Built Tone Bender Mk 1 biasing question

[philbone]

Greetings all,

Had a lot of fun reading through all the posts here.

On to my question.

I have built a three tranny Tone Bender Mk 1 from the original Sola schematic. I selected three germanium NPNs from a batch and I have the following:

Q1 hfe 53, leakage 112uA
Q2 hfe 68, leakage 80uA
Q3 hfe 70, leakage 120uA

Bias resistor on Q1 is 1Mohm
Q2 bias is the voltage divider plus 50K pot
and Q3 has been replaced with a 10K pot.

My voltages are rather odd and may be a little out of whack:

Vcc 9.03V

Q1 E 1.13, B 1.11, C 9.03
Q2 E 0.2mV, B 0.11, C 0
Q3 E 0.1mV, B 20mV, C 7.45

My question is; how would one go about calculating the correct bias resistances for optimum setting using the hfe and leakage values?  ...and what should those ideal voltages be?

Many thanks!

Phil

[PRR]

Link to exact schematic you used?

[LucifersTrip]

there's no optimum....for most this is a trial and error nightmare. you probably don't want to use trimmers until you're very close to what you like, then use them for the final tweaking.

obviously, Q2C shouldn't be 0
Q3C is usually > 8V

[mac]

Do a search, there are recent post about biasing.
Most members agree that this intended for leaky germs.
Yours are in the lower limit.

Q1E should be above a volt.
Q2C should vary from 6v to 9v.
Q3C above 8v.

My question is; how would one go about calculating the correct bias resistances for optimum setting using the hfe and leakage values?  ...and what should those ideal voltages be?

There is a calculator in my gallery. You can play with it to get aproximate resistors values.

mac

[Arcane Analog]

Mac: I tried to have a look at your calculators a while back and there was only a short description there - no files could be found.

[mac]

Mac: I tried to have a look at your calculators a while back and there was only a short description there - no files could be found.

Try this link,

http://www.aronnelson.com/gallery/main.php/v/mac/apps/

Items have no thumbnail, just a description. Clic on "no thumbnail", you should see a "download file" link on the next page.

mac

[Arcane Analog]

Same thing. I do not see any links.

[LucifersTrip]

[mac]

Thanks LT 

By the way, those were made using realbasic & mac running OS9 

mac

[Arcane Analog]

I hover over the download link and but does not appear as a link.  

[LucifersTrip]

try a different browser


http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=41760

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=42777

[Arcane Analog]

I deleted my history and it seems to have fixed the issue.

[philbone]

Hi Everybody and thank you for all your information!

I was a little busy but finally got to it.

By the way, Luciferstrip that is the exact schematic used - thank you for posting.

Thank you for your figures Mac. After some more adjustments I now have the following with a supply voltage of 8.56V  Do you think that I should try to source some higher leakage Qs ?

       Emitter     Base     Collector
Q1 1.08           1.06      8.55
Q2 0.2mV        140mV   5.57-8.49
Q3 1.5mV        19mV     7.6

These look a little better but my problem is still in the bias of Q3. The sound of the unit while great with a stratocaster, just craps out with a higher output Les Paul.

I wish that I could find a detailed description of how to go about calculating the biases for these circuits :-)

Thanks again all,

Phil

[LucifersTrip]

You can find detailed info on biasing circuits, but you picked the one circuit that is more about trial and error. Even if you have the right voltages, it may not sound good.

It is easy to get any voltages you want by adjusting the base to ground or collector resistors, but the big problem will most likely be with noise and ultra fizzy decays. Check out  the other threads here and on the fsb and you'll see what nightmares people run into.

For Q3, if you use something leakier, the C voltage will drop, so in your case, it would seem you'd need something with less leakage.

I would use a 9V supply instead of 8.5 so you can compare your voltages with others easier, since most report voltages with a 9v supply.
Your Q3C would probably be closer to 8V.

The lower the Q3C voltage is (closer to 4.5V), usually the fizzier it will become, but the MKI is biased up there near 9V, so there's less noise and it borders on gating....or does gate. The problem is when you get near gating, you have the splattery decay (which many people like and is a part of the MKI sound)....thus, the trial and error balancing act.

[philbone]

Thank you Mac,

Yes, definitely trial and error! 

The first time I put this together it sounded great off the bat!  It was with a Gretsch and I just thought it would be better with the Les Paul.

As I push Q3C toward 8 plus volts it starts to sound worse so I will just go by my ears. I am finding that the "Attack" control seems to be too extreme right off the left scale, in other words most of the action happens from & oclock to 10 o'clock. Perhaps a smaller resistor for the 1K8 in the schematic above , I already have a 470K for the 180K in the voltage divider.

Anyway, lots of fun and driving the wife mad in the wee hours of the night!!

Phil

[mac]

Anyway, lots of fun and driving the wife mad in the wee hours of the night!!

It's very pleasant to me to turn on my amp at sun down, make a coffee and milk while tubes heat up, and then annoy the whole building with wierd sounds like those of the MKI!

mac

[LucifersTrip]

I am finding that the "Attack" control seems to be too extreme right off the left scale, in other words most of the action happens from & oclock to 10 o'clock. Perhaps a smaller resistor for the 1K8 in the schematic above , I already have a 470K for the 180K in the voltage divider.

remember, that attack is nothing more than a variable resistor. so, if all the action happens at the beginning (it is a common problem), just try a smaller attack pot, like 10 - 20K

...or measure the exact distance (in K) you want the attack sweep and put a resistor in parallel across the attack pot to get that exact value

[mac]

If your are building for yourself you should consider an external pot to set Q3C,
and a trimmer at Q2 to fine tune the attack pot as LT noted.
Leaky germs with emiter grounded are unstable.

If you still have issues with your LP try some resistance between stages and at the input.

mac

[philbone]

Thank you guys... especially Mac and LT !

Well... a 56K in parallel with the Attack pot and another 56K on the signal input and we are slamming!  (I measured the "back off" on the guitar pot where it sounded great and that turned out to be about 50K)

The unit really sounds good, workable and most important totally usable, so thank you for all your words of advice

I appreciate that the next one I build will use a whole new set of values!

Phil