amp until it sounds right to you. This can take hours or weeks, but nothing satidfying in life is easy!

P.S. doing it on a circuit board will be next to impossible without cutting traces or using terminal strips for the reworked stage. Even then it might be problematic. The best bet is to get one of George's PTP boards.

Brentsp · Post by **Brentsp** » Mon Apr 25, 2005 5:13 pm

oh wow thanks soooooooooooooo much. Doug Roccaforte mentioned that the black/silver face fenders had no CF either. I have a Metro amp with turret board that I'm in the middle of building. I haven't started on the board yet just finishing up the pwr section. I wish I could get some pics of Roccaforte's HG100.