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

[tubenit]

Finally got this amp compelted. Sort of a complicated build for just 3 tubes and single ended.

I am VERY happy with how it turned out. Similar blooming and sustain to the Tweed Overdrive. Harmonics are great.
I am as happy with this as the TOS and TBM builds.  

However, it has more chime and less creamy smooth than the TOS. The chime is more like the HoSo56 type chime but with lots of sustain. Very touch sensitive to play. The overdrive is ALOT of fun!  The OD is comparable to the Tweed BluezMeister and Tweed Overdrive but with more chime.  Possibly the 6BM8 pentode being the factor??

My guess is Geezer would really like the chime in this amp along with the solid state rectification. That feature makes it pretty touch sensitive.  I think this "small" amp sounds better than the 56T with 6BM8's and VVR.

I expected the amp to be noisy being a small chassis, sloppy wiring and cramped. However, it is VERY quiet at idle. Both the clean channel and the OD are extremely quiet. Much quieter than the VibroChamp I owned.

Yet this 3.5 watt amp is quite loud when cranked!  Surprisingly so.  I can dime all the controls and NO oscillation!

The clean channel is reasonable clean and has plenty of contrast to the overdrive channel. So it is genuinely useful to build both and not just the OD.

The digital delay sounds excellent both in the overdrive and clean channel. My guess is that is because of the CF mosfet and the CF triode/6BM8?

THANKS for all the help with this amp design especially with the mosfet CF which I view a success.  

The updated/current schematic and layout are here: http://www.el34world.com/Forum/index.php?topic=11607.0

I'll post soundbits and voltages sometime in the next several wks.

With respect, Tubenit

[tubenit]

I am running the amp thru the cab with a Emminence  Cannabis Rex.

The digital delay makes the amp sound HUGE at 3.5 watts 

And again, despite the messy wiring and not neat layout ............. the amp is very very quiet.

Tubenit

[tubenit]

You can't see it but there is a 12v zener with the mosfet CF like on the schematic.

[Willabe]

tubenit,

Another great set of ideas, and another great build.

I was waiting to hear how this amp was going to turn out. Mosfet in the signal path,       But.... you say it's great, so I'll have to let this sink in.

I will say this, believe it or not, one of --- the best --- live guitar tones I have ever heard was a guy named Carlos Jonson, using a SS Acoustic amp (early 70's?) with a late 50's Gibson 335 with humbuckers. He's a Chgo. blues man.

I walked in a usual haunt called B.L.U.E.S., holds no more than 150 people tops and got a beer and started talking with a buddy of mine Bob Levis who played guitar for Lonnie Brooks and Otis Rush for many years. He was very excited about Carlos' playing and tone that night. As I sat there and listened I was floored, the man was tearing that guitar up. He was bending those strings as good or better then Albert King, Otis Rush, or S.R.V. and the tone was out of this world. Another friend of ours was there too, a guy named Mark Skyer, great guitar player and singer. He played a short time with Canned Heat and played for the great Etta James for a while among others. He said to me "I think Carlos is the guy who's wearing the Chgo. string bending crown right now."

So I said to Bob whats the amp he's using? He said an Acoustic (SS) amp.        I said no way, I can't believe it. So I walked up front and sure enough it was a SS amp.  

I've resisted telling this story here but with you putting a mosfet in the signal path in this build and it turning out so good sounding, I thought I'd go ahead and tell it now.

Glad it worked out so well for you.


             Brad                  

  

[Geezer]

Looks great, T! Hope to hear some sound clips soon....

I'll be building this into my "new" VJr head as soon as things slow down (this fall/winter, I suspect  ).

Thanks for the updates!

G

[Fresh_Start]

That gorgeous curly maple on top of everything else just kills me.  Beautiful work all the way around.

I'm sure that the MOSFET cathode follower is necessary to drive the effects loop, especially where there is no recovery stage between the Return and the power tube.

Cheers,

Chip

[theundeadelvis]

HOLY Sh&! 

[eleventeen]

The maple is beautiful, really nice looking!

Where do you get your chassis?

[Ritchie200]

I don't believe that it even works.   Until I hear a sound clip, I think you are LYING!

Yer pal,
Jim

[DummyLoad]

nice work mr. tubenit. can;t wat to hear some clips.

 

[Tone Junkie]

Cool glad it turned out so well, I like how your always thinking outside the box and bringing new ideas around.
Thanks Tubnit cant wait to hear it.
Bill

[John]

Just beautiful. Love the wood!

[ncusack]

Congrats on the build tubenit. I'm glad to hear it worked out for you. When you get around to posting your voltages it will go a long way in helping me with my 5879 Champ build.

[Tone Junkie]

T   I have been admiring your woodwork , Ok jealous would be the right word that is just plain fine.
Bill
                                                       

[LooseChange]

Nice work... It's far from sloppy!
You may need to move this to the solid state forum due to that Mosfet. 

[kagliostro]

Very nice build

thanks for share schematic

Kagliostro

[tubenit]

You may need to move this to the solid state forum due to that Mosfet. 

YIKES!  Maybe that wasn't such a good choice for someone called "Tubenit".   

Guys, thanks for the nice comments!  The inspiration and help from this forum made this build (& many others) feasible for me.

I'll get soundclips and voltages up soon.

With respect, Tubenit

[JBP]

Looks great , anxious to here sound clips. Tubenit where do get the solder lugs riveted under the eyelets on your board ? I'd like to try this and can't seem to find them at the usual places. Thanks, Jeff

[tubenit]

where do get the solder lugs riveted under the eyelets on your board

I have no idea? That was a cheapo tag board. When I have built Hoffman's turret boards, I find Hoffman's boards are MUCH MUCH sturdier. Even using a solder sucker on the solder lugs can rip them right off. That's how flimsy they are.

with respect, Tubenit

[tubenit]

I discovered an "issue" with this amp that I want to resolve. Ironically, I would play the amp with NO digital delay ....   OR I would plug in the delay prior to turning the amp on.  Well, I finally did a A/B comparison and discovered that the delay pedal in the passive effects loop lowers the volume somewhat.  The amp without the delay is impressively loud for a 3.5 watt amp.

The digital delay in my other two amps with LTPI does not lower the volume at all. I am thinking the 220k grid resistors to the power tubes which are after the digital delay in those amps prevents a problem?

So, I was puzzled by that?  Then I got to thinking that either the Mstr Vol on the clean or the OD2 pot on the overdrive was acting like a psuedo grid resistor to ground for the 6BM8 pentode when the delay pedal was not in place.

So when I insert a delay pedal between the pentode and the Mst Vol or OD2 pot, I think that maybe this is messing up the needed psuedo grid resistor (at some level)?

So now I am thinking I need to add a grid resistor to ground AFTER the passive effects loop?  Does this sound like I am headed the right direction in this thinking?

Thoughts???  I realize that the two amps with LTPI have an additional gain stage following the passive effects and this amp doesn't.  So it could be that maybe?  However, it is plenty loud without anything in the passive effects loop.

I've also listed an option #2 that would move the passive effects loop prior to the master volume and prior to the 5879 OD which could also act as a recovery in the FX loop?

Thanks, Tubenit

[LooseChange]

The effect may just be loading down the power tube.
Just guessing here... add a grid leak to the power tube. Try something between a 220k and a 1m.

If you turn the amp volume down and play through the effect is the volume drop as noticeable?

[tubenit]

If you turn the amp volume down and play through the effect is the volume drop as noticeable?

Great question! I'll check that out. And I'll try a grid leak resistor.  I am thinking since the OD pot is 500k, that I'll start with a 470k and see what happens?

Thanks for your help!  It is greatly appreciated.

With respect, Tubenit

[Fresh_Start]

This is just an uneducated guess, but IIRC a higher grid return resistor should produce more gain from the power tube.  Is is possible that the end of the delay effect circuit has something lower than 500K going to ground and that's what is reducing the volume?  If that is the case, is there a way you could put an AC coupled cathode follower after the effects loop return and then have a 470K grid return resistor after that cathode follower?  You've already let the genie out of the bottle, so what's one more MOSFET?

HTH

Chip

[tubenit]

Well, I am learning that a SE amp is different than a push/pull amp when it comes to a passive effects loop insertion point.  
 

I tried numerous grid leak resistor values to no avail. The Boss digital delay still had less volume and after careful listening I noticed it colored the tone with some slight distortion. In other words, the tone did not sound as transparent to me.  Ironically, I could NOT tell any difference adding a grid leak resistor from the psuedo grid resistor of the master volume and/or OD2 pot.

So, I tried the delay between the guitar and amp. It sounded nicely transparent but it had too much white noise for me. I am somewhat picky on this issue and really like a quiet amp.  It also had no volume loss.

So I thought, I wonder if I bypass the V1a input gain stage and put the passive effects between V1a & V1b if it would work?

It does !!!!  Sounds fantastic to me!  Very transparent with no volume drop at all. It adds delay to the tone without coloring the tone that I can tell.  Works perfect on both the clean and OD. The white noise is cut way way down on the delay after the 1st gain stage.  I can't tell that it adds any more noise between V1a and V1b then it did inserted prior to the 6BM8 pentode?

When I lowered the 220k down to 22k going into the grid of the 6BM8 pentode, that opened up the amp some also. I think it added to the transparency.

I am super pleased with the results of all of this! This is an ideal bedroom type amp for me. I can get the blooming and sustain at very reasonable levels. It is a very fun amp to play.

Would I rebuild this with the mosfet CF and the 6BM8 triode as a CF after the 5879.  Yeah, I would.  I don't know if the mosfet CF matters that much?  I'm thinking it allows a somewhat clearer clean channel but I don't know for sure.  I am fully convinced that the CF after the 5879 OD stage is pretty important to get the smoothness in tone that I want.

Updated schematic and layout are here:  http://www.el34world.com/Forum/index.php?topic=11607.msg107840#msg107840

With respect, Tubenit

[Willabe]

Hi tubenit,

I wonder what it sounds like if you would jumper across the mosfet CF, to bypass it? Maybe it does'nt need it now that you have moved the FX loop?


       Thanks,    Brad       




 

[jojokeo]

So I thought, I wonder if I bypass the V1a input gain stage and put the passive effects between V1a & V1b if it would work?  It does !!!!  Sounds fantastic to me!  Very transparent with no volume drop at all.

Now you're thinking inside the box and outside the box 

Congratulations on your amp T!

[Fresh_Start]

Hi tubenit,

I wonder what it sounds like if you would jumper across the mosfet CF, to bypass it? Maybe it does'nt need it now that you have moved the FX loop?


       Thanks,    Brad       

I was wondering the same thing.

So you like having the delay before the overdrive section?

Cheers,

Chip

[DummyLoad]

i was thinking you'd want the FX loop global as well - as in either OD or clean then the FX loop.

attached is what i was thinking about...

respectfully,

--DL

[Tone Junkie]

This is all very interesting good work T.
Bill

[tubenit]

I think I'll try bypassing the mosfet CF and see what tone change (if any) there is?  And report that.

This amp is somewhat chimey-er than I prefer. I prefer the real creamy smooth tone more. It's more EL84-ish chimey, IMO.  And I have typically not been a fan of EL84 amps for some reason.  I think my son & son-in-law would love the chimey tone of this amp alot.  I think both the 5879 and the 6BM8 bring some chimeyness to the table?

I may eventually  possibly put the mosfet CF behind the 5879 and convert the power tube from 6BM8 to a 6V6?  Not sure? I'll record it as is prior to converting IF I do change it.

With respect, Tubenit

[jojokeo]

i was thinking you'd want the FX loop global as well - as in either OD or clean then the FX loop.

That's what I was thinking also D/L.

In prior (beginning) posts T said he really like the clean tone one way and the OD tone another way regarding the ss CF approach. This was when only one of the switching positions he stated that he liked and was perfect w/out the ss CF. Then when he developed/perfected the ss CF stage further, he incorporated both stages to the ss CF. I questioned this (why mess w/ something you like and say "is perfect") but he decided to do it his/this way anyway. I'm curious how this will all play out especially w/ T being the master of trial and error that he is!? 

[tubenit]

The reason I changed the insertion point of the FX loop from after the clean/OD to before the clean/OD is that after the clean/OD distorted & colored the tone somewhat & the volume dropped compared to no pedals plugged in.

Moving it between the V1a & V1b has maintained the same volume, much better transparency and less distortion.
Not sure why that is the case, but it is?

I had it wired exactly like DummyLoad suggested with the exception of the coupling cap after the passive FX loop prior to the grid leak resistor.  Maybe that cap is needed & that's why I was having problems?  I tried a variety of resistors and none made any difference to the volume drop & colored/distorted tone.  Maybe that additional coupling cap would resolve that concern? 

I think this is what you're referring to?:    On my TOS with  the tube CF after the 5879, I discovered the delay pedal sounded excellent with the OD on.  However, on the clean channel of that amp with the CF, the delay doesn't sound as good.  That is what prompted me to try a SS CF on the clean and use the 6BM8 triode behind the 5879.

However, doing that didn't work on this mini-TOS SE amp for some reason with the Boss digital delay? I don't know if the LTPI on the TOS made the difference or not?

With respect, Tubenit

[jojokeo]

Yes, I understood why you changed the effects loop point and also said "way to think inside the box" referencing the possible loading affect by the effect. :) I thought this was very creative on your part.

The coupling cap "may" not be needed or make any difference but it's always a good idea to include and to be safe - not for safety's sake but for any possible DC that could find it's way from something being plugged in to the return jack. Effects boxes usually do this on their inputs & outputs as a matter of "good form" so that it also prevents any small DC from messing up something that it's plugged in to. Also this is similar to a coupling cap used sometimes before a tone stack or even before a pi stage when the signal is coming directly from a tone stack for two examples. There should be no need for the extra coupling caps used in those places but they don't do any harm.

[tubenit]

Jojokeo,

OK, thanks!  I appreciate the info about the coupling cap post the FX loop.


I thought I'd post a sound clip before I start messing with really experimenting with the amp. Excuse the mediocre playing, please.
 

http://www.soundclick.com/player/single_player.cfm?songid=10784031&q=hi&newref=1


I had a 12AV7 in V1 in this clip. A 5751 gives it more sustain and blooming.

The volume was at about 5.5, the OD1 was about 5.5 and the OD2 was 6.  There was plenty of room to increase blooming or sustain.

I don't think this amp is as smooth as the TOS or TBM amps?  It's pretty chimey live. I have some friends that I'd think might prefer the chime more, but my personal preference is to get it a little smoother in tone. So I'll experiment a little more.

With respect, Tubenit

[Ritchie200]

I don't believe that it even works.   Until I hear a sound clip, I think you are LYING!

Yer pal,
Jim

Ok, I guess I believe you now....

Sounds really nice!  What geetar were you using?  I'd also like to hear some chords blasting outa that thang!

Yer NEW pal,
Jim

[VMS]

I think the problem with the fx-loop right before the power tube is that 6BM8/ECL82 tube needs driving voltage of 9,5Vrms for full power. Your delay probably gives the standard +4dBu line level output which is only about 1,2Vrms. So you need a gain stage to up that voltage. In push-pull amps LTP PI usually has enough gain for the passive loop to work.

[tubenit]

Ok, I guess I believe you now....
Sounds really nice!  What geetar were you using?  I'd also like to hear some chords blasting outa that thang!

WHAT !!!!   A compliment??!!   Gee, I know you must be up to something now.  I will be vigilantly on my guard.  

Well, prior to my smashing the guitar into smithereens at the end of the show .............. I was using the Tele (Note it was NOT a strat) on the left.  However, I discovered people are less impressed smashing a guitar into a 3.5 watt amp then they are a 100 watt amp. Go figure!
 

With respect,  your sort sometimes pal back ...........Tubenit

[Ritchie200]

Oh....a tele......

[tubeswell]

That's a very compact wiring set-up tubenit! (You might think your own work looks cramped, but its actually very tidy).

[tubenit]

I've drawn up a mini-TOS with a 6V6 version.  This would use the mosfet CF following the 5879 pentode.

Haven't built it (yet).    The editable version with a layout is here:  http://www.el34world.com/Forum/index.php?topic=11607.new#new

With respect, Tubenit

[jojokeo]

T, what does the 470k + 250pF in parallel w/ the 250k trim pot & treble bleed cap being used for or how does it help and/or work exactly? I've seen this w/ the HRM & other dumble-ish builds also.

My initial thought or theory is that it's bleeding a tad bit of highs to ground through the 470k resistor thus helping to shape the frequency response of the trim pot for the signal going into that 1st OD stage?

[tubenit]

Is it possible to add negative feedback to this mini-TOS amp?  I am wondering how that might work doing it as I have
drawn it?  Any thoughts?

With respect, Tubenit

[DummyLoad]

it will become + feedback when you kick in the OD. maybe switch it out in OD.

[Fresh_Start]

tubenit - if you read Keen's "MOSFET Follies" which discusses using a MOSFET as a cathode follower, he was careful to note that the solid state device would be more linear than a tube triode and would not provide the compressive effect of a tube triode cathode follower.  It's worth trying, but I'm guessing that you won't get the nice compression that you noted with a tube triode in the CF spot after the 5879.

Regarding NFB, if you used a tube triode for the CF after the pentode in the 6V6 circuit, then you would have an additional gain stage which could be used as an insertion point for NFB.  A low gain, cold biased triode might add to the amp anyway.  Maybe a 12AY7 or 12AU7 in that spot?

Just a Sunday morning thought...

Chip

[jojokeo]

Is it possible to add negative feedback to this mini-TOS amp?  I am wondering how that might work doing it as I have drawn it?  Any thoughts?

Are you trying to lessen gain? Maybe try a conjunctive filter accross the OT primary and see if this helps w/ what you're looking for?

[tubenit]

OK, I tried a few things.  Let me start with what did NOT work out.  What didn't work was removing the mosfet CF after the V1-b. I did not like the tone on the clear OR the OD ............. so the mosfet CF went straight back in. The CF seems to add some smoothness.

I changed the V1-6 plate coupling cap from .02 to .01.
I changed the 5879 cathode resistor from 820R to 410R.

Biggest improvement was a 22pf from 6BM8 pentode plate to pentode cathode. It cut down on the chimeyness somewhat.

With respect, Tubenit

[Fresh_Start]

tubenit - thanks so much for going through this process with us!  Knowing what didn't work for you is just as important as knowing what does work.  Most amp designers never mention all of the things that don't work out - all we see is the end result of their experiments.

Cheers,

Chip

[jojokeo]

T - I agree w/ Chip, but I'd personally would like to see what you think of the conjuctive filter instead of the 22pF cap? It would be a real quick and easy change-out to see how it works or not regarding your chiminess issue - that's part of what they're for and making things more linear which is also what NFB does (and you mentioned wanting earlier)?

[tubenit]

I'd personally would like to see what you think of the conjuctive filter instead of the 22pF cap?

Can you point me to a schematic with the conjunctive filter for a SE amp?

With respect, Tubenit

[tubenit]

OK, I found a website with some great information on a conjunctive filter for a SE amp.  It has some excellent soundclips giving a clear demo of the differences.  I like the smoothness I am hearing that it adds in the soundclips.

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

I will give this a try and report back.

With respect, Tubenit