when making a stompbox version of an amp....

[pinkjimiphoton]

is there a rule of thumb regarding coupling caps?
working on an emu of an ollllllllllllllllllllllllllllllllld (in fact, one of the first) transistor amps, and it has like HUGE coupling caps...
50u electros...

from previous experience, i'd expect that to be pretty unstable... should i drop it by a factor of 10, 100, 1000 even? just curious...

will post the prelim layout i'm working on shortly, it's a really simple implementation of a kay 700 vanguard  amp... the first commercially available transistor amp,
and ALL GERMANIUM. i used to have one, it gets a nice distortion...

basically, i took the preamp from the schematic of the kay, and grafted a fetzer valve on the end of it.

advice appreciated, thanks!

[Davelectro]

50uF coupling cap doesn't make any sense, unless impedance of next stage is extremely/stupidly low.

[pinkjimiphoton]

well, remember where this is coming from... this is a solid state amp, the first.. and it's  almost like a tube amp the way it's set up.
thinking the impedance may be why they did that,  haven't looked up the original transistors yet, but will post the schematic as soon as i get a chance.
thanks bro

[pinkjimiphoton]

ok, here we go...
EDIT.... TOTALLY SCREWED THIS UP, DON'T MESS WITH IT!!!
here's the original schematic:




here's my adaptation of it... basically just the preamp with a fetzer valve on the end of it with switchable diode clipping (there's a deet of the switching, i drew it as 3pdt on/off/on, in case i wanna add led's to it... pretty lazy tho...)




and finally, the vero i layed out, obviously not verified or anything...



tho the diode clipper is shown on the  vero here, i'll mount it on a switch as per the schem i'm working up. why so many ge diodes in the clipper? cuz i've found that it takes that many to equalize most of the volume out between silicon and germanium diode clipping....it accounts for the difference in voltage drop i think... if ya use the/on/off on switch  as i did, you have ge, none, or si diode clipping, which hopefully will give it a little more versatility. we'll see. i have the weekend off, so hoping i can maybe breadboard it... it's REALLy just a handful of parts!!

i'm thinking i'll make the huge coupling caps 1u instead of 50... in my experience anything over about 2.2u just lets too much bass thru, and is unstable...

[Keppy]

Hey Jimi, I think you missed a couple of things. The original has Q2 collector connected to the supply voltage through the transformer. Your schem has no supply on that collector. You've connected the Q2 emitter to the supply through a resistor, but that's different too. In the original, Q2 emitter connects to a voltage supply 1 diode drop from ground through a 1k resistor. Plus you connected Q2 input signal to collector rather than base. Did you do all that on purpose?

[pinkjimiphoton]

nope, i totally screwed it up. oooooooops...


i'll fix it.... thanks keppy!!

[pinkjimiphoton]

again, thanks keppy for spotting my screw up!!!

i believe this is "right" as it were now.. i can't believe i missed the q2 c connection!!

i figured 2 back to back 1n4001's should give me close to the 1v drop to the c of q2, so i added that. not sure if i did it right.. would led's be better?

gonna try using 1u coupling caps instead of 50u (47u) and see if it flies... probably will socket and see what seems to work best.

advice appreciated!!!!

[Keppy]

Check one more time, Jimi. I think it's reversing the polarity of the supply that's messing with you.

Q2:
Base is fine now.
Collector should be connected to the power supply through a resistor. In the original, the resistance was supplied by one winding of transformer TF1. A 2.44v drop over R6 works out to 2.44 mA current. Apply that current to the 1.5v drop across the transformer winding and it appears you need a 615 ohm resistor.
Emitter should connect to ground across a diode with a .16v drop. I've been working with some 1N5818 Schottkys (Schottkies?) that would be about right, but I wouldn't be surprised if any old diode (or even a straight connection to ground) worked.

EDIT: I didn't mention the emitter resistor, which you already have in as a trimmer. The emitter connects through the resistor/trimmer to the diode to ground.

[midwayfair]

10K pregain voltage divider? Unbuffered? That's a lot of loading.

[pinkjimiphoton]

jon...

are you saying dump the pregain control? only added it cuz it's as the original schem was.... this is very much a work in progress, so... if it's unnecessary, all the better!

or are ya talking about the bright/normal switch thing? remember, you guys know WAYYYYYY more about this stuff than i do!!!!
or... should i add a buffer stage to it? i didn't think it would be necessary, as i'm trying to catch a little of the sound/mojo/vibe of the original one..

keppy, thanks for the help bro...

so, are you saying i shouldn't try and drop the voltage to c of q2 with the back to back diodes, and instead go with maybe a 1k trimmer?

also, just to get this straight.. add a diode to ground from the 1k trimmer where it connects to q2 e? why? i don't understand, i don't think... sorry to be so dumb!!!!!

like, looking at the schematic, wouldn't the b+ be connected to the e of q2 directly thru the 1k resistor (that i replaced with a trimmer)?

i am definitely missing something here i think!!! do you think it will work as i laid it out, or will it work out to being a fatal error?
i figured the trimmers would hopefully give enough leeway to  "tune in" the circuit...
but i'm not afraid to admit it when i have no clue, like now!!



anyways, here's the latest vero attempt, which i believe matches the schematic i cobbled together, tho if i have to make revisions still, that's all cool!!!

thanks brothers!!!!!

[Keppy]

so, are you saying i shouldn't try and drop the voltage to c of q2 with the back to back diodes, and instead go with maybe a 1k trimmer?

In the original schem, the voltage drop is over the winding of a transformer, which relative to the collector functions as a resistor. The diodes might just clamp your output to 14v DC. I think the trimmer's a good idea.

also, just to get this straight.. add a diode to ground from the 1k trimmer where it connects to q2 e? why?

No, Q2 E connects to the trimmer, and the other end of the trimmer connects to the diode to ground. The emitter in the original schem is connected through a 1k resistor to a -.16v source. If you look at the power supply section, the -.16v is achieved with a diode to ground. I don't know why, but that's how it was done in the original schem you posted. It'll probably work fine to connect that trimmer to ground without the diode.

like, looking at the schematic, wouldn't the b+ be connected to the e of q2 directly thru the 1k resistor (that i replaced with a trimmer)?

Not b+. E of Q2 connects through the 1k resistor to a -.16v voltage source. Well I guess you could call that b+, but your handwritten schem has that trimmer connected to 9v, which is way off.

i am definitely missing something here i think!!! do you think it will work as i laid it out, or will it work out to being a fatal error?

Connecting the Q2 emitter trimmer to your main voltage supply will not work, I think. Connecting it to ground should work, connecting it through a diode to ground copies the original.

i figured the trimmers would hopefully give enough leeway to  "tune in" the circuit...

The trimmers are fine, it's what they're connected to that is the issue.

I think the way the original schem is presented is messing with you. You know, the whole positive ground to negative ground, PNP to NPN swap, supply voltages written at the bottom instead of the top, kind of thing.

If I may make suggestions, I'd put a 1k resistor on the Q2 emitter instead of a trimmer. Connect the other end of the resistor to ground instead of 9v. Use the trimmer on the collector in place of the diodes.

I hope this helps, and I hope it sounds as good as you remember!

[Keppy]

Oh, and Jon's right about the loading. If you're keeping things clean, a <10k input impedance will have TONS of treble loss.

However, that's not always a bad thing on the input of a fuzz. The harmonics created make up for the lost treble.

Also however, the first transistor is emitter grounded, so removing the volume control still won't raise the input impedance that much. Just try it as is, and buffer it if the loading is a problem.

FWIW, the Fuzz Phazzer has ~30k input impedance, and the fuzz sounds great. The clean is horrible, though, because of the treble loss.

[tca]

basically, i took the preamp from the schematic of the kay, and grafted a fetzer valve on the end of it.

And why not building an amp out of it, replacing the output stage with a class ab capacitor coupled stage?

Cheers.

[Gus]

This is nothing like the amp.  

The big missing part is the transformers.
The amp has a first gain stage
then a single ended Ge transformer gain output driver stage
Then a push pull  transistor transformer output stage.

Gain stages don't make an amp emulation when you have transformers in the amp.

Also look closer at the input
The feedback bias resistor R3 in the Kay schematic also forms a inverting amp with the series input resistors and/or caps so the base is a low impedance summing junction. So the 10K is not that high a value so it can function as a volume control at that place in the circuit.

[pinkjimiphoton]

This is nothing like the amp.  

The big missing part is the transformers.
The amp has a first gain stage
then a single ended Ge transformer gain output driver stage
Then a push pull  transistor transformer output stage.

Gain stages don't make an amp emulation when you have transformers in the amp.

Also look closer at the input
The feedback bias resistor R3 in the Kay schematic also forms a inverting amp with the series input resistors and/or caps so the base is a low impedance summing junction. So the 10K is not that high a value so it can function as a volume control at that place in the circuit.

diggit, remember gus, i generally am about 8 miles beneath you... say what?



don't really mind if it doesn't sound all that much like the original.. this is just kinda loosely based on it, and probably won't work without some serious revision.

speaking of which, i revised the vero to add the trimmer and e to ground resistor keppy had suggested... thanks kep!!

we'll see what happens. worst case, YAFF or variant, right?

gus, should i maybe try adding one of them little audio transformers that rat shack sells maybe?

seems like overkill...  but again, i am a hack <<<< (that pink guy ain't kidding, he's a hack)

see, it's like this...

when i was a ween, i got my first electric guitar, a 3 pickup tiesco del ray that was vaguely stratish.
my dad was like NO @#$%ING ELECTRIC GUITAR AMPLIFIERS IN MY HOUSE!!!!

so the pawn shop guy felt sorry for me, and gave me a little tiny practice amp built into a book shelf speaker cab.

flash forward slightly, and i discovered the 12" jbl bass speakers my brother wally had made this thing MUCH louder.

then one day, i was at the pawn shop drooling over tiescos and sekovas and kays and stuff, and i noticed a 42$ amp...coincidentally, the tiesco i bought there was also 42$. i believe it was kismet.

it was a kay vanguard something or other, not sure which model... with tremolo, and a 12 " speaker. it was love at first site. i had to have it so i sold a lot of joints worked my ass off like a good young entreprenour, and bought it. i had also bought two individual particle board 1x12 cabs about the size of the old 2x12 traynor cabs. hey... it was the early 70's...lol..

into these i loaded the 2 12" jbl's. they were 8 ohms. ever being just smart enough to be dangerous, i applied ohm's law and discovered the pair, in parallel, was 4 ohms.

the speaker in my kay amp was a 16r 12", so i immediately used the flawed logic of the day, and figured...well, 8 ohms will be twice as loud as 16, so 4 ohms thru 2 speakers would make it 4x as loud (in my mind) and give me twice the speaker area.

in theory, it wasn't quite that, but MAN, did it get loud. the back of the amp said 150 watts, so i figure if that's what it drew, it probably was pumping about 30 watts into 16r. now i was pumping the same amount of power into 4 ohms. surprisingly, the amp didn't blow up.

and it was @#$%ing LOUD.

this was where kid photon learned about FEEDBACK. i had a hollow body jap double cutaway with a wigglestick on it and two hot single coils (looked like sekovas, if that means anything to you you're old too) and i would crank that amp full blast and have at it with the tone controls almost off and the wigglestick throbbing.

i learned to control feedback and distortion (all germanium, unknown to me at the time) before i had actually learned my first chords.

my family, and the neighborhood, and the cops were thrilled. you could hear me at mcdonald's, one mile from my house.

that's why dad had to do the circuit breaker trick.

anyways, the amp took a hit at my first gig (at which my rig was a red tiesco with 2 pickups, a double cutaway with the black pickguard with white flowering vines) my ludwig phase II and the kay, thru them two jbl's.

it was louder than god until the generator blew my kay, which sadly never was to run again.


that's why i bought another kay on evilbay, and why i'm even considering messing with this circuit. i wanna see if i can find any of the magic that was there 40 years ago, and if i still like it. so success...failure... it's all good.

every new fuzzy noisemaker is always a reason to celebrate.

i re-did the schematic and vero, tho i have to re-draw the schematic so i can scan it... it got some oil on it or something, don't wanna foul the scanner.. i expect some sound will pass thru it
as it sits, and expect it will need some tweaking in to get it to sound good...
schem will be up when i get a chance to draw it again... here's the vero, with changes as suggested by keppy.

also changed out the input pot to 100k, hoping that may make the circuit react better. most likely that will change too, i'll mix and match until i find something toneful hopefully.

anyways... the vanguard fuzz (as yet) unverified:



if anyone is crazy enough to build this before me, let me know if it works.

[Gus]

I have heard older solid state amps like this and they can sound interesting.
IIRC the pignose also has transformers.
R.G. has written about the Vox amps.

Note the output transistors have collectors connected to the primary side output transformer.

The input resistance
Looking at the bass input.  The bass connects to a 220K series resistor then a 50uf cap and volume control then to the base of the first transistor.
Note the value of the feedback bias resistor at the first transistor it is 150K This means even with the volume control at max the gain is under -1

So one way to design around a low input resistance is to make the circuit an inverting type, with the series input resistor most of the input resistance(you should think about the passive guitar or bass and input cable that connects to the circuit and adds Ls,Rs and Cs).

When you think about the input resistance of the Kay think like an inverting opamp (a transistor with C to B resistor and a input series resistor to the base is an inverting amp)not a non inverting opamp
http://en.wikipedia.org/wiki/Operational_amplifier_applications

Have you tried a 47uf cap instead of the 1uf you show?

[pinkjimiphoton]

i haven't done anything yet at all with it. just trying to work out where what will be pretty much, thanks for the tips gus.

way WAY over my head most of it, but i'll investigate it some and see what happens.

the only reason i was figuring not to use the 47u caps was the last time i tried to use big caps like that the circuit was completely unstable.

and remember too, i'm just trying to get some of the sound from it..

the ge's are pnp's with a minimum gain of 65 ( http://alltransistors.com/pdfview.php?doc=2n408.pdf&dire=_rca  ) so i figured conversion to npn would just make things easier.

there's no transformer in a pignose that i remember, i've had the classic original and still own a 30/60.

so... i will experiment some and see what happens, maybe if i graft in a 1m resistor for the 150k one i can find a decent value to change it to?

thanks for the help guys!!!!

[Gus]

http://www.diystompboxes.com/smfforum/index.php?topic=21063.0
http://www.gitaarnet.nl/forum/showthread.php?151138-bedrading-Pignose-7-100

Google Bing etc "Pignose 7-100 schematic"

[pinkjimiphoton]

ok, so it's using a center tapped transformer as a driver stage and an output transformer.
cool.

too complex for what i'm trying to do tho bro. one of the things i dug about this original schematic was how low the parts count was, and how nice my old one sounded.

thank you tho!!

[Gus]

http://www.pritchardamps.com/prs_history.cfm