Hiss in Tone Bender...

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Hiss in Tone Bender...

Postby HappyCat » 26 Aug 2014, 17:49

Well, I've just finished a TB, and I'm suffering a pair of troubles that are getting me crazy. It's a TB MKii, with a DPDT switch to change between MK1.5 and MKII modes. The wiring of the switch is verified, and I tested in other buildings succesfully. I followed the well-known schematic for the OC75 version, traced (I think) by Electric Warrior:

Image

In this case, I mounted GT308b russian germanium transistors. Gains are specified below. Leakage zero, are usually happens with these old soviet germs. At the same time, I mounted an external bias pot (100k) for Q2 collector, and an internal trimmer (20k) for Q3. Let's see what happens...

The sound in MK1.5 mode is acceptable, even with the wild voltages for Q3. In MkII the sound is very noisy, hissy, and a bit gated. With extreme settings in the bias pot an trimmer, the sound can be avoid, but the sound is not very acceptable. I tried different transistors, and the result is the same.

About the readings:

Q1: Hfe around 45
E 0.00
B 0.01
C 10.40

Q2: Gain 59
E 0
B 0.20
C 0.70

Q3: nearly 70
E 0.440
B 0.68
C 8.080

Battery: 10.90V

Thanks!

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DrNomis (26 Aug 2014, 18:48)
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Re: Hiss in Tone Bender...

Postby DrNomis » 26 Aug 2014, 18:52

HappyCat wrote:Well, I've just finished a TB, and I'm suffering a pair of troubles that are getting me crazy. It's a TB MKii, with a DPDT switch to change between MK1.5 and MKII modes. The wiring of the switch is verified, and I tested in other buildings succesfully. I followed the well-known schematic for the OC75 version, traced (I think) by Electric Warrior:

[ Image ]

In this case, I mounted GT308b russian germanium transistors. Gains are specified below. Leakage zero, are usually happens with these old soviet germs. At the same time, I mounted an external bias pot (100k) for Q2 collector, and an internal trimmer (20k) for Q3. Let's see what happens...

The sound in MK1.5 mode is acceptable, even with the wild voltages for Q3. In MkII the sound is very noisy, hissy, and a bit gated. With extreme settings in the bias pot an trimmer, the sound can be avoid, but the sound is not very acceptable. I tried different transistors, and the result is the same.

About the readings:

Q1: Hfe around 45
E 0.00
B 0.01
C 10.40

Q2: Gain 59
E 0
B 0.20
C 0.70

Q3: nearly 70
E 0.440
B 0.68
C 8.080

Battery: 10.90V

Thanks!





Try reducing the 47k Q2 collector resistor to 33k, this will reduce the voltage on Q3's collector and will reduce the gating........ :hmmm:



Also, check this thread out:


viewtopic.php?f=10&t=23674
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
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Re: Hiss in Tone Bender...

Postby HappyCat » 26 Aug 2014, 19:57

Thanks a lot, Dr.! I'll try the trick of the 100k resistor of that post. About the 47k resistor, I mounted a 100k pot instead, so I'll go for a 33k value. Do you find the voltages as good? The high Q1 collector seems strange, but I didn't find any bridge to -9v rail supply.
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Re: Hiss in Tone Bender...

Postby Electric Warrior » 27 Aug 2014, 17:01

When it's gating it should be dead quiet. You may have one or more noisy transistors, but I can't be sure. I only tried low leakage transistors once or twice.

Your collector voltages are alright. That's what MKIIs with stock values measure. I'm not surprised to see over 10V on Q1C with nearly 11V supply voltage. And 8V for Q3C isn't high at all.

Here are some voltages from my vintage unit:

Battery: -9.67V
Q1 C -9.02 B -0.03 E 0
Q2 C -0.17 B -0.08 E 0
Q3 C -8.44 B -0.17 E -0.11

Have you tried swapping transistors around before messing with the trim pots? Using increasing hfes for subsequent gain stages is not always the best way to set up this circuit.
I usually start out with a rather closely matched set of transistors and then start swapping things around.

With that little leakage your build may suffer from problems I'm not familiar with.. You might want to replace Q1's base resistor with a trim pot as well. Low leakage trannies may or may not work better with a larger resistance there.
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Re: Hiss in Tone Bender...

Postby HappyCat » 29 Aug 2014, 00:56

Well, I solved the trouble a bit increasing the value of the base to ground resistor in Q1. But I have to try a pair of things yet! Thanks, guys! By the way, about the 100k resistor between Q2-Q3... If a use a lower than 100k resistor, I get a cleaner, less fuzzy sound. Interesting, but I think that I need the opposite... more fuzz! I suppose that the mix of low gain transistor and zero lackage has their own rules...
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