SE-M & FX 6V6 single ended

[tubenit]

OK, here's the situation .......................

Sometime back, I made a tube preamp for someone for an acoustic guitar to plug into a PA system.  It worked great.  However, it is going to be given back to me for a conversion to then pass on to someone else. 

I am planning on making as few changes in the amp to get it where I want it to be so that's partially why I am going to leave the FX on the already constructed layout board.

So, it's an already built amp that I am adding a single ended 6L6 or 6V6 to.  I've had a 10w single ended OT around for yrs.  The chassis is very small and won't easily allow a push/pull  conversion.

Q1:  I'm really not that familiar with single ended amps.  The OT has an option of 5k or 7k on the 6L6 side of it.  Which would be better to hook up
       to for a 6L6?   

Q2:  And would that change, if using a 6V6?   I want to set it up so the guy can interchange a 6V6 or 6L6 easily if he wants to?

Q3:  Can anyone point me to a single ended Marshall inspired schematic?   I've found some but would be interested in a proven design.

With respect, Tubenit

[shooter]

fwiw, doing "multi-choice" tube PA doesn't buy much, maybe even loses some since bias, drive, and impedance change tube to tube.  I've found you get a better outcome dialing in 1 tube, leave the tonal magic in the preamp.

for a clean amp, I try for ~~90% dis by having a higher B+ and less current, for a lead/distortion amp, I shoot for ~~ 100+% dis with a lower B+ and higher current.

In a clean amp, I try and limit PA drive to ~20% over bias, for lead/distortion, I go as high as twice bias.
 

[92Volts]

I partly agree with shooter. On one hand, I have an SE amp that works with multiple tube types, and I've enjoyed trying different ones to get my preferred tone.

On the other hand, my amp doesn't really have features to accommodate different tubes... it's built for a 6V6, and when using larger tubes, I run them cold and with a high (7k) impedance. In theory, 5k and higher current is "better" for larger tubes. I miss out on potential power running big tubes this way, but it sounds great. FWIW an old 6CA7 is my favorite tube in this amp. 6V6s have a noticeable amount of excess treble. 6L6s are a bit too dark.

I was not expecting tube selection to matter because I'm not overdriving the output (high-gain preamp and master volume). I think part of this is the lack of NFB on my SE output (and many SE circuits), since it can't be applied to the PI like it is in most PP amps (including Marshalls). If you wanted to get this in check, I think you could apply NFB to the powertube cathode. I haven't minded this variability since I found a tube that I liked, but I didn't plan/expect the poweramp would color the sound so much.

On that note, I'm not sure if there is a "real" Marshall style SE stage... they are all pretty simple/similar, they sound like Champs and other well-liked SE amps, but they don't sound like PP amps. Feedback to the cathode is definitely one opportunity to change things up.

In my experience if you want to bias 6V6s conservatively, you need a lot more cathode resistance than the 470ohm used in Champs. Once you find the right ballpark for your preferences/voltage, you can ditch pots/test points and use a simple switch to change the cathode resistance to suit a 6V6 or 6L6.

[tubenit]

Guys, 

Thank you for the help and thoughtful responses and guidance!  I appreciate it.

I don't mind experimenting at all. In light of what has been said, I think I'll try the 5k winding on the OT and see what NFB might sound like per schematic.  I have a decade box, so running thru different resistor values on the NFB is pretty simple and quick for me.

With respect, Tubenit

[SILVERGUN]

A common improvement to SE would be to add an additional cap node to the power supply before the plate supply.
Not sure if you have room for that, but if not, you might be able to just reconfigure your current lineup.
Keep the dropping resistor value low cause it will be passing all of the current of the amp.

[92Volts]

No problem! I'm curious how your build goes because I'm considering mods to mine too.

I'm not sure you want the volume control inside the feedback loop-- the feedback ratio will change with the volume setting. Might sound OK, but I don't imagine it would make the poweramp behavior more predictable.

I ruled out something similar on mine because the last preamp stage is also followed by a tonestack, and applying NFB around that would be even weirder/counterproductive.

[sluckey]

Are you aware that PT and FWB as drawn will put out 735VDC unloaded?   

[tubenit]

The PT doesn't have a center tap.  I incorrectly wrote 260-0-260.  I guess it should've been written 260-260 ?

I used Hoffman's 1000v bridge rectifier VG20.

I don't have the amp currently in my possession but should within a couple of weeks. I'll check the voltages on node A.  It is wired as shown with a full wave bridge rectifier & a PT with no center tap.  I looked for a voltage chart from when I built it and couldn't find one? 

The amp has worked and has sounded good since Feb 2015.        https://www.soundclick.com/html5/v3/player.cfm?type=single&songid=13020598&q=hi&newref=1



with respect, Tubenit

[2deaf]

Definitely delete the volume control inside the NFB loop.  There is already a 100K pot.* on the return that serves the same purpose.

There is a possible problem with the low gain of a 12AY7.  By the time you knock the open-loop gain in half with NFB, the 12AY7 may not be able to fully drive the power tube.  A 12AX7 will work fine for the CF and would be much more suitable as the FX recovery stage when using NFB.

*Can anybody explain to me why the wiper is tied to the top of the 100K pot.?  I mean, it allows the end-user to short the output of their FX device.

I vote for the 7K winding.

[tubenit]

Definitely delete the volume control inside the NFB loop.  There is already a 100K pot.* on the return that serves the same purpose.

There is a possible problem with the low gain of a 12AY7.  By the time you knock the open-loop gain in half with NFB, the 12AY7 may not be able to fully drive the power tube.  A 12AX7 will work fine for the CF and would be much more suitable as the FX recovery stage when using NFB.

*Can anybody explain to me why the wiper is tied to the top of the 100K pot.?  I mean, it allows the end-user to short the output of their FX device.

I vote for the 7K winding.

Thanks for the information.  I think I have every variation of 12A_7's to experiment with including 12DW7 and reverse 12DW7.

My guess is the level pot would impact the NFB much as a PPIMV does in a push/pull amp?  IF that is the case, so far that's not been problematic for me tonewise.

SG, thanks for the comment about the extra filter cap.  Not sure how much room I'll have and I don't currently have the amp in front of me. I would like it to be a quiet amp, so we'll see.

With respect, Tubenit

[shooter]

I would like it to be a quiet amp,

SG's suggestion is good, I've found that 100uf-R-100uF works well, any ripple >2vac at the plate tap will probably be heard.  You can find the small modern 100uf/450vdc pretty easy, 2 will take up about the same footprint as a single normal 47uF/450.

I've had a 10w single ended OT

something to consider, 10W audio in SE needs a tube cooking >20W diss. at idle and a pretty good AC swing.  I would shoot for 300v or more swing, so something close to 380/400 plate.  That depends a lot on what type amp, mostly clean or mostly distortion at normal operating volume.

[tubenit]

Shooter,   thanks for the information!

I got to thinking about the Level pot after the FX and the NFB,  ................  there's already a level pot on the front chassis of the amp.  So, the "hole" is already drilled out for one. 

I think I'll experiment with something like a 100ka pot with a 220k resistor into the pot and a 33k to ground.  That would still allow some adjustment perhaps without being problematic.   Again, the small range I use for adjustment on my PPIMV doesn't seem to lessen the effectiveness of the NFB on my amps with PPIMV.

With respect, Tubenit

[jjasilli]

Q3:  Can anyone point me to a single ended Marshall inspired schematic?   I've found some but would be interested in a proven design.

Looks to me like you're building a Champ variation.


Also checkout how the Blackface /SF Champ or VibroChamp uses NFB.  My suggestion is to build the amp 1st; then consider NFB.  Also, the amount of NFB might be speaker dependent.  This calls for variable NFB. 


Attached is my hotrodded VibroChamp schematic which might offer some ideas. 

[pullshocks]

I have been thinking about re-purposing my SE build along the lines of a single ended Marshall, and came across this one.  I am not that knowledgeable about "real" Marshall tone, and the demo sound quality is not pristine, but it sounds  ok to me.


http://chasingtone.com/dvnator/the-seplex/


Will be watching this thread with interest.

[tubenit]

Jjasilli & pullshocks,   thanks guys!  I appreciate the schematics.  Those are helpful.

with respect, Tubenit

[jjasilli]

Pullshocks schematic is spot-on! 


I thinks that's close to a Marshall tone for an SE amp.  But, IMHO, no cigar.  No one's fault.  I think the true tone requires a split tail PI > PP power tubes.

[tubenit]

I did a poor ineffective job in communicating in my first post when I said:

Can anyone point me to a single ended Marshall inspired schematic?

I'm actually NOT interested in getting a "Marshall tone" from a single ended amp.  I was looking more for a schematic that had topology similar  to Marshall with a cathode follower, then tone stack, etc ...........  into a 6L6 or EL34. 

The preamp and FX is already built,  I just need to marry it to a SE power amp. I don't anticipate it or need it to sound like a Marshall.  I'll build it and then probably spend quite a bit of time tweaking it to something I like. Ideally, I'd like a reasonably clean tone that is reasonably touch sensitive.

Once I get the amp in my hands (a few weeks out),  I'll repost what I'm coming up with and any more questions.  THANKS for all the help, guys!  I genuinely appreciate it.   

With respect, 10thtx.

[pullshocks]

Another Marshall-inspired SE amp is the AX84 SEL http://ax84.com/static/sel/AX84_SEL_101004.pdf



Marshall itself has the Class 5 amp  http://www.forum.metropoulos.net/viewtopic.php?t=33575


Sorry, here is a working link http://cdn.tonegeek.com/wp-content/uploads/Marshall-Class-5-Stock.pdf

[tubenit]

Pullshocks,

THANK you!   Both interesting and useful schematics.  I appreciate it.

With respect, Tubenit

[tubenit]

OK, I built this!  Love it!  Super quiet at idle and surprisingly  loud with a 10"  Emminence 1058 when cranked.  I tried a bunch of different tubes and liked the 12AY7 in V1,  12AX7 in V2 and a JJ 6V6 in V3.  Tried a JJ 6L6 that sounded pretty decent also.

The active FX is actually a nice feature on this amp with a delay pedal.  And you can crank the preamp and lower the FX level for a very low volume warm overdrive tone.  And it has enough headroom for what I am wanting.

The switches have pretty subtle changes.  The NFB switch seems less effective when the FX level is turned down?   The bright switch seems more effective when the volume is turned down.  The mid-boost works quite well & is more substantial unless the mid pot is turned way up. 

Thanks for your help, guys!  I truly appreciate it. I am VERY happy with how this turned out.   I will try to get some sound clips done sometime soon. 

With respect, Tubenit

[sluckey]

I'd be a bit nervous using that 10W OT with a 6L6.

[tubenit]

OK, thanks for the head's up on the 6L6 concern!  I only tried a 6L6 in it for a few minutes and it sounded good.  It currently has a JJ 6V6 in it which sounded the best to me.  Also tried an old stock Zenith 6V6 that sounded decent.

Since this is a "converted" project,  I have a few more holes to plug up.  I've added one more mini-toggle since the attached photo was taken.

with respect, Tubenit

[shooter]

Nice lookin cab, will probably be doing mine similar, clear edges, I did my revib cab that way.

[davidwpack]

Very nice!

[kagliostro]

COOL 


Franco

[tubenit]

I am super happy with how this amp sounds!  I've put some time in playing it with both single coils and humbuckers.  Everything sounds good to me. 

It initially seemed somewhat silly having all the switching features on this, but switching guitars and wanting reasonably clean to just starting to break up to single ended amp "pushed" overdrive ………….. the switches really actually are useful dialing in tones.  Far more then I anticipated.

This is going to be a gift to someone and it was determined a combo cab would be better then a head cab, so I built this. Had some left over figured maple, Sapele front panel wood, left over grill cloth and then reused the leather handle and rubber feat off the head cab.  Essentially, only $30 more out of pocket to build the cab.  Used half blind dovetails and the cab got an Emminence 1058 10" speaker.

With respect, Tubenit

[tubenit]

IF any of you would like to hear a soundclip of this amp covering different playing styles, I posted a link to a sound clip here:

http://el34world.com/Forum/index.php?topic=23540.0

With respect,  Tubenit