TOS Build Log

[dennyg]

I spent couple months planning this build, with lots of great advice from Tubenit and others on the forum, and it's coming along great! I've tried to apply everything I learned in my first three builds, designing the layout from scratch using terminal strips and gothic ring principles - if you're not familiar with g-ring, see last pic with the PI circuit - the b+ and grounds are on one side, signal wires on the other.  Thought i'd post some pics as i progress.  I like to test circuits as i go along and so far so good thru the PI/PA, and the layout seems to be holding up.  Only thing I've missed thus far is OT orientation - I assumed same position on previous builds would yield least hum but ended up having to rotate the OT a bit - but didn't have to move it which would've blown my whole layout!   Anyway, comments / suggestions / questions much appreciated and if I get in a jam I'll post for help.

[purpletele]

Nice looking work!

What is it that you are building, I might have missed something?

Is this a design that Tubenit whipped up with his eggs in the morning.

[Tone Junkie]

That would be the tweed overdrive special.

[dennyg]

OK guys let the troubleshooting begin.  Good news is I like to test each circuit before building the whole thing out.  I've got the PI/PA built and just built the first two stages to test the PI - only with the first gain pot - not yet added the tone nor the FAT stage.  Using a scope, fired it up with a 440 test signal-  80mvpp - set the preamp vol at 9:00 feeds 16Vpp into the PI, yielding 70Vpp output signal from each side of the PI which is heavily distorted (square wave). 
Being my first PP build I'm just learning about all this PI stuff but that seems like a darn hot signal feeding a small triode.
The voltages for the PI seem reasonable but please verify:
B+ 377
82K plate is 236
100K plate is 230
cath 34

Thanks in advance for insights!
Denny

[tubenit]

Those look reasonable to me.  With one of my amps that has a somewhat similar LTPI, I have 247, 240, & 55 with a B+ 366v on 1st node. Mine is cathode biased 5881's.

With respect, Tubenit

[dennyg]

How much signal voltage should this PI take before it clips? 
Not sure how to do a load line with such a cold bias.

[tubenit]

I don't know the answer to your question regarding signal voltage.  You can look at the Amp Garage files on Dumble layouts and schematics and perhaps see what signal voltages they are getting?  Or look at amp schematics/layouts like a Super Reverb for comparison.

Another option is to use a 5751 or 12AT7 in the LTPI if you want.

With respect, Tubenit

[dennyg]

I think I figured out the load line calc and was doing it wrong - referred to the PI chapter in Merlin's book on load line and it's drawn using Rb (470R) vs the total cathode resistance including the tail. So if I'm reading it correctly, each triode of the PI behaves similarly to any other triode stage.  The TOS  design is warm biased and would grid current limit around 2.5V+ and cutoff around 3.5V-

So running the signal straight out of the second stage at 16Vpp is overdriving the PI - so I think the PI is fine - I just need to install the other other components for the clean channel which should provide the proper attenuation before feeding the PI. 

[dennyg]

I finished stages 1-4, added back in the level control for the clean channel (I originally left it out but do need it to attenuate signal to PI for total clean), and wired up the switching circuit.  One of my buddies at AX84 gave me a good summary of PI headroom - with this bias it should take up to 8-10Vpp before clipping - he explained that since the signal is split, each triode will take 4-5V before clipping.  I love this stuff.
As for the tone for the clean channel - glorious! I've never owned a fender but this sure sounds like my friend's super reverb.  I love the simplicity of single tone knobs, and the fat control moves the tone toward my 18Watt marshall. So much versatility and I haven't even gotten to the core pentode circuit of this bad boy.  The amp is dead quiet too. 

A pic with the adds - hope to finish up the pentode channel tomorrow.  Can't wait. 

[tubenit]

Watching this with great interest!  Glad it's quiet at idle 

With respect, Tubenit

[SILVERGUN]

Watching this with great interest!  Glad it's quiet at idle 

With respect, Tubenit

+1
I like seeing the alternative approach youve taken here

[dennyg]

OK guys - i finished it up tonight - pics below.  Fired it up, voltages are reasonable.  The clean channel is dead quiet at full throttle.  The gain channel is a bit fizzy and squeals when MV cranked past 1:00 or so.  Think I've got some oscillations to tame.  I realize that I set out on my own with this layout so will have some tweaks to make, but sounds good enough that I'm likely in striking distance.  A few notes I'd appreciate some comments on:
* With output to dummy load, input grounded and scope on the PI, there are no oscillations within the preamp - everything dimed.  Turning up the PPIMV sets off the oscillations
* I don't drop as much voltage into the preamp as Tubenit's schematic shows - my B+2 (after the screens) is 370V, vs 340V on schematic.  I'm using 1.2K at B+2 and could increase to 4.7K or so if preamp needs to run at lower v's
* I daisy chain the HT in a different order than schematic: B+2 > PI > Pentode > stage 3/4 > stage 1/2.  voltages cascade down accordingly.
Schematic has pentode at the end with lowest B+.  Any issue here?
* The yellow wires are all unshielded signal wires.  I may need to shield those.

[SILVERGUN]

I would post the schematic that accurately reflects what you've got there.
Did you build it 100% to tubenit's schematic. Which one?
This attached pic is the top one on the build thread that Doug links to for his board build.
 
You could try a "snubber cap" across the plates of the PI to help with the oscillation
 
If you used different value dropping resistors in the B+ line that would explain why your seeing different preamp voltages
 
 
 

[sluckey]

Have you verified that the OT ic connected properly to provide correct phase for the NFB loop?

[dennyg]

thanks soooo much for chiming in - the amp is dead quiet with clean channel so focusing on the OD section for the oscillations.
responses to questions:
1) I used the same schematic Silvergun posted.  Only a few variations:
- did not wire up the effects loop - didn't want that variable early on being my first PP amp
- due to layout considerations, the pentode follows the PI in the B+ chain, so it's higher B+ than if at the end; easily remedied if need be
- coupling cap between trim stage and OD pot is 5.1uf vs 6.8uf  - forgot to include 6.8 in my mouser order
- added variable bias (1K 2W) for power stage so i can dial-in between 6V6 and 6L6GC if need be
- running 6V6 at the moment with plates at 396v, screens 372v, Vk 31 (.023ma per tube =9.1W at idle); i have a new pair of matched 6L6GC's to try later
2) I'll try anode bypass caps on PI this eve and report back
3) I only have 6 wires on the OT: primary, 2 leads for the plates, 3 secondary leads.  I didn't think there was a proper order for the plate leads with a PP OT but let me know if otherwise.
4) I originally forgot to add the NFB connection; adding it made big improvement in how far I could crank before squeal so assume everything is in proper phase

Observation I could use comment on:  with the PPIMV off, I get no oscillations within the pre-amp via scope - everything dimed.  Since I'm using PPIMV, the PI should be pumping it's full output only varied by the OD and clean level pots - i.e. if it were triggering oscillations I'd think it would oscillate within the pre-amp even with PPIMV at zero.  I've got 6" leads from PI to PPIMV - was thinking of shielding them.

The tone of this amp is amazing - i just want to be able to crank it to it's full potential w/o squealing

[tubenit]

Try a 220p cap across the entrance triode's plate in the LTPI.  I call this an "enhance" cap. I have found it to smooth out the high end without eliminating the high end tone.  This would be the 82k on the schematic.

I would try shielded wires to the PPIMV for sure.

Moving signal and heater wires gently with a non conductive  stick.  Changing out the LTPI tube for a 12AT7?  Maybe increase the 10K grid resistors to the power tubes to 22K?

Out of curiousity,  IF you dial the OD tone pot to where there is no treble tone coming thru,  does it still oscillate?  I removed the 500p cap to the OD tone pot on one of my amps and it seemed to help smooth out the OD tone.  I am only having a .0047 cap to ground.  And I am dialing the tone pot at around 1pm.    The OD tone pot may make zero difference in the oscillations but I am curious if it would at all?

with respect, Tubenit.

[tubenit]

I'm thinking more about Sluckey's comment.  As I understand it, the PPIMV renders the NFB useless the more the PPIMV is turned down.

I am wondering if you should remove the NFB  wire and simply ground that juncture and then turn the PPIMV all the way up? Does it still oscillate then?   IF not, then I am thinking maybe your OT wires need to be switched?  Sometimes, there is just significant oscillation instead of the "normal" ear splitting squeal if the OT wires are backwards?  Since you stated the NFB wasn't hooked up prior, maybe this idea has already been ruled out in practice? I can't tell for sure what the chronology of information is?

My thinking on this may be incorrect and hopefully Sluckey or someone else knowing can chime in.

with respect, Tubenit

[SILVERGUN]

Once you do determine that the NFB phase is correct and if you still have the oscillation, here are the 2 versions of cap placement that we suggested:

[SILVERGUN]

Since it looks like you already cut your blue OT lead too short to swap it if you had to, you can also just swap the 2 yellow wires that you are going to switch to shielded anyway.
I would just start there.
-Swap those 2 yellow wires feeding the PPIMV to change the phase and see if that fixes it before you go just switching out to shielded wire.
 

[dennyg]

Thanks guys - I did two things that nearly eliminated the squeal.  1) Increased the dropping R after the PI from 4.7K to 10K which dropped another 20V to the pentode, cascaded on down.  That made bit of difference and got my voltages (except the pentode) in line with schematic.  Let me know if there's a reason the pentode is at the end of the B+ chain in the schematic - my plate is 60v higher than schematic but it's easy enough to rewire.
2) Then swapped the PI leads and that pretty much knocked it out.  Didn't think of phase being an issue with PP but good to know. 
I can now push the MV to past 3:00, but beyond that it may just be that I'm in a small room - my other amps can't handle full throttle in that room w/o feedback.  The high end is a bit bright past 12:00 on MV so i might remove the 500p cap from tone control as suggested, but that's minor at this point. Lots of options to strap bypass caps to taste.
I'm just sooooo stoked that the layout worked! This is my first layout from scratch, and took my time doing it, considering all the notes I took on previous projects with lessons learned.  I'd really appreciate any suggestions about what I could have done differently.  I made liberal use of shielded cable but with the 2 channels and effects loop (which I plan to add later with tube buffer), there was just no avoiding it. 
Thanks again Jeff for an amazing design.  I'll record and post clips soon. 

[tubenit]

Glad you're having fun with it and enjoying some success!  Will look forward to the sound clips.   I love it when someone does something innovation like your layout and has it work out well.  Bravo!   

I would highly recommend trying an "enhance" cap across the 82k  LTPI plate resistor. It has been a significant help in smoothing the overdrive tone in my experience.  It removes the high end "hash" harshness without removing the high end.

With respect, Jeff



As an FYI,  DaGeezer was the primary designer around the TOS.  I was corresponding with him early on but it was his creation for the most part & he should get the credit for the innovation of the design. I was simply building one right on the heels of his. 

[sluckey]

2) Then swapped the PI leads and that pretty much knocked it out.  Didn't think of phase being an issue with PP but good to know.

It's not a PP issue. It's a negative feedback issue. Same thing can happen with a single ended power amp.

[dennyg]

I grabbed my phone during lunch break to take a quick clip of the TOS - ok so i'll do something of better quality soon but thought I'd prove that the amp actually works, and with plenty good blues demos around i thought I'd dial-in the high-gain side of this beast as I had no idea of just how high gain this thing could go.  So much for it being only a blues amp!  MV set on 10:00 and I'm playing right next to the phone so even though it's really loud in the room, you can still hear my pick on the strings.

https://www.soundclick.com/html5/v3/player.cfm?type=single&songid=13648055&q=hi

[PRR]

> Same thing can happen with a single ended power amp.

+1.

I very carefully accounted the polarities around my single 6550 amp, and it still came out wrong.

I believe the transformer winders do not look at the transformer designer instructions for start/finish identification. So there's a 50:50 chance it will be wrong. Even two "same" OTs wound by different workers.

[purpletele]

That's a serious amp and a serious tone!

[tubenit]

Thanks for the sound clip!  Sounds great.  What speaker were you using?  I alternated between a Cannabis Rex (the majority of the time) and a Red, White and Blues.

The Tweed Overdrive Special was a very unique amp for me because it had a strong vowel/wah tone in the overdrive that I loved. A friend of mine who opened for Derek Trucks once and also Dave Mattews ended up buying that TOS amp from me.

Please post some more sound clips when you get a chance!

With respect, Tubenit

[dennyg]

Thanks guys - I'm playing thru a pair of Celestion G12H75's in a deep open back Avatar cab.  I've never played the C-Rex but I've had good experiences with Eminence speakers.  They tend to have an 'organic' woody type of tone that's awesome for blues and classic rock stuff.
A few changes I made to the TOS:
* removed the 500p cap from drive tone - it's a bright amp in general and makes the knob more usable - great suggestion!
* added the 220p anode bypass cap across PI 82K side
* replaced the PPIMV leads to PI with shielded - even quieter now at high volume with the drive knobs dimed - i'll never play it that way but any improvement in noise reduction a good thing

Do you think it would be helpful to post my layout for anyone who might want to build using term strips?

Denny

[tubenit]

Do you think it would be helpful to post my layout for anyone who might want to build using term strips?

Yes, I think that would be an excellent idea!  Please post it.   Thanks.

With respect, Tubenit

[SILVERGUN]

Nice job, sounds great!
I'd be interested in seeing that layout

[dennyg]

I'd be more than glad to provide a layout - but need some advice - I did this layout on paper and tweaked as I built the amp - so let me know software to use and copy of existing layout that has all the objects I need and I'll get to working on it.  Would be a pleasure to give something back to this community that's been so generous with time and knowledge to help me along. 

[tubenit]

Visio is the best program. 

ExpressSCH is a FREE download program you can easily use to illustrate it reasonably well.

So, I've attached an editable starting place for SCH layout of your amp that you can use IF you want to.  I am NOT pushing you to do this at all. You can simply post your drawing if you want to.  Having said that, I personally have found drawing schematic and layout to help my understanding of amp designs.

Once a SCH is done,  you export to BitMap then open with PAINT then resave as a GIF.  I can help with that IF you provide a SCH layout.

With respect, Tubenit

[dennyg]

wow - thanks for the jump start!! downloaded SCH and i'm on my way...

[tubenit]

One thing that may or may not be useful to you could be to print the "jump start" off and pencil (with an eraser)  in the components til you're satisfied with how it looks and then redraw those components with SCH. Whatever works best for you .................

Remember on things like the circles, rectangles etc. that you can click on to those and resize them smaller or larger for your needs.

with respect, Tubenit

[tubenit]

We have an entire SCH library with 100's of editable schematics and layouts.  An advantage is one can grab a schematic or layout and easily modify it into something else.  There is "almost" always something that can be quickly edited instead of starting from scratch.

What I've done is the following:

1)  I downloaded from the Library of components in Favorites the following:  fuse, speaker, transformer, small transformer, capacitor,
     polarized capacitor, inductor, pot, resistor, ground, diode, zener, LED, DPDT, SPDT, SPST.  I can draw 98% of anything I want with those.

2)  Looking at previous schematics,  I grouped large areas and download them as custom components such as:  active FX, Fender style
     reverb, one tube reverb,  AB763 clean preamp,  LTPI and power amp,  Dumblish style overdrive channel,  HoSo56  preamp,  Marshall preamp,
     James tone stack,  Marshall tone stack,  Dumble tone stack, Fender tone stack,  PT and B+ rail,  VVR, triode with cathode follower, triode with
     mosfet follower, etc....... etc.....

     All of those can quickly be put into a custom library by clicking on the schematic in our SCH Library that use those.

     So all I have to do is either quickly modify an existing schematic or grab large sections and tie them together.   For example,  I can click on
     the HoSo56 preamp, click on to an active FX,  click on to an LTPI into the power amp,  click on to the PT with B+ rail and I'm done (sort of)

With respect, Tubenit

[dennyg]

I'm slowly working on the layout for using P2P term strips, but meanwhile I'm compelled to report on this amazing amp for those who've either built it or considering their next build. 
* I can't stop playing this thing! The versatility is unparalleled - from sparkling fender cleans to darn near metal distortion, and everything in between.
* I've made a few mods vs schematic:
- I was getting slight buzz (not noticeable while playing but I have OCD) so I added DC heaters conversion for the pre-amp tubes.  Kicking myself for not baking into the original layout because it's only $3 in parts and my PT can handle the increased amperage just fine.  With g-ring, the heaters run right down the middle of the pre-amp tubes and DC eliminates any noise due to heaters.
- Added a 100pf cap across the grid of the pentode - the gain channel is very bright past noon on the MV, even with the existing filter caps, and this mod, coupled with Jeff's mod of removing the 500pf tone ctrl cap did the trick for my taste
- I added a parallel speaker jack and connected a couple of 2x12 cabs to this thing and it is thunderous.  I bought couple of 6L6's which I'll try at some point, but I could easily play for 100+ people as is.  I'm curious as to what this thing would do at double the wattage. 

The PPIMV adds to the versatility. This is my first PP amp and the PI is my new best friend.  The beauty of PPIMV (vs standard MV) is I can use the channel level pots to overdrive the PI - which adds another dimension to the gain tone options - and w/o blowing the roof off.   By diming the PPIMV you basically have a standard non-MV config where you can use the channel levels to overdrive the power tubes, if your ears can handle it. 

If you are considering what amp to build next, BUILD THIS AMP!!!

[dennyg]

I've been reading up on PI options - curious as to why a presence circuit wasn't considered in the TOS design? There's lots of focus on reducing high-end hash and harshness on the OD channel with using some pretty hefty anode bypass caps so would seem a presence circuit would be a natural addition.  If no one has done that yet, I'm willing to be the test subject!  Also just for kicks I removed the anode bypass caps from both the trim and pentode stages and got no parasitic oscillations - yay - but did add more high end on the higher PPIMV settings - so I added in a couple 500pf caps grid-to-ground (which Mr Blencowe prefers over anode bypass caps) and find the tone to be a smoother attenuation.  I'll be adding that to my layout-in-progress.

This amp is so flexible and amazing that I found myself struggling for what to build next 

[dennyg]

Another lunch break demo with my phone - this time to demo the crazy feedback this amp can produce - pardon the hum - my guitar is only 3 feet from the tubes and should've stood back a bit but with the MV only at 11:00 the feedback and sustain is amazing.  I pause between riffs so you can hear that the feedback is controlled - for the most part - I still need to practice with this amp to learn how to keep the f/b under control at the higher settings -  i've just never played such a touch-sensitive amp - continues to impress

https://www.soundclick.com/html5/v3/player.cfm?type=single&songid=13654431&q=hi

[dennyg]

Tubenit - I've been studying the OD section schematic for COS Plus since it's same config as TOS but with a few  tweaked components.  Do you consider these refinements to the TOS OD section or were those tweaks made to support the very different clean channel design feeding the OD in the COS Plus?  Just curious as to the evolution there. 

[tubenit]

These are refinements to have the OD section sound good with the COS clean preamp channel.  Having said that,  I think you would do well to move the tone stack to be prior to the 5879 instead of after the 5879.  I found this to be MUCH smoother.  This is very easy to do and
I would be surprised if it didn't make the TOS overdrive sound significantly smoother.  The other tweaks may just be different and not better.

Just unsolder the wire going to the non grounded side of the OD level from the wiper of the tone control .  Now safely get an insulated alligator clipped wire and try connecting it to the non-grounded side of the OD drive pot.  I A/B-d these trying different tone control settings and different amp volume settings and different guitar volume knob settings.  Across the board, the tone stack prior to the OD drive pot sounded smoother in every case.  It may be different on your amp, but that is what I experienced.

I think there is something about the 5879 pentode that benefits from having the tone control prior to the pentode instead of after it.  I doubt there is any advantage to moving that in front of a 12AX7 triode.

Here is what I currently have in my COS as of today.

With respect, Tubenit

[dennyg]

I've been tweaking the TOS for the past month and found that the beauty of this design is in the endless possibilities to tweak to taste.  Initially I gravitated toward making this a higher-gain amp than Tubenit intended - and made a few mods to protect against blocking distortion, top-end harshness etc - but ended up gravitating back toward the unique mid-gain tonal sweetness provided by the pentode.  Note that this did not squelch the characteristic blooming/feedback/touch sensitivity that you hear on my demo! I finally put a bow around the schematic of a version that to my taste, is tonal ecstasy.  I also included voltages.  Mods to Tubenit's most excellent design in summary:
* 6V6's vs 5881's and updated the schematic w/voltages
* used a different PT and OT, updated voltages
* used a cap multiplier in the power supply. i use this on all my amps now as it nearly eliminates ripple at B+, enabling use of small 10uf decoupling caps and creates a dead-quiet amp
* added DC for pre-amp heaters
* 10k grid stopper at input plus 100pf bleed cap - lower noise than larger stoppers with same miller effect high end filtering
* added 68K grid stopper to stage 2 plus 1m grid leak to reduce input into stage 2 for greater sweep control for clean channel
* added 100pf bypass to clean channel level pot - offsets the high end damping of the 100pf snubber i ended up adding to the PI to tame high end harshness for the OD channel
* reduced coupling cap from 3rd stage into pentode to 5.1nf - originally to eliminate potential blocking distortion (although I don't think BD was really a problem but liked the tightness it provided so left it)
* after much experimentation with screen voltages for the pentode (which controls compression), i ended up with 405K Rg2 and screen voltage of 72 - slightly more compression than the 330K used on the Carolina OS but less than the original TOS - sweet spot to my ears
* strapped 500k across OD level pot for greater sweep control before overdriving the PI
* strapped 100nf cap across the 10nf coupling cap from pentode to PI - added lots more bottom end to OD channel
* increased Rk on PI to 1k to prevent BD (i.e. center vs warm bias)
* reduced second bypass cap on PI from .1 to .047 to reduce chances of BD
* added 100pf snubber to PI - I tried both a 220pf bleed cap with and without the snubber and ended up keeping the snubber w/o the bleed cap for the OD tone suiting my taste - but added a 100pf bypass to the clean channel level pot per above to compensate for the high end loss

I also used a star ground and gothic ring layout - still working on the layout with SCH and will post when complete. 
If you're on the fence for your next project - BUILD THIS AMP! you will not be disappointed and it's very easy to tweak this amp to get it right in your ideal tone zone. 

Schematic in SCH below. Also pic of the amp - just waiting on the faceplate

[tubenit]

The TOS is a great amp and it's DaGeezer's design.  It truly is a "tweaker's amp" and I would encourage anyone who builds it to approach it that way.  Build the original design and then make it your own.

Denny, thanks for sharing your success and the mods you are using. Very much appreciated. Glad you are enjoying your amp!

I put this in a GIF format for forum members who do not have ExpressSCH.

With respect,  Tubenit