Reworked the power supply. Now too much gain?

Built a tube instrument preamp a while back, using a similar topography to most tube guitar amps out there.  Both halves of a 12ax7 with a gain pot between them, with a final stage built around a 12au7.

The original power supply put out around 250VDC.  The new supply is sending around 300VDC.  Now it seems like it overdrives much more readily.  Got out the scope and the tone generator and ran a sine wave through it.  First stage seems fine.  Second stage starts to clip as the gain pot is turned up all the way, as one might expect.  But the whole thing seems to distort quite readily.

I guess this was to be expected, but I'm unsure as to the most effective way to deal with it.  Changing plate resistors?  Cathode resistors?  Changing the power supply again to reduce the output voltage?

(The new power supply is a simpler design compared to what I had earlier.  Got it from a Youtuber who goes by the handle of "Uncle Doug" and has a ton of videos about guitar amp tube circuits.)

Oops!  Attached the schematic in the wrong file format.

> a similar topography to most tube guitar amps out there

Such as??

It would be more common, for a non-headbanger amp, to take a tone-control loss between 1st and 2nd stage.

If you plot the gain diagram for a low 20mV input, the 2nd stage is near clipping. For a more typical 200mV input it is about 10X over-driven, clipping 90% of the wave, gross fuzz.

Also: the output of that (most) tone-stack runs >200K in bass but 20K in treble. With anything less than ~~1Meg load, the response will suck.

Yikes!  No wonder!  At least now I know where to start.  I wasn't particularly fond of that tone stage anyway, so out it goes.  And whatever I do put in will go between the two halves of the 12ax7.

So, I made a couple minor changes that seem to have helped....

Replaced R5 on the 12ax7 with a 56K.  (Wanted 47K, but I couldn't find any on the bench...)

Added a 68K grid resistor to the second stage.  (Not sure if this was necessary...)

Took out the tone stack.  (Will likely go with something much simpler anyway once the basic stuff is cleaned up...)

Much better.  Second stage (after gain pot) still crunches at high settings, which I'd expect, but at least it's at higher settings than before.  Output still clips too, when I turn it way up.  This doesn't seem right....

Do I need a plate resistor on that 12au7 B+ supply now that the supply is putting out an extra 50VDC?

The other thing that concerned me a little....
I was poking around with an oscilloscope at various points, trying to determine where things were getting particularly ugly.  Nice clean sine wave at the output of C1.  Nice and clean coming out of C2 as well.  But, when I turn the gain up into the overdrive zone, the wave definitely squashes, but it's not at all symmetrical.  I would expect some asymmetry, but the bottom of the wave is flattened out, but still round, while the top of the wave looks very sawtoothed.  Is this anything to be concerned about?

The same goes for the output stage.  If the output is turned almost all the way up, the tops of the wave start to go completely square.

At least it's starting to clean up a little bit. 

that's the James Jamerson tone stack,  got one of those feeding a pair of KT120's not too shabby if you like mellow,

maybe experiment with some of the resistors in stage 1 and 2,  and try out the bass boost circuit,

headbangers in this town use the Sunn Beta Bass when you can blend clean and dirt together to make the termites crawl,

CJ said:

headbangers in this town use the Sunn Beta Bass when you can blend clean and dirt together to make the termites crawl,

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Oh, man!  I used to have a Sunn Model T an old friend actually GAVE to me!  That sucker was tasty loud! 
Gave it back to him a few years later.  He loaned it to some kid who then went and pawned it. 
Been kicking myself ever since.

As for that Ampeg schematic....
Wow!  Those resistor values in the first couple stages look like they'd give me way more of what I'm trying to get rid of.  Are they using a different tube?  Ah, I see.  Second look at the schematic, and I noticed the penciled note specifying a 6SL7.  I'm starting to wonder if I should be listening to the output on something a little more robust than my bench headphone amp.  :-[

if you remove the bypass cap from any of the first two stages will reduce gain and also increase output Z.

the secound stage might wanna be able to drive the load of the eq... so i would try removing the bypass cap of the first stage atleast and see how that would sound.

reducing the plate resistor in the first stage will prob set the bias higher and perhaps to high for what the triode is rated for and shorten the life time.

5v333 said:

the secound stage might wanna be able to drive the load of the eq... so i would try removing the bypass cap of the first stage atleast and see how that would sound.

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Per PRR's advice, I'm likely going to put the tone stack between the first two stages.  If I do that, I suppose I should make the plate resistor for that first stage at least 100K.

here is a tube simulator where you can play around with tube parameters.

http://bmamps.com/ivds.html


Fender tone stack might have more loss if you are trying to slow signal down, and you get a middle control,

just put a 470 K resistor above that gain control if you wanna kill voltage,  maybe a small cap across it if it kills too much high end,

Plagiarize, plagiarize, plagiarize!!!

Most of the good plans (also many bad plans) have already been found and plans leaked to the world.

It IS possible to start from a clean sheet and design a guitar preamp. Odds are, unless you have very strange design tastes, or love excess parts, you will wind up near one of the Classic Designs.

Well, what a difference a few days -- and a much better headphone amp -- make!

The headphone amp I was using to listen to this beast on the bench is a piece of crap.  So, I purloined a decent one from my day job and tested again, after changing a few things.

It actually sounds pretty good.  I went ahead and experimented with adding a simple tone control and settled on placement for that.

Now to put it all back together in the box.

CurtZHP said:

Now to put it all back together in the box.

Click to expand...

You might want to consider a resistor from the output of C2 to ground.

Gene

Gene Pink said:

You might want to consider a resistor from the output of C2 to ground.

Gene

Click to expand...

That was a big help.  Thanks for the heads-up!

Got it all tested, and it sounds great.  Now to put it all back in the case.

Start by losing the cathode bypass capacitors, change the first stage cathode resistor to 2.2k; you might find that the second stage wants to go to 2.2k also.