Compensating for zero leakage Ge transistors in a MKII circuit?

[Nitefly182]

So this is the opposite problem of most Ge transistor threads here.

I've got some killer high quality military Mullard OC42 yesterday. They all measure up with fantastic gains (60-130) but they almost all have zero leakage. None. Aside from one or two with 40ma they are all high quality devices with no leakage. Great for rangemasters but not so great for MKIIs. I dropped some in my test rig and the attack pot range was decent. Went from very low fuzz which is nice to a decent amount of fuzz but not massive sustain. I didn't take voltages but I suspect the voltage drop on Q1C wasn't enough because there was no leakage to help the circuit bias up.

So my question for anyone who has run into this problem is, what is the preferred method of dealing with the problem? Raising the base resistor to ground above 100K or adding a resistor across the B and C? I've read that a 1M resistor across Q1 B and C will give the right voltage drop but I haven't tried it before. Anyone ever deal with this type of "too good" germanium issue in a MKII?

[DDD]

Some leakage in MKII trannies is essential, vital for the circuit operation.
Leakage acts kinda bias to provide for proper operation point.
I.e. good trannies for MKII are leaky trannies.
(Get the pedal warm enough with iron, and you'll see the drastic sound change due to the leakage increase).
Also, 1 MOhm or so resistor from base to ground may improve the situation.
At the same time base-to-collector resistor will kill all of the MKII essence.

[Nitefly182]

Ill have to experiment a little. I had read that the CB resistor was a really good solution here or another forum. Perhaps these might just be better suited for fuzz face type circuits. Or just for Q2 and Q3 in a MKII.

[jrod]

There are a couple of MKII type circuits that use silicon transistor floating around. One is called Hot Silicon and Dragonfly has another one in his gallery.

You might check these out see how they compensated for no leakage.

Edit: Here is the Hot Silicon

and here is Dragonfly's Si TB

[Nitefly182]

OK so Andrew definitely used the BC resistor on Q1 but the 470K is half the value I have heard people suggest. Time to experiment

[deadastronaut]

There are a couple of MKII type circuits that use silicon transistor floating around. One is called Hot Silicon and Dragonfly has another one in his gallery.

You might check these out see how they compensated for no leakage.

Edit: Here is the Hot Silicon

and here is Dragonfly's Si TB

aha, just what i was after, i didn't fancy heating up trannies ......   cheers!. i'll knock up a layout!...

[mac]

I have some low leakage Toshibas 2sa49-52-53, less than 30ua. Really good devices Toshibas.

The problem is Q1 because as the leakage is so small it will bias near to Vcc. You can add a big resistor in the order of one or two Mohms either from B to C or B to Vcc to set bias near 7 - 8v.

The FF section will bias easily, and since the leakage is small the thermal runaway will also be small.

mac

[DDD]

They say that MKII has a very special "mojo" just due to the C-B leakage of the 3-rd transistor.
So, shunting the C-B junction with the external resistor may reduce or damage this special effect.
As far as I know MKII enthusiasts prefer to use leaky trannies instead of any artificial compensation. Moreover, on their opinion the leakage is higher - the sound is better.

[mac]

Leaky germs at Q2 will put Q3 collector closer to 7v or higher. Q3, leaky or not, has a very small impact on bias voltages. The FF sections depends on Q2.
Many users, included silicon builders, like to set Q3 near 7v. Leaky germs bias this way naturally. But IMHO it is a bias thing. You can set Q3 C at 7v using non leaky devices by adjusting the trimmer.
RG wrote something about "the sound of leakage". Do a search.

If you want to play with FF bias go to my gallery and download a small app I wrote to calculate FF and other circuits bias.

Q1: The use of a big resistor from B to Vcc when using non leaky germs, reduces the 100k by 10% or so because it is in parallel with it. The big cap at the input makes this reduction negligible.

mac

[Earthscum]

How leaky should Q3 be? Q1? And Q2 doesn't matter, essentially?

[mac]

How leaky should Q3 be? Q1? And Q2 doesn't matter, essentially?

No, Q2 does matter... a lot!!!

Q1: there is nothing feeding the base, so the leakage is what turns the transistor on (DC speaking) and sets the collector voltage.
for example if you disconnect the 100k then 0.1ma=100ua of leakage will put the collector at vcc-10k*0.1ma=9v-1v=8v
that's what you do when you use RG method to test germs.
but if you connect the 100k then there will be 0.1v/100k=0.001ma through the 100k assuming vbe is 0.1v.
this current comes from the collector and is hfe times this value. if hfe is 50 then the base is sucking 0.001ma*50=0.05ma from the collector current, ie, from the leakage.
so the transistor will sit at 9v-(0.1ma-0.05ma)*10k=8.5v

I'd say that 100 - 300ma is fine is hfe is in the range 50-100. (70, 100ma) is my personal choice.

Q2: FF bias depends a lot on this transistor. Finger touch it and you'll notice a quick increase of the voltage at Q3 collector. As I explained before, more leakage means a higher collector voltage at Q3. Too much leakage makes impossible to bias the thing!!!

No more than 200ma here IHMO.

Q3: FF bias is ALMOST INDEPENDENT of Q3 leakage and gain.
put whatever you want here, leaky, non-leaky, an ECC83, a brick, it won't change Q3 collector voltage. Finger touch it without changing Q2 and see the results.

mac

[R.G.]

Having too little leakage is easier to deal with than having too much.

Leakage in a transistor comes from the reverse-biased base-collector junction letting current through. If you have nearly zero, but want more, you can simply put a reverse biased diode from the supply to the base. This lets the diode's leakage get through to the base as well, and act exactly like leakage from the collector would. A diode's leakage is very nearly constant from a few volts reversed up to the point it starts zenering or avalanching. A resistor doesn't act the same way.

Any diode will work, as long as you select it for leaking the amount you want. Silicons, even "high leakage" silicons like the 1N914, don't leak enough. A selected 1N34, 1N270, or the collector-base of another germanium device might be perfect.

[Earthscum]

Thanks very much for the explanation! (I actually meant Q3 not mattering much, as per your post above, but that's kinda inconsequential given your breakdown of it. I was typing mobile).  

I have about 15 oddball transistors that are leaky, and in the low range. Guess I'll have to measure those again and pick a couple... and find the NPN's again.

Thanks, R.G., as well... you posted while I was typing.  

ETA: hmm... I have leaky diodes... including a couple transistors that only one junction appear to be conducting, but they still measured for leakage across those 2 legs, and diode action. They just appeared as an open circuit between base and the other leg.

[mac]

Having too little leakage is easier to deal with than having too much.

Leakage in a transistor comes from the reverse-biased base-collector junction letting current through. If you have nearly zero, but want more, you can simply put a reverse biased diode from the supply to the base. This lets the diode's leakage get through to the base as well, and act exactly like leakage from the collector would. A diode's leakage is very nearly constant from a few volts reversed up to the point it starts zenering or avalanching. A resistor doesn't act the same way.

I did just the opposite to deal with too much leakage, using a very leaky diode or transistor to shunt some leakage from base to gnd.
BUt as you noted dealing with leaky devices is more problematic because temperature make them even more unstable.

I was typing mobile 

Blackberry microscopic keyboard?

mac

[DDD]

Wow!
I'm very sorry for my faulty messages in the present topic.
I was speaking of the MKIII circuit instead of MKII.
(I've spent some time playing with MKIII and leaky germs a long ago, hence my mistake)

[alarm]

Hi, I've just built a MK1 with transistors with too little leakage. I've used the resistor B to C method to get the basic sound right but now I have a high piched whining noise even when the pedal is bypassed!!! Help Please.