Need help with TBM2P biasing

[mat]

Hi!

I just complited the negative ground Tonebender Mark II Professional from GGG
http://www.generalguitargadgets.com/diagrams/tonebender_m2p_lo_pn.gif

Here is the patient :

http://tinypic.com/5tis8


I have three measured ac128's (low leakage) in there and the two collector voltages seems to be quite high:

Q1
c-9,05  b-9,03  e-9,14

Q2
c-9,04  b-9,08  e-9,14

Q3
c-0,308  b-9,03 e-9,05

I'm not quite sure how to get those voltages between 5 and 6 volts. Should I (first remove the 10k, 47k and 1k+8k2 resistors) put a 50k trimmer on each one's collector and tweak until it sounds right and the replace them with resistors ?

One thing confused me when looking at the ggg layout. There is red + marks on there that suggest that the electrolytes should be put with the + side on the ground ? Could someone clear this for me.

Big Thanks!
mat

[RDV]

You mean you didn't?! You got some high voltage readings then! Put the + to ground and it should work if everything else is right.

RDV

[mat]

Hi RDV! "the fastest man alive   "

I first tried the +'s to the ground but no sound. Then I reversed the electrolytes and now the circuit is hotter than hell  :evil: , I mean it has way too much gain. Eaven with the volume and attack pots rolled back it screams like.... something..

mat

You mean you didn't?! You got some high voltage readings then! Put the + to ground and it should work if everything else is right.

RDV

[mat]

Hello,

I soldered the electrolytes in right way (+ to the ground). Thanks RDV for pointing that out for me.

I get sound but it is still too hot. I cannot back it up enough with the pots. The bias is the problem I think.

With 9.13V battery i get:

Q1- 1.093
Q2- 8.9
Q3- 0.73

Could some one help me with this one,

Thank You,
mat

[mat]

so I got the thing to have:

Q1 - 8.68 V
Q2 - 8.86 V
Q3 - 8.86 V

are these values ok?

Thanks,
mat]

[petemoore]

There are some good notes on that...
 Runoff Groove>Articles and Schematics>FAQ>homewrecker.com>TonebenderMkII notes.
 You may have to do a little scrolling here and there...
 Q2 and Q3 should be closer to 4,5v I think.

[mat]

Thanks petemoore, i'll check these out !

mat

There are some good notes on that...
 Runoff Groove>Articles and Schematics>FAQ>homewrecker.com>TonebenderMkII notes.
 You may have to do a little scrolling here and there...
 Q2 and Q3 should be closer to 4,5v I think.

[mat]

Hi,

I've been strugling with this TBMK2P ( http://www.generalguitargadgets.com/diagrams/tonebender_m2p_sc_pn.gif ) one quite a lot. I removed the outboards off of the circuit.

The readings with the 8.98V battery are:

Q1
C-8.8
B-8.8
E-8.8

Q2
C-1
B-8.8
E-8.8

Q3
C-1.3
B-1
E-1.6

The emitters of the Q1 and Q2 should be 0V ?? plus the bases should very much something else i have..

The Q2 gets very hot and i cannot keep the battery plugged very long time. I have maybe lost the Q2 getranny. I've got two measured spares but am not willing to put them in before i figure out what is causing the warming of the Q2.

I checked the http://home-wrecker.com/tbmk2.html and there were very valuable information there, but i think i have some serious error in my circuit to have readings like those above :cry:

I've checked the components so many times i've lost count.

Well, i'm not giving up but could use some advice what to look next.

Thank You,
mat

[RDV]

Start over. You'll figure out what happened the 1st time if you do it again.

RDV

[mat]

Maybe that would be the best thing to do..

but i'm just wondering what could cause the voltages on those emitters to be so high ?

Well gotta hit the sack.. new day tomorrow.

Thanks RDV for Your advice, i think thats the next thing i'll do.

mat

Start over. You'll figure out what happened the 1st time if you do it again.

RDV

[phillip]

On the ToneBender MKII, the collector voltages should be roughly as follows:

Q1 = 8.5V
Q2 = 1.1V
Q3 = 4.5V-5V

the collectors of Q2 and Q3 should have the same voltage readings as Q1 and Q2 in a good Fuzz Face.  I would highly recommend installing a 25K trimpot on the collector of Q3.  Once the voltage there is between 4.5-5V then the collector voltage of Q2 should also swing into place.

Germanium transistors are a real PITA to bias.  Also make sure not to take the voltage readings right after handling the transistors...the heat from your hands can cause their gain to go way up...so the voltage readings will be off from what they'd normally be in the box when they're cool.

The MKII Professional is a screaming hot circuit...gobs of fuzz, sustain and even feedback.

Check out this page:  http://fuzzcentral.tripod.com/benderII.html

Phillip

[mat]

Hi philip,

I have trimmers on all transistors but cannot get the Q3 collector voltage high enough.  I'm curious why the emitter Q1 and Q2 has so high voltage ? They are supposed to be 0V.

One thing (also) i dont understand. If this  (and this is supposed to be) negative ground version, why are the electrolytes put so that the + is on the ground ?

Thanks,
mat

On the ToneBender MKII, the collector voltages should be roughly as follows:

Q1 = 8.5V
Q2 = 1.1V
Q3 = 4.5V-5V

the collectors of Q2 and Q3 should have the same voltage readings as Q1 and Q2 in a good Fuzz Face.  I would highly recommend installing a 25K trimpot on the collector of Q3.  Once the voltage there is between 4.5-5V then the collector voltage of Q2 should also swing into place.

Germanium transistors are a real PITA to bias.  Also make sure not to take the voltage readings right after handling the transistors...the heat from your hands can cause their gain to go way up...so the voltage readings will be off from what they'd normally be in the box when they're cool.

The MKII Professional is a screaming hot circuit...gobs of fuzz, sustain and even feedback.

Check out this page:  http://fuzzcentral.tripod.com/benderII.html

Phillip

[petemoore]

Emitter of Q2 should be At Ground and read 0V, check that that connection is made, you're dmm should Beep with black to ground and red to Q2 emitter, and read 0V from ground.

[Torchy]

The emitters  of Q1 & Q2 are tied directly to the +9V rail so how can they be "at ground"

Also NONE of the electrolytics have the + connected to ground (ground is negative).


Check the schematic against what you have wired up.
http://www.generalguitargadgets.com/diagrams/tonebender_m2p_sc_pn.gif

[phillip]

I'd go back and build the circuit as a pure PNP positive ground circuit.  There seems to be some problems when using PNP transistors and a negative ground:



Some obvious things there...the 5uF capacitors could be 4.7uF and the 50uF capacitor can be a 47uF.

I also almost hate to ask, but do you have the pinout of the transistors correct?  Here's a page at GEO that shows the pinout of that type of transistor:

http://www.geofex.com/FX_images/to1dwg.gif

Phillip