fet boy

[BR]

I actually have a 200K trimmer there. I'm just wondering if I'm actually measuring it right?
Am I supposed to be measuring between the two legs of the resistor??
Pardon the stupid question, but that is what I understood from "measure V across R5".

thanks
Gil

 

[skipwave]

I was confused by that direction too. Since one leg of the resistor is grounded, shouldn't that be 0V, and the 4V should appear on the other leg, at the junction with the emitter?

 

[dissonantstring]

hello everybody,
i'm building fabio's fetboy and i've run into the same problem several have had. my unit passes signal and gain works until a little more than half way and then suddenly breaks into oscillation. at the very end of the pots' travel (around the 4 o'clock position) the signal completely cuts out - no sound! :? i measured hfe of all BD139s and calibrated the FETs per Matta's build page on Kev's site.

i've tried the 22K resistor between the gate of Q8 and the wiper of the 10K pot, which helps a little, but there's still oscillation. i tried pulling C1 as Fabio suggested and now my unit does not pass any audio. does anyone have any other ideas to get rid of this oscialltion? it seems like my unit has a lot of gain even before the ocialltion. when plugging a bass into the DI the signal begins to clip pretty early (around the 9 o'clock position).

so far the units sounds really good, until it begins oscillating. :grin:
any help would be greatly appreciated.
thanks.
grant

 

[skipwave]

[quote author="dissonantstring"]i've tried the 22K resistor between the gate of Q8 and the wiper of the 10K pot, which helps a little, but there's still oscillation. [/quote]

I used a 22k not for its value, but because it was the only big carbon comp resistor I had at the studio when I was testing it out. I was looking for a smaller value, but since this design just has heaps and heaps of gain it didn't do any harm, and it stopped the oscillation completely in my build. I can turn the pot all the way up, however it never gets turned up past 9 o'clock due the onset of clipping with any normal mic and source.

It's not very versatile, but I love the way this pre pushes a distorted guitar to the front of the mix.

[quote author="dissonantstring"] i tried pulling C1 as Fabio suggested and now my unit does not pass any audio.[/quote]

:shock: That doesn't make sense. That should only affect the feedback around the first stage. That cap is around part of the source resistor,and the signal is being taken from the drain to the output transistor. It shouldn't have killed your output. That's truly weird.

Here's a thought, variable feedback using a pot in place of the source resistor. Hang the cap off the wiper to ground , so you can adjust how much of the source resistor is bypassed, if any.

 

[dissonantstring]

hi skipwave,

[quote author="skipwave"]
:shock: That doesn't make sense. That should only affect the feedback around the first stage. That cap is around part of the source resistor,and the signal is being taken from the drain to the output transistor. It shouldn't have killed your output. That's truly weird.[/quote]

that's what i thought (but then again, i'm a newb )! i was surprised when i didn't get any signal.

[quote author="skipwave"]
Here's a thought, variable feedback using a pot in place of the source resistor. Hang the cap off the wiper to ground , so you can adjust how much of the source resistor is bypassed, if any.[/quote]

so you're saying, leave C1 in and replace the 1K82 resistor with a trimmer pot (maybe 2.5K or 5K) with wiper to ground and adjust til oscillation goes away?
thanks for the help skipwave. truly appreciated.
-grant

 

[skipwave]

[quote author="dissonantstring"]
[quote author="skipwave"]
Here's a thought, variable feedback using a pot in place of the source resistor. Hang the cap off the wiper to ground , so you can adjust how much of the source resistor is bypassed, if any.[/quote]

so you're saying, leave C1 in and replace the 1K82 resistor with a trimmer pot (maybe 2.5K or 5K) with wiper to ground and adjust til oscillation goes away?
thanks for the help skipwave. truly appreciated.
-grant[/quote]

I think you got it. Just for clarity, C1 would be connected from the pot wiper to ground. I think I've seen this in guitar preamps. It just occurred to me that the ability to introduce more feedback as needed to tame this beast would be a good thing. We might have to tweak the value of the drain resistor as well, if we increase the value of the source resistor to a 2.5k pot.

Anyway, I think you have some other issue that needs addressing first. Removing C1 shouldn't have killed the output, so it makes me want to figure out why that happened. You can always up the 22k resistor to try and kill the oscillation and still stay true to the design. I'm afraid to yank C1 completely, because it might harm the sound I've come to love.

 

[dissonantstring]

hi skipwave,
i'll investigate the loss of signal with the removal of C1 again.
i put C1 back in so i'll place a trimmer pot in line with the wiper of the gain pot and see if i can dial in a fixed value to rid me of the oscillation and if all goes well then replace i'll it with a fixed resistor. i'll report back if I can get this sorted out.
thanks for the help!
-grant

 

[Xopek]

Hello!
Look pls, whether correctly I have connected the output trafo (OEP A262A2E, 1:1, 600:600)?

http://ft.fotoplenka.ru/ft/99/82/48299/111216/199daf42.jpg

In original clause one of pin the output trafo is not connected... :sad:

 

[clintrubber]

[quote author="Xopek"]Hello!
Look pls, whether correctly I have connected the output trafo (OEP A262A2E, 1:1, 600:600)?

http://ft.fotoplenka.ru/ft/99/82/48299/111216/199daf42.jpg

In original clause one of pin the output trafo is not connected... :sad:[/quote]

Looks good to me w.r.t. the 600:600 side of things.

W.r.t. polarity you might have an intentional or unintentional polarity inversion going on, see
http://www.canford.co.uk/commerce/resources/catdetails/2739.pdf
You've dotted both your primaries otherwise. The secs are OK.

BTW, what's the drawing program you're using ? For some reason I like strange fonts & 'different' looking schematics. Nice. Does that program work outside Russia ?

Bye,

Peter

 

[Xopek]

I have finally got confused... :?
I used data from official site OEP.
( http://www.oep.co.uk/audio_transformers_high_performance/audio_transformers_high_performance.html )
I thought that F1, F2 is the beginning of windings; S1 and S2 - their end. And under your link - on the contrary?
http://www.canford.co.uk/commerce/resources/catdetails/2739.pdf
In general I have created a new pattern (the point designates +)

this variant correct?

PS
Which program I use - pcad 2002 + russian fonts
PSS
W.r.t. - what is it? :roll:

 

[3nity]

Just fire mines today.....they work really fine....although my PSU was heating a bit causing some oscilation but that's gone.

They're not racked yet cuz i need to get more 2n5457 so i cant match them....i got 15 last time from digikey none of them was around the 12 volts.

 

[skipwave]

Or you could select a new drain resistor value that better matches your JFETs, like chrissugar did.

If you do buy more, go for the OnSemi brand, instead of the Fairchilds. Apparently they have lower beta on average which makes more of them bias properly in this design with the given drain resistor value.

 

[dissonantstring]

wait! 3nity are you using fabio's fetboy boards or something else (perf, etc.)?
fabio's boards have the trim pots to dial in the 12V which may drift a little, but wait til the channel is good and warm and then trim. if you're not using fabio's boards then please disregard this message.
kind regards,
grant

@skipwave,
i forgot to thank you for your help with my fetboy build ages ago. both channels work like a charm thanks for giving me insight to that osc cure!
sorry for my much belated offer of thanks. i couldn't have got it working without your help. :thumb: :guinness: :guinness:
cheers,
grant

 

[rotation]

Hi, i'm buying parts for Fetboy and i'm not sure about capacitors at the output. They should be poly 10u, but here i can only find nice 15u poliester and 10u tantal. I could get 10u polypropylene but they are very very expensive.
So, can i use 15u poliester and not have problems with it? And can you tell me more precise what effect would tantal caps have there? It was mentioned that midrange becomes darker...

Miha

 

[jensenmann]

http://www.diyfactory.com/projects/fetboy/fetboy.htm

15uF is fine, polyester surely too. Tantals read link above or ask matta

 

[rotation]

Thanks

 

[skipwave]

Grant - You're welcome!

Miha - I used Sprague Mylar that I got cheap on evilBay.

Can you get Solen? They should be cheaper in EU: http://triodeelectronics.com/somepofica.html

Shipping from the states seems to be prohibitive these days. If not, ApexJr has a nice selection:

http://www.apexjr.com/capacitorA.html

I've got a couple of those Aerovox 20uF Met-poly. I should try one with the Hamptone circuit.

 

[3nity]

Grant i'm using Fabio's fetboys. Actually the FET give like 16-18V... not even close to 12v so at warming no major change.

I'm gonna get some locally by NTE components.

 

[rotation]

Well, Solens are a bit expensive for me at the moment, Apex's are cheaper, but posting is expensive...
I found some 15u poliester from Iskra (local company that makes some good caps; poliseter, polypropylene..) that are around 1 euro and very cheap tantals. I will buy both and try what comes out.

 

[buschfsu]

ok, im a bit tired but i have it hooked up and im using scotts voltage regulator so i get 23.94V dc to the board. problem is i get 23.94v everywhere. all of the bias points, R5, before C2, can this be right? i don't have another meter here to check, but it appears to be working ok with a 1.5v battery. won't pass signal through DI either

any suggestions?