Mini-TOS evolution ........

[jojokeo]

Is it something that's only limited to pp amps? I would think you could put a cap or cap+resistor across the pri tranny B+ and plate. I still think it'd be easy to try don't you think?

[tubenit]

The link I posted above gives both schematic and soundclips of SE amps with conjunctive filters.  Yes, I plan to try it.

With respect, Tubenit

[jojokeo]

Thanks for the link as I'm not on my home 'puter. BTW, good sound samples & I like the difference it made.

[tubenit]

OK, I have played the 6BM8 version for a few hrs and it's too chimey for me. It's more like an EL84 & I've gotten rid of every EL84 amp that I've owned or built. Just not my tone. I am gonna skip the conjunctive filter at this point.

I'm going a significantly different direction with this.

1) While the passive FX loop sounds good between V1a & V1b,  I think it would sound even better prior to the V2 gain  
    stage. So I plan to move it.

2)  I also discovered with the Mini-TOS that I can dial the controls with the OD engaged to get a very clean tone. Not
     much difference between the clean and "OD dialed clean".  However, the OD can also be dialed to get that blooming
     sustain that the clean doesn't have.

3)  So I am going to eliminate a switchable OD gain stage.  Instead, I'll do some cathode cap switching to go from a
     cleaner tone to a more overdriven tone.

4)  I decided to go back to a PAB on the tone stack.

5)  I am convinced some of the chimey-ness is from the 6BM8 pentode & the 5879 pentode also. So, I am going to
     go with a 6V6 for a smoother tone.  And I am going to lose the 5879.

6)  And I am going to use a 12A_7 with a CF in the V2 position. I did this on my friends  "Double C" amp and it sounds
     excellent.  This way I can use everything from a 12AU7 to a 12AX7 & dial in different gains with different tubes. I
     think it will actually make the amp more versatile from clean to OD than the switchable clean to OD approach.

Don't know if I will have to get it all done this wkend, but I'll keep you guys posted.

With respect, Tubenit

[sdp1234]

http://www.cs.cmu.edu/~ehn/ax84/#filter


Same link I posted a few posts up.  Tubenit

[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).  

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.

Editable SCH schematic and layout are here:  http://www.el34world.com/Forum/index.php?topic=11889.0

With respect, Tubenit

[Willabe]

Man tubenit, you are an idea machine! Hats off to you sir.       

Thanks for sharing not only the finished thing, but also the whole thinking and build process.

Great name too. Look forward to hearing the sound clips. (BTW, FWIW, I think your a better player than you allow yourself credit for.)


        Brad         
         

[Fresh_Start]

And now for something completely different!

I love the fact that our friend tubenit has no hesitation about saying "I just don't like this, so I'll change it by..."

Really interesting circuit evolution.

Cheers,

Chip

[Tone Junkie]

Good job T Im glad you found the sound you really like. and gave us interesting things to think about along the way.
Bill                                   

[tubenit]

Here is the Carolina Songbird amp played reasonably clean. Only about 3 watts with 6K6 tube. Nice low volume. The song did not record real well but you can get the idea. There is some sustain and overdrive towards the end of the sound bit.   The clean to the OD shows the picking dynamics because I never changed the volume. It sounded smoother live, IMO.
 

http://www.soundclick.com/player/single_player.cfm?songid=10829392&q=hi&newref=1

I like the sustain at the end of the soundclip.  That's with a 12AV7 in V1 and a 12AY7 in V2.  Obviously with a 5751 or 12AX7, you'd get ALOT more.

And we are all gonna pretend that I didn't really do this one (same amp played pretty clean):

http://www.soundclick.com/player/single_player.cfm?songid=10829410&q=hi&newref=1

 

And with the mid-boost switch engaged. Excuse the very mediocre playing on this one.

http://www.soundclick.com/player/single_player.cfm?songid=10819072&q=hi&newref=1

With respect, Tubenit

[John]

Well I would post a few soundclips, but while I don't mind you all snickering at me bumbling my way through the build, I'm not brave enough to put up my playing. You want mediocre, by gum I'd show you mediocre! 

[Tone Junkie]

Tubnit I like the bite and pick attack when you dig in the amp sounds good in all 3 segments. I always Liked the lead sound with a good mid boost .
Thanks Bill

[moonbird]

Tubenit --

Watching you go from great design to great design has my mind spinning -- thanks for sharing it all with us.

Have couple of small questions - I looked carefully  but could not find the primary impedences for the mini-TOS and Carolina Songbird.

Thanks again for the great ride!!

[tubenit]

I think it's 7k primary to 8 ohm speaker.
With respect, Tubenit

[moonbird]

Tubenit -

Checking my trafo stash I came up with a couple of Hammond 269GX PTs - they are 225-0-225V 75mA. If I used them, I guess I could put in a SAG resistor and/or choke to get back down to 269EX voltages. What DCR would you recommend I shoot for? thx much!!

[moonbird]

Just read the ongoing thread about dropping PT voltage with a zener or power resistor on the center tap. Think I am more interested in a SAG resistor on the B+ unless it is too big and causes too much sag in which case I will probably use both methods until the right SAG level is reached.

Tubenit -- Do you have any idea how much current to use for the calculation for both the mini-TOS and the Songbird?? thanks much.

[tubenit]

What power tube? Use tube rectification if needed

[moonbird]

I would like to try both the 6v6 and 6BM8 designs you have posted. My reasoning (or madness) for using a sag resistor instead of a tube rec is that in Class A my understanding is that tube rectifier SAG cannot not happen. The SAG resistor is the only way to simulate the effect on a Class A amp. Thanks for the response Tubenit.

[tubenit]

Just an update.

I've got one of Doug's 12AY7 in V1, an old 5751 in V2 and now I've got an NOS RCA 6K6 in.  With that combination, I can dial in both a reasonably clean or very smooth overdriven tone.  The digital delay makes this small amp sound huge at a very low volume. This is working even better than the VVR'd amps for playing at home.

This is proving to be one of the most fun amps to play that I've ever plugged into. It just has a really sweet musical feel & the RCA 6K6 gives this amp a really smooth tone. Again, the picking dynamics really change the clean to OD tone significantly. It is very touch sensitive,IMO. 

I am convinced the mosfet CF helps with the digital delay and the 12A_7 triode helps with the OD tone control allowing it to be less edgy and somewhat smoother. 

FWIW,  I think the 6K6 is a smoother tone than the 6V6 tube.

With respect, Tubenit

[sdp1234]

Tubenit,

I have been planning a dumble style se amp for a while now but I think this may be a better way to go.  Just a couple questions.  If I have room is there an advantage to adding another tube and replacing the mosfet with a tube or would you stick with the mosfet?

I really would like to be able to retain the clean channel.  I know you said that you could dial in the overdrive channel to get clean but I really want to be able to go cleaner with a foot switch.  Should I try to add this back in or would you recommend staying away from the switching?

Thanks for your design

[tubenit]

I think it would be fine to use a tube for the CF instead of mosfet, but it's certainly not necessary.  The reason I am not doing the switching is that to get the tone I wanted using a passive effects loop with my Boss digital delay, I found I needed another gain stage prior to the power tube.  IF you are NOT using a passive loop, then I'd definitely go for the switching.

Having said that ................. this recent tweaked version has more clean headroom but still maintains excellent sustain and nice harmonics. In other words, it is a comparable sustain but has a cleaner less slightly distorted tone.

With respect, Tubenit

[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. I put a SCH editable schematic and layout in the SCH library.

With respect, Tubenit

[sdp1234]

I think it would be fine to use a tube for the CF instead of mosfet, but it's certainly not necessary.  The reason I am not doing the switching is that to get the tone I wanted using a passive effects loop with my Boss digital delay, I found I needed another gain stage prior to the power tube.  IF you are NOT using a passive loop, then I'd definitely go for the switching.

What if I had the third preamp tube using one triode for the cf and the other as a gain stage for the clean channel after the passive loop?  Would that maybe give me the best of both worlds?

[tubenit]

I only had about 5 min to draw this for you. IF I am understanding you correctly ....... then yes I think something like this
would work just fine.  I would use a 12AU7 or 12AY7 for that CF & following gain stage. That would sort of be like installing a built in FX loop (kind of).   Since I was cutting and pasting the values of that gain stage may need tweaking?

With respect, Tubenit

[Geezer]

 I would use a 12AU7 or 12AY7 for that CF & following gain stage. That would sort of be like installing a built in FX loop (kind of).   Since I was cutting and pasting the values of that gain stage may need tweaking?

With respect, Tubenit

Yes, that looks right (all the parts are there), but as T said, after you actually get it wired in, there will probably have to be some tweaking of the values (biasing the extra gain stage & coupling caps & such)....or it may be just perfect "as is". You'll have to get it done & see if you like the results or if it needs some tweakage.

G

[sdp1234]

exactly what i was thinking.  Thanks for the help guys.

[sdp1234]

What is the purpose of the first pot (100K)?

[tubenit]

Mid pot. You could use a resistor and a spdt center  off for 3 different values.

With respect, Tubenit

[sdp1234]

Mid pot. You could use a resistor and a spdt center  off for 3 different values.

With respect, Tubenit

But isn't there already a mid switch?  I thought the switch with the 390P cap was the mid switch?  What would be the purpose  2 mid switches?  Sorry for the questions but all this tone shaping is difficult for me and I'm trying to learn.  Most of my builds had the traditional tmb off a cathode follower.

Thanks

[Geezer]

Mid pot. You could use a resistor and a spdt center  off for 3 different values.

With respect, Tubenit

But isn't there already a mid switch?  I thought the switch with the 390P cap was the mid switch? 
Thanks

That's the mid-BOOST switch, it increases the coupling cap to the treble pot for a big boost in mids ....the mid pot gives you control over the "normal" mids

[sdp1234]

Thanks Geezer,
Eventually I'll learn

[tubenit]

My goal with this amp was to increase blooming and clean overdrive and sustain.


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 which of the mids are being overdriven and this sounds signficantly smoother to my ears.  I A/B'd over and over setting the controls on the amp and guitar in different positions and it consistently gave a smoother tone with more bloom.

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. The 82p took off some high frequencies when the amp is set to a cleaner tone. IF you like chime, then leave the 82p off completely.

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. It may have boosted the mids and highs just a tiny bit?

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 very pleased with it. Sounds great with 6K6 or 6V6. It is the "biggest" SE amp tone that I've heard in person.

With respect, Tubenit 
 











http://www.el34world.com/Forum/index.php?topic=11889.new#new

[tubenit]

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 have a resistor box and a capacitor box that made tweaking a quick process.

I tried changing component values in about 6 different areas. Most of the experiments made no difference in tone ......OR worsened the tone. Everything I experimented with in blue was useless or worse.  I simply tried different values in the coupling caps where indicated in tone stack and/or between gain stages.

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.  What I did is I paralleled the 250p with the 100k & 120p.


I would say this amp is the most touch sensitive of any that I've built and has the most transparent harmonics. It's just got a very musical tone to it (for lack of a better description).  I feel like the bass, mid and treble frequencies are very very nicely balanced now!

I now have added some voltages to the schematic.  I am using a 12AY7 in V1 and a 5751 in V2 & continuing to use the 6K6.

I have updated the schematic and layout in the SCH library. At this juncture, I can't think of anything else to try? So this should be the "final" version.

With respect, Tubenit

[jojokeo]

...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.  What I did is I paralleled the 250p with the 100k & 120p.

With respect, Tubenit

Thanks for the update tubenit. So you have a very small cap paralleled w/ a resistor and even smaller cap. I personally don't think all of this necessary as you're simply making a very high pass filter which should be able to be accomplished w/ only a single capacitor? Have you tried just doing that instead? The reason I say this is because when I used to do my tone stack bypass circuit sort of in the same way. I used a larger one for blocking all DC and the resistor to block some low end w/ another cap paralleled to allow a certain amount of high end through. But, simply using a smaller coupling cap did the job even better and I was only over-doing something and making it more complicated when it didn't need to be w/ three parts instead of one. It ended up sounding better when I simply found the right cap. (I got a cap decade box which helped also). I've been finding that the least resistors or resistance used in the signal path the better as far as "tone" is concerned.
In your circuit you're feeding the second triode w/ such a high end signal it's very similar to feeding a reverb input circuit where you're only boosting the extreme high end, but if it's giving you what you want then bueno. These are just my thoughts and experiences and giving you some input.

[tubenit]

Sure a single 390p cap would be better. 

I simply paralleled the 250p with the 120p because unsoldering two caps and soldering back in one was more work then soldering the one leg of the 250p to the 120p. If you look at the schematics I posted, the 250p was already in there but going to ground thru a 470k.

With respect, Tubenit

[John]

Tubenit, the switching shown in your schematic... are they simply switches or relays? I keep meaning to utilize the relays in the amp I built, but it's sounding so good I hate to mess with it anymore. Thanks again for sharing your work!

[tubenit]

They are just switches unfortunately.  I wish the midboost & PAB was a relay because I'd definitely use it for leads and leave the midboost off for chords. 

The amp chassis is really small though & it would be a challenge to install a relay.

With respect, Tubenit

[jojokeo]

Sure a single 390p cap would be better. 

I simply paralleled the 250p with the 120p because unsoldering two caps and soldering back in one was more work then soldering the one leg of the 250p to the 120p. If you look at the schematics I posted, the 250p was already in there but going to ground thru a 470k.

With respect, Tubenit

T - I did look at the schem and you mentioned the 100k resistor in there too. As long as it's in there it's not the same thing. Respectfully, J

[tubenit]

I personally don't think all of this necessary as you're simply making a very high pass filter which should be able to be accomplished w/ only a single capacitor?

OK, got it now.  You're right, I wasn't understanding you correctly. You're meaning eliminating the resistor and the cap and just having a cap.  I thought you were meaning having one cap & resistor instead of two caps & resistor. My error.

Yes, I tried that & did not care for the tone. I did try a cap there with no paralleled resistor. I felt like it was overdriving that gain stage more than I wanted it to.

However, perhaps I may have not tried it in the fashion that you are suggesting & didn't experiment enough with different caps?  I used the lower value caps like 500p to 47p alone. Maybe a larger value would have worked?  I don't know.

Or maybe if the grid resistor was larger, then the single cap would have worked?

With respect, Tubenit

[jojokeo]

When cascading a stage or two, I almost always end up needing or wanting to "siphon off" some signal. Similar to what's going into the OD stages. Something you might like to try would be to use split load resistors on V1B's plate? But you have your "clean" channel to worry about and you'd lose some signal from this point and it may not drive the output tube as much. However, it's a low output amp anyway and you may be okay w/ this? You may have a lot gain in reserve already? Then you could go w/out any voltage divider all together and w/ less resistance in the signal path, you'd retain better clarity and tone into your OD section. You'd still have your gain/vol control and grid resistor's resistances which would lessen too much high end and you would still have a smoothing cap on the incoming OD stage or feedback cap to further bleed off highs in those ways if you wanted. Again, just "thinking out loud" & throwing ideas out there.