5879 Preamp With Ax84 P1 SE

[tubeswell]

If you want to use it with a CF, try hooking the morph control pot input to the CF cathode (better load impedance for the pentode). I've done this and it works good.

[pullshocks]

I just got my copy of Merlin's preamp book and I am still on chapter 1, a long way from morph controls, but I notice a similarity to the "squish" control on the AX84 Blues Preamp, which I have built.  It has one less capacitor and the pot is wired differently.   http://ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf. 

As you turn it up it gets louder, more aggressive sounding, and a lot noisier.  I don't know that I would describe it as a compression effect, but then, I have never used a compressor, and I'm not sure I would recognize a compressed signal.  I pretty much keep it half way up. 

[llama]

WOW! Page 3...

From Advice I received from Jazbo8 on the ax84 forum (he originally sent me here when I was looking for a 5879 build- Thanks Jaz!), I chopped-sticked my build last night with the knobs dimed to find the source of the High pitched wailing.  I scared my wife half to death when I moved the wire coming from my Bass pot to the mid switch.  It shook the house.  It got way worse, with all the settings giving osculations. I finally rerouted the wire and now the noise is gone all together! 

I was going to closer 'er up, but now with this new information from JojoKeo and Tubeswell, I'm going to have keep digging in. 

Jojo, I guess I can get away with a mini switch since the voltage is less than 2V.  That would be nice to try.  I did try to 10uF bypass, but I don't recall hearing too much of a difference from the 2.2uF ( but the 10 was and old NOS...so...),  Never heard it lifted.  Looking forward to trying it out!

Tubeswell,  In looking at your DWG, I think I only need to move the input of the 470nF cap from pin 8 of the 5879, to pin 3 of the CF.  Is that correct?  Is the 1000pF cap on the output part of the morph?  I go into the Tone stack at that point.
 What does works good mean in "I've done this and it works good"?  I guess I'll find out for myself. 

Thanks again guys.

pullshocks,  I looked at the blues preamp, and looks different to me, but I'm not qualified to answer your question.  I'm sure your find your answer here.

-LLama

[tubeswell]

Tubeswell,  In looking at your DWG, I think I only need to move the input of the 470nF cap from pin 8 of the 5879, to pin 3 of the CF.  Is that correct?

Yup

Is the 1000pF cap on the output part of the morph?  I go into the Tone stack at that point.

The 1000pF is for AC coupling to whatever comes next. If your tone stack provides AC coupling, then go with that instead. I used 1000pF as the coupling cap because its sufficient to pass just about all the frequencies you need for guitar with that 12AU7 CF.

[jojokeo]

Jojo, I guess I can get away with a mini switch since the voltage is less than 2V.  That would be nice to try.  I did try to 10uF bypass, but I don't recall hearing too much of a difference from the 2.2uF ( but the 10 was and old NOS...so...),  Never heard it lifted.  Looking forward to trying it out!

You won't hear a whole lot between the 2.2uF & 10uF but you will with lifting it completely then switching one of them in. You will get a big more bass response with higher cap value is all. Depending on mood, volume, and style of playing - I like it w/out the cap at all but then I'm not quite using it in the position you are on most of my amps. Mine are the first tube of the preamp. You could use a 2-way or 3-way switch and get creative here. 

[llama]

Jojo,

I lifted the Bypass cap with a SPST mini switch and did hear the difference between the lifted and 2.2uF (and then the 10uF).

The lift quieted the amp a bit and took off some edge, but only when the Morph knob was at or near 100%.  In my case the morph actually made a bigger difference when turned to 25% to 0% rotation. 

I ended up restoring the 2.2uF cap to it's original soldered spot.  Since I now have a switch on the front of my amp, I decided to lift the Power tube Bypass cap, instead.  That made a huge difference at all knob settings.  The switch cuts some bass an maybe compresses the sound for a more rhythm setting.  This is turning out to be a more versatile amp than I had hoped for.

Thank you for getting me to try something new.!

I'm trying to get to a buddy's studio for some real sound recording, My Chinese condenser mic does not cut the mustard with my amps.

BTW, Jojo,  What is your Profile picture of?  Is that you and Buckethead?  What are you doing with your hand?    The pic is small on my laptop!

[llama]

Hey guys.  Sorry to revive this post, but I have a question about the pentode grit.

I tend to have guitars with lower output pickups (strat and LP with AlNiCo2 pickups).  With those guitars, the amp sings and has a beautiful deep drive that out of this world, but If i play any higher output humbucker on it (even a LP with PAF's), the tone gets very gritty and looses a lot of what makes it sound good.  I guess  that the circuit needs to be tweaked to accommodate a wider range of guitars.

My mind tells me to look at the pentode Cathode load and bypass, but then think of attenuating the signal going into the pentode.

I love the sound of the amp with the lower output guitars, I'd hate to loose that sound.  Any suggestions?

i hope you can see the attached schem.


Thanks - LLama

[jojokeo]

put a switch on the cathode cap or make it variable, I much prefer this to the triode / pseudo triode or whatever other manipulations are out there

[SILVERGUN]

Jojo, what do you think about an input trim pot ?.....

[jojokeo]

Not a fan of loading the guitar like that, I'd rather:
1) lower gain of V1a via:
   a) decrease load resistor, increase voltage, and/or utilize split load resistors
   b) remove or switch bypass cap
   c) create voltage divider on output
   d) lower output load resistance/grid leak (which is in parallel w/ plate resistance) such as simply using a 50k (or 100k) gain pot then eliminate the treble bleed cap (wouldn't be necessary any longer).

2) bail the CF set-up, create voltage divider on the output of the pentode (two 220k) AND still switch or make variable the bypass cap

3) utilize NFB on pentode

4) make batter use of using a single 12a_7 for V1a and V1b thereby eliminating the extra (12au7) tube OR if you have two more triodes to play with???

I forget if there's something paralleled or not? But it's not shown on the schem so it appears two stages are being wasted? Personally I like the pentode first in V1 then a normal CF stage driving the tone stack/vol > output tube. This is plenty of gain and then I'd run two inputs for the typical Hi/Lo set-up and plug the HB's intothe Lo if wanted.

[llama]

  JojoKeo,

Thanks for all of the input!  I'm likely to go with the lowering of gain at v1a. to start.  This amp is all buttoned up, But I'm likely to build another one soon enough.  I especially like the two input idea.  I believe the idea for having the pentode as V2 comes from Merlin,  but I'm sure your experience counts for a lot.  I appreciate all of your help with this build!

  Silvergun,

Thank you for throwing gasoline on the fire of knowledge!! 

Thanks guys!