Author Topic: Tonebender mk1  (Read 344 times)

JebemMajke

Tonebender mk1
« on: August 22, 2019, 02:09:36 PM »
Hello

Some time ago i accidentally stumbled upon a small box filled with black glass Mullard Philips transistors and Valvo transistors.

oc44-oc75, and some others included ( many different AC141/142/188 ....,, tfk's oc602-oc614 ).

oc45 rangemaster is very very nice and very very dark <3.

So, I was wondering what would a tonebender mk1 sound like with different transistors.

oc75/oc72's, for example and so on.

Can anyone give me any advice on the target voltages, and which resistors are used for biasing?



I see that attack pot is biasing q2, but which resistor is biasing q3? 8,2k or 15k?

PRR

Re: Tonebender mk1
« Reply #1 on: August 22, 2019, 10:47:26 PM »
By conventional design thinking, Q and Q3 have NO active bias, and will not amplify well.

However in Germanium, device leakage gives an (unpredictable) bias.

You have to try it. Be prepared to select transistors.

JebemMajke

Re: Tonebender mk1
« Reply #2 on: August 23, 2019, 04:36:46 AM »
Well, I guess I could just put trimmers almost everywhere.

Can you suggest some target voltages?

q1 c=9v, obviously.

q2c = 5-8v(8.5)

but q3 ... I can not even imagine .... 7v, like some sort of a Rangemaster? But that would be silly I think because q2 is a Rangemaster.

Also, what is the point of 1M resistor before q1? I could replace 8.2k with a trimmer to help the output bias of it.

Rob Strand

Re: Tonebender mk1
« Reply #3 on: August 23, 2019, 04:56:55 AM »
Try these,
VE1 =    1.3V to 3.4V reported range  ; more than 2V best.
VC2 =   3.5V to 8.8V ; maybe 5.5 to 8.6 more usable
VC3 =    6.8V low  8V typ.
The mind often distorts without gain.

Re: Tonebender mk1
« Reply #4 on: August 24, 2019, 06:25:10 AM »
OC75s worked well for me, especially with a 470k on Q2b (but you should probably tweak that depening on the transistor/hfe/leakage and the range that you would like to have on the attack pot - vintage units used a range of different values here). No surprise, some early units had three black glass transistors which are usually assumed to be OC75s - now way to tell for sure, though. Lots of parts numbers and values were scraped off..

Auditioning transistors is very important, indeed. Lots of tone shaping potential here. Give your OC75s a try and report back with voltages. You'll probably find that you won't need much tweaking if any at all..


Quote
but q3 ... I can not even imagine .... 7v, like some sort of a Rangemaster? But that would be silly I think because q2 is a Rangemaster.

Just because it uses the same topology as a rangemaster does not mean it was biased to the same voltages  ;)
I measured a little over 8V on Q3c in my clone. Sounds alright to me: https://picosong.com/8r8G/
« Last Edit: August 24, 2019, 06:31:13 AM by Electric Warrior »