FETboy transistor choosing & biasing questions.....

[gary o]

One of the biggest hurdles was to overcome the fact that 'original' MPSA14 Darlington has an hFE of 20,000! The away around this was to use a pair of either BC550A or BD139's transistors that when, multiplied together would bring you close to the original value.

Since BD139's were easily available I bought a few to try and find a few pairs. What I found is the stock BD139's didn't give me high enough hFE and I had to switch to BD139-16's, which have a higher hFE.
How I went about doing it was testing the hFE values of each transistor and drew a series of circles on an an A4 piece of paper and placed each transistor into it's own circle and wrote down the hFE of each one. I found this method very useful and was then able to extract the values at created pairs that would when multiplied would give me close to the 20,000 hFE so for example a pair consisting of say, 130 x 150 which gave me 21,000 hFE

from the Fetboy page

What happens  if we didnt get a pair of transistors to bring us up to the 20 000 hFE mark....Is this to bring the output level to the same as the original Hamptone pre ? will effect the sound if not right or near value ?

I dont have a means to test the transistors...I have sockets & loads of transistors BC & BD types....I can plug and play I suppose..

Will a cheap digital multimeter with transistor test socket give me an idea ?

I never realy made any transistor kit till now , Iv vero boarded fetboys 2 seperate stages for a mono mic pre , I have 100K trimmers and sockets for all transistors.

To my happy supprise they worked straight away...tho in one of the two circuits I can get the correct FEt bias voltage of 12V ...thats if I understand it right... in the other gain block board the nearest I can get from my little bag of FETs is 9.6 volts.....thats pretty bad right ?.....Is this bad for sound , I dont need to match two channels for stereo I just want mono....

will the other transistors in the circuit affect the Fet bias ?

Just quick tests it sounds ok unless turned right up, kinda valve fuzzy distotion...also the RL resitance load part of circuit the 100K 22K and 820PF cap doesnt seem to do all lot...is that because very subtle ? like I say I havent had a real good listen.

Im running on 24V regulated by LM317T I have variable resistor there, is running it at higher voltage bad idea ? ...I was wondering if that would help me to get both my FETs to around 12V ...if that matters that is

Thanks for reading.

 

[gary o]

Ok ...on my own here ......trying to understand why FEts need to match other than in stereo unit.....I have my two little boards on one i can find a 3 fets out of 20ish that set up up at 12 V ....good right....the other board the output boards fet I can only find a couple that go down to 16V...bad right....there seems to be bit of strain on this output board 30R res gets hot transistors bit warm too....so is this why we need to match ?.....& and in my case 20x transistors none match my two gain stages......In this state it does sound ok Im not sure how it should sound as I dont have another....compared to DIY V72 Sowter in /out trans & choke the V72 is more detailed more guts nice bass, this maybe unfair comparison as FETboy only has input tran cheap OEP no Op tran too plus as u read my circuit maybe set up wrong...& il add the fetb is a usable sound il still use it even if I cant get it running smoother

 

[Ptownkid]

You can test hfe with any old multimeter

 

[gary o]

Ah thanks ...Is it the diode test thing ? ....do you teast as 2 diodes ? .....if so youd have two number...how do you calculate the over all Hfe value that way ? ....cheers, thanks for patience , Im still learning.

 

[Ptownkid]

see the little grid type thinger in the bottom right, you need to use the row appropriate for the type pf transistor you're testing and insert the pins into the correctly labeled holes (emitter, collector, and base). Then you take the values of a pair of transistors and multiply them to see if you get close to 20,000

 

[Kingston]

You can test hfe with any old multimeter

I've found only the really cheap ones have that feature. My fluke 179 don't for example (which was $200 and my cheap five dollar one does!)

I've been meaning to set up a really simple test rig. I wonder what the simplest way is?

 

[Ptownkid]

The simplest way would be to buy a cheap DMM, lol.

 

[Kingston]

The simplest way would be to buy a cheap DMM, lol.

well like I said I have one, just that this Fluke is a hell of a lot more accurate, and I'd prefer measuring everything with it.

 

[gary o]

Thanks for that guys ...I had one of those cheap meters but its now broke ...i will get another soon...I tried to do the test with the diode test thing on my other cheapish meters..... I get numbers like 890 1000 ish for BC550 can I do it likr this ? for now ...prob maybe for us is three meters I have all had different results for diode ...I would guss tha the cheap tran test meters would vari too which would further bugger up the calculation.

Cheers

 

[Silvas]

By the way,as i have learned with time and experimentation,  biasing the fets is crucial in this circuit, i followed crhissugar's advice (on replacing the 1.82k resistor with a SOT resistor but i use a trimmer) and now i custom-bias each fet. I mean, i select 4 fets who bias at the same exact voltage, then i'll bias them to get exactly 1/2 whatever voltage is the power supply (i use 28v) , but that's only for the first stage. For biasing the second stage, i do somewhat the same way i custom bias the fet on a neumann u87,feeding signal and watching the distortion and levels, then adjusting bias for the lowest possible distortion and maximum level, with the output level being very close to 0dBFS (that's +18 on my digi interfaces calibrated for 14dB headroom). This has worked wonders for me, and has allowed me to get more voltage swing from the pre's output before it clips. But if i measure the voltage before the coupling cap...well, it's a couple volts below 12v, but i don't give a s**t, me and my fellow engineers really love this pre, and we have neve, amek, hardy, scully, hhb....as i said before, probably not the best or correct way of doing things but it works really nice for me. Go for it, this pre is really worth the effort !

 

[gary o]

Silvas, thanks intersting stuff Id like to try similar approach I have much to learn tho.....I too have to ahave 2K trimmer in place of 1K82s ...dont seem to dp much to sound & Im not sure where to measure the voltage as I trim.....How do you watch the distortion as you bias? with scope ? I have big old scope sitting next to me can I do it with that sadly Ima complete beginner when it comes to scopes.

Cheers.

 

[skipwave]

Way to go Silvas! There is something mighty special about this design. I use mine a lot. I gotta try out some other input transformers, just for comparison's sake. The passive components are so important in a simple amp like this.

What happens  if we didnt get a pair of transistors to bring us up to the 20 000 hFE mark....Is this to bring the output level to the same as the original Hamptone pre ? will effect the sound if not right or near value ?

Gary - Just to address your original question. The current gain of the emitter follower output stage will not affect "level," as this stage has no voltage gain, just current gain. It's a big 'ole buffer.

 

[Silvas]

Hello

In fact, i use a 5k trimmer plus a 100r, and i measure just before the coupling cap.

I watch the distortion in pro tools using waves PAZ.

But this is a pre you can't judge by its specs or distortion....only by it's sound. To me,  it's magic, and a complex sound, not "clean" or boring, just magic. But if built properly is really quiet with tons of gain. Ah, i think a 20dB pad is more useful in this pre than a 14dB pad btw.

 

[gary o]

Thanks guys......cant wait to get it workin right.....Skipwave you'l have to excuse me I still dont understand all the terminolgy & the way things work, I usually deal with valve /tube amps not that I fully understand what Im doin...but tubes seem less fussy more forgiving design wise.

When you say current gain...what does that translate into...the sound but not level ?....

Think of me as total newbee thanks sorry to be pain I wanna learn

 

[gary o]

Aha I got a 2nd hand digital meter a lead wise MT600 prob crap but hopfully good enough, only a fiver....my lowest two Hfe for BC550C is 523 & 519 ? 523 x 519 = 271437 ? miles & miles away am i doin this wrong?.....the lowest 2 Hfe for BD135/16 s are 158 & 161 158X161= 25438 better right ?.....why is there such a massive difference ? ...I did read that is doesnt matter....is that really so.

Cheers.

 

[Conductor]

BC550C + BD135/16 = Darlington
(523 x 158 = 82634; 519 x 161 = 83559)

 

[Silvas]

use the BC550A, which has lower hfe...around 100-150 iirc

 

[gary o]

Ok il get some BC550A s then , thanks .....Im still confused & a little more confused after reading about 20 pages of fetboy problems this is just about the fiddlyist most time consuming thing iv ever built ive built 4 differnt tube pres  during this build...ok they havent got cases but all working fine.Anyway Im bit confused as I said , I have 100K trimmers for R4/9 I trim for 4 V across R5 without FET in socket...yeah..when I go thru that that set up process one gain block of my mono pre I get bang on 12V with one of my fets the other gain block I can only get down to 18.6V, I even borrowed the FET from the 1st gain block but thats slightly higher, I read we can match our resistors to our FEt & thought I could do this for the 18.6V gain block,I already have 2K trimmer for R 3 is that correct ?.....If so it doesnt do any thing when I turn it all volts stay the same ?....it should tho right ?
Does R5 100R have to be replaced with a trimmer ? is that how to trim to 12V ?
I read some folks FET boys were left with wrong voltages so may not sound as intended, I wanna hear how it should sound....Thanks for help.

 

[gary o]

Ok...it oficial Im a cock ....I had my FETs pins wrong  :-[ red face dont no how, what a knob... It worked that way tho haha....now I have more level too much really but I can trim the 2K pot to get 12V at the cap now so hopefully I have that part done.

Not sure I understand the darlinton stuff tho...the darlington is Q2 & 3 right ? for Q 4 I have a BD135-16 soldered in there.

I had a pair of BC550C  for Q2/3 according to my meter they have HFe s of 523 x519=271,437 miles from the 20 000 as required.... the lowest BD135-16 s I have are 158 X 161 = 25,438 much much nearer to the required 20 000.....

Conductor are you saying I should use one of each ?
82,634 still miles away tho ?

Anyway I tried both combinations....

A pair of BC550Cs sounds ok lots of level tho ...the lower pair of BD135-16 s are s**t sound awful ...distorted to hell.. I thouht they shgould have been best ?
Conductors BD135 & BC550  sound nice same sort of level

I have a ELa250 DIY mic plugged in poly output capacitors on fetb (about to try tants flavour)pre plugged into my DiY RCA Ba6A limiter the limiter has tons and tons of level,I dont know how to work out db and stuff yet but I seem to have a lot of level from fetboy my other valve DIY pres I have the Ba6a input attenuattor at about 3 or out of ten for fetboy has to be 1 or lower and fetboys level pot would be about 1or2 out of ten

 

[skipwave]

Think of the FET as a 12AX7 in common cathode (plate out) config, direct coupled to a cathode follower (the darlington emitter follower).