bc108c in BYOC tone bender MKii

[idgit]

anyone know if swapping the OC75's for BC108c's is a straight swap?  my OC75's are so mismatrched that the feedback is horrible.  any other suggeestions welcomed too

Thx

[Scruffie]

BC108 is first of all silicon, so it wont work in the MkII circuit without alterations to the circuit and second of all... I think it's NPN? The circuit needs PNP transistors.

Have you adjusted the circuit bias? And also have you tried swapping the OC75 transistor positions, that can make A LOT of difference in this circuit.

Failing that, best bet is to buy a Tone Bender MkII set from smallbearelec, they'll be properly matched to work in the circuit.

[idgit]

thanks for taking the time to respond.  yes the germaniums are horribly mismatched so biasing is fruitless.  I saw some pieces at smallbear I may invest in.  did not know if silicon was swapable.  perhaps i will look into rebulding the circuit for silicon

It was my last kit before I found out about this forum and although I don't know much about electronics, I am good at following directions and reading instructions.  we'll see how I do with the projets I find here.

[Scruffie]

thanks for taking the time to respond.  yes the germaniums are horribly mismatched so biasing is fruitless.  I saw some pieces at smallbear I may invest in.  did not know if silicon was swapable.  perhaps i will look into rebulding the circuit for silicon

It was my last kit before I found out about this forum and although I don't know much about electronics, I am good at following directions and reading instructions.  we'll see how I do with the projets I find here.

Not a problem 

Did the transistors come with the kit? BYOC claims on the MkII Page that 'The BYOC E.S.V. 2-Knob Bender kit comes with a set of N.O.S. Philips OC76 germanium transistors that have been tested for leakage and gain selected to produce the best tone.'

If that's the case and you think your transistors are unsuitable it might be worth shooting them an e-mail to see if they can rectify it.

It's probably not worth rebuilding it on the stock PCB for silicon, I think to get a MkII sound there are some substantial value changes made to the circuit besides fake leakage and smoothing caps or whatever it uses.

I'm sure you'll do fine with the projects here and find people are very helpful at getting them running.

[Effectsiation]

If that's the case and you think your transistors are unsuitable it might be worth shooting them an e-mail to see if they can rectify it.

This. I know they did this in the past. I think you had to not shorten the leads though. Regardless, Keith and the crew at BYOC are very easy to work with, I'd contact them.

Also, you said the transistors are misbiased, have you tried them in different positions in the circuit? The kits that BYOC puts out have fixed component values, so I'd imagine all of the transistors they provide have to have had the hfe checked.

[marauder]

Try biasing at around 7-8V on Q3, its not correct at 4.5V, this seems to be a carry over from biasing a fuzz face.  Dave Main of DAM has tested loads of vintage Mk2s, and the good ones are in this range.  My OC75/OC76/OC75 Mk2 has a 4.7K resistor where on the stock circuit is a 8.2K, and it sounds great.  I assume the BYOC has a trimmer.

To make that circuit work with silicon still requires a germanium in Q1,  it needs the leakage to bias Q1 correctly.  the other two can be PNP silicon, but wont sound as good as germaniums.

[Cortexturizer]

could you post a picture of the board and the soldering side as well? feedback and squeal, oscillations and such, it's enough to have your pet's hair between the legs of the transistor and the infinite squealing begins...

clean your board like crazy maybe it will go away, cheers

[Effectsiation]

  I assume the BYOC has a trimmer.

Yeah the BYOC has the usual bias trimmer for Q3. I completely second the higher bias suggestion too, I don't know why I forgot to mention that, the full quote is: David Main said 7.6-8.9v for Tonebender MK IIs. My first BYOC kit was a TB MKII, before they did the ESV version, that thing sat in a cabinet for years before I read that different biasing advice and now it's really come to life.

[Scruffie]

  I assume the BYOC has a trimmer.

Yeah the BYOC has the usual bias trimmer for Q3. I completely second the higher bias suggestion too, I don't know why I forgot to mention that, the full quote is: David Main said 7.6-8.9v for Tonebender MK IIs. My first BYOC kit was a TB MKII, before they did the ESV version, that thing sat in a cabinet for years before I read that different biasing advice and now it's really come to life.

Yeah I shoulda mentioned that too, i'd forgotten about that annoying 4.5V rumour, gets stuck on rangemasters too sometimes.

To make that circuit work with silicon still requires a germanium in Q1,  it needs the leakage to bias Q1 correctly.  the other two can be PNP silicon, but wont sound as good as germaniums.

You can make it work with a silicon in Q1, just fake the leakage required to turn the transistor on, a reverse bias germanium diode from base to collector or a resistor from collector to base will simulate it.

Small value caps (22 - 220pF range should do it) across collectors and base of the transistors might help with any fizzyness from the silicons too. There are quite a few tricks to getting it to 'sound' germanium.

[idgit]

Great info from everyone, thank you

I got the kit from Geargas, Ernie had the second half of the OC transistor batch that BYOC had, what Ernie at Geargas told me.  BYOC has been out for a while.  They are OC75's.  Ernie seems like a nice and accommodating guy, I'd just rather have a go at fixing it myself

He offered to take a look at the pedal and replace the transistors if needed, but It's been tough getting in touch and getting this done as he only communicates through email, so I wanted to just move on and fix it myself.

The values of the OC75's are
Q1----65, L=247
Q2----72, L=221
Q3----91, L=239


I'll try the bias with a meter at the higher value,. I've tried many positions by ear, none produce enough volume without unruly feedback.  I've read that a well matched set is in the 70's for gain, I know nothing of leakage matching.

Thanks for the great feedback



here are some shots of the board

[idgit]

I will add that the transistors are soldered into the sockets per some suggestions i read, i burned one socket removing them once to trim the leads

[pedalman]

don't waste your time...go to smallbear and include a note to steve that you are looking for Jimmy Page growl. steve will make it come to life! best 20 bucks I ever spent   

[idgit]

yep, probably will do that.  probably ought to change the sockets and not solder the new ones too.

Thx