Fender'ish build question - input gain too high?

[markmalin]

Hey guys,


I've finished this build - (thanks for all the input on brightening up the circuit).  Anyhow, it's a really interesting sounding amp...but something's bothering me. 


If I turn the first volume up beyond maybe 3. and leave the Master anywhere sane, the amp overdrives.  It's a very loud amp, which is fine because the MV takes care of that, but I'd like more clean headroom.  If I bypass the master, for example, and just use the "Fendery" volume control it breaks up around 3....so with the Master in the circuit you really don't get much chance to have a clean tone above livingroom volumes.  My question is, is there something I can do to reduce the gain in the first (and/or) second gain stages to give me more clean headroom?  Or is this just the nature of the beast?  It's designed after a 67 Vibrolux...maybe that's to be expected??


Incidentally I tried a 12AT7 in V1 and it gave it much more clean headroom.  I guess that's a solution, but I miss the brightness and punch of the AX7 in that first spot!


Here's a schematic and a schematic and a diagram of the MV circuit I used ( as well as shameless self promotion photos of the finished product )


V1 plate voltages are 204 and 201 V.  1.5 on the cathodes


Thanks for all the help, guys!


Humbly,
Mark.

[HotBluePlates]

If I turn the first volume up beyond maybe 3 ... the amp overdrives.  ... I'd like more clean headroom.

How "driven" is the overdrive? How clean do you need the cleans?

I've had original blackface Princeton Reverb and Super Reverb amps. The Princeton subjectively stayed a bit cleaner a hair higher on the volume control, but both amps started getting a little "hair" on the clean notes by about 4 on the volume control. The Super Reverb distorted earlier on the volume control, which is backwards from conventional wisdom as tube plate voltages are higher throughout.

I didn't really notice how fuzzy Fender clean can be (outside of huge amps like a Twin or a Showman) until I built a super-clean amp.

Assuming it's not a matter of unrealistic expectations, there are plenty of ways to reduce gain. Omit a cathode bypass cap. That'll cut gain by half in a typical 12AX7 stage. Make a cathode resistor larger; the larger bias voltage that results will help allow a larger input signal before distortion, unless you carry the idea to an extreme by using something like a 10kΩ cathode resistor.

[2deaf]

Make a cathode resistor larger; the larger bias voltage that results will help allow a larger input signal before distortion, unless you carry the idea to an extreme by using something like a 10kΩ cathode resistor.

With a 100K plate resistor, the negative side of the incoming signal gets clipped before the positive side.  A larger cathode resistor does allow a larger positive input signal before clipping, but it decreases the absolute value of the negative input signal necessary to clip.  At typical voltages, 1.5K on the cathode and 200K on the plate cause the negative and positive sides to clip at about the same point.  Deviations from this combination can affect gain, but will also decrease headroom.

[HotBluePlates]

Yes, but we also can't look at the problem in a vacuum. If the input signal is only 100-200mV, maybe a bias change from 1.5v to 2v doesn't make much difference.

I suppose that argues against what I'm proposing, at least for the 1st gain stage. We probably ought to draw a load-line for the 2nd gain stage and assume an input signal level.

[2deaf]

If I turn the first volume up beyond maybe 3. and leave the Master anywhere sane, the amp overdrives.

I take it that this is with the DPDT switch in the up position and the Master is the OD Level since there is nothing you could call a Master with the switch down.  If so, you have the equivalent of four stages and that would have to be turned way down in order to clean up.

If I bypass the master, for example, and just use the "Fendery" volume control it breaks up around 3.

Here it sounds like the DPDT switch is in the down position and the only volume control is the one after the first stage.  If so, this is the standard Fender setup that you see all the time and they are what I would call very clean at 3 and only mildly distorting at 10.  If the distortion is anything like the DPDT up position, you may have some problem other than circuit design.

 

[2deaf]

Omitting a cathode bypass cap. has a huge effect on gain as compared to changing the plate and/or cathode resistor(s).  You can vary the gain by putting a resistor in series with the bypass capacitor.  Most of the variation in gain occurs with 0K to 5K in series with the capacitor. 

If you eliminate a bypass cap. completely, the gain drops about in half and the input signal necessary to clip about doubles.  From this you can see that eliminating the cap. on the first stage will have about the same effect on clipping as leaving it intact and eliminating the cap. on the second stage.

The first thing to clip with increasing signal is the mixer stage followed by the second stage.  You could reduce the mixer stage clip by eliminating the common cathode bypass cap. for the recovery stage/mixer stage, but it could still clip a little at the second stage at 10 with a hard whack on a humbucker.  If you want to reduce clipping even more, you could fool with the cathode bypass cap.'s on stage one and/or two.   

[dude]

I'm no expert but try lowering that first stage cathode cap, I think you have 25uf /1K5. Keep the 1K5 and lower that cap to around 5.6uf or the like. Try the amp, this should tighten up the amp somewhat.

If that doesn't work for you, keep the 5.6uf in the first stage and take the 25uf out with another 5.6uf cap in the  mixer stage.

This is typically done on Fender Tweeds and takes the floppy bass response out, tightening up the amp which may work for you.

Simply lift one leg of the cap and add the 5.6uf, try. Then the other if needed. Not much work. You could even go lower than 5.6 but any higher you won't hear any differences. I've went as low as 1uf. Marshalls sometimes use .68uf there.

You could raise the first stage resistor, up to 2K and try...

al

[blues man]

You could try a plug solid state diode rec instead of the 5u4. If that helps that may tell you you need a little higher voltage on the plates. I have been trying to get more headroom out of my own amps. Increasing the mains up to 40uf and the first 16uf after the choke to 40uf I find helps. I just finished a super reverb I converted from a fender 75 and I have 233v on the first preamp plates. This is the first amp I tried 2 220uf caps in series for the mains and with a diode bridge and now finally I am getting somewhere and getting the tone I am looking for. I personally wont  use the 5u4 any more. I have never been totally happy with them and have blown 2 of them because I didn't use the standby switch when i turned the amp off and didn't notice it and when I turned the amp back on they blew.

[markmalin]

Hey guys!  Sorry to be a bit AWOL here on my post ;) 


HotBluePlates -- you asked:


How "driven" is the overdrive? How clean do you need the cleans?


Your comments on the general Fender topology being more "dirty" is probably something I need to take into consideration.  I'm used to building D'clones which are just dead clean way up!! -- I just don't remember Fenders as starting to break up so early, but maybe that's the nature of the beast and I'm OK with that. 


Basically what I was getting at is, if I turn the MV to about 4 - 5 so that the overall volume isn't that loud, and turn the Volume (first volume) to like 1 or 2 or so, it's nice and clean.  If I turn the first Volume up toward 3 there's "hair" on the signal - it's breaking up...overdriven.  It's a good sound, but I'm not sure I want that so early -- I wish I could have more clean headroom.  I have the Dumble OD circuit in there for when I want overdrive.  If I turn the MV down to 2'ish and turn the Volume up beyond 4 or more, it's just obnoxious clipped/fuzzy, kind of muddy distortion.   If I turn it up to 7 it's down right crackly and bad sounding!  Bear in mind, Fender's don't always sound that good way overdriven.


What I was hoping was, maybe having clean headroom with some breakup as the gain increases, then using the OD switch to add in the extra gain stages to get the creamy overdrive I'm looking for.

If I turn the first volume up beyond maybe 3. and leave the Master anywhere sane, the amp overdrives.

I take it that this is with the DPDT switch in the up position and the Master is the OD Level since there is nothing you could call a Master with the switch down.  If so, you have the equivalent of four stages and that would have to be turned way down in order to clean up.

2deaf - sorry for the confusion.  What's not shown in the schematic is the "master volume control" in that other diagram.  I haven't drawn that in my schematic yet ;).  So my questions about overdrive and such all refer to that circuit being included.  Sorry.  And they all pretty much ignore switching in the extra OD stage

[HotBluePlates]

Gotcha. Try any/all of the suggestions so far.

But be disciplined: 1 change at a time, then listen. Otherwise, you won't know what's making the change or get a sense of how much impact each change has.

I think you'll get there. However, the 25L15 has a level of clean blackface Fenders can't match. The reason is the structuring of the gain stages: 12AX7 -> 12AT7 -> 12AU7. Each tube can handle a progressively larger input signal very cleanly. Not just "not clipped" but very low THD at every stage. And that's why it doesn't get "furry" as you start to turn it up (that, and there's not a lot of extra drive available beyond what pushes the output tubes to max clean power).

[2deaf]

And they all pretty much ignore switching in the extra OD stage

So now I think you are talking about a standard Fender with a post phase inverter master volume.  It's only my opinion, but those post PI Master Volumes just don't sound good.  The Fender circuit in front of it has too much bass for overdrive and PI clip is sharp with a harsh, fuzz-box sound to it. . . in my opinion.  Without the post PI Master, the output tubes clip way before the PI so that you are hearing output tube distortion and not PI distortion.  You can design circuits that will get rid of the muddiness, but the obnoxious/fuzzy distortion is there to stay with that MV.

What I was hoping was, maybe having clean headroom with some breakup as the gain increases, then using the OD switch to add in the extra gain stages to get the creamy overdrive I'm looking for.

A Master Volume in front of the PI and using switchable OD stages can easily do this.

[markmalin]

And they all pretty much ignore switching in the extra OD stage

So now I think you are talking about a standard Fender with a post phase inverter master volume.  It's only my opinion, but those post PI Master Volumes just don't sound good.  The Fender circuit in front of it has too much bass for overdrive and PI clip is sharp with a harsh, fuzz-box sound to it. . . in my opinion.  Without the post PI Master, the output tubes clip way before the PI so that you are hearing output tube distortion and not PI distortion.  You can design circuits that will get rid of the muddiness, but the obnoxious/fuzzy distortion is there to stay with that MV.

What I was hoping was, maybe having clean headroom with some breakup as the gain increases, then using the OD switch to add in the extra gain stages to get the creamy overdrive I'm looking for.

A Master Volume in front of the PI and using switchable OD stages can easily do this.

Hm.... that's very interesting.  So maybe I' hearing the output tubes distorting.  I kind of makes sense.  I wonder if I should try a pre PI master volume??  What's a good circuit??

[markmalin]

HBP - I tried a few things...here are the results.  Check out 2deaf's last post in relation to my results.  (?)


(in all these tests, I had the volume at 3 and was dialing the Master Volume up and down to keep it at "livingroom" volume.  It started out pretty overdriven)


- swapped in 5 uf for V1's first cathode cap (with the 1.5k cathode resistor)
   > same OD/overdrive at low volumes, but sound is brighter...which I like
- put a 2.2k on cathode of V1's first gain stage
  > slightly cleaner ...slightly better
- put 2.2 on the second gain stage cathode of V1
  > maybe a "bit" cleaner, but subtle
- At this point I swapped back and forth between 1.5k and 2.2k to see if I could tell the difference.
  > If I'm honest..it's very subtle change
- put both 2.2k and 1.5k in V1 first gain stage.
  > still subtle change
- Pulled up one leg of the cathode cap from V1 first gain stage to remove it from the circuit
  > this actually had significant effect.  I can have the Volume on 3 and there's a faint amount of "hair" on the sound if I dig into it, but if I back off it's clean. 


So - pulling the cap actually was getting a nice, clean tone.  Is it legit to leave that out??  I actually like the way the amp is acting.
On the flip side, I wonder about 2deaf's comment that maybe the issue is I'm hearing OUTPUT tube distortion due to the post PI master volume! 


Thoughts, guys?

Gotcha. Try any/all of the suggestions so far.

But be disciplined: 1 change at a time, then listen. Otherwise, you won't know what's making the change or get a sense of how much impact each change has.

I think you'll get there. However, the 25L15 has a level of clean blackface Fenders can't match. The reason is the structuring of the gain stages: 12AX7 -> 12AT7 -> 12AU7. Each tube can handle a progressively larger input signal very cleanly. Not just "not clipped" but very low THD at every stage. And that's why it doesn't get "furry" as you start to turn it up (that, and there's not a lot of extra drive available beyond what pushes the output tubes to max clean power).