5879 Preamp With Ax84 P1 SE

[llama]

That's a nice offer, Thanks.  I'll let you know!

[Willabe]

So, for best smoothing on an SE, I should connect the Inductor's second lead to the OT where the signal is coming in (plate connection), and then connect the second filter cap to the first directly , where lead on of the inductor taps off?

No.

Choke goes between B+ PSU nodes A and B. OT then connects to node B but not where you have it. It goes to the OT primary leg shown as A in your drawing.


            Brad    

[llama]

I'm glad I asked.  That made no sense to me, but a senior tech where i worked said that was right.  Guess I have to stop listening to him!

Thanks!!!!

[Willabe]

Here's a drawing with plate and screen node shown. It's for an octal (8 pin) power tube but it's the same thing just different pin numbers.


You don't have to use a separate B+ screen node for SE amp but you can, it will only help with quiting down any B+ ripple noise.

Depending on what you use for your PSU rectifier, SS or tube you can increase the filter cap values.


               Brad    


 

[llama]

You don't have to use a separate B+ screen node for SE amp but you can, it will only help with quiting down any B+ ripple noise.

Depending on what you use for your PSU rectifier, SS or tube you can increase the filter cap values.

Thanks Brad!

The pictures sure helped.
If I understand it all correctly,  I can use the node B for both the plate and screen, but it may be noisier that using the screen on a later node. 

I used my choke differently for my ax84HO build (that this PSU is based on).  I put it in front of the first cap in series, and that does a great job filtering noise, plus it dropped my B+ to a better voltage (in my case).

I'm using SS rect.,  Should I go bigger caps to keep noise down.  I recall reading, maybe not, that 22 - 47 uF filter caps do a even job, Is that incorrect?

I appreciate your help.  Thank you!

[Willabe]

If I understand it all correctly,  I can use the node B for both the plate and screen, but it may be noisier that using the screen on a later node.

Yes.

I'm using SS rect.,  Should I go bigger caps to keep noise down.  I recall reading, maybe not, that 22 - 47 uF filter caps do a even job, Is that incorrect?

There's a limit of how much capacitance you can use at the 1'st cap (A node) if you use a tube rectifier. The larger the caps value the more initial inrush charging current is needed to charge the caps. The inrush current is at least double to triple the normal current needed once the filter caps are fully charged up. The tube rect. can't handle rectifying this extra current and will die sooner or later from stress. Depending on how large the uF value (60, 80, 100uF) is that you use at the 1'st node, it could die the 1'st time you power it up.

IIRC there's a couple of things you can do to get around this?

1. Because you have a choke in series with the B+ PSU leave the 1'st (A node) within the limit on the rect. tubes spec sheet and add any extra uF's needed after the choke at node B. The choke will slow down the inrush by it's nature.

2. Put a stand-by switch after the 1'st filter cap but before the choke. To do this just break the connection at the A node and choke, insert switch. This way only the A node filter cap/s will draw current. Then once the cap/s are charged up and amp is warmed up flip the switch to on/play. Now the A node caps will not need any current to charge up only the caps down stream will need charging and they are after the choke.

3. Install SS diodes in series with each tube rect. plate. IIRC they do the work of rectifying the AC to DC and protect the rect. tube from wall voltage line spikes? I don't recall if they allow the rect. tube to pass more current at start up? Tubeswell knows.

4. Do all 3 of the above.

I added how I'd wire the ground for the circuit with a single connection to the chassis. The other chassis connection would be at a different area on the chassis for the power cord green ground wire.

Here's another drawing of it with what I've listed.


                  
                             Brad      


Edit; They add the SS diodes as a back up if 1 of the tube rect. goes out. The SS will keep working.

[llama]

I pulled my PT last night to investigate the "deadness" of it.  Turns out that it was only mostly dead (thanks to Billy Crystal's line).

One of the HT secondary leads was severed about a 1/2 inch outside of the end bell port.  I pulled the endbell off and had 2 inches of wire to work with and soldered a new lead and shrink wrapped the junction.  I did that same for both the the Primary connections, as they were very short and giving intermittent readings.  Both the Primary and secondary DC resistances are reading within spec, now.

Just a small setback, but what a great lesson, in how to test, troubleshoot and repair a transformer that someone would have left for dead.

I haven't reinstalled it to see if it "works", so no victory dance yet, just in case there is another issue!  It is, after all, a 51 year old transformer.  It's ugly inside, See pic below.

I will dispense with the crimp connectors and solder it all!

But, I'm hoping for speaker tests by the weekend.....

Willabe,  I'll make that change to my circuit as well, Thanks for your help.

[Willabe]

One of the HT secondary leads was severed about a 1/2 inch outside of the end bell port.  I pulled the endbell off and had 2 inches of wire to work with and soldered a new lead and shrink wrapped the junction.  I did that same for both the the Primary connections, as they were very short and giving intermittent readings. 

Good job!   



           Brad       

[SILVERGUN]

Just a small setback, but what a great lesson, in how to test, troubleshoot and repair a transformer that someone would have left for dead.

Way to go L,,,,,thats great news and a commendable job of troubleshooting
 

[llama]

And now the Ubiquitous Good News / Bad News scenario.....

The good news is that I fixed the tranny and she's pushing 529Vac between the Secondaries.  

The bad news is that I have a short somewhere towards the input side of the standby switch.  

-I'm using the Paul Ruby Startup guide.  Now with the PT is in order, I move onto soldering the PT to the rectifier (SS).  
  
-I'm also using a Lightbulb limiter in series with the amp.  

-When the unit is on Standby, i see 6.8Vac across the heater wires, 300V -dc after the rec, and the lightbulb is glowing dimly.    When I go to full on, the bulb gets bright and i only measure 12V dc  at the rec. -Sounds shorted to me.

-Just to be clear - the tubes are not installed yet.

- I suppose I could unsolder the path half way toward the input, and see what that tells me.

-Anything that I'm missing here?

[sluckey]

I would suspect filter caps first. Disconnect them one at a time and see if the short dissapears. You don't need to power up to do this. Just connect your ohm meter between the rectifier output and ground with the STBY switch in operate mode.

[SILVERGUN]

I would suspect filter caps first.

Are you still trying to save a buck and use those NOS caps?

Bring it here and I'll show you how to figure it out........whatever you do, DON'T DIE

I'd hate to have to explain to your wife that you were trying to use 10+ year old caps, in a brand new amp design

[llama]

Are you still trying to save a buck and use those NOS caps?

Nice!

No the the filter caps and all electrolytics in this build are new.  The NOS's i've used are ceramic.or poly.

I might have already performed the test that Steve mentioned, but will do so as he described. 

how low of a resistance reading should raise a flag.  I tend to look at the value of the resistive components and see if that is close to what I'm reading.

Divide and conquer is what I'll try next - thanks for your concern SG!

[SILVERGUN]

Here's a quickly done visual explanation of what Sluckey is telling you to do.....please forgive the slop
I tried to write things out as simply as possible, so please excuse me if it's too simple,,,,but I'm not sure what you know

how low of a resistance reading should raise a flag.  I tend to look at the value of the resistive components and see if that is close to what I'm reading.

An ideal capacitor would have infinite resistance

[llama]

Holy cow SilverGun!  That was AWESOME!!!  I loved you use of colors.  Very Festive!

Thank you.  Your picture made more sense out of it for me!

I never did have to do that though.....I found the culprit, once I started asking intelligent* questions.  The fist question i asked, was why is B+ to ground 100R?  Then i saw the 100R bleed resistor next to C1.....  Somehow I installed a  100R resistor instead of the 220K for the bleeding.  My amp was a hemophiliac.! It bled out all if it's current!!!!  Good thing I'm not building humanoids.

Once I switched it out for the correct value the Voltages were all OK - but very low.

Second intelligent question:  Why is my voltage reading 188 at B+ for a 280V CT PT?  -- Then I saw the light....it was dim.  My Light bulb limiter was dropping the voltage in series.  Thank you Ohms Law!  I felt comfortable to remove the limiter and my voltages at B+ are now 316Vdc.  That's better.  I know that's a bit high for the tubes, but it was what I was expecting.

I took all the measurements as per the Ruby guide and played guitar for the rest of the night, while the family slept. 

It sounded good.   More to come!

*Note:  The use if the word intelligent does not indemnify me as one who is.  Just used as how someone who has a clue might think.

[llama]

I have edited my schematic to show the changes and mods I've made up to this point, from the original.

I hope you can see the voltages at each node, plate and cathode.

right now, the amp how nice growl at medium gain settings.  if I turn gain and master Volume up i get a flubbing sound that i guess is an oscillation. kind of a rapid thub thub thub sound, not too loud, but definitely there.

also the tone stack makes no change in sound.  (assuming CF load resistor (more on that in a later post))

could more trained eyes tell me if my voltages seem OK and.

 BTW  I am using a 7189 instead of a EL84 because it has a 400V plate limit vs the 300V in the EL84.  But i hear that 316V on the plate of an EL84 is usually OK!

Thanks for all of your help through this
-Scot

[Willabe]

You have B+ nodes C and D fliped at their tubes.


              Brad         

[llama]

You have B+ nodes C and D fliped at their tubes.


              Brad        

Hi Brad, Thanks,

That's the way Firemedic had it.  I thought it was for the fact that the pentode likes lower voltages.  I realize it is more noise for the input.

If you don't agree, then I could change it around, but that's how it's wired!

-Scot

[Willabe]

That's the way Firemedic had it.  I thought it was for the fact that the pentode likes lower voltages.  I realize it is more noise for the input.

OK, that makes sense. Thought it was just a typo.


              Brad        

[SILVERGUN]

How did it go last night?

[llama]

I didn't have much time to do a whole lot, but I was able to remove the CF cathode resistor (56K) and add a 10k in series with a 100K pot.

I played through it while adjusting the pot and sound hardly changed (except for it quieted at the lowest settings and lit got much louder at the highest settings.  91K sounded loud and clear (plenty of distortion-for me).

The tone stack was completely useless at all settings.

I followed PRR's advice from a much earlier reply on this topic.
Beginning of quote----------------------------------------------------------------------"    
Re: 5879 Preamp With Ax84 P1 SE
« Reply #7 on: October 04, 2012, 08:37:10 pm »
   Reply with quoteQuote
> The mu of the tube doesn't matter for a CF application

In a way it does.

The cathode resistance (1/Gm), *at the same current*, is about the same for all the small tubes. Roughly 1,000 ohms at 1mA.

The plate resistance is Mu times higher. So higher for high-Mu tubes.

The maximum current at a given voltage is limited by plate resistance.

So a low-Mu tube can pass more current at a given voltage.

llama's plan shows perhaps +250V at node D, and 171V at CF cathode. 250-170= 80V across the tube. The 56K cathode resistor must be flowing 3mA. The tube must flow like a 80V/3mA= 27K resistor. "
End of Quote----------------------------------------------------------------------

I used his method for calculating the best cathode resistor, and came up with 21K (+243V at node D, and 153V at CF cathode. 243-153= 90V across the tube. The 56K cathode resistor must be flowing 4.3mA. The tube must flow like a 90V/4.3mA= 21K resistor.)  Obviously I just copied PRR's text and used my values - don't know what I'm doing here!

So I set the pot to measure 21K across the pot and 10k resistor, and no change in sound, nor tone stack...

I'm bewildered!...actually, I'm too NooB to be bewildered....More confounded, but thanks for asking, Silvergun!

-Scot

[llama]

As Promised.

I recorded the 5879 amp for your critique,  Sorry that I only used my iphone, and the video points at my ceiling (but the mic is at the bottom).

The guitar is a Epi LP with A2 pickups, and the speaker is an Emenace pulled from a 96 Fender HRD.

The vid lives on my Facebook fan page (eh..)

https://www.facebook.com/LLamaTone

I plan on doing better recoding once the amp is in good shape to travel to the studio.

-Scot

[SILVERGUN]

Thanks for the update....

Keep up the good work,,,the amp sounds great and of course I like it best with the gain cranked!!

[llama]

Fellas,  I glimmer of understanding has illuminated my brow.  I get it now.  the Tone stack is a voltage and frequency divider.?.!

I never added the PAB? switch from Firemedic's original drawing.  I figured that I'd leave it out and then see what it does. 

1.  I have added a rotary switch with different R values to be in parallel with the existing 330K to ground (straight wire, 100R, 4.7K, 470K, Open).  I did start with a potentiometer before, but thought this was more elegant.
    -I couldn't give it too much time to nail down my observations but her's what I recall -the wire and the 100R sound vintage-like - less volume, softer sounds,  the 4.7K has a great sweep for both treble and base knobs & maybe more mids.  The 470k has limited sweep for both knobs, but has a louder, rapier sound, and open (no paralleled resistor to the 330K) sounds loud, and in your face!

Is this type of switching used much for voicing changes?  are there better locations?

2.  I ended up soldering in 92k of resistance for the Cathode follower Cathode load (say that 3 times fast).  the sound was richer to my ears.  I need to reread the Merlin chapter on CF's.  My tone stack seems to work fine at that and any load i tried with my 100K pot.  Am I creating much current amplification with such a high load?  Would i be better off switching to a normal gain stage (low gain)?

I know I've been riding the coat tales of Silvergun, but The mad scientist thing is really cool...

Thanks for all your help!

-LLama

[SILVERGUN]

1.  I have added a rotary switch with different R values to be in parallel with the existing 330K to ground (straight wire, 100R, 4.7K, 470K, Open).  I did start with a potentiometer before, but thought this was more elegant.
    -I couldn't give it too much time to nail down my observations but her's what I recall -the wire and the 100R sound vintage-like - less volume, softer sounds,  the 4.7K has a great sweep for both treble and base knobs & maybe more mids.  The 470k has limited sweep for both knobs, but has a louder, rapier sound, and open (no paralleled resistor to the 330K) sounds loud, and in your face!

Is this type of switching used much for voicing changes?  are there better locations?

That's a cool idea, and as long as the results are noticeable enough to deserve a dedicated switch, that looks like a great way to achieve a few different flavors.........better locations?,,,,only if your experimentation deems them to be better
If you're going to keep the switch you can eliminate the parralel 330K (just so I don't have to do the math on your newly paralleled resistor values)

2.  I ended up soldering in 92k of resistance for the Cathode follower Cathode load (say that 3 times fast).  the sound was richer to my ears.  I need to reread the Merlin chapter on CF's.  My tone stack seems to work fine at that and any load i tried with my 100K pot.  Am I creating much current amplification with such a high load? 

Where is that reflected in your schematic? (56K-R14?).......that value is going to affect the overall distortion character of the amp, and I'm hoping one of the big boys here will help you figure it out...it's my understandeing that lowering that value can help warm up a fuzzy sounding distortion...?(merlin)

One thing to keep in mind....If the amp sounds great,,,it may be done!.....if that CF circuit was designed around the 56K value, it might be in your best interest to keep it there and look elsewhere for enhancements (play it safe with the bias point).........dont forget about the point i hit where I was "boosting" the signal at every possible location, and it always sounded "better", BUT I also could've just increased the Volume and gotten a better sound (pushed more signal)...make sense?

[jojokeo]

The 330k resistor location on bottom of the stack serves the midrange part of the frequency. With nothing (bare wire) you get the least amount of mids with the famous deep notch in low mids famously used by metal heads. The greater the value, the more mids and increase in signal strength. That's why it's called a "boost" too because it's also called a tone stack "lift". Meaning when the stack is not grounded it is lifted as if it was out of the circuit and it gives the boost because it's not loading down the signal. This gives the "tweed" effect of the entire full signal w/ all the mids included. I've made amps w/ the mid pot containing a switch to switch it out of circuit and w/ a DPDT switch to give varying degrees of mid range content and frequency response. It's up to you whatever you like.

Generally the CF stage has a matching resistor to the plate's but Merlin suggests 'lowering' this value for smoother operation/grind like what SG said.

[SILVERGUN]

The 330k resistor location on bottom of the stack serves the midrange part of the frequency

So consider just putting a mid pot in there, if it makes more sense to you than the wiring and extra resistors for the switch

The switch could be viewed as a cool little "trick", and maybe incorpoated as a 2 position slide switch (vintage - boost) OR (good Llama - bad Llama)

[llama]

Thank you JojoKeo,

It took me a bit to adsorb your comments, plus a little interpretational help from Silvergun.  I see what your saying.  I could use a 500K pot instead of switching resistors as well.  Actually, I tried it last night, and I'm amazed by the sweeping change the mids make.  Thank you.

I decided to move on with the switch since it took me a while to find a switch that would fit in the chassis.  I'm using a Jamco military rotary switch that is used in aircraft, ships and tanks..and for free....why not!  It's all in the name of mad science!  I have 4 positions so I plan on going with a 100R, 5K, 50K and 330k, or somewhere in the neighborhood.

I plan on removing the 330k from the board and adding it to the switch so it all is in series. no more parallel resistors.  I'll have a bit of time to get the R values figured out tomorrow.

...and now I know the the Tweed effect is all about...full mids and more signal -- thanks for your help JojoKeo.

BTW, Silvergun here's what I'm talking about-  Mine has 4 positions but I found some with up to 12 (now that's overkill!)  and I need to save a bit of room for one more knob for Merlin's Pentode Blender thing.  But I'm sure that your right!

[jojokeo]

Boy that's too nice of a switch to use for midrange controls. If it was me, I'd use a 25k or 50k pushpull pot or one w/ a SPST switch.

[llama]

The issue is that I'd have to actually pay for a Pot with a pushpull.  My company has military grade components.  My amp is filled with them!

I though that i should use a 500K pot as a resistor (lug 3 not used) because of what you said about the high resistance.  I guess the spst switch would cover the 330K then the pot would cover the lower end?  I think i see said the blind grasshopper!

BTW let me know if your interested in any components,  my company does not yet sell to the consumer market, nor does it market these components i find so useful in amps.  I was going to contact Doug about selling off some neat things....eventually .

Thanks again JojoKeo
 

[jojokeo]

lucky you, you get nice components for a great "price"! That would be expensive normally. I don't know the power ratings but it looks pretty robust, you could use it as an impedance selector switch maybe where they're 12 to 15 bucks ea., the power supply area to adjust voltages, fixed power scaling, etc...

[llama]

lucky you, you get nice components for a great "price"! That would be expensive normally. I don't know the power ratings but it looks pretty robust, you could use it as an impedance selector switch maybe where they're 12 to 15 bucks ea., the power supply area to adjust voltages, fixed power scaling, etc...

Now Your talking!  I know the one I have has a 1 amp rating,  Some smaller one are a 1/4 amp.  Dunno the  Voltage ratings, but I assume I'm good.  I think that they go for about $50 to Uncle.

Tell me more about power scaling...

[HotBluePlates]

if I turn gain and master Volume up i get a flubbing sound that i guess is an oscillation. kind of a rapid thub thub thub sound, not too loud, but definitely there.

Did you ever fix this "flubbing"? It's really motorboating...

[llama]

Did you ever fix this "flubbing"? It's really motorboating...

Hi Blueplates,  no I still have the issue at higher signal levels and bass levels. I haven't yet looked into it but now I'm nearing the finish line with this one, and would love to get it right. 

Btw just so we all are aware,  last night installed Merlin's pentode morpher from the end of chapter 3 in his preamp book. Sounds neat.  So my schematic is not up to date.

Thank you for inquiring,

Scot

[HotBluePlates]

Okay, then...

Two things to try:

1. You have a 1k resistor between your output tube plate/screen node and the following node. 1k and 16uF is 3dB down at ~8Hz, which seems reasonably close to how fast the "flubbing" was. Since you're only pulling a couple small tubes' current through that resistor, make it 10k and see if the noise goes away.

2. If it doesn, add a resistor and cap between R2 and C2, and shift your preamp power supply to the left on the schematic. Essentially, you're just adding another R-C filter section to your power supply, duplicating R2 and C3. This new node will feed your screen and keep it separate from the plate node for the outputt ube. Again, due to low current demand, the "old R2" should be bumped up to ~10k, while the "new R2" might be something like 4.7k.

So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.

But we'll try the simple fix first before adding a power supply node.

[jojokeo]

So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.

Good suggestions HBP but I think you may have forgot one last capacitor after the last 10k to 12ax7 node. Should be: ...->20uF (5879 node) -> 10k -> 10uF -> (12AX7 node).

The morph control just makes a transition into triode mode w/ a pentode tube rather than only being a pentode-only/triode-only option. It lessens gain doing this which should help lessen oscillations / motorboating. Do you notice this to be the case llama? After many pentode amps I don't include a triode mode option anymore, why use/design for a pentode tube if you're going to run in triode mode? Doesn't make any practical sense, should just have made or remake using a 12ax7 then. What I do like and prefer is using a pentode w/ variabe gain control. The "Squishy" control is also pretty lame/weak and a waste of time and faceplate area but I digress...

[llama]

HBP,

Thanks for your input.  I will swap out the 1k resistor with the 10K as soon as i'm able, then will repost.  I found your second option very enlightneing, as I can understand how the motorboating may come about from my the plate and the screen supply coming from the same node.  (but of course, I will try the resistor first).


JojoKeo,

I definitely like reading your input and opinions.  Keep them coming.  This is an experiment in understanding and tone.  I can see why you dispense with the gagetry and go for the proven.  A pentode gain control would be a much simpler and useful.  But at least I can see where your option comes from.  I'm like my son,  I can try to save him the time and pain in his journeys based on my experiences, but he feels the need to try it himself and learn from the pain.

Which brings me to my latest lesson learned.  After dicking around with it and adding and removing elements, the amp sounds fizzy and the bass is flubby, especially with my single coil (lace sensor) Strat- yuck.  Now I'm going to have to go back to basics.  Plus since the chassis in non standard, I cannot have a super clean build.  I had a series of shorts yesterday, from moving soldered components around so i could solder in a tight spot.

I must have damaged a component or two in my zeal to get it up and running before an event....

I will test and learn and will be better for it,... eventually.  Thanks guys!


Thanks guys. 

[SILVERGUN]

So now this is the point where you can't let yourself get down.....you WILL figure it out and it WILL be another AH-HA moment, and it's what everyone here has gone through.....

Be patient and work from input jack wiring, straight through the circuit, and follow along with a highlighter on a printed out schematic to eliminate what you have tested, and what you must.....don't take anything for granted

On my wreck, I plugged it one day and it was GONE,,,so I remembered having tugged on the input jack with a cable plugged in , and sure enough, I got lucky,,, and that was it.......1M resistor to ground buss (that I had been messing around with previously) came loose off of the input jack......soldered it back in and wham,,,,back to melting faces

GOOD LUCK...........Llamatone is for winners.......winners dont quit............not all winners are Llamas, but ALL Llamas ARE winners.....little known fact

[llama]

GOOD LUCK...........Llamatone is for winners.......winners dont quit............not all winners are Llamas, but ALL Llamas ARE winners.....little known fact

Thanks for the cheer leading, Silvergun!  You are a good guy, I don't care what the other guys say about you...

It turns out that my amp just did not like the JJ El84 i switched to from the 7189, but then I went with a 6005 in the seven pin spot - sounds really nice in most of it's settings. 

I'm figuring that since This is an experiment, that not all of the settings will be winners, but many of them sound really nice with both my strat and LP.

I still have a strange chassis grounding issue that i can tap on the amp to get working at times - like the old Tvs _not so hard though.  and the 5879 is really sensitive to vibrations and osculations. 


HBP,  I replaced the 1K resistor with a 10K and the motorboating still occurs, and now I have a high frequency noice that I can describe as the tubes screaming.  all of these noises occur at high gain, high volume. 

_I guess it's time to think about the extra RC node - After the holidays!

I asked my web expert how to take vids with good sound quality - I hope to do that sometime this week.

Thank you for everyone who helped and those also who read and followed my plight!!

[llama]

Hi all.  THe holidays are over, and I'm digging into my amp again.

Below are my updated schematic and layout (not exactly a matching set, yet) reflecting changes I've maetiond here, as well as HotBlue Plate's suggestion for the extra PS node.

I Plan on adding the new node very soon, and finalizing values soon.
-LLama

[SILVERGUN]

Hey...welcome back

We've missed you around the disfunctional round table...

Here's that link I was telling you about:
http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

[jojokeo]

Hi Llama,
After looking at the schematic I have concerns regarding how you drew that 5879 going into the 12au7. The pot is doing "double duty" a) for the signal voltage divider going into the 12au but also b) the .1 capacitor for the 5879 - which affects the respsonse of the 5879. Is that truely what you want going there instead of being seperately controlled?

[llama]

Hi Llama,
After looking at the schematic I have concerns regarding how you drew that 5879 going into the 12au7. The pot is doing "double duty" a) for the signal voltage divider going into the 12au but also b) the .1 capacitor for the 5879 - which affects the respsonse of the 5879. Is that truely what you want going there instead of being seperately controlled?

Now your making me think above my understanding level....  I believe the Morphable Merlin Pentode control does two things.  It affects the Screen Compression through the screen grid voltage amount and used as a triode/pentode control in either end of the dial.

I'm pretty sure that I wired and drew it according to the Merlin pic (attached below),  But I've been wrong before!

Since Merlin usually escapes my full understanding, here is an except from his book.  (I hope this is not in violation of this site or of Merlin's Copyright)

But overall I like the effect with a single knob  - Thanks again JojoKeo!!

----------------------------------------------
Triode/pentode ‘morph’ control:

Taking this even further it is not hard to combine both pentode/triode switching and variable screen bypass in a single control, if we appreciate that for triode mode the screen only has to be connected to the anode as far as AC is concerned. With the addition of one capacitor we obtain the circuit in fig. 3.29. We now find that with the control fully clockwise, the screen bypass capacitor, Cg2, is connected directly to ground and we have normal pentode operation. P1 is large enough that is does not heavily load the pentode.
When turned fully anticlockwise we see that C1 and Cg2 appear in series and effectively connect the screen directly to the anode, as far as AC is concerned, resulting in triode operation. (C1 is included simply to keep DC off the pot’.)

The actual DC voltage on the screen remains unchanged, and this causes the ordinary triode characteristics [fig. 3.6] to be shifted to the right by an amount equal to Va-Vg2;6 we effectively create a unique set of triode characteristics depending on the screen voltage chosen!
At intermediate settings we obtain the screen-compression effect described earlier and ultra-linear operation, all through the setting of one potentiometer! Assuming we want the same variation in effects at all frequencies, then C1 and Cg2 must be large enough to pass all useful frequencies between anode and screen.

C1 must be large enough that the reactance of the series combination of C1 and Cg2 is less than Ra∥ra(triode) at the lowest desired frequency. Therefore C1 should be at least five times larger than Cg2, if possible, for triode operation down to around 100Hz. The frequency response of the circuit in fig. 3.29 is shown in fig. 3.30, and it is interesting to note the stage has the lowest gain at an intermediate (ultra-linear) setting of P1.

EXCERPT FROM
Cover Art    
Designing Tube Preamps for Guitar and Bass
Merlin Blencowe
Category: Engineering
Blencowe, Merlin. “Designing Tube Preamps for Guitar and Bass.” Lulu.com, 2012-02-17. iBooks.
This material may be protected by copyright.

[llama]

add a resistor and cap between R2 and C2, and shift your preamp power supply to the left on the schematic. Essentially, you're just adding another R-C filter section to your power supply, duplicating R2 and C3. This new node will feed your screen and keep it separate from the plate node for the outputtube. Again, due to low current demand, the "old R2" should be bumped up to ~10k, while the "new R2" might be something like 4.7k.

So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.

Hi Hot Blue Plates.  I performed your advice and added a new node as you and JojoKeo described.  And that sure fixed the motor boating issue I had.  I returned the Screen resister back to 1K also. Thank you Thank you Thank you!!

I also reduced the Fist Coupling cap from 0.22 to 0.1.  That brought the bass to a fine, fine level.  Still enough to shake the walls with a wicked growl, but much more tight.  Dropped D with a Les Paul in high gain sounds Deep and tight - very modern.  But with a Strat, I have a nice bassy blues bottom end with plenty of vintage feel, especially on the Neck. Pure Silky drive. It's like getting kicked in the balls while wearing silk boxers!  Kick me again!  

The only ugly noise left is a high pitched drone that appears when all of the knobs at dimed.  The frequency lowers when I roll off the gain a bit and stops after a 1/5 turn of the Gain Pot.  My guitar signal has no affect on it.

With a scope app I can see two frequency peaks; one near 3200 and lesser at 6300 KHz (all knobs on 10).


Soo Close to Perfect (my description of it).   Thank you to everyone who responded to my queries.  I am in indebted to you all!!!

Tube Amps Rock!!!

[tubeswell]

Here is a 5879 with triode-pentode morph control from a circuit I have built. Works fine. (The input is where I had the vol control - so to make this schematic technically correct, it'd need a grid leak resistor there)



The only thing I would review is the morph control pot - it doesn't need to be 1M (not enough useable turning range of the pot). 500k or even 250k would probably suffice.

[jojokeo]

"I believe the Morphable Merlin Pentode control does two things.  It affects the Screen Compression through the screen grid voltage amount and used as a triode/pentode control in either end of the dial.

I'm pretty sure that I wired and drew it according to the Merlin pic (attached below),  But I've been wrong before!"

You don't have it drawn like it's supposed to be. If you drawing is correct then you are doing it incorrectly. The morph control only affects tube compression btwn the .1uF cap & Ground. If it was to involve "morphing btwn pentode/triode modes" then your drawing doesn't do that either. Lastly, your volume pot should be by itself and not mixed with these other two controls as you'll never be able to control them independantly.

[llama]

OK Thank JoJoKeo,  

But I'm a little confused.  are we all seeing my updated schematic?
 

***Note * Saw that my CF section was disconnected from the B+ node.  Redrew line.  Hope this was not a point of contention.

I replaced the image

[jojokeo]

sorry, comparing w/ tubeswell's schem I see it now...

[llama]

sorry, comparing w/ tubeswell's schem I see it now...

.....Whew!  I was getting worried that I F#%&ed up something royally...  Thank you for even reading my posts!

[jojokeo]

No, you're fine Llama! What messed me up was the morph control combining with the cathode follower use. I was going to give you another thing to experiment with but in seeing the tube & it's use, I think it's okay the way it is? But - as with all CF's for grins, see how much gain you add/remove by lifting the bypass cap on the 5859. It can clean up the tube even more & also differently than the morph control does. It's only a little switch if you like the option and want to add it? Sometimes the triode mode doesn't do enough and this would help give a bit more clean tone/less CF gain for a little smoother response too.