The SS Project.....AB763 with PPICF

[SILVERGUN]

Here's my take on what I'm seeing as I experiment with the voltage divider for the bias, and the reason for needing the increased negative supply......posted so that just in case I'm missing something,,,you guys can correct me....that seems to be working out for me so far  
Also, in case someone is following along and needs it laid out for them like I do.....
These results seem to be in line with what Gj and jazbo have figured out and posted above  

EDITED 7/24---FIXED CATHODE VOLTAGES TO READ NEGATIVE,,,ORIGINALLY POSTED AS POSTIVE.....

[terminalgs]

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

Do you know wreck of the old 97?

[Glennjeff]

SG,
Couple of things.

When you swapped valves around, I think different valves settle with quite different voltages between grid and cathode. That could affect output valve bias dramatically. As you did not do any voltage checks during the valve swapping exercise we have no way of coming to a conclusion about what different valves really sound like. You rushed that one a little.

Also, when you put in the new, more negative bias, you can pull out the power tubes and check that you can obtain a suitable negative voltage on the cathodes of the CF's before inserting the power tubes. Pulling the power tubes should not affect the measurement of bias voltage, and you may need up to -40V with 380 B+. Don't want to stress those 6L6's.

This experiment is very educational and entertaining by the way, thank you.

All the best.

[Willabe]

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

Good point and your right, it is dangerous. I thought you might have missed Sluckeys reply.


                       Brad     

[terminalgs]

Here's my take on what I'm seeing as I experiment with the voltage divider for the bias,

with a single knob, you are trying to balance two bias's,,  one for the CF grids, and one for the 6L6 grids/CF-k's.

perhaps this would solve the problem?  remove the 82K-R,,  and isolate the two neg. bias's:



The bias on the LTPI is about -40v,   the bias on the 6L6's is about -40v,,   I'd shoot for a bias on the CFs of ... -40v !!!  (so that would be -80vdc on the CF grids).

but then, whats your plate voltage on the CF's?  300v? what happens to a 12at7 if you drop 340V across it? (isn't max plate voltage 300V?) and if the plate to grid is 380v ??

[SILVERGUN]

When you swapped valves around, I think different valves settle with quite different voltages between grid and cathode. That could affect output valve bias dramatically. As you did not do any voltage checks during the valve swapping exercise we have no way of coming to a conclusion about what different valves really sound like. You rushed that one a little.

You are 100% correct Sir,
So I stayed tonight and did the right thing.....got your results
I did increase the negative supply and was able to bring -142vdc to the table after all was said and done.....it didn't have the effect that I thought it would have with that 12AX7 in there, and the most difference I was able to dial in was close to 2 volts.....

So I shut it down and did 2 separate tests with each other tube type I had here,,,,because I want to know the real deal too....
These next 2 charts are accurate readings with those 2 different tubes,,,and you're dead right..........the voltages were way different

SERIOUS NOTE:
I noticed that just by swapping from the 12AT7 to the 12AU7 (AND NOT ADJUSTING ANYTHING) it immediately increased my cathode current in my 6L6s to 130ma EACH  
SO BE CAREFUL OUT THERE KIDS.....DON'T TRY THIS STUFF AT HOME!  

EDITED 7/24- CHANGED A RESISTOR VALUE IN THE VOLTAGE DIVIDER TO REFLECT ACTUAL VALUE (82K NOT 870K)..IN THE AT7 EXAMPLE
   UNFORTUNATELY I DIDN'T MEASURE WHERE THE POT WAS SET

[SILVERGUN]

OK, so thats the scientific stuff,,,,,but my other reason was to put the different tube types in and listen
Here's the facts as I heard them: (with the amp re-biased with each CF tube changed)
1)The original 12AX7 stays pretty clean through 5 on the volume, and gets more overdriven by 7, but still tolerable, and just brings more bite and push to the sound (it already has a lot of bite and push at 3).....above that I don't hear an increase in volume,,,just distortion

2)The 12AT7 was fine around 3, but seemed a little more harsh than the AX,,,and as I turned the vol. up to 5 it was overdriven to the point that I wouldn't want to go any further......but for the sake of experimenting, I went to 7, and lost clarity, and it was just too overdriven and sloppy sounding.....not where I want to head with this build

3)The 12AU7 was more overdriven on 3 than the other 2, and lost the "controlled clean definition" that I loved about the 12AX7, 5 was terrible and 7 was unbearable.....just bad distortion, that lost the crystal clear glassiness completely,,,,and felt wayyy too pushed (maybe that had something to do with how badly balanced the triodes seemed to be) 

Hope that helps 

[Glennjeff]

Leaping Electrons Batman !


Those differences in 12AT 12AU tube bias are HUGE aren't they. The results of the listening tests are the exact opposite of what one would expect from those valves and I suspect that each valve needs to be run at much different current values to get them to work well in that particular circuit. Just drew a few lines on plate characteristics graphs and confirmed that (somehow, without knowing what I'm really doing)

Don't mind me SILVERGUN, I'm using this great effort of yours as a learning exercise. It was sounding great so, maybe that was it.
I've got just a touch of Mad Scientist Syndrome and will certainly be trying out any ideas put forward here when my parts arrive.

Having said that, terminalgs's schem looks good to me, you could set it up so that the AX is biased exactly the same as original and there should not be much of a difference in sound. PLUS, that will give me a better platform to experiment with different valves and impedances.

SERIOUS NOTE:
I noticed that just by swapping from the 12AT7 to the 12AU7 (AND NOT ADJUSTING ANYTHING) it immediately increased my cathode current in my 6L6s to 130ma EACH  
SO BE CAREFUL OUT THERE KIDS.....DON'T TRY THIS STUFF AT HOME!  

Sorry bout that but I did mention to test (voltages on cathodes of CF's to be more explicit) with 6L6's out of socket first.

[SILVERGUN]

The bias on the LTPI is about -40v,   the bias on the 6L6's is about -40v,,   I'd shoot for a bias on the CFs of ... -40v !!!  (so that would be -80vdc on the CF grids).

I don't know term,,,,,I'm starting to lean back towards the way we originally had it..
that's the best it's sounded....

Actually, I'm leaning more towards jazbos drawing from reply #69

[SILVERGUN]

Sorry bout that but I did mention to test (voltages on cathodes of CF's to be more explicit) with 6L6's out of socket first.

I would never blame someone else for me being a knucklehead 

Luckily I've left a second meter hooked up to measure 6L6 cathode current constantly, and it's how I make some quick adjustments....

Just thought I should let people know, that if you build it,,,don't think you can just safely swap tube types 

[Glennjeff]

Regarding jazbo's #69, if you're referring to the SS section it looks real good, lots of headroom.

[terminalgs]

The bias on the LTPI is about -40v,   the bias on the 6L6's is about -40v,,   I'd shoot for a bias on the CFs of ... -40v !!!  (so that would be -80vdc on the CF grids).

I don't know term,,,,,I'm starting to lean back towards the way we originally had it..
that's the best it's sounded....

Actually, I'm leaning more towards jazbos drawing from reply #69

the original circuit sounds good at 3/10 volume.  the bias on the CF  was -1,  so the AC voltage was going through that stage unclipped because it  was a small Vpk signal,, maybe 8V? 10V,,   when you turned up to 5/10,,  you exceeded the -1 biased CF's ability to pass signal unclipped,, and further up the dial, between 7/10 and 10/10,,  there is no volume increase,, and not much distortion increase because the signal is so heavily clipped, it probably looks like a square wave instead of any semblance of a sine wave.

your original goal was "great clean sound that will stay crystal clean as you crank the amp",  and it doesn't sound like you there yet...

a hallmark of an AB763 is it continues to sound better and better between 7 and 10!

Go back and look at the bias's on some of the CF's of those referenced known to be solid amps.   I'd look no further than the most stable, rock-solid clean amp I've ever loaded in and out of a Dodge van: the SVT:    the 5/69 rev.B sch I've got shows plate at 220v, grid at -77v, and cathode at -47v.

BTW:  re: tube rolling. each of those tubes has a different plate resistance,  so each one would change the DC current flow and thus the cathode voltage,  thus changing voltage at the cathode, thus changing the bias of the 6L6's and the CF stages. those changes effected distortion, etc.  but you aren't looking for distortion in this part of the circuit...  and...  IMHO, at this point in the development of the circuit, tube rolling 12xx7s isn't good tuning step (the circuit seems to still need tweaking...)..

press on!   

[jazbo8]

the original circuit sounds good at 3/10 volume.  the bias on the CF  was -1,  so the AC voltage was going through that stage unclipped because it  was a small Vpk signal,, maybe 8V? 10V,,   when you turned up to 5/10,,  you exceeded the -1 biased CF's ability to pass signal unclipped,, and further up the dial, between 7/10 and 10/10,,  there is no volume increase,, and not much distortion increase because the signal is so heavily clipped, it probably looks like a square wave instead of any semblance of a sine wave.

If you are talking about a grounded cathode stage, then yes, hard clipping will occur when the input voltage exceeds the Vgk bias voltage; but that is not the case here with a cathode follower stage, which has another name - the grounded plate amplifier. So to see what is happening, flip the tube upside down, now the real clipping point is actually between the grid and the plate (which is at AC ground) and not the grid and the cathode, to illustrate this point, please see the following chart: the input voltage is set at a ridiculously high 80Vpeak (do not try this on the bench!) and look at the output waveforms: the SSS section with a low ~-3V bias remains un-clipped and the SS with ~-16V bias may appear clipped but only because of its low bias supply (-100V). The SSS waveform also shows possibly why Mr. Dumble designed the CF stage with such high bias supply - it provides a ton of headroom!

X1: 12AX7, X2: 12AU7


 

[SILVERGUN]

Thanks for pushing term,,,,,and thanks for pushing back jaz!
Great stuff with a great explanation.....priceless

If you are talking about a grounded cathode stage, then yes, hard clipping will occur when the input voltage exceeds the Vgk bias voltage; but that is not the case here with a cathode follower stage, which has another name - the grounded plate amplifier. So to see what is happening, flip the tube upside down, now the real clipping point is actually between the grid and the plate (which is at AC ground) and not the grid and the cathode, to illustrate this point, please see the following chart: the input voltage is set at a ridiculously high 80Vpeak (do not try this on the bench!) and look at the output waveforms: the SSS section with a low ~-3V bias remains un-clipped and the SS with ~-16V bias may appear clipped but only because of its low bias supply (-100V). The SSS waveform also shows possibly why Mr. Dumble designed the CF stage with such high bias supply - it provides a ton of headroom!

It sounds like we're onto something here......Where would you like to see me go from here?
I have the ability to take the negative supply up to approx. -300vdc pretty simply and with minimal modifications and no safety concerns....could be done today...
Maybe I can benefit from keeping a lower B+, by not stressing the CF tube as much as the sss design?,,,,but stil use a high - supply V

Does this explanation partially explain why it may be true in the original hand drawing of the sss that the cath. was marked as measuring 1 volt less negative than the grid?(+1v bias?)....or do you think that was an error and it has to be the other way around?

[SILVERGUN]

Here's the quote from Merlin's website......"the triode gain stage" chapter that is available for free download (so I don't think he'll mind me using it)
http://www.valvewizard.co.uk/gainstage.html
I had mentioned this concept earlier in the thread, but rz shot me down pretty quick and I wasn't informed enough to fire back.....

Am I possibly taking it too far out of context by imagining that it applies to this application?

This is an excert from page 12---Biasing section

[jazbo8]

It sounds like we're onto something here......Where would you like to see me go from here?
I have the ability to take the negative supply up to approx. -300vdc pretty simply and with minimal modifications and no safety concerns....could be done today...
Maybe I can benefit from keeping a lower B+, by not stressing the CF tube as much as the sss design?,,,,but stil use a high - supply V
Does this explanation partially explain why it may be true in the original hand drawing of the sss that the cath. was marked as measuring 1 volt less negative than the grid?(+1v bias?)....or do you think that was an error and it has to be the other way around?

Good catch, I missed that completely, from the simulations and your bench measurements, I do not think those readings are correct - at least not the grid... Also, in one of your earlier posts for the 12AX7, the cathode voltages were mis-labeled, they should be negative not positive 24V, I hope...

As for which way to go, you need to lock down the output stage first, are you sure you want the low power output? You are leaving quite a bit of power on the table sorta speak, sorry to be a nag... Do you already have the PT?

[Glennjeff]

My first ExpressSCH piece of art. Could you humour me and plug it in jazbo8.

[SILVERGUN]

Also, in one of your earlier posts for the 12AX7, the cathode voltages were mis-labeled, they should be negative not positive 24V

I went back and fixed that pic in that post.....thanks again

As for which way to go, you need to lock down the output stage first, are you sure you want the low power output? You are leaving quite a bit of power on the table sorta speak, sorry to be a nag... Do you already have the PT?

I don't take it as nagging.......your help is priceless

6L6s are staying, and I like the low B+ just for the sake of running cooler across the board....
I'm fine with the lowered power output (can't see ever needing this amp to be louder), and will just make up for the lower voltages by running a little more current.....the bias calculator says 65mA per tube @ 320V plates is right on @ 70%
I don't have the PT yet, so thats still up for discussion.......it might be difficult to find a perfect choice that fits these voltages
Maybe I should look at a more generic 120 to 240vac xfmr?.....and then just run a separate filament xfmr

Maybe this one with it's 260-0-260 secondary taps: .....that'll obviously bump thing up a little
http://www.classictone.net/40-18065.pdf

[SILVERGUN]

What about HBP's concern about a high voltage accross that CF tube at initial power up?
If we wind up with 320V on the plates and a -240 supply, does that seem a little more reasonable/ acceptable to expose it to 560V? (instead of 700+)obviously it is a slight improvement....and is my thinking correct?

Is there a 12AX7 that will tolerate this better than others, that you know of?.....and are we still thinking AX7 is the best choice in my application?

[jazbo8]

What about HBP's concern about a high voltage accross that CF tube at initial power up?
If we wind up with 320V on the plates and a -240 supply, does that seem a little more reasonable/ acceptable to expose it to 560V? (instead of 700+)obviously it is a slight improvement....and is my thinking correct?

Is there a 12AX7 that will tolerate this better than others, that you know of?.....and are we still thinking AX7 is the best choice in my application?

When I checked with the builders over at TAG, not one said they had an issue with the HV at start-up, so perhaps it is a non-issue? Perhaps HPB could give some guidance on this. Some have suggested that the new JJ 12AX7s have no problem with the high voltage, so that's worth a look. In any case, you now have several options to lower the plate and the negative bias supply if HV across the tube is indeed an issue. Also because you are running at a lower power, the negative bias requirement could be reduced, so that should help as well.

My first ExpressSCH piece of art. Could you humour me and plug it in jazbo8.

Yup, your design works well. With the 50k pot max'd out, the CF's cathode is at ~-52V, which is ample for this design, when set to 20k, the cathode is ~-24V, just about right for the SS. I would also insert a "stopper" resistor in series with the pot, so no one will accidentally turn the pot all the way down and kill the tubes.

 

[Glennjeff]

My thinking is that the supply rail + and - are fairly close so it should like handle 300V peak to peak input.

Means setting up special supply rails, easy with a few R's, a couple of FETs and a couple of electrolytics though.
This would solve all overvoltage concerns, especially if there were some caps hanging off the gate resistor of the FETs controlling the rails, 10 second soft on if one was so inclined, like me.

As I'm merrily setting up multiple B+'s and B-'s, had the menacing thought of direct coupling the PI as well upon waking this morning.

From experience, how well does your SIM track reality?
Hows this?

[jazbo8]

In simpler circuits, my experience with the simulations were quite good, but since I don't have a test bench at the moment, I am relying on SG to give us feedback on the accuracy of the simulations, I wager they will be pretty close, at least within the tolerance of the tubes used. The stopper looks fine, SG can always set it to where he feels comfortable (or needed).

So if the voltages stay where they are now, then my earlier plate characteristic chart could be used to estimate the output power and distortion, I have not looked at the preamp section yet, it may need a few adjustments to make sure it does not overdrive the PI and output, since this is suppose to be a clean amp with good headroom... more fun ahead

[Glennjeff]

Here is a little mod that will enable precision calibration of the PI balance, well almost.

82k may need to be tweaked up or down a bit.

A 68k resistor and a 50k pot will give a greater range of adjustment but probably overkill.

B+ is arbitrary, the more you can put on the valve, the greater the headroom.

DC coupling the PI presents a LOT more difficulties with +180V / -150V than I initially imagined.

[SILVERGUN]

Last night I ran some more tests and just pushed the boundaries around to see if I could hear any differences...

I changed voltages, and went as high as 380v B+ and messed around with bias setting from low to high and back,,,,in search of a noticeable sweetspot...
I couldn't seem to find any reason to go in any other direction than where we have been going, because nothing really made that much of a difference....
I tried the AU7 and AT7s again and just wanted to go back to the AX7,,,,because even though those tubes work fine and I can manipulate the setup to get an acceptable sound,,,,,the AX7 still sounded the best to me

My thought about this is in line with something that PRR had mentioned on the last 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.

And maybe thats the point......even though the AU +AT may be better suited to do more of the job (especially if your running more output tubes),,,,,we only need so much work done, in this smaller application,,,,and the AX7 covers it
SO here, the AU+AT might just be too much of a good thing.....?...make sense?

[Glennjeff]

As I have already stated, YOU MAY HAVE IT! If you want to push the 6L6's up to 380V, will the current configuration handle that? Do you want to design an amplifer that can meet real life professional requirements, and does the current platform satisfy that engineering specification?

The issue of potential over voltage at power up may still be an issue, also, it may not really be an issue in the real everyday world.

Depends! Possibly on how often you like to replace 12AX7's, possibly not.

The schemata I derived for solving that issue, by the manipulation of supply rail feeds to the CF, can be applied to a 12AX7. I see no reason  that it would change anything about the small signal performance of the stage. That is if you call 200 volts peak to peak small signal. Leaves the question of switch on stress addressed. I'm going to try it out when the mailman arrives. Some would call it an un-necessary waste of components.

DO NOT TRY MY EXPERIMENTS WITH A 12AX7. Love this emoticon, been waiting to use it. AT or AU only. I'll post a 12AX7 version shortly, just for you, even if you choose not to use it.

It all depends on how much fun / fulfilment you get out of playing with your spaceport breadboard setup from my philosophical perspective. (Wish I had one, working on turning my tool-shed into a physics lab at the moment)

On a side note, I think I've cracked the - DC coupled to PA phase inverter - , so CF may be redundant. That depends on weather the phase of PS ripple of (-)ve supply  is IN or OUT with phase of (+) supply from a hum perspective. Which I will have to measure in a "Physics Lab" environment. That means nails n screws n fitting woodcuts n sealing spider cracks n applying long term pesticide against ants n termites n running mains power cables n determining lighting requirements n ventilation n fume cupboards  n n n ....l.  

I should point out, I LOVE MY D'MARS ODS, but why stop there. What a fascinating hobby.

All the best and MORE SOUND CLIPS PLEASE SILVERGUN.

[jazbo8]

Hit the pause button! Found a discussion on MEF that's relevant to the SS design, the SVT has a similar CF design like the SSS and it seems that some 12AU7 failures could be related to the HV across the tube and/or the cathode and the heater, like HPB described... But since the SVT has a HV timer, it would point to the latter case as the likely candidate. Whatever the reason, it seems putting in new production tubes gets rid of the early failure.

[Glennjeff]

jazbo8,

Any links on how to elevate (or depress in this case) the 6.3 V AC heaters to a higher or lower DC level.

Guess you could tie either the 6.3 V AC ground tap to  a DC supply voltage, but you need it at ground zero for the rest of the circuit.

All the best.

[jazbo8]

jazbo8,

Any links on how to elevate (or depress in this case) the 6.3 V AC heaters to a higher or lower DC level.

Guess you could tie either the 6.3 V AC ground tap to  a DC supply voltage, but you need it at ground zero for the rest of the circuit.

All the best.

No links offhand, but this CF design is a special case, where a separate filament supply might be useful. As you said, you don't really want/need to lift the heater up for the other tubes, so two independent filament supplies is an option, of course, it would also mean higher cost and more real estate taken up... Alternatively, may be just go with more robust new production tubes and call it a day, since Enzo said in his MEF post that he never had one SVT came back in for service after the tube replacements.

[Tone Junkie]

SG you can get Edcore iron in about any voltage you might want and if you don't see it on there web site call and ask . Only down side 4 to 5 weeks for delivery, I love there pretty blue color .
Bill

 

[SILVERGUN]

Thanks TJ, I'll check their stuff out.....

Hit the pause button! Found a discussion on MEF that's relevant to the SS design, the SVT has a similar CF design like the SSS and it seems that some 12AU7 failures could be related to the HV across the tube and/or the cathode and the heater, like HPB described... But since the SVT has a HV timer, it would point to the latter case as the likely candidate. Whatever the reason, it seems putting in new production tubes gets rid of the early failure.

Thanks for continuing to do your homework on this one jaz,,
I'm hoping that the lower voltages in the SS will calm things down a bit too...I haven't heard a reason why they should be higher....the Cf makes the amp plenty punchy @300 B+
I guess, if anything,,,it just appears that it's more difficult to find lower voltage/higher current xfmrs, which is probably partially why it's not very common to see  
People don't usually put this much effort into a 25w single channel clean amp, or deviate much from the AB763 stock numbers  

I did modify my bench bias supply and was able to get to -280vdc for testing, but was unable to hear any significant difference.......To me it doesn't make any sense to have a huge negative supply if it's not needed to make an audible difference........
I sent my scope out about about 3 weeks ago, so I'm hoping to get it back any day so maybe I can see whats going on.........I'm just starting to get the hang of it, so I might have to ask for help to know what I'm looking at/for, in this case

GJ, your interest and excitement in this project help keep me going and I appreciate all of your ideas.......can't say I'll use em all,,but it makes for interesting reading

[Glennjeff]

SILVEGUN,

You have built a breadboard system to try out ideas.

What is the point of that exercise if you are reluctant to try stuff?

Not saying my ideas are guarenteed to work, but trying to put a man on Mars takes a bit of calculated risk.

I will use the ideas generated here, and I have you to thank for being TEAM LEADER, so keep up the pursuit of what it is that you want to achieve.

What is it?
The ultimate clean punchy amp?

Have you defined your engineering objective?

You got a great buzz out of pushing through your uncertainty about direct coupling the CF to the PA. So which mountain shall you climb now. Given that you've got a great sounding amp it's got to be somewhat disconcerting to tackle the next summit (metaphor).  

So, I have to ask, where are you, "we" , on this project, what do you think we should put our energies into?

You appear to have control of bias, so why not put 380V on the 6L6's?

Anyhow, it's been a lot of fun and very educational. I'll refrain from further suggestions until specifically requested to do so.

All the best.

[SILVERGUN]

GJ, I hope you didn't take what I said there as anything negative.....(that's the problem with words on a page,,,they are up for misinterpretaion)
You tend to think on more levels than I am capable of (at least with my limited education) ,,,and I was letting you know that I appreciate you throwing stuff out there, and just want to let you know, that I may not be able to keep up with you 
It is frustrating for me to be somewhat intelligent, BUT not be able to grasp some of the concepts that get presented here...I've still got a lot of reading to do...
It can be embarrassing to get an education on a public forum  ....
I've numbered your questions again, and consolidated them...
I am listening 

SILVEGUN,
You have built a breadboard system to try out ideas.
1) What is the point of that exercise if you are reluctant to try stuff?
2) Not saying my ideas are guarenteed to work, but trying to put a man on Mars takes a bit of calculated risk.
I will use the ideas generated here, and I have you to thank for being TEAM LEADER, so keep up the pursuit of what it is that you want to achieve.
3) What is it?.....The ultimate clean punchy amp?
4) Have you defined your engineering objective?
5) You got a great buzz out of pushing through your uncertainty about direct coupling the CF to the PA. So which mountain shall you climb now. Given that you've got a great sounding amp it's got to be somewhat disconcerting to tackle the next summit (metaphor).  
6) So, I have to ask, where are you, "we" , on this project, what do you think we should put our energies into?
7) You appear to have control of bias, so why not put 380V on the 6L6's?
8) Anyhow, it's been a lot of fun and very educational. I'll refrain from further suggestions until specifically requested to do so.

1) I'm not reluctant to try stuff,,,,just have a limited time schedule,,,and an already large list of stuff to try
2) I am learning as I go, so I try to stick to one specific thing at a time,,,,and that alone is difficult,,because I want to try everything, but am not experienced or imaginative enough to want to continue pushing further for the sake of experimentation.......just get this one done, and then on to the next
3) AB763 with PPICF safely and solidly installed, and then migrated into a combo chassis (that I still have to build) with one 15" speaker,,,for me to eventually use on stage as part of an A/B configuration,,,,,,with this amp running the "super clean" part of the program
4) To eliminate the mystery surrounding this PPICF circuit as it was installed in the SSS, for the purpose of reducing my fear (and maybe some fear of others) to actually some day build a SSS or derivative..........I am still considering the idea of laying out the entire SSS circuit on the breadboard while this part is already done.........once we get the SS off of here
5) I don't want to build complicated amps,,,and I hope I'm not addicted to the buzz of discovery.....it was just a side effect of feeling like I accomplished something I have been thinking about for the last 4 months leading up to this......the first time I heard that amp I thought "I'm gonna build that someday".....so to take some of the mystery out of that is HUGE, and cause for celebration
I left my original project hanging to start this one because I was moved by the spirit to do so.....If you read a little bit of that thread, you'll see where I came from and how quickly I jumped in over my head;
http://www.el34world.com/Forum/index.php?topic=14575.0
6) I was truly hoping that a couple of the really big fish would swim along here and confirm/deny some of our thoughts.....BUT, if they don't and we continue down this path, I'm looking forward to getting my scope back and hooked up, and taking a closer look at what's going on here, and stop speculating by merely hearing and/or not hearing a difference
7) Good question....but I think---"why PUT 380v on the 6L6s?".....if we're worried about high voltages, why continue to try to inject high voltages? ......I don't need 50 clean watts, and I like the idea of a lower voltage, high current design (I like to build "special" amps)
8) Yes it has, and continues to be......you learn a lot about yourself when you "put yourself out there" like I have ......it can be difficult to "juggle personalities" that arise in a thread.......if you notice we seem to have lost a few along the way, possibly somewhat due to jazbo's fact finding 

Please don't let one misworded post chage your opinion of your place here.....here's the correct wording:
Gj, your interest and excitement in this project help keep me going and I appreciate all of your ideas.....thanks for all of your continued help!

[tubenit]

This has been a very very interesting thread! 

And I like the results I hear from SilverGun's video. It's positively impressive. I think SG has been pretty gutsy and innovative on this.

What are the remaining questions to be answered at this point?  Can someone list those?  (if there are any?)

It seems like there is a dynamic of something unfinished or unanswered, but I can not tell what that would be?  I pick up on that unfinished or unanswered theme because there continues a plethora of more info and perspectives? 

Do we have reasonable closure for this project on this specific amp?  I can't tell where things are at this point?

With respect, Tubenit

[SILVERGUN]

What are the remaining questions to be answered at this point?  Can someone list those?  (if there are any?)

1) Should I be concerned about the fact that when using a 12AX7 in the CF I only wind up with the grids being approx. 1 volt more negatine that the cathode?....that seemed to be a concern when you look at this from a standard biasing scheme (as terminalgs brought up earlier),,,,but if jazbo is correct about this being a "plate grounded" design, does that over-ride the way we would normally view those measurements for biasing this stage? (sorry jazbo, it wasn't obvious to me)..

2) Can anyone confirm/deny Merlin's quote above as being relevant to this design?.....
Is it possible that it might be OK to have the grids even slightly more positive than the cathodes in this stage?....not that I need them to be that way, but it'll just help me confirm/deny the validity of the hand-drawn scematic of the sss

3) How can I confirm/deny the actual negative supply voltage needed to satisfy the CF for what is was designed to do?....I don't think I have heard a difference between -85v or -280v and in each case, the voltage divider just knocks it down to -25ish volts.........so what was the point of Dumble putting -328 there?......and, should I be aiming for the -40vdc to be making it through to the CF?

3) Am I missing anything by concluding that the 12AX7 is "THE" tube for this job, based on the smaller scale of this design?........the perception being that we don't "need" anymore current handling in this stage

4) Is HBP OK?

Do we have reasonable closure for this project on this specific amp?  I can't tell where things are at this point?

We're close....once I get my scope back and hooked up, I'm hoping to trace down the point in the circuit that is causing clipping at high volume...
I was getting some "breakup" in the amp before the CF install, so I tweaked some of it out by reducing gain in V1....
Once we nail down the specifcs of the negative supply and cofirm that it's not the CF that is clipping, I'll go back thru the preamp and try to find it and reduce it without killing the tone and response we've created......I didn't want to change anything in the preamp while we were tweaking the CF...
While at a standstill the other night I tried a 5751 in V1, which didn't change the audible OD much,,,and then a 12DW7 that did, but made the amp too quiet, and took away too much ...

With the 12AX7 in the CF spot,,,there is a spot on the volume control (around 7) where the amp seems to begin to clip (audibly distort---which my friend liked) hard, and it is immediately recognizeable by an associated low hum, that gets introduced into the normally quiet output signal, when I hit 7-8
With the AU7+AT7 that spot gets lower on the vol. control......(approx. examples - AU7-3     AT7-5)

I'll probably never have to turn it up past 7.......I'm 44 now
 

[SILVERGUN]

OK, had a couple minutes tonight, and since I'm tired of claiming ignorance,,,I decided to use my head and try something different  

So I hooked up 2 meters at the same time to see what happened to the relationship of the grids to the cathodes on the CF as I fed it a signal and turned it up past 7......
The first phenomenon that I noticed was that the grids now appeared 2v more negative than the cath.--------so maybe the impedance of the meter has an effect on the readings when you take one reading, take it off, and take the other reading, when you're only workling with one meter at a time    

So at idle we have a 2v spread (-21v grid; -19v cath.)   , and as I increased the signal I retained that 2v spread throughout the entire span of the volume control (up to -55v grid; -53v cath.)

Just a note, while it was fresh in my head  

[jazbo8]

Something odd with the reading when crank'd up, how is the signal applied and how are you measuring the voltage?

I was writing when you posted...
===
I tend to get into analysis paralysis sometimes, sorry if that slows things down for some... The fact is, there seems to be conflicting info on the PPICF, not so much on how it works but rather its safe operating conditions, unfortunately no amount of simulations by me or the others can give a definite answer, since you have a breadboard with variable supplies, I was hoping that you can observe the waveforms/voltages on startup and during over-drive, i.e., when it's the most stressful for the tube to see what we are dealing with. Where is the scope when you need it?

I think the reason that you do not hear much difference between the high vs. low bias supplies, is simply that you are not pushing neither the PI, CF nor the PA to their limits. Do you have a dummy load? It would be interesting to see the waveforms when the gain and volume controls are turned up, you don't want to do that with a speaker, it will be too damn loud...

Although, I am still somewhat puzzled by the performance of the AX vs the AT & AU. If the AT & AU are properly setup, they should perform equal or better as a power amp driver than the AX, so much for being an armchair designer But don't let that stop you from completing your build, just curious, that's all...

[Glennjeff]

I phrased that a bit clumsily. Sorry if it came across as a dummy spit.

What tubenit said, about closure - that's what I was kinda trying to express.

Too many different ideas lead to confusion, so I'll stop with the experimental suggestions.

I believe that the hard clipping you are hearing at 7 is the CF clipping as per the graph in jasbo8's reply #112.

I can SIM 12AX7's. Can't get them to cope with much more than 5 V RMS with the SS's current bias setup (+315 / - 85). I think the PI is capable of putting out about 50 V RMS. Got notification that my parts have been dispatched from local warehouse so should get them Monday. Then I'll build my AU suggestion reply #120 above and splice it into my D'Mars temporarily (I'll do AX, 5751, AY,AT, AV and AU versions). If it works, I'll ExpressSCH the CF and it's MOSFET controlled power rails and post here.

To address your point form query;

1) and 2) Don't be too concerned about grid bias anomalies at this point. Although the new readings are more in line with expectations.

3) regarding negative supply, it's a complicated juggling act that in practical terms determines at what point the hard clipping occurs. (What your hearing at 7, what you see in the graph of jasbo8's reply# 112.) That is why I'm going for + 170 / - 130 or thereabouts. It's a fun juggle that provides maximum headroom without overvoltaging the valve (in theory).

4) HBP seemed enthusiastic on the previous thread, let's hope it's a well deserved vacation in some tropical paradise.

It is kind of unfortunate that some of the big fish haven't rocked up to lend a hand. They have their reasons no doubt.

[jazbo8]

It is kind of unfortunate that some of the big guns haven't rocked up to lend a hand.

I was wondering about that as well, does anyone know what happened to HPB? He was an active participant on the old thread, and many of the work-arounds came from his suggestions... We can sim all day, but nothing like someone with actual building/servicing experience with the amps.

[Glennjeff]

3) How can I confirm/deny the actual negative supply voltage needed to satisfy the CF for what is was designed to do?....I don't think I have heard a difference between -85v or -280v and in each case, the voltage divider just knocks it down to -25ish volts.........so what was the point of Dumble putting -328 there?......and, should I be aiming for the -40vdc to be making it through to the CF?

Try to determine if changing the negative supply from -85 to -280 changes the point at which hard clipping occurs, Volume at 7 with -85V  or volume at 8 with -280 V.

(Foam earplugs or wait till you get the oscilloscope back and use a dummy load.)

May have misunderstood your reference to -40V DC making it through to the CF BUT;

The CF design should end up being a "building block" that can be spliced into any guitar PA IMHO.

Given 6L6's, depending on the B+ voltage used, anywhere between 280 and 400 V DC. This CF design should be able to provide adequate bias. So that means it should have an adjustable range of say -15V to -50 V DC at the cathodes. Maybe an even greater range if you want to include other output valves EL34, KT77, KT88 etc.

[SILVERGUN]

Something odd with the reading when crank'd up, how is the signal applied and how are you measuring the voltage?

Signal is going into main guitar input, and just quickly tried to dial in something that sounded like an "A" 440hz
I cranked the amp up to 8 and then took the level of the s.g. to a point that sounded as loud as the guitar signal when cranked

I had one DMM connected to the grid of one side of the CF, and the other one connected to the cathode of that same triode
At idle I was reading -22vdc on the grids and -19vdc on the cath.,,,,I had to get the signal pretty loud to get those voltages to move but once they did, they stayed evenly spaced at 2 volts apart all the way up to -55 grids ...-53 cath.

I thought that was a good sign.....shows you how much I know

I was hoping that you can observe the waveforms/voltages on startup and during over-drive, i.e., when it's the most stressful for the tube to see what we are dealing with. Where is the scope when you need it?

Me too.....I will get a scope hooked up as soon as possible, and try to follow your instructions more closely, and just let you guide me through some of the measurements we need

I do have a dummy load but I just haven't wired it up for use yet (8 ohm- 200watt "corncob" resistor),,,,and you read my mind...I kinda felt like I could blow a speaker when I was doing the V measurements earlier....LOUD

I went back and edited my post because I had reported a difference of 3 volts when it was actually 2....math is not my friend 

[SILVERGUN]

Got notification that my parts have been dispatched from local warehouse so should get them Monday. Then I'll build my AU suggestion reply #120 above and splice it into my D'Mars temporarily (I'll do AX, 5751, AY,AT, AV and AU versions). If it works, I'll ExpressSCH the CF and it's MOSFET controlled power rails and post here.

Very cool, I'm looking forward to hearing your results...

Try to determine if changing the negative supply from -85 to -280 changes the point at which hard clipping occurs, Volume at 7 with -85V  or volume at 8 with -280 V..

Good plan.....it will make sense to do all of these checks with the scope hooked up, so I'll make a list of request that you and jaz have mentioned, and get answers instead of my opinions or guesses.....including confirming/denying some of the previous supplied sims

[jazbo8]

I had one DMM connected to the grid of one side of the CF, and the other one connected to the cathode of that same triode
At idle I was reading -22vdc on the grids and -19vdc on the cath.,,,,I had to get the signal pretty loud to get those voltages to move but once they did, they stayed evenly spaced at 2 volts apart all the way up to -55 grids ...-53 cath.

I see... but I do not think that's the correct way to measure it, since the cathode is the output and has large voltage swings, instead you should take the measurements with respect to a fixed reference like the ground. If you look at the plot I posted earlier, you can see the bias voltages remain at ~-40V (center line), and those reading are with respect to ground.

[Glennjeff]

jazbo8,

I think SILVERGUN is measuring wrt ground. I THINK (don't know) that what is happening is that once clipping sets in the whole DC environment is becoming compromised and pulling down. That may be shutting down the output tubes as well, giving lots of crossover distortion. Be interesting to observe if the DC shift corresponds with the onset of clipping.

All the best.

[Tone Junkie]

Good stuff guys it will be interesting to see what scope shows .
Bill

[jazbo8]

jazbo8,

I think SILVERGUN is measuring wrt ground. I THINK (don't know) that what is happening is that once clipping sets in the whole DC environment is becoming compromised and pulling down. That may be shutting down the output tubes as well, giving lots of crossover distortion. Be interesting to observe if the DC shift corresponds with the onset of clipping.

All the best.

I think you are right after re-reading the post, so the simulation is bogus?! Must get to the bottom of this - if like you said, the bias is being pull down to -52V, the tubes are in complete cut off

[Glennjeff]

I think you are right after re-reading the post, so the simulation is bogus?!

Have you actually SIMmed the PI, CF and PA and bias supplies as one unit. If not then the SIM is not necessarily bogus. I've seen this paradigm of behaviour before in all sorts of "out of boundary" situations.

 We need to wait until it can be measured before we throw our computers in the fish pond.

[jazbo8]

I think you are right after re-reading the post, so the simulation is bogus?!

Have you actually SIMmed the PI, CF and PA and bias supplies as one unit. If not then the SIM is not necessarily bogus. I've seen this paradigm of behaviour before in all sorts of "out of boundary" situations.

 We need to wait until it can be measured before we throw our computers in the fish pond.

Trust but check as they say 

[SILVERGUN]

and those reading are with respect to ground.

One meter was connected so that I was reading just the grid to ground, and the other meter was measuring cath. to ground....
Just a silly experiment that I thought up, that I was hoping might show me something.......I'm not really working out of a book, so I'm learning by doing

Must get to the bottom of this - if like you said, the bias is being pull down to -52V, the tubes are in complete cut off

jaz, did you mean to say bias voltage there?

This is one of the questions that I need to clear up in my own head:
Do we still look at biasing these triodes the same way we see bias in a cathode biased stage?.....so what I'm asking is does -21 on the grid and -19 on the platecathode still imply a -2vdc bias?
I'm sorry, I just want to make sure I understand what is being said.....
I don't always understand what I read from other sources,,, and then try to apply to this (odd) application
Do my results partially explain (or make us question more) why Dumble might have started with -40vdc on the CF at idle ??(as seen on the hand drawn schematic)


Just for the record...my workbence is at my place of full time employment,,,,,so (usually) I only work on it Mon.-Fri. 5pm-?pm
So I won't be able to supply anymore test result until then.......today is golf  

Feel free to give me specific test instructions so that I can give you the answers you need.......you don't ever have to say please  

EDITED to reflect the wording error sluckey pointed out (plate=cathode)

[sluckey]

so what I'm asking is does -21 on the grid and -19 on the plate still imply a -2vdc bias?

I'm sure you meant to say "does -21 on the grid and -19 on the plate CATHODE still imply a -2vdc bias". If so, the answer is yes.

Tube bias always refers to the voltage difference between grid and cathode. To say I have zero volts on the grid (typical preamp gain stage) or I have +50v on the grid (think LTP or bootstrapped PI) or I have -50v on the grid (your particular CF or fixed bias power tube) tells me nothing about the bias of that tube... until I also know the voltage level on the cathode.

[terminalgs]

2) Can anyone confirm/deny Merlin's quote above as being relevant to this design?.....
Is it possible that it might be OK to have the grids even slightly more positive than the cathodes in this stage?....not that I need them to be that way, but it'll just help me confirm/deny the validity of the hand-drawn scematic of the sss

Merlin's quote make the point that the absolute voltage of either the cathode or grid is not critical, but the difference in those voltages is critical. (sluckey's point).  So,  the actual bias is important to the stage.  in that doc, Merlin gears the examples to the first gain stage of the amp, where a -1 or -2 is his target.

If you read his AC-couple CF page ( http://www.freewebs.com/valvewizard/accf.html ), in those exmaples, his input signal voltage is higher, and he chooses a higher bias:

"In this case a bias of about -3.4V is chosen, giving about 50Vpk before clipping, into easy loads" (from linked doc)

regarding, plate voltage , check the datasheet for the tube you are using for this rating.   If its 300, then the difference between the plate and cathode shouldn't exceed 300.   If you have 350 @ P,  and -47 @ K,,  then 397 exceeds that number.

The SVT schematic I referenced earlier:

the 5/69 rev.B sch I've got shows plate at 220v, grid at -77v, and cathode at -47v.

has a  bias of -30, and a plate voltage of +297.  ( the 220v supply comes from it's dedicated 6146B's screen power supply that provides 220v).