The SS Project.....AB763 with PPICF

[DummyLoad]

SG just for S&Gs, what's the DCV measurement across the Rk (2.7K) of the LTPI?

--Pete

[SILVERGUN]

SG just for S&Gs, what's the DCV measurement across the Rk (2.7K) of the LTPI?

6.5v @ idle drops to 5.9v with full signal.....with 22K in the tail

Thanks for looking DL

On my other subject...
Is a small amount (a few volts) of bias drift normal/acceptable as current increases in the PA?.....is it "the right idea" to use those grid leaks to prevent it?.....or is it OK to just let it happen,,,so that as cathode current increases, bias voltage increases to keep it in check?

How much grid current is too much? (I've been thinking that zero is ideal,,,but where's the boundary?)
If I've got 2mA-5mA grid current at max signal,,,can I stop obsessing about it and just get on with my life, knowing that the amp will probably never see 9 or 10 on the volume control?

[SILVERGUN]

Now if you recall HPB's point on the CF's function to handle the onset of grid current as oppose to AB2 operation, then you need to control the amount of drive at the power tubes' grids, with little or no grid current when the volume is fully dimed - you can do that by adjusting the overall gain in the preamp stages.

I did notice one other thing last night....in my effort to reduce/eliminate g current, I measured the 12AT7 (CF) as pushing about twice the current that an AX7 did
The AT7 was causing about 7mA on one side and 5mA on the other,,,,while the AX7 brought it down to an even 2.6mA per side
I had a matched triode Sovtex 12AX7-LPS that I usually use for a PI (in a different amp),,,and I put that in there, and not only did it drop the g.c., but it evened it out side to side......so I think the imbalance in g.c. was being caused by an imbalanced signal coming through the CF.
One side was just getting hit harder than the other.....

Maybe this explains why AX7s are an OK choice for the CF....
Like PRR said in the other thread:
> why not use an even more capable driver tube?
How much can you shove into the 6L6 grids before they melt?
It's not clear on the datasheets. With all the many different 6L6es made, and variations in grid-wire processing, it may be best to be conservative.

[Tone Junkie]

SG did you try it out with your guitar and see how it sounded with AX7.
Bill

[John]

SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?

[SILVERGUN]

SG did you try it out with your guitar and see how it sounded with AX7.

Yeah,,,It sounds about the same to me as having the AT7 in there....it's just that the AX7 dropped the grid current readings I am getting on the 6L6s...
It's really hard to tell if there is a difference in the point where OD kicks in because my ears are beat, and at that vol. I can't stand next to it for very long..
It does sound like a really bad-ass Deluxe/Super Reverb that wants to blow some speakers  ......
I'm thinking some more efficient speakers might actually help it stay clean....I think I'm hearing power tube and speaker breakup combined (plus a little blocking distortion)....I've tried dropping some signal but it looses fullness and way too much volume if I lower the signal to the point where I get ZERO grid current

SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?

Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up

[Tone Junkie]

Yeah,,,It sounds about the same to me as having the AT7 in there....it's just that the AX7 dropped the grid current readings I am getting on the 6L6s...
It's really hard to tell if there is a difference in the point where OD kicks in because my ears are beat, and at that vol. I can't stand next to it for very long..
It does sound like a really bad-ass Deluxe/Super Reverb that wants to blow some speakers  ......
I'm thinking some more efficient speakers might actually help it stay clean....I think I'm hearing power tube and speaker breakup combined (plus a little blocking distortion)....I've tried dropping some signal but it looses fullness and way too much volume if I lower the signal to the point where I get ZERO grid current

SG, I have been following this project with interest, but I must admit to skimming a lot. (a lot of the calcs and theory is above my level) But it seems as though one thing the PPICF has done for you is allow more clean volume before clipping? Where as with many high gain amps the aim is to overdrive earlier, you have been aiming for more clean volume?

Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up
[/quote]
Sounds like a good combination right were you have it, now does your last schematic have your latest values in it . I wouldnt mind throwing it out on the breadboard and hearing it for myself. Gotta love a fairly clean poweramp with good bass responce. Sounds like heaven to me .
Bill

[John]

Yup John,,,this one is supposed to be about how clean I can keep it further up on the vol. control
And the whole project was really started to just try to nail down the design of the PPICF,,,,,and take away my concerns about eventually trying to build an SSS
But this AB763 amp has a quality that makes it unique in itself.....the CF adds the bass response that you always wished for in a DR plus an overall glassiness at higher vol.,,,well past where the DR would start breaking up

Thanks for the response, and I congratulate you on forging ahead!

[SILVERGUN]

Thanks for the response, and I congratulate you on forging ahead!

Thank you John.....some nights it feels like I'm out here on my own, and it's good to hear that someone (besides the regulars) is actually interested 
I'm hoping to complete the thread with a full schematic, possible layout, and bias supply solution,,,,,and last but not least a couple good sound clips  .....I gotta give some more thought to speakers

Sounds like a good combination right were you have it, now does your last schematic have your latest values in it . I wouldnt mind throwing it out on the breadboard and hearing it for myself. Gotta love a fairly clean poweramp with good bass responce. Sounds like heaven to me .

I agree,,,it does sound good right where it is now,,,and I'd love to hear your thoughts if you do lay it out
I got a little too obsessed with trying to keep it clean all the way up to ten,,,,and completely eliminating grid current
On 5 I think you'll agree it's a very strong clean sound............single coils sound much better to me
The preamp schematic in the 1st post of the thread hasn't changed (much),,,and the power amp portion in reply #182 is accurate
The 12AU7 in the PI has really helped me clean things up,,,and the only tube that is still up for debate is the AX7/AT7 CF

Keep in mind that I have a separate bench bias supply, so that does not match the schematic  (currently about -210vdc)
Have fun with it and please let me know what you think.....it still needs some tweaking to complete, and I'd love to get your input

[DummyLoad]

6.5v @ idle drops to 5.9v with full signal.....with 22K in the tail

thanks, tube cad sim indicates with 290V B+ and Vgk of -6.5V 12AU7 can deliver an output of around 55Vpk before clip. with a gain of roughly 8.5 that's about 6.3Vpk of input threshold.   
does that compare to you're measurements?

Thanks for looking DL

you're welcome. :-)

On my other subject...

...deferring to the experts.

 

--pete

[jazbo8]

I think notwithstanding my obsession with the grid current and bias shift , it's time for you to wrap it up and build the darn thing, especially if you are happy with the sound and the response of the amp. And thank you for letting me be a part of the "debugging" processing 

[SILVERGUN]

thanks, tube cad sim indicates with 290V B+ and Vgk of -6.5V 12AU7 can deliver an output of around 55Vpk before clip. with a gain of roughly 8.5 that's about 6.3Vpk of input threshold.  
does that compare to you're measurements?

I'm gonna take a couple days off and then take a fresh run at it on Mon....I'll let you know

I think notwithstanding my obsession with the grid current and bias shift , it's time for you to wrap it up and build the darn thing, especially if you are happy with the sound and the response of the amp. And thank you for letting me be a part of the "debugging" processing  

Jaz, I'm gonna stay at it and tweak it down a little further before I start cutting wood  
It has been my pleasure to have you here,,,and I must thank you again for asking some great questions that have prompted me to take a crash course in tubes 101....I have learned a ton in the last couple weeks, and I'm gonna give my ears a little break, and get in some golf, and go back at it as a more educated experimenter.....
I'm not happy with some of the dead end threads over at TAG, so I'm very tempted to go after the SSS full steam ahead  

I found this interesting thread here: (while googling AB2).....happy reading
http://music-electronics-forum.com/t197/

Oh yeah,,,,and then there's this: 
 https://www.youtube.com/watch?v=TB465GMTsjQ

[Tone Junkie]

Nice sound on that Hartman amp. Anything going on SG or are you taking a little break.
Thanks Bill

[SILVERGUN]

Yeah TJ, taking a break,,,,,marinating in information a bit
I'm a little stuck at the moment and not sure which way to go....all of my free time is being spent helping my son get his house ready to live in   

There was one thing that I wanted to add as a follow up to something I had mentioned earlier.....
I used those two 500K pots that I had put in place of the 470K grid leaks to dump some signal just before the grid stoppers,,,, I kept my eye on the bias voltage and monitored the drift from -25v to -30vdc,,,,with the vol. cranked and the bias reading @ -30vdc, I dialed the grid-leak pot down to get the bias voltage back down to -25, and wound up at 180K on the pot....I did the same thing on the other side and wound up with the same 180K over there...So that got me NO bias shift
Well that was a bad idea....although it did help with the bias drift,,,,it also cut off the bottom of the signal on both sides 
I didn't see it until I looked at the waves going into the grids of the 6L6s and saw a completely squared off bottom half on each side   
I dialed the values back up and watched the wave "reform"  into it's original shape,,,and that happened right around 420K, so the original 470K value was good....so I'll leave em there

I'll work on a completed schematic, so we have a reference....

[SILVERGUN]

Here's an updated full schematic as of this point......it's nothing more than what we have been talking about, all cut and pasted together

The negative supply values have been left blank to protect the innocent.....if you can't figure it out,,,you probably shouldn't build it 

I'm gonna veer off the path a little and try to incorporate some Dumble type mods to the preamp, similar to what I have seen called Ultra-phonic modded AB763....since I'm not so far away from there now, it seems like a fun direction to take,,,,,and I'm thinking this could morph into a channel switchable amp, with the CF power stage staying in place.........we'll see
The inclusion of an OD channel will also force me to figure out how to tame the grid current on those 6L6s,,,,or melt them trying

I'll take that project to a new thread and call it Ultra AB763 with PPICF.......coming soon

[rzenc]

HI SG!


Couple of questions to you Sir:



1) I would like to ask if you tried boost/OD/Dist stomb boxes in front of this amp?


2) If so, how did it sound?


3) Did you try a high gain preamp?


I'm asking these questions because a fellow studio owner got interested in building this amp. But he also asked whether it was possible to build a high gain channel inside it too.


Many thanks in advance!


Best Regards


R.

[SILVERGUN]

1) I would like to ask if you tried boost/OD/Dist stomb boxes in front of this amp?

2) If so, how did it sound?

3) Did you try a high gain preamp?


I'm asking these questions because a fellow studio owner got interested in building this amp. But he also asked whether it was possible to build a high gain channel inside it too.

Hey R,
I have some pretty strong opinions about your questions

1+2) I tried pedals out front and Boost/OD distortion was only good at lower dist/OD settings, and more just plain clean boosted
The thing that makes this amp so special is how clean it stays, and 'for me' any clipping of the wave created a 'not so desireable' result...
I believe that this is partially a result of the driver tube being too capable of creating grid current in the output tubes. (grid current does not sound good  )

3) I did and hated it immediately, and just wound up doubling back and taking a time out

If I was going to build this amp,,,it would be because I wanted the best possible clean sound I could get...it really is awesome.
If I was doing a gig where I need some high gain, I would use a separate/different head.

[SILVERGUN]

Then again R,,,I've learned a few things since then...
One of those, just recently,,,while contributing to the Major thread...
Based on this info posted by Willabe on that thread, maybe I shoulda used a UL OT?...and some 6550s

From TUT5, chapter 7, page 3;

"Since the UL connection reduces the effective gain or sensitivity of the tube, much more drive voltage is needed to get full output. Referring back to Mullard's own spec; in tetrode, -34V of bias on g1 with 67Vpp drive, versus -75V and 140Vpp in UL.


So don't let my strong opinions fool you into thinking I know what I'm talking about   
There 'may be' a way to make it work, by taming the grid current , and using the UL OT hookup to allow a bigger signal voltage. 

[DummyLoad]

SG,

Looks like you are ready for your new build, congrats. I checked with the guys over at ampgarage that have built the SSS with the 12AT7 CF and they said they had no issue with the high voltage across the tube, but of course, most of them use NOS tube not the new production from the East. So you may want to start with the breadboard "as is" and see if any issue crops up. Looking forward to see more on your R&D results 

Jaz

use a 12AZ7 instead of 12AT7 if that's worrisome.






use the cap coupled CF driver - it's much less trouble than dicking around with a high volt neg PS to direct couple. GEC figured that out in the 50's.


http://www.triodeel.com/gec400w.gif 


referring to the GEC400 scheme; replace 6SN7 driver/cathodyne with 12AX7 or 5751 or 12AY7, use a 12AT7 or 12AZ7 for the CF driver. the cathodyne and pre-cathodyne gain stage can be any tube. maybe copy a fender deluxe or super 5F4 type plan? bet it sounds cool. if you want to copy the GEC 400W plan but use novals, then use 12BH7 or 6CG7/6FQ7.


--pete

[rzenc]

Thanks guys!!


SG: I was asked to build a clean sounding amp and immediately remembered this project. But I was also asked whether it would be nice with pedals in front of it...Let me talk to the guy - studio owner - and see what happens. I would like to thank you for you kind attention!!


Pete: I already own a 1968, Linden made, Ampeg SVT...man this thing is heavy..It hurts just thinking to carry it outside.


However, I also have handy a kit of iron which may be used with a 6x6L6/KT66/EL34. Specs:


P.T.


B+(1): 340 - 0 340 @ 1A.
B+(2): 250 - 0 250 @ 250mA.
Bias: 60 - 0 @ 100mA.
Fil. 3,15 - 0 3,15 @ 10A.


O.T.
U.L. @ 40% winding turns
150WRMS @ 30Hz
1750 ohms plate to plate Z
0-4-8-16 speaker out


Choke 10H @ 100 ohms @ 250mA.


I got interested in building a power amp only with the GE schem you provided. If going KT66, you can call it Kattie66six 


Once again, thanks for your kind attention!


Best Regards


R.

[DummyLoad]

cool. that's some nice iron. are they custom winds? post your progress.


respectfully,


--pete

[Ed_Chambley]

Here is info I had from a while back.  You may have seen this, but it is still worth kicking around on Fender tone stacks.


There may be a couple of things in here to help with the Fender/Dumble flavor.

[Willabe]

Hey Ed did he (Steve Ahola) also draw up a schemo for those? Way to hard to figure out the circuit by looking at a layout drawing IMO.

           
                     Brad          

[rzenc]

are they custom winds?

Yes.


I got them sometime ago to a project, but customer changed his mind and instead of 150Wrms, he decided to make a 50Wrms. So we ordered new iron and I kept it for myself.


I've been looking forward to give this set of iron a "special" home.


To tell you the truth, I'm very interested in marrying this output section to a Standel type of preamp.


Any inputs on that??


Thanks guys!!!!


Best Regards,


R.

[Ed_Chambley]

Hey Ed did he (Steve Ahola) also draw up a schemo for those? Way to hard to figure out the circuit by looking at a layout drawing IMO.

           
                     Brad        

No Schematic.  I will try to find more info on this.  What this is part of is doing the modes Dave Funk did on Fenders when he was working in Nashville.

It is hard to tell.  Let me check.  I have done many of what was called the "Funk" mods.

Here is a link at TAG fore more info on Dumblizing a fender.  Look at the first post where it verbally explains a lot of the mods. 

http://www.ampgarage.com/forum/viewtopic.php?t=5762&postdays=0&postorder=asc&start=0

I guess Dave found out it was more profitable to writ the book "How to Hot Rod Your Fender amp.

I actually have the book in PDF form.  I will check to see if there are any schematics in it.

[Willabe]

I looked up the book and it says it's written by Jeffrey Falla, who builds Tonetronic amps?


                           Brad   

[Ed_Chambley]

I looked up the book and it says it's written by Jeffrey Falla, who builds Tonetronic amps?


                           Brad   

Sorry, wrong book.

Here:
http://www.amazon.com/Dave-Funks-Tube-Amp-Workbook/dp/1500115185/ref=cm_cr_dp_asin_lnk

[Willabe]

    Ok.

I have that book.


               Brad       

[SILVERGUN]

Let's take this over here KIDs:
http://el34world.com/Forum/index.php?topic=17586.new#new