Suitable transistors for Tonebender Mark 1?

[Mark]

Hi

I bought a TB MK 1 clone a while ago as I didn’t want the hassle of going through auditioning a multitude of Ge PNP transistors in order to build a TB MK 1. However, the pedal does the Velcro/spluttering thing in spades and I was disappointed as that’s not a sound I have a lot of use for. Rather than sell it, I thought a mod here and there coupled with transistors might remedy the situation.

The build quality is good, and as shown in the circuit diagram is fairly typical of a TB MK 1/Zonk Machine, I suspect my problem is the transistors coupled with the biasing of the second stage.

The transistors are as follows:

Q1 CV7007 Hfe = 30, leakage = 100uA, emitter voltage is -0.688VDC.

Q2 CV7355 Hfe = 118, leakage = 80uA, collector voltage is -2.33VDC.

Q3 OC 71 Hfe = 49, leakage = 101uA, collector voltage is -7.6VDC.

I noticed that the pedal used 180K instead of the typical 470K. I put a resistor in series with the 180K to make up 470K and I found it sounded a lot more to my liking with this value. I found that the gain/bias/attack control using 470K bias resistor didn’t do anything past 9:00.
I read posts that stated the emitter voltage of Q1 should be -2VDC, I substituted another Ge transistor with a Hfe = 90, leakage = 160uA, emitter voltage of -1.5VDC. I found that the pedal sounded the same but seemed better, I also found that the gain control actually worked (it worked with the 180K resistor but it provided more Velcro/spluttering).

I have gathered that Q1 needs a Hfe of 60 - 80, and the leakage around 200uA.

Q2 can have very low leakage as there is a bias network to bias this stage. I don’t know what the ideal gain/Hfe would be for this stage. I would imagine 80 - 100 might work, but I don’t know that for sure.

Q3 I imagine would also need 80 - 100 would be good, but the leakage would also have to be something like 200uA too as the leakage biases the transistor in this circuit.

Given that I’m after the Jeff Beck Yardbirds and Mick Ronson sound. What are your thoughts and what would you recommend I do regarding transistors?

I have heard of people putting trimmer pots into fine tune the circuit and I have heard people who are vehemently opposed to trim pots. What is the consensus on this one?

For reference, this sounds pretty close to the mark for me.

https://youtu.be/HMoowjhnKm0?si=-b9BG6OHLGWwO0Un

IMG_2338.jpeg

Thanks for reading my post I appreciate your time and patience.

Regards

Mark

P.S. this is pretty much what I’m trying to avoid though it isn’t without its charm, though my clone sounds much worse.

https://youtu.be/SZX0SUZAYj4?si=9U11jvLLejhok_EM

[statorvane]

Trim pots? Go for it man. Once you've found your bias settings you can either leave them in, or replace them with suitable resistors. Once it is buttoned up who cares?

Kelly

[Mark]

Thanks for your reply Kelly.

I did check this out as well.

http://www.geofex.com/Article_Folders/z ... onkbst.pdf