Carolina SongBird SE

[tubenit]

This started off as the Mini-TOS using a 5879 in overdrive with a 6BM8 triode CF  going into a 6BM8 pentode. The mini-TOS was too chimey for me ............... so I converted into the Carolina SongBird.

This amp is much smoother tonally. Still does the sustain and blooming thing but it has a clearer tone. I think the 6BM8 was being somewhat overdriven & the tone was not as clear as on the Carolina SongBird.  It does NOT have the jangly chimey tone of the mini-TOS.

This amp may be the most touch sensitive to picking attack of any that I've built. Without  changing any setting on the guitar or amp, light picking will almost sound like I switched to a clean channel and turned off the overdrive. Diggin in, the overdrive returns. It has very nice harmonics to it.

While the amp doesn't have a switchable OD, you can dial the amp from clean to light overdrive to more overdrive by chaning the pot setting and engaging the PAB and/or midboost.  The midboost switch would be a nice feature to have
footswitchable. The midboost increases both volume and sustain.

With both V1 and V2 being 12A_7 wiring, you can substitute all kinds of tubes for different tonal options.

I am using a 6K6 in the amp currently and love it!  It's about the same loudness as the 6BM8 was.

The passive FX loop between the V1 and V2 seems to have the best balance of transparency that I've tried (since I tried 3 different placements). I think the Mosfet CF on V1b helps the Boss digital delay sound at it's best.

Between the mini-TOS and Carolina SongBird, this amp is much closer to the tone that I like. I definitely prefer a smoother tone to a chimey-er and more jangly or distorted tone of an EL84 tube.

With respect, Tubenit

Schematic and layout changed and more current versions are in following posts.

[sluckey]

Great name for a great amp! Early on in this project I was reminded of the Carolina Wren. Huge voice for such a tiny bird! One day I'm gonna build one of your creations if I can ever catch up to you!

[tubenit]

I finally had time to do thorough tweak experiments.  This version has more headroom & sweeter harmonics.  However, I lost a little of the blooming feature at the previous volume.  I can recapture the blooming by notching the volume pot from 6-7.5.

I have found that on my Tweed BluezMeister and other push/pull amps I prefer a lower value cathode cap such as 10uf/100v.  So that is what I started out with on the Carolina SongBird. However,  I tried a 50uf paralleled with the 10uf and it sounded better to my ears. Then I tried a 100uf and it gave more headroom for chording, so I went with that.
A 60uf cathode cap still allowed for considerable blooming which was slightly loss with the 100uf.  I think the 60uf would be a good compromise if someone played more lead and very little chording.

I changed how I did the PAB and like this version quite a bit more. The 22M resistor keeps the switching quiet. I originally had a 82p across the volume pot but changed that to a switchable 120p & 250p which I like better.

If you're looking for a bedroom type volume with sweet harmonics and a blooming sustain, this amp is worth considering.

With respect, Tubenit

[tubenit]

I made 3 more changes.  The sustain, blooming and harmonics are superb, IMO!!!  This small amp sounds HUGE with the digital delay.

I changed the mid cap from .02 to .03.  This allowed more blooming & lessened the minor distortion on mids. The mids bloom better and are somewhat cleaner. The .03 apparently made a shift in the mids being overdriven and this sounds smoother to my ears.

I added an 82p on the mid pot wiper to ground.  This is only engaged when the midboost is OFF. The 82p goes away with the midboost on.

I lessened the resistor going into the OD section from a 200k to a 100k. This also increased the blooming and sustain. It also made the bass notes sound clearer but changed little on the mids and treble frequencies from what I can hear.

I have never spent this much detailed time tweaking an amp. I will make a tweak or two, then play it for hrs before going to any other experiment.

I think this amp really does have the "wow" factor going for it. I'm very pleased with it. Sounds great with 6K6 or 6V6.

EDIT:  

I have been resolved to continue experimenting to see if continued tweaking really does make a significant difference and eventually end up yielding something really remarkable in tone. I tried changing component values in about 6 different areas. Most of the experiments made no difference in tone ......OR worsened the tone.

However, I did find something that worked well and that was eliminating the 250p &470k to ground going into V2-2. It opened the amp up some more and gave it a more transparent tone with richer harmonics.

8-14-11  I tried upping the 6K6 cathode cap from 100uf to 147uf and liked that and left it in. You may prefer the 100uf?

EDIT:  I eliminated the other schematics and left these two versions. I am re-attaching a Hoffman style layout BUT note that the component values have changed. So edit the Hoffman style layout to match the schematic you are using With

3rd EDIT 8-28
Turns out the coupling cap between V1-6 and the mosfet CF grid/gate is not needed.

respect, Tubenit

[Jack_Hester]

I've been following your build on this and other amps.  I'm not a musician and am envious of those here.  Put my guitar down 30 or so years ago, as it didn't travel well cross-country on my motorcycle.  But, my re-learning of tube electronics is coming along nicely.  So, maybe guitar will follow suit. 

My question is: why use a MOSFET where a single triode tube might do the job?  Is this a place where a triode won't work at all as a CF, or is there that much difference in sound?  Or am I way off base, and don't see it's use?  I'm so engrossed in re-learning tubes that I'm trying to avoid SS components as much as possible, leaving efficiency and ease in build out, for the most part.  My old sponge just doesn't soak up the obvious, more times than not. 

Thanks

Jack

[tubenit]

Jack,

I don't see anything special about using the mosfet other than it saved me a tube. A tube would be just fine, IMO.

With respect, Tubenit

[Jack_Hester]

I don't see anything special about using the mosfet other than it saved me a tube. A tube would be just fine, IMO.

Great!  Another question about your power supply.  Did you use diodes for the same reason, to save a tube?  The 6X5 has the same voltage drop as your diodes appear to have.  Any idea if a rectifier tube would make a major difference in the sound of the amp?  I've read about 'sag' from a tube, but have no idea what it's like.

And, a question about the zener (found it spelled zenor, also) diode between the gate and the source.  It's purpose?

Thanks for the fine work.

Jack

[tubenit]

I used solid state rectification because I did not have any 6 volt rectifiers & had very limited room and wanted the B+ as high as I could get it.

With respect, tubenit

[tubenit]

I decided to go for it and try something more Dumblish and changed the V2 from having a CF to a V2a & V2b gain stage.

I love this amp!!! The blooming and sustain is very musical with great harmonics. It is still very touch sensitive and very articulate with a smooth clear overdrive.

I do not feel this amp needed a switchable gain stage. The transparency and clarity of the overdrive is so good that a desiginated "clean" (different than clear) channel is not needed, IMO.  

Voltages using 12AX7 put about 151-158v on the plates.  A 12AY7 and 5751 have lower voltages.

EDIT:  
I had been having some minor frustration with the mid pot making the mids too gritty sounding beyond maybe "3".  I decided to remove the 2.2uf cap off the V1-8 cathode and reduce that resistor from 2.2k to 1.5k.  I feel like this added considerable smoothness to the amp with no negative downside.  I can easily crank the mid pot to 7-8 with the mids staying smooth sounding now.

2nd EDIT

Turns out the coupling cap between V1-6 and the mosfet CF grid/gate is NOT needed.


With respect, Tubenit

[overtone]

while I was checking it out I took the liberty of exporting the pdf files,
the links should be below,
best, tony

EDIT: eliminate the coupling cap between V1-6 and the mosfet CF grid/gate.

[tubenit]

I wanted to keep experimenting to see what would happen IF I moved the tone stack after the mosfet cathode follower?

Well, it gave the amp quite a bit more headroom but still with decent sustain and nice harmonics. It also gave it more of a Marshall amp overdrive tone than a D-style amp tone.

It kept the touch sensitivity of the CSB. However, it lost some of the vowel tone and the blooming. It has more crunch to it than
the CSB did.

Moving the tone stack changed how the trim, drive and level on the OD interact also. I think this version actually has the most versatile tone range of between the Carolina SongBird, Carolina SongBird Plus and this one.

I'm naming this one the Carolina Hummingbird OD.

EDIT: 9-16-11  I added a 22p silver mica cap from the 6K6 grid to the pin 8 (cathode). This significantly smoothed out the high's and upper mids withOUT eliminating the highs (from what I can hear).  Smoother & no loss of highs.  I tried a 47p, but it began to cut into the high frequencies. 

With respect, Tubenit

[Jack_Hester]

Did your component values stay the same, on this build?  I'm just now opening it, and haven't made a comparison.  Great work.

Jack

[tubenit]

No, some of them changed.  This is a nice sounding amp. Just different & not necessarily superior to the CSB or CSB-Plus.  It has a different feel and tone to it. If you want more headroom, then it would be the one to build.

Tubenit

[JBP]

How about combining the CSB with the Hummingbird for a two channel monster ?!!

[JBP]

Ok, I'm taking the plunge on building a CSB SE. Question about the schematics - 8/22 shows NFB , 8/18 none. I didn't see any discussion. Whats the verdict ? Also I take it the schematic is the gospel, layout dosn't quit match up. Thanks, JBP

[tubenit]

There is no NFB.

Please post how you like the build when done! 

With respect, Tubenit

[LooseChange]

Tubenit, I've been wanting to ask this for quite some time.  (Understand I ask this without attitude)

What's up with the Mosfet CF in your amp designs? What's wrong with a plate driven tone stack? Have you done significant testing and tweaking to try to eliminate it?

If the CF is really needed why not add another tube like a single triode 7 pin mini tube?

Just my opinion.... I won't build an amp with silicon in the signal path.

Thanks Man!!!

[tubenit]

Great question! I discovered these things:

Having a CF prior to the passive effects loop helps with my Boss digital delay.  In fact, with the CF ....... the digital delay sounded as good or better than in the Dumblator that I built. 

Having a mosfet CF saves me space in a chassis with limited space.

Like on the TOS 2CF, I was a triode short getting a CF in front of the passive effects loop for the clean/OD switching. The mosfet met that need without needing another triode.

And I discovered that the CF after the 5879 pentode in the TOS made a BIG difference in smoothness of tone.

And I can not tell any difference tone wise between the triode and mosfet in the CF position. I would be stunned if anyone could tell a difference in tone in a blind A/B test. (refer to R.G. Keene's mosfet follies article )

They're cheap.  

Hope that helps answer the question.    With respect, Tubenit

[Attila]

Tubenit,

a quick hello from South Africa,  would you happen to have a BOM for the CSB + 8-22.

I can't get JSchem to export a BOM from a .sch drawing, all I get is file headings... am I missing something

peace

Attila       

[tubenit]

No, sorry, I do not have a BOM for the Carolina SongBird.

with respect, Tubenit

[Attila]

No worries, I composed a CSB 8-22  BOM  yesterday will post once I have re checked

Noticed 2 discrepancies between Sch and Layout - nothing to serious   

1) Trim resistor on layout in marked 22k vs 14.6k on sch. 
2) Mid Pot marked 250k on the Layout and 100k in the sch.

peace

Attila 

[tubenit]

I am not familiar with replacing mosfets with tubes

I don't understand what you're asking here?   Did you mean to say replacing tubes with mosfets?

Regarding the mid pot value,  I would use 100k not 250k.  However, you can use 250k and if need be ......... put a 220k resistor across the pots.

With respect, Tubenit

[tubenit]

You mentioned replacing a tube with a mosfet. So I was asking what kind of tube would have you used if you had not included the mosfet in the design.  Related to this would be what additional caps and resistors would use to effectively use the tube? 

Typical 12A_7 type tube. Look at Geezer's HoSo56 for an example.

With respect, Tubenit

[Attila]

OK so I finished building my CSB +  on Friday night

I can only say WOW I am totally blown away with the dynamic response of this amp and it firkin loud too - A job well done to Tunenit  

Over the next couple of weeks I will see if there is any tuning to be done but I suspect I will leave this to the Gurus

I fixed up a few minor things on the CSB -8-22-11  Layout, created a BOM and a Voltage Check sheet. See attachments


Attila
 
Edit:  just sorted out standard naming conventions and included .jsch/sch files
 

[tubenit]

Hey, glad you like it!   

IF you have pictures, please post some and your comments on the Tweaks thread!

Thanks for sharing your info and the BOM

With respect, Tubenit

[Attila]

Jeez sorry Gents I forgot to post pics of my CSB build, I tweaked small things, to suit either what I had on hand or to taste most of which were ideas shared by the kind folks from this forum.

I used a 5751,  12A7 & 6l6 valves respectively,  I also wired up a socket for an EL84 tube (Either/Or) and switch up to 16 Ohm on the OT to compensate for the mismatch not ideal, but doable
   
AND  also time to fess up my dyslexia got the better of me .     Middel instead of Middle ........will fix that during the tear down.
 

[tubenit]

Beautifully done!  BRAVO!  Thanks for sharing the photos. 

With respect, Tubenit