Mini- Tweed Overdrive 6BM8 & 5879 CF

[tubenit]

I haven't built this amp but would sure like to.  Maybe someone on the forum would be interested in giving this one a try.

It is a hybrid of Mat's 86 Special and the Tweed Overdrive Special that uses the 5879 with cathode follower.

I'll draw up a layout for it when I can find the time.

5751, 5879 and 6BM8.  And only 5 full size pots.  Very simple amp that I think would sound killer!

With respect, Tubenit

[tubenit]

Layout and schematic are in the editable SCH file here:

http://www.el34world.com/Forum/index.php?topic=11607.new#new

[topbrent]

This would be a lot of fun to build in to a Epiphone Valve junior head.

Just add the 125ESE OPT and drill for the extra tube and pots and you are ready to go.
 

[Geezer]

This would be a lot of fun to build in to a Epiphone Valve junior head.

Cool looking circuit, T!

On the Valve Junior, anyone else noticed how scarce they are nowadays? Since they've been discontinued, if you do find one for sale, they usually want "an arm & a leg" for it.

I should have bought a bunch when they were $84 shipped (brand new) @ MF! 

If anyone has one to sell (even just the head cab, or not working) let me know.........

G

[Geezer]

Just an update on my Valve Jr search.....I found a NOS unit (still in the original box) for $90 shipped, so I got it for close to what I paid for my 1st one.
I'll probably build a Tubenit TOS 5879CF into this one, using a pair of PP EL84's...

G

[tubenit]

Wow!  $90. Very cool.  Will look forward to hearing what it sounds like with the EL84's.  I bet that will be a great sounding amp.

Tubenit

[tubenit]

OK,  I have the board built for this amp!  Starting to get fairly enthused about this project.  

Since I found out that the CF works so well with the passive effects loop & my digital reverb, I am gutting the FX-Reverb unit that I built and converting it to the mini-TOS.

A couple of questions ...............

Q1)  This amp has solid state rectification with the SE 6BM8. Does it really need a standby switch?  

Second question has to do with the mosfet.  I used some of Hoffman's turrets and soldered the mosfet into them.
Two of the turrets are NOT touching but really close to each other. I'm not familiar with mosfets & certainly not as cathode followers.

Q2)  As long as the legs of the mosfet are NOT touching, is it OK that they are pretty close to each other? I didn't know
       if there is any concern that something would arch over? I am guessing there is a 1/64" space?

Q3)  It looks like I have both a 500kl and 500ka pot for the OD level.  I am thinking that I want the 500ka?  That sound correct?

Attached is the current schematic and layout that I'll use for the build prior to tweaks.

With respect, Tubenit

[TIMBO]

Hi Guys, My idea of the standby sw. was to allow the valves to warm up prior to the inrush of power to them.So if this is not a concern and it does not damage them i can't see why you would need a standby.As for the mosfet i would not think it would arch over but to on the safe side would you be able to place a small piece of plastic or similar between each leg. On any PCB the dobs of solder are going to be close anyway.I glad that the mosfet was a success and looking forward to rebuilding my TOS with it added. Thanks

[tubeswell]

A couple of questions ...............

Q1) Two of the turrets are NOT touching but really close to each other. I'm not familiar with mosfets & certainly not as cathode followers.

Q2)  As long as the legs of the mosfet are NOT touching, is it OK that they are pretty close to each other? I didn't know
       if there is any concern that something would arch over? I am guessing there is a 1/64" space?

The IRF820 is designed to plug into perf board, so no danger of arc-over due to the electrode spacing on the package itself. But I still think you need to put a ~12V zener between the source and the gate (cathode/banded end pointing to the gate) to prevent gate/source insulation breakdown. I read somewhere that the gate/source insulation in these is only rated at 50V or something like that IIRC.

[Fresh_Start]

http://www.mouser.com/catalog/specsheets/91059irf.pdf

Looks like the gate/source voltage max is +/- 20 volts on this variety of IRF820.

You'll notice from that spec sheet that there is an internal zener diode between the source & the drain, but not between the gate and the source.  Immediately upon turn-on, the 5879 won't be drawing much current so the DC voltage at the plate of the preceding 5751 plate will be B+; however, there's a capacitor between that plate and the gate of the MOSFET.  Therefore, IMHO you don't have to worry about a big voltage differential between the gate and the source - it's only the AC signal which shouldn't spike at turn-on. 

If you used a DC coupled topography for the MOSFET cathode follower, then the Zener tubeswell is recommending would be necessary (I think).  Here's an alternative MOSFET available at Mouser which has back-to-back zeners built in between the gate and the source which was recommended over on AMPAGE:
http://www.st.com/stonline/books/pdf/docs/8830.pdf

The specs still show a max +/- 30 volts between the gate & source though, so I'm confused.

PLEASE correct me if I'm wrong in this analysis.

I know this belongs in the "Solid State" section, but here are Keen's "MOSFET Follies":
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

http://music-electronics-forum.com/t3509/

Respectfully,

Chip

[tubenit]

Any concerns about how the CF mosfet is mounted on this tagboard?

The turrets are not touching & I'll put an insulating rubber strip between them for extra protection.

Tubenit

[tubenit]

I don't think that the OT being close to the 6BM8 would matter in "OT placement #1"?

Any thoughts about that?  The 6BM8 triode will be the CF to the 5879 pentode.

I do have the option of going with "OT placement #2" if needed and punch a new 9 pin socket hole.

With respect, Tubenit

[Fresh_Start]

tubenit - this may not be feasible since the PT is already mounted, but my thought would be to rotate the PT 90 degrees so that its endbells are facing front-to-back instead of lenghtwise down the chassis.  Then mount the OT perpendicular to the way shown in #2 so that the OT endbell isn't pointing straight at your 6BM8 tube.  Might not be feasible and/or sensible, but that's my thought for the day...

Respectfully,

Chip

[Willabe]

That's what KOC shows in his TUT books, (TUT 3) with the OT pri. facing towards the PT.


            Brad       

[tubenit]

Man, it would be some effort to rotate the PT because of the chassis interior wiring etc.....   

I'd have to undo ALOT of stuff inside the chassis! I'll have to think about this.

It may be more realistic and easier to simply build a aluminum shield between the OT and the 6BM8?
I may go that route if it's a problem.

I have looked at previous build pics I've done and I have done the PT and OT orientations both ways but mostly on tweed style chassis.

With respect, Tubenit

[Fresh_Start]

I was afraid that rotating the PT would be a bear.

Would this location for the OT work physically?  IIRC the interference between magnetic fields goes down logarithmically with distance.  If it seems to generate noise, a shield between the PT and the OT might do the trick.  Steel might be a better choice of material for the shield though. I remember an old discussion on Bill Machrone's Blues Jr. site which showed a peculiar shield between the PT and OT but can't find it now - the site's moved & been updated a lot.  The shield Bill claimed had the best results was kind of a Z shape (looking at it from the side).

Respectfully,

Chip

[tubenit]

I had a chance (over a lunch break) to measure the distance IF I rotated the OT.

Only 1/8" difference.  I'll try the shield route if there is an issue (which I doubt).

Thanks for the input and help!


With respect, Tubenit

[jojokeo]

tubenit, in my building w/ the PT both directions/ways and from also what I've read - you have the PT mounted correctly, IMHO. In fact I've been mounting them this way for a while. The reason is that the radiated wave directions emminate out through the laminations direction and not from the bell pri and sec sides. This is especially true if you mount your power tube(s) near the end of the laminates as they'll get affected by this. In my tranny listening tests this also proved correct. There's also reference to this "I think?" on Merlin's site but his site is down as of the moment because of "bandwidth limitations reached". I'd suggest that you try plugging in the PT while taping off or wire nutting the loose secondary ends. The get some headphones and connect them to the secondary of your OT. You can hear the EMF this way as you move around your trannies, or at least your OT. I don't suggest that you put it diaganol to the PT and you'll hear why if you do. My disclaimer is that not all tranny's are the same or react the same and why it's such a good idea to do this little part before you build if you can. You just might be surprised on what you hear?!

1) I was looking at the OD section's tone and volume circuit. It looks like the 500pf of the tone pot is in parallel with the 100pf of the vol pot.

2) Earlier you said you liked the sound of the clean channel and was perfectly happy with it. That was w/out any mosfet CF. But now, you're going through the mosfet w/ BOTH the clean AND the OD channels. I'm wondering why? You could have it so that only the OD's channel goes through it.

[tubenit]

Thanks for the PT & OT orientation information. It is appreciated.

Yes the 500p and the 100p are in parallel.  I think I noted that in my previous post about the "secret sauce" cap and that I was puzzled why the ceramic 120p (value?) made such a huge difference in the blooming effect.  It truly made a remarkable difference for some reason.

Regarding the mosfet CF..............  
What I discovered was that with a CF behind the 5879 pentode,  that my digital delay sounded every bit as good (and perhaps even clearer/transparent) than using the FX-Reverb unit.  However with the OD disengaged and the clean engaged without a CF prior to the digital delay, it did not sound as good.  So that is what the experiment is using the mosfet CF.  

In other words, since I won't need the FX-Reverb unit, I decided to convert it to the mini-TOS & try to preserve the great digital delay effect by having a tube CF after the 5879 and a mosfet CF after the 12AX7 triode.  

I'm thinking this will work?  

IF I don't like the mosfet CF, I set up the layout so I can very easily bypass it with a 5 min tweak.

So that's my thinking so far.  Again, if I have any noise with the OT orientation, I'll just build a shield wherever it's needed.  Some of the old HiWatt amps seemed to do that IIRC.

With respect, Tubenit

[Fresh_Start]

I'm unsure now about where the magnetic field is in relation to a transformer's endbells and laminations.  In The Ultimate Tone Vol. 3 (p 4-12), KOC wrote "most magnetic leakage radiates from the end-bell face."  That's what Brad/Willabe must've been thinking about too.  I seem to remember a drawing showing the magnetic fields being shown as two ovals with parallel sides going through the laminations and encircling each endbell, but my memory is not perfect anymore (as my wife and daughter like to remind me... ).  If someone can enlighten us on this subject, I'd sure appreciate it.

Please understand, I know that jojokeo's practical experience is worth more than me theorizing.

Respectfully,

Chip

[jojokeo]

Chip I remember us talking about this about 3 or 4 months ago and I also remember the same lamination drawing. If I was home I'd have the info since Merlin's site is down. I distinctly remember the EM leakage going from the end-side of laminations and told to be careful of your power tubes being too close and directly in-line with this (the PT not the OT). My listening tests confirmed this on several pairs of trannies, and the subsequent builds have been dead silent even at the highest gain & full volume settings.
I'll try to provide an update to this later this evening.

Yes the 500p and the 100p are in parallel.  I think I noted that in my previous post about the "secret sauce" cap and that I was puzzled why the ceramic 120p (value?) made such a huge difference in the blooming effect.  It truly made a remarkable difference for some reason.

I can see where the brightness and signal high end gets through the vol w/ anything less than full up on the volume and same for the two caps being in parallel only when the the tone and vol controls are full up. Those two things are maybe unlikely to happen but even if so, they'd both be shorted anyway so that's a moot point. But, if you want to compensate for the vol being turned down, you'll still get the same advantage of a normal treble bleed - the more the tone is turned up full like the normal Tweed tone & vol controls. Humm, sort of need to hear and play with it to see if there's really much difference?

[Willabe]

The reason is that the radiated wave directions emanate out through the laminations direction and not from the bell pri and sec sides. This is especially true if you mount your power tube(s) near the end of the laminates as they'll get affected by this. My disclaimer is that not all grannies are the same or react the same and why it's such a good idea to do this little part before you build if you can.

I have not tried mounting a PT in different ways to hear the difference but...

I seem to remember a post not to long ago here on this subject, ie; core orientation and noise. IIRC, Z mounting of the PT was said to have the least noise. Yes I realize that's not the issue here but again IIRC, a pic. was posted from Merlins site showing and saying that most of the EMF radiates from the -- corners -- of the laminations, which struck me as odd. I had never heard that before, not that I know much about it. Then IIRC someone, (HBP's?) brought up something about the core windings in a EI PT can be wound in a different orientation from another EI PT. Which I still don't understand, not saying it's not true.

Now jojokeo relates his tests to show the most EMF waves radiate out through laminations....  which I take as not the corners but 90 degrees from the end bells.

Now in TUT 3, 4-12, KOC writes, "most magnetic leakage radiates from the end-bell face" 90 degrees from the laminations. Now a lot of guys there have built a lot of amps according to this placement of the PT core and are very happy.

Now I don't profess to know which is best from experience, but I do have a question.    

If most of the EMF waves radiate from the laminations, be it from the corners -- or -- off/from the flat ends, then why would any one bother to use end bell shields  --- and/or ---  copper flux bands as EMF shielding?                                                                                      

The price of the PT (or OT, choke) would go up and the "bean counters" would never go for that if it didn't make a difference. At a certain point in production Leo switched over to using all shielded PT's, OT's and chokes. Even Fenders Z mount PT's had end bells after a certain point in production.

Leo was a good businessman, if all he had to do was turn the core 90 degrees. why would he spend the extra money on end bells?

Marshall used them after a certain point also.

    
           Thanks,    Brad        

            

[tubeswell]

I always mount the PT and OT cores oriented in different 'planes'

[tubenit]

I mounted the OT at a 90 from PT and I think it will be OK?  I found a small piece of steel sheet metal that I forgot that I had.  So if there is a hum issue, I'll use that.  However, I am not anticipating a problem & never had a placement issue in any previous builds.

I went ahead and installed a standby switch cause I found one in my soldering box.

Got the board installed and pots installed and new sockets installed.  Ready to solder on buss wire and begin wiring the thing up.

Thanks for the help!  This is proving to be an interesting experiment for me.  The idea of a 3.5 watt Dumblish tone is a cool idea if it works out.

With respect, Tubenit

[jojokeo]

I mounted the OT at a 90 from PT and I think it will be OK? The idea of a 3.5 watt Dumblish tone is a cool idea if it works out.

I think the tranny orientation will be fine. Maybe a better point to make is that it's not really that critical afterall? Just like grounding. Follow a basic method w/out doing anything or newbie-ish and you're usually fine. My more preferred way, which maybe over rides the PT orientation, is mounting the OT as far away as practical & possible. Many major companies do this like Mesa & others.
You'll be liking the lower powered fun. I'm still blown away by the volume of a firefly-ish amp using a 12bh7 in split load. Smaller amps are fun to run various speakers & cab combos.

[jojokeo]

Well, I have to come correct - on one of my last amps I did mount the PT with the laminations going toward the power tube. But - this didn't cause any problems at all. I will stand by my earlier comment about the proximity of the PT & OT to each other. It can only assure no interactions.
In Merlin's newest book, right in the beginning he explains the PT emissions radiating through the lamination ends and corners even going into permeability and grain structure too. But this isn't the same picture Chip & I remember seeing and reading about.

[Willabe]

jojokeo, that's a beautiful build. Neat as a pin.


          Brad        

[Fresh_Start]

jojokeo, that's a beautiful build. Neat as a pin.


          Brad       

+1 on the compliments!

Chip

[John]

Wowza. The only reason I'd post pics of my  build is to give you guys some comedic relief!

[Tone Junkie]

Wowza. The only reason I'd post pics of my  build is to give you guys some comedic relief!

Thats funny John thats how I feel to.
Bill

[jojokeo]

T/Y but you guys are always so complimentary, modest, and most everyone has mucho talent galore.
This amp turned out the be something it wasn't quite intended in the beginning, but this isn't the first time for that little(?) thing to happen. In fact, I'm maybe 50% in this regard? This is involving the preamp section this time. It was to have among other things a soft clipping circuit to give a little extra sizzle at lower volumes but because it was my first VVR build I quickly realized this wasn't necessary at all. So then I had to remove a p/p pot, misc parts while creating empty eyelets and then the switch was still staring at me for something better to do!? I had it on the first gain stage like many other amps do but the old light bulb went off  Now, it biases two gain stages at once. No more empty eyelets and the switch and owner are very happy.