The SS Project.....AB763 with PPICF

[SILVERGUN]

Thanks guys,
Gj, I really do appreciate ALL of your help....I also appreciate the fact that you would care enough to come back and edit your post if you thought there was a problem with it or you changed your mind about something  
I have done that on numerous occasions in other threads
I had read some of your earlier post,,,and the one thing I remember is that I wanted to share this.....
I always use the weber bias calculator as a guide for setting my output tube bias....found here:
http://www.webervst.com/tubes1/calcbias.htm

Pretty clean sounding with "amp on 3", what happens when you crank it up?

I start to bleed from my ears and my component storage bins fall off the wall...
40 some clean watts is definitely LOUD!!!.....but I'll try to tough it out and give you a clip, because I agree,,,,the amp should retain the headroom when you crank it.......and we will have to confirm that to call this a true victory

I think the biggest problem with cranking it up is the placement of the volume control,,, tends to increase preamp distortion as I turn it up, (even without the PPICF) and I was considering putting a master volume right before the PI, similar to Sluckey's Tweed Deluxe Reverb

Before I changed some component values and "cleaned up" the front end, I couldn't believe how easily this amp would distort, even at low volume...........It definitely felt like my pickups were too high output, and I was just crushing the front end.......sounded best with my guitar vol. on 8 instead of 10.....not really the "cleans" I expected from the AB763
Almost bought some low output strat pickups,,,,but I waited....for now....I have a feeling that is going to be a real winner in this amp

The amp does not sound just like an sss....obviously, BUT, the reason I am so excited about it is because only a month ago we were questioning the design of the PPICF,,and now I can confirm that it does make a difference and now this amp sounds like a Deluxe/Super Reverb with a PPICF
I'm counting on guys like you to help me tweak it to it's potential,,,,and then we can experiment with changing out the preamp.....of course I'm thinking about how it will sound with OD and what kind of difference I can expect if I add this to the TurboWreck, TOS, etc., etc  

TDR master shown below

[jojokeo]

Pretty clean sounding with "amp on 3", what happens when you crank it up?
I start to bleed from my ears and my component storage bins fall off the wall...
40 some clean watts is definitely LOUD!!!.....but I'll try to tough it out and give you a clip, because I agree,,,,the amp should retain the headroom when you crank it.......and we will have to confirm that to call this a true victory

Just wondering if you have to use 6L6s instead of 6V6s? You could save your ears until things are better dialed in then sub the 6L's, re-bias, and test those in the end? 

[SILVERGUN]

Wow SG! So impressed. I just now saw the thread and started reading tonight. So in three days it goes from concepts > to building it > to you AND a sound demo!??? 
This is just plain awesome my friend!!! Keep it up (which I know you will) and god speed as they say. Is this something suspected or actually documented from the D-man & his stringer?

Thanks jojo AND Timbo,
It's 3:45 AM here in Philly so that's how you know I'm excited about this  

This "research" is for all of us,,,,and you can trust me when I tell you that I won't stop until it's right

I am working from mhartman's sss layout from TAG,,,,and a hand drawn (scribbled) schematic from some guy in Japan that "had one on his bench" (see above posts)
When we originally started talking about this on another thread,,,I was pushing hard for it,,,,,and HBP inspired me to build an amp that I could try to incorporate it in first,,,,because he thought (and I agree) that it would be easier to install it in a known working amp.
That was a great idea, and since I was looking for an excuse to do an AB763, it was a no brainer for me to try to enhance those cleans with this addition............there had to be something special about that sss circuit for it to sound so unique.....THERE IS

[SILVERGUN]

Just wondering if you have to use 6L6s instead of 6V6s? You could save your ears until things are better dialed in then sub the 6L's, re-bias, and test those in the end? 

As a semi-delusional guitar player,,,,I like to imagine that bigger bottles = bigger tone 
I had some 6L6s here and an OT that was perfect for the job...I could go back to 6V6, but I would have to buy them,,,and if I want to buy tubes, there's better places to spend my money right now.............I still have to explain this stuff to the old lady, ya know 

This thing is all about "balancing" the bias between the voltage divider for the CF and the output tubes,,,,,so I guess it's easier to tweak with the actual setup....

Thanks for showing up my friend,,,,,this is pretty cool stuff,,,,,,please take a look back over the thread when you get a chance and make sure I didn't say anything too stupid

Off to bed.......zzzzzzzzzzzzzzzzzzzzzzz

[jojokeo]

I thought you went back and forth w/ 6V6s & EL34s when working on the 'wreck? But that's okay, don't let a couple of little ol' bottles get in your way :)
Yeah I saw the mhartman reference (didn't know who or what?) and entire thread but didn't know you were discussing this on another thread before this one. So when I saw "d-man" and "sss" I immediately thought about Dumble and his Steel String Singer.
I'm not trying to distract here but if looking for ultra cleans I'm surprised ultra linear OT is not being considered also?

[Glennjeff]

SILVERGUN,

Yeah, I had a real bad hangover this morning and the brain just was not working right.

Could you tell us what valves you are using for PI and CF, you said in the thread somewhere that you were going to use 12AT7 for CF but the circuit diagram with voltages has a 12AX7.

Still wondering why the CF is biased so low on the valves transfer curve.

Seems d-man had a preference for 12AU7 PI and 12BH7 CF in the sss's, from hours spent at TAG today. Tempted to make my next build a Singer, but your AB763 sounds real nice.  

Take care of the ears, I use foam earplugs when practicing drums and recording real loud guitar parts.

All the best.

[SILVERGUN]

I thought you went back and forth w/ 6V6s & EL34s when working on the 'wreck? But that's okay, don't let a couple of little ol' bottles get in your way :)

When I built that little custom Champ I cooked one (for research sake  @18 watt dissipation) and then used the other to finish the amp at a much more reasonable output.....EL34s all the way in the Wreck,,,the 6V6s couldn't keep up

I'm not trying to distract here but if looking for ultra cleans I'm surprised ultra linear OT is not being considered also?

No distraction taken....that's the beauty of the breadboard....we CAN try it, if you think it'll make enough of a difference...
Supposedly the sss was built on Twin iron,,,so I wasn't real concerned about it, and I had this Hammond 1760L sitting here..
I mentioned it briefly while we were looking at the Fender Studio Bass schematic, earlier

[SILVERGUN]

1)Could you tell us what valves you are using for PI and CF, you said in the thread somewhere that you were going to use 12AT7 for CF but the circuit diagram with voltages has a 12AX7.

2)Still wondering why the CF is biased so low on the valves transfer curve.

3)Seems d-man had a preference for 12AU7 PI and 12BH7 CF in the sss's, from hours spent at TAG today. Tempted to make my next build a Singer, but your AB763 sounds real nice.  

4)Take care of the ears, I use foam earplugs when practicing drums and recording real loud guitar parts

I numbered your questions/ comments again so I could address each:
1) PI is AT7 and CF is AX7.....schematic is correct,,,,and is a result of some reading over at TAG
But all of these are options that we can try
2)Me too, but I just feel lucky that I was able to get this far, this quickly,,,and I don't question it too much because it seems to be in line with the scribbled schematic.........I was hoping that one of the real mathmeticians would weigh in on that one for us  
3) I missed that,,,and probably because I promised myself that I was gonna try to not do hours of reading over there....the more I read, sometimes, the less I get done
4)Yeah,,,,thats a big problem for me right now, because I already have some hearing damage from years on stage with a drummer who broke huge sticks regularly,,,,and I have to walk a fine line between hurting myself and hearing results...........when I do the cranked demo for jazbo, I wont be in the picture  

Please stay tuned....this isn't anywhere near over....it's just installed and working.....step one complete...I'm sure i'm missing a lot of the magic of the sss in the preamp,,,but you're right, this one sounds pretty cool too, and I wouldn't build another AB763 without it

[SILVERGUN]

There is one other thing that I would like to mention ,,,for the record....
If you look back at Tubenit's original post on this subject and compare it to my adjusted schematic, you'll notice that mine is an exact copy of his.....
So, once I compared his schematic drawing to the Japanese scribbled version and mhartmans layout, I actually used his schematic whilst I was "cleaning up mine" as a template......if I was any good with express sch, I would edit it there, but it's still much easier for me to cut and paste into sluckey's original print

As always, thanks for all of your help T  

[jojokeo]

Once you get a feel for using the .sch program you'll be happy you took the time. It's a short learning curve but you just have to dive in at some point...

[jazbo8]

Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with... Congrats again on such speedy and successful build (even for bread-boarding it is impressive).

Purple: Class A Load Line, Turquoise: Class B Load Line:

[Glennjeff]

Here is a quote from HotBluesPlate, from the previous thread, that mirrors what I was thinking about yesterday (In the deleted comment):

I'd probably drop the 220kΩ cathode load to a smaller value (since the negative supply is less and we won't need to drop as much voltage), which also works better with a lower-mu tube with less resulting output impedance (12AT7 or 12AU7), allows the cathode follower to run at a little higher idle current and probably sink/source grid current much better. In fact, thinking about it now we could probably fine-tune this circuit element to be superior in every way to the Dumble original, at least for the purpose I believe it serves.

Guess I'll have to build it myself. Don't want to blow up someone else's amp. I'm convinced the biasing (and impedance) is not optimal around the CF region. Don't have the academic skills to back up my assertion however. Have put some industrial strength P channel MOSFETS on order to build a VVR style negative supply rail. If in doubt, use the empirical method, suck it and see.

[Glennjeff]

Apparently different versions were built differently in the PPICF department.

[tubenit]

I'm pretty sure #1 is a drawing error.

Tubenit

[Glennjeff]

Looked dysfunctional to me too tunenit.

So this might be worth experimenting with (for me at least).

Think I'll have to take the plunge and start learning ExpressSCH and get some sim software.

Does anyone know of some open source / freeware / shareware simulation software?

[SILVERGUN]

Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with...

Thanks for all of that jaz.....I like the idea of the lower variant sss, and wasn't concerned with trying to get the most out of the 6L6s, other than good tone....It's cool to see it graphed out like that and I look forward to the day that I will figure out how to acquire/use that type software.......
I can't imagine I'll ever be on a stage again where I'll need this to go past 5...max 

What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?

[SILVERGUN]

Apparently different versions were built differently in the PPICF department.

Thanks for sharing all of your research Gj
This aint over,,,,,and i'm looking forward to getting it to a point where it makes sense to everyone...

The first hurdle was just getting it hooked up without blowing anything up, and confirming that it actually would have a noticeable improvement......check 

DL and PRR seemed to like the potential of the 12AT7 for this application, so that will be step 1.....try it,,,and then try it with lower Rk

From HBP's post you mentioned:
(12AT7 or 12AU7), allows the cathode follower to run at a little higher idle current and probably sink/source grid current much better.
In Merlin's description of using a high negative voltage applied to the grid of the CF tube, he claims that it eliminates grid current,,,
how high of a negative voltage is "required"?

[terminalgs]

Ran some sims for you, based on your plate voltage and the bias as compared with the original SSS. You are not getting anywhere near the 40W output, perhaps that's what you want - a low power variant of the SSS. But besides the output power difference, I think your preamp can over-drive the power tubes easier (have not sim it, so just speculation...) may be that's what you are hearing when you crank it past 3. It could still sound very good because of the PPICF, which was the whole reason for the trial. But it is something to think about and play with...

Thanks for all of that jaz.....I like the idea of the lower variant sss, and wasn't concerned with trying to get the most out of the 6L6s, other than good tone....It's cool to see it graphed out like that and I look forward to the day that I will figure out how to acquire/use that type software.......
I can't imagine I'll ever be on a stage again where I'll need this to go past 5...max 

What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?

12AT7 and 12AU7 make good CF candidates. they handle the higher current better than 12AX7.  12AX7 can do it, but it might lead to a shorter tube life. see transconductance ratings on the data sheets.  a normal gain stage might a total of 1-2ma, a CF might be 7-10ma. same reason you see both triodes in a 12AT7 or 12AU7 driving a reverb tank or transformer.  They are up for the harder work load.  for your application, 12BH7 is really no different than a 12AU7,  and they go for a bit more $$$, whereas AU7s are given away practically.

If I were you, I would try to keep both of the tubes the same (The LTPI and the CFs),  12AT7 seems like the one to make work.  This suggestion comes from a practical point of view:  When it comes time to convert from breadboard to real amp,  having a 1/2 LPTI and CF in the same bottle will let you  keep the coupling cap and plate resistor on the tube and save eyelet board space. (couple the plate to grid with a cap from pin 1 to 7, B+ direct to 6,  LP's plate resistor from 6 to 1,, etc...).

Even if you operate the the amp at 3 or 5, you want it to stable with any combination of knob turnings.  If troubleshooting is difficult at max volumes, maybe build a dummyload for the purpose of initial stability testing, or build a 'dead box' with a real speaker in it..    Also,  as far as 'how loud it is in the next room' or for the neighbors,,  I think there is little difference in a 12W amp "dimed" and  a 40W amp "dimed".  If they can hear it,, its annoying them just the same :-)

[rzenc]

Another tube to try: ECC88.

If you get brave enough, you may even try a pentode CF - I have planning it for some time . It would unleash some possibilities regarding compression effects.

Hope this helps
Best Regards

R.

[jazbo8]

What are your thoughts about how that CF tube is biased?.....I'm planning on lowering the cathode resistor values, and trying to balance the bias a little better to give me a little more headroom there.......and trying different tube types,,,as you had mentioned in the other thread AT7, AU7, BH7............what should I be looking for?

As I suggested on the old thread (before reading the TAG threads), that the AU, AT and BH were superior than the AX. But those have built the SSS had zero issue with the AX, so may be my concern were over-blown... especially since HPB suggested that the key was to deal with the onset of the grid current and not purposely driving the output tubes into A2 operation. The remaining concern (if it should be there at all) is the high plate-to-cathode voltage that the tube sees with the SSS arrangement, so below is a safer version for you to consider, the downside is more parts count, since you need to create an extra voltage node off the B+ and properly bypass it, ditto for the bias supply, you need to add a resistor divider to get a lower bias voltage. In the SSS design, the supplies were just tap off the existing ones - less parts.

[SILVERGUN]

As I suggested on the old thread (before reading the TAG threads), that the AU, AT and BH were superior than the AX. But those have built the SSS had zero issue with the AX, so may be my concern were over-blown... especially since HPB suggested that the key was to deal with the onset of the grid current and not purposely driving the output tubes into A2 operation. The remaining concern (if it should be there at all) is the high plate-to-cathode voltage that the tube sees with the SSS arrangement, so below is a safer version for you to consider, the downside is more parts count, since you need to create an extra voltage node off the B+ and properly bypass it, ditto for the bias supply, you need to add a resistor divider to get a lower bias voltage. In the SSS design, the supplies were just tap off the existing ones - less parts.

Thanks AGAIN for taking the time to do that jaz, I really appreciate it..... 
I was actually considering taking the negative supply right off of the 120vac input power (in the actual amp build), so this will be easy for people to duplicate, and it will be just a little different than what i'm doing now (running a separate bias supply on the breadboard).....so your  suggestion for -100vdc is perfect for that .......

Also, yes,,,,I remember HBP's concern about the high voltage going across the tube at startup, and that's why I choose to use lowered voltages to begin experimenting....
When I first fired the amp up I think I had approx. 250vdc B+,,,,,,and I had plenty of bias supply voltage until I started to crank up the B+ and ran out of bias supply....and that's when adjusting the voltage divider became mandatory,,,,so now I see what he was talking about when he wanted me to give him the specs of my PT ahead of time

[jazbo8]

Also, yes,,,,I remember HBP's concern about the high voltage going across the tube at startup, and that's why I choose to use lowered voltages to begin experimenting....
When I first fired the amp up I think I had approx. 250vdc B+,,,,,,and I had plenty of bias supply voltage until I started to crank up the B+ and ran out of bias supply....and that's when adjusting the voltage divider became mandatory,,,,so now I see what he was talking about when he wanted me to give him the specs of my PT ahead of time

You are quite welcome, I am learning something new each time running a sim as well, so it is time well spent. In this particular setup, there is more inter-play between the bias supply and the B+, thus HPB's question on the PT that you plan to use, he is usually a few steps ahead of us anyway.

Notice that the design still has the bias at ~-40V just in case you need the headroom, if not, you can always dial it down. Anyway, let us know if the lower supply voltages work or not, perhaps some adjustments are still needed.

[SILVERGUN]

Posted another short clip, just to emphasize the punchiness of the current configuration......... same settings as before,,, same session, different take

SSdemo2

[tubenit]

I like that! Very percussive without being too compressed. That is a very cool tone.  I think that's getting towards the SRV Dumblish tone.

Thanks for sharing your findings and success on this amp build!

With respect, Tubenit

[Willabe]

Yeah you sound real good SG.


              Brad       

[SILVERGUN]

Thanks a lot guys....the biggest thing that stands out to me now, is how it stays clean even when I really dig in and I kinda get a rubber band feel off of the strings (they are 11's).....even though, like T said--without being too compressed
Whereas with the original build, it was constantly breaking up, even with the amp on 3,,,and I was surprised with how much "dirt" was in the tone of the AB763.......and because of the SS rect and large filter caps, it was VERY stiff
I honestly expected it to be "more clean" when I originally wired up the stock circuit,,,,so this is a very pleasant surprise
I did make a couple tweaks to the pre-amp and PI,,,, and then with the addition of this CF it just all came together 

Now that I see how easy it is to record on my smartphone and that it sound OK, I'll be taking requests 

[rzenc]

Very nice tone!

I did make a couple tweaks to the pre-amp and PI

Mind sharing a schem?

Best Regards

R.

[SILVERGUN]

Very nice tone!
Mind sharing a schem?

Thanks.... I think it's a cool middle ground between an sss and a blackface DR with 6L6
The schematic for the preamp and PI still match the first schem. on post #1
I also slightly tweaked the reverb by removing the cath. cap from the driver tube (as the result of my other thread)...and thats on there
What I was saying was that, when I built the stock circuit, it needed a little tweaking to get it to the point where I was ready to try the PPICF
That's where the month went between the original thread and this one.....

That schem. (in post #1) is my "cut and pasted" adaptation of sluckeys original "AB763 single channel, reverb, no trem" print...look at the cathodes on V1 and the 1K in the PI, thats about it, besides the reverb driver
.....pretty good for a counterfeit print 

The one disclaimer that I should make is that I never changed anything on the schematic to reflect the fact that I'm running a separate bias supply for testing.........once we get it all narrowed down I'll fix the schem. to reflect all changes and how/where I tapped the negative supply...still up in the air

[Willabe]

Now that I see how easy it is to record on my smartphone and that it sound OK, I'll be taking requests  

Ooh, ooh, Louie Louie, no wait, Wild Thing, no, no, In a Godda Devida, no, Whippin Post, no, no, Free Bird!

No wait, I've got it! Little Green Bag, but you need an electric 12 string and a good pair of maracas. Cool bass part too!

 
http://youtu.be/5ixrvc-YDI0


Sorry, couldn't help myself, here allow me.     
                

                                                 Brad        

[SILVERGUN]

Sorry, couldn't help myself, here allow me.     

Yeah,,,thanks for taking care of that for me 
Well,,,, if you didn't post I never would have known that song existed,,,so it isn't all bad.......but Louie Louie?????
And,,, I wouldn't know that you're actually paying attention to my thread...soooo.....I'm honored by your mere presence Sir 

Now don't you have a new workbench to go straighten up? 
I'm gonna expect to see a PPICF in that GA-77,,,,,so ya better get busy,,,,none of us are getting any younger around here 
 

[SILVERGUN]

I spent this evening trying a couple of different tube types and messing around with the voltage divider,,,trying to figure out how to increase the bias on that CF tube...
Here's the results:
1) Tried a 12AT7 and it actually introduced more distortion at a lower volume setting, so I can only imagine it's pushing the grids of the 6L6s too hard........forgot to take V measurements while in there, so maybe it was just my haste that ruined the results...
2) Tried a 12AU7 and hated it,,it seemed to have less drive, but also distorted and kinda killed the freq. response across the board.....bleh....couldn't pull it out fast enough
So when I get back in there i'll take V measurements and see if it was something that I was/wasn't doing wrong.....
I couldn't wait to get the 12AX7 back in there and put the amp back to where I love it......it makes me wanna play  

But after that, I gave my ears a break, and messed around with trying to increase the bias on the CF tube and then balance it with the power tube bias......interesting
It's truly a "balancing act" and I can see why a higher negative V supply would be helpful,,,because then your not limited by that one puzzle piece.....as it is now, I'm maxxed out at -85vdc
I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power, and just using that as a separate "highly adjustable" fixed bias supply.....sound good?.....I can't see needing more than that 
EDITED- CROSSED OUT A REALLY BAD IDEA!

[Willabe]

Yeah,,,thanks for taking care of that for me 

You said any requests? 

Well,,,, if you didn't post I never would have known that song existed,,,so it isn't all bad.......but Louie Louie?????

That song, Little green bag, is cool, so is Louie Louie, your too young to get it. AM top 20 stuff.

And yeah I read your posts, there good.



                 Brad     

 

[SILVERGUN]

And yeah I read your posts, there good

Well alrighty then  ,,,,,I was afraid I was bringing down the collective IQ of the group

Man if I could turn back the clock and pay attention to this stuff when my brain was still pliable,,,I'd be much better off
It is infinitely interesting to me.....
I remember seeing the TUT books for sale (at reasonable prices) and thinking "man that stuff is cool,,,but there's no way I'm smart enough to get it"
Now I have to read and re-read everything, and my reading prompts more reading,,,because I forget what I read a month ago 
I had to purge my brain of every last Top 40 hit I ever memorized just to make room for this stuff.......some days it actually physically hurts....that's how I know i'm getting old 

[Willabe]

Well alrighty then  ,,,,,I was afraid I was bringing down the collective IQ of the group

Not at all, if anything your helping bring it up.


             Brad    

[sluckey]

I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power

Now why would you do that? We go to extra trouble to keep the AC line separated from the chassis. Just steal some AC from the PT HT winding.

[Glennjeff]

sluckey

Could you explain that a little more please.

Sounds like SILVERGUN is going to make (-)ve DC straight from the wall mains. Which sounds wrong and / or dangerous at first glance. I just can't explain why.

Why not develop a (-)ve rail from one of the HT taps (340V or whatever) on the PT ?. That tap is only drawing a couple of milliamps at most and could be varied with a VVR as necessary. Can a bridge rectifier be used instead of a single diode, this will enable much better filtering of the (-)ve bias, especially if an inductor / capacitor combination is used. This will stop 50/60/100/120 Hz hum being injected into the CF and 6L6 grids, as always, I don't know nothing.

All the best.

[Willabe]

Why not develop a (-)ve rail from one of the HT taps (340V or whatever) on the PT ?.

Just steal some AC from the PT HT winding.

                  Brad        

[Glennjeff]

Willabe,

I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power,

That scared me, I felt it was explained too casually,so I wanted it reinforced.
I'm neurotic, MmmmK.

Just steal some AC from the PT HT winding.

All the best.

[Glennjeff]

SILVERGUN

Got hold of an evaluation version of a SIM.

Built your CF in it and it came up reasonably close to your DC conditions. The SIM says each side of the CF will handle about 5 V RMS signal before clipping. It also said the CF will not drive an impedance below about 330k, which is probably not a problem if we stay in Class AB1. That is with 315V B+ and 85V (-)ve bias.

With 380 B+ the SIM seems to think you'll need 120 V (-)ve bias, fiddling with resistor values didn't get me too far with less negative bias. SIM suggests that CF will handle 7 - 10 V RMS input on each side with the extra volts. Some of the experienced amp builders may know the signal level here offhand.

Tube datasheets for 12AX7 do give plate characteristics for up to 460V at low current. Which means they've been tested at those voltages. With higher volts the AX7 will have about 420V across it.

TheValveWizard (you've probably got his books) here, http://www.valvewizard.co.uk/dccf.html,  has a diode-resistor protection circuit for startup grid overvoltage in preamp cathode followers. Could something similar be devised to protect from cathode over-negative-voltage at start up?

Like a 50 Volt zener in series with resistor, zener anode towards valve cathode, zener cathode towards ground.

Don't know if that is of any use or not.

Unfortunately, to get any valves other than 12AX7 you gotta buy the Pro version of the SIM - $1500. Not anywhere near within my budget.

All the best.

[sluckey]

That scared me, I felt it was explained too casually,so I wanted it reinforced.

It is scary. Here's a little more info...

We all like to use a proper 3 wire power cord so we can connect the amp chassis (and guitar strings, microphone body, etc.) to earth ground for safety reasons. And most amps (all of mine) also connect the circuit common to chassis ground.

Looking at the whole picture, you see that Neutral and Ground are connected together in the main power panel. Now look at the bridge and two things should be noted. First, D2 is totally short circuited by the neutral to ground bond in the power panel. Second, D1 has 120VAC directly across it due to the neutral/ground bond. D1 will pop as soon as the power is applied. Hopefully it will just blow apart, leaving an open circuit. As drawn, the circuit cannot work.

There are some easy/safe ways to get safe AC to power the bias supply. The easier/cheaper way is to steal AC from the PT HT winding and use a common half wave rectifier. Another way would use a small 6.3VAC filament transformer, connecting the 6.3 secondary to the filament string. This will produce an isolated 120VAC on the primary winding. Now you can safely use a full wave bridge.

[SILVERGUN]

I'm thinking about creating a bias board that I will hope to use in the actual amp build ,where i'll use a FWB rect, to get a potential max. neg. v of approx. -170vdc,,,,by tapping off of the 120 vac input power

Now why would you do that? We go to extra trouble to keep the AC line separated from the chassis. Just steal some AC from the PT HT winding.

Just keeping you sharp sluckey....I gotta make sure you're still with me  
But this is a good topic to cover, because I have had this thought for different applications

Now,,,, what about the center tap of the HT winding that we connect to the chassis?......that's where my confusion comes from

Coming from welding machines I tend to like to keep things separated, and overbuild for current handling,,,etc.

I have no problem tapping off one of the HT legs, but i'm just curious why this other way is a bad idea? ....since this bias supply is so critical in this application, I want to make sure it's rock solid

EDITED:
I was typing while you were posting,,,so I'll just go ahead and post this so you know where I'm coming from.....but, now I do understand what you're saying

[SILVERGUN]

Looking at the whole picture, you see that Neutral and Ground are connected together in the main power panel. Now look at the bridge and two things should be noted. First, D2 is totally short circuited by the neutral to ground bond in the power panel. Second, D1 has 120VAC directly across it due to the neutral/ground bond. D1 will pop as soon as the power is applied. Hopefully it will just blow apart, leaving an open circuit. As drawn, the circuit cannot work.

Well here's the ugly truth.....I did it already  

My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....and when I first hooked it up, one of the diodes did go,,,,and I thought I had overheated it during soldering
So I replaced it, and it has held steady for the time being,,,but probably because the bridge is sooo overrated for the application (400vac--8amp)

I'll take this conversation to another thread so as not to detract from the original intent of this thread.....thanks for throwing flags guys!
EDITED AGAIN...NO NEED FOR ANOTHER THREAD,,,SLUCKEY STRAIGHTENED ME OUT

[sluckey]

My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....

No, that's not the same. Your little step transformer changes everything. It provides total isolation from the wall.

[SILVERGUN]

My current bench power supply consists of a separate bias supply that is wired up exactly like you're drawing (except I have a little step transformer that is taking the 120vac coming out of the wall and knocking it down to 60vac).....

No, that's not the same. Your little step transformer changes everything. It provides total isolation from the wall.

So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN

Now I really get it,,,,because of the isolation that the xfmr provides....of couse I should know that
Thanks so much for taking the time to explain that!!!!!.........talk about having just enough knowledge to be dangerous  

So what I will do now is:
Create a negative supply that taps off of one leg of the HT, with caps that are sized appropriately (right now my - supply tops out at -85vdc,,,so I have 100v caps in there).........

The main reason I didn't do it that way in the first place was so that each supply would stay independantly adjustable,,,,buit since we are designing for an actual amp build, I'll start down that path 

[llama]

Got hold of an evaluation version of a SIM.

Hey GVlennjeff, what's the name of the SIM?  I'd like to try the eval out.

Thanks

-LLama

[Glennjeff]

http://www.spectrum-soft.com/demoform.shtm

Micro Cap 10 by spectrum Software. Oh and it's $4.5k for the full version, sorry about that.

[sluckey]

So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN

Connecting the positive tab of the bridge  to chassis ground was never a problem, either on you proto board or in the amp. The only problem was connecting the AC mains (line and neutral) to the AC tabs on the bridge.

Now, you can safely connect the line and neutral to a bridge in a circuit that is totally insulated (isolated) from the user. The positive tab would have to be floating and not connected to any earth ground. Not a good idea for an amp though.

[SILVERGUN]

Quote from SILVERGUN:
So, then it's Ok that I connected the positve tab of my FWB to the main common ground connection on the chassis?.....on my bench it's Ok because of the isolation,,,but in the amp layout, It's a really BAD PLAN

When I read this back it sounded a little smart-assed if you read it a certain way.....please don't take it that way

Take it like I'm an idiot whose trying to re-explain it to himself to make sure he completely understands  

Just for the record, here's what my current bench bias supply looks like in case anyone was wondering:

[SILVERGUN]

OK,,,,back to work....
I am going to start by updating the bench bias supply,,,by feeding it an isolated 120vac (instead of the 64vac currently),,,and upgrading the caps to 350V.
So this will give us up to approx. -170vdc to continue experimenting with.....for now

I figured it would be easier to just change my bench supply to continue experimenting (for ease and range of adjustment), and then when we have a solid design, we can adjust everything for the final HT voltage we come up with....use a smaller value bias adjustment pot,,,etc......sound like a decent plan?

[sluckey]

Head 'em up... Move 'em out...