Author Topic: Transistor type or HFE ?  (Read 681 times)

Sparky

Transistor type or HFE ?
« on: July 30, 2022, 01:59:28 PM »
What's more important to the sound quality in a FF or Tonebender circuit?  Comparing 2 different transistors of equal HFE doesn't always yield the same results in the sound of the pedal.  I've been swapping 2N3053 with other trannies of equal HFE for a slightly "softer" distortion [depending on the particular circuit they can sound very good].   I guess both factors need to be considered but was wondering if the type [and maybe the brand] of transistor means more than the HFE.

Mark Hammer

Re: Transistor type or HFE ?
« Reply #1 on: July 30, 2022, 04:22:51 PM »
Keep in mind that one of the hallmarks of a "good" Fuzz Face, is that it cleans up nicely when you turn the guitar volume down a bit.  Don't judge a FF solely on the basis of what the max distortion tone is like.

Sparky

Re: Transistor type or HFE ?
« Reply #2 on: July 30, 2022, 07:10:30 PM »
Keep in mind that one of the hallmarks of a "good" Fuzz Face, is that it cleans up nicely when you turn the guitar volume down a bit.  Don't judge a FF solely on the basis of what the max distortion tone is like.
Absolutely!  It needs to sound good when the volume is backed of.  It also needs to be as quiet as possible for Strats and Telecasters.   I also think I'm going to keep the  330 ohm resistor  [on the silicon models]  right where it is.  I've been experimenting with raising it to 1 K  for more volume but it also increases the noise. 
One of the mods I may use is Gagan's Pre Gain control  instead of the stock fuzz control.
« Last Edit: July 30, 2022, 07:13:33 PM by Sparky »

Rob Strand

Re: Transistor type or HFE ?
« Reply #3 on: July 30, 2022, 08:12:01 PM »
What's more important to the sound quality in a FF or Tonebender circuit?  Comparing 2 different transistors of equal HFE doesn't always yield the same results in the sound of the pedal.  I've been swapping 2N3053 with other trannies of equal HFE for a slightly "softer" distortion [depending on the particular circuit they can sound very good].   I guess both factors need to be considered but was wondering if the type [and maybe the brand] of transistor means more than the HFE.

When you measure hFE with some test equipment you get a hFE at a particular collector current.  What that collector current depends on the tester.  Most testers test with a fixed base current so the collector test current goes up with hFE.  A typical test current might be 2mA or so.

A good deal of effect pedals operate at lower collector currents.    Some transistors have a hFE which doesn't vary much with collector current.   Others vary a lot.    So while your tester shows the same gain for three samples the actual hFE of three samples in-circuit can be different to each other at the lower operating current.

You cannot assume three transistors of the same part number behave the same at different currents.   It varies from device to device, manufacture to manufacturer and even era to era.   I see a lot more gain drop-off at low currents in old 70's transistors.

The way you would determine if the hFE is varying is to measure the transistor gains yourself using a circuit like RG's transistor gain tester except you need to tweak the base current so all the transistors are operating at the same collector current.    Another sign the hFE's are off is the DC voltages in the circuit are off - however circuits which have some DC feedback will hide this issue.

hFE isn't the only thing that varies.  The transistor capacitances can vary.  In fact these can vary a lot between manufacturers.  Not so easy to measure directly.    Adding small caps between b and c on the transistors can often make one transistor sound closer to another.   If you like the sound of a transistor with low capacitance it's not possible to remove capacitance from the higher capacitance units - luckily most people like a rolled-off sound!


A major difference is when the DC bias voltages aren't the same for each transistor.  It is inevitable the circuits will sound different.  That's where tweaking resistors to change the DC bias voltages can help push one transistor to sound like another.

« Last Edit: July 30, 2022, 10:17:08 PM by Rob Strand »
Send:     . .- .-. - .... / - --- / --. --- .-. -

pacealot

Re: Transistor type or HFE ?
« Reply #4 on: July 30, 2022, 10:08:39 PM »
I only regret that I have but one like to give to this reply...
"Stupidity has a certain charm; ignorance does not." — Frank Zappa

"I hate guitar players. Whatever." — Scott Thunes

Sparky

Re: Transistor type or HFE ?
« Reply #5 on: July 30, 2022, 10:33:19 PM »
What's more important to the sound quality in a FF or Tonebender circuit?  Comparing 2 different transistors of equal HFE doesn't always yield the same results in the sound of the pedal.  I've been swapping 2N3053 with other trannies of equal HFE for a slightly "softer" distortion [depending on the particular circuit they can sound very good].   I guess both factors need to be considered but was wondering if the type [and maybe the brand] of transistor means more than the HFE.

When you measure hFE with some test equipment you get a hFE a particular collector current.  What that collector current is depends on the tester.  Most testers test with a fixed base current so the collector test current goes up with hFE.  A typical test current might be 2mA or so.

A good deal of effect pedals operate at lower collector currents.    Some transistors have a hFE which doesn't vary much with collector current.   Others vary a lot.    So while you tester shows the same gain for three samples the actual hFE of three samples in-circuit can be different to each other at the lower operating current.

You cannot assume three transistors of the same part number behave the same at different currents.   It varies from device to device, manufacture to manufacturer and even era to era.   I see a lot more gain drop-off at low currents in old 70's transistors.

The way you would determine if the hFE is varying is to measure the transistor gains yourself using a circuit like RG's transistor gain tester except you need to tweak the base current so all the transistors are operating at the same collector current.    Another sign the hFE's are off is the DC voltages in the circuit are off - however circuits which have some DC feedback will hide this issue.

hFE isn't the only thing that varies.  The transistor capacitances can vary.  In fact these can vary a lot between manufacturers.  Not so easy to measure directly.    Adding small caps between b and c on the transistors can often make one transistor sound closer to another.   If you like the sound of a transistor with low capacitance it's not possible to remove capacitance from the higher capacitance units - luckily most people like a rolled-off sound!


A major difference is when the DC bias voltages aren't the same for each transistor.  It is inevitable the circuits will sound different.  That's where tweaking resistors to change the DC bias voltages can help push one transistor to sound like another.

  Lots of times I've used the same type of transistors from the same company [BC 108, BC 182, etc..]  and measured them out to get the lower HFE  for the Q1 slot.   I used my old Tech volt-ohm meter and the HFE 's  were all over the map.  Years ago I noticed that the specs on newer stuff were different from lots of the older original brands.   I also noticed that the sound of a [decent] Fuzz Face or Tonebender onstage with a band didn't need to be tweaked to the ultimate voltages in order to get the job done  [enter lots of silicon versions of the old classics].  I just started measuring the HFE of the transistors I had laying around in order to get a decent sounding pedal.   Since covid I had more time alone to ponder how some of these things actually compare to others with the same specs.  More pondering has ensued .......
[Years ago I noticed my original Tonebender had transistors of different values for Q1 and Q2  ...... first time I saw that.]

« Last Edit: July 30, 2022, 10:39:02 PM by Sparky »

Rob Strand

Re: Transistor type or HFE ?
« Reply #6 on: July 31, 2022, 07:14:25 PM »
Quote
Years ago I noticed that the specs on newer stuff were different from lots of the older original brands. 
Yes, it does look like things have changed.

Quote
I also noticed that the sound of a [decent] Fuzz Face or Tonebender onstage with a band didn't need to be tweaked to the ultimate voltages in order to get the job done  [enter lots of silicon versions of the old classics]. 
None of my transistors old or new will bias the standard silicon fuzz face with an 8.2k collector resistor to 4.5V, the voltage is always much lower.   However other people seem to state it does.  I don't know what to make of it.

I posted some results here (IIRC the hFE are nominal values from a DMM).   You can also see Electric Warrior's post.
https://www.diystompboxes.com/smfforum/index.php?topic=127926.msg1229614#msg1229614

Quote
I just started measuring the HFE of the transistors I had laying around in order to get a decent sounding pedal.   Since covid I had more time alone to ponder how some of these things actually compare to others with the same specs.  More pondering has ensued .......
Yes, the only way you can make sense of this stuff is to measure everything and see what patterns show up.  At this point I think there's a lot of info posted on the web about good bias voltages and gains for the older classic pedal.  The results aren't always consistent across posters but you will see groupings of some "good" values.

« Last Edit: July 31, 2022, 11:21:11 PM by Rob Strand »
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Sparky

Re: Transistor type or HFE ?
« Reply #7 on: August 02, 2022, 11:44:55 AM »
There is the problem of some transistors just being noisier than others, too.  I have a bunch of 2n3053  [metal canister ] trannies that I like the sound of [kind of soft and grainy ]  but they have a bit more inherent noise than others.

pinkjimiphoton

Re: Transistor type or HFE ?
« Reply #8 on: August 02, 2022, 06:25:12 PM »
imho, the only thing that really matters is the gain being in the ballpark ratio between q1 and 2.

type matters less than hfe.... silicon? ge? both can sound and work fine.
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Sparky

Re: Transistor type or HFE ?
« Reply #9 on: August 03, 2022, 10:14:01 AM »
imho, the only thing that really matters is the gain being in the ballpark ratio between q1 and 2.

type matters less than hfe.... silicon? ge? both can sound and work fine.

Agreed.   It always seems to sound better when Q2 has a higher HFE.   There are certain transistors that have a particular unique quality that can be used to enhance the clarity or distortion of a circuit, too.

mac

Re: Transistor type or HFE ?
« Reply #10 on: August 03, 2022, 10:57:42 AM »
I like some silicon power transitors (Tip41, Bd237) for two reasons,
1. hfe falls faster at low ic, a cool feature when turning the guitar volume down.
2. higher internal capacitance --> less highs (but this is a personal taste)
3. they have the "standard" gain.

mac
« Last Edit: August 03, 2022, 04:01:28 PM by mac »
mac@mac-pc:~$ sudo apt-get install ECC83 EL84

Sparky

Re: Transistor type or HFE ?
« Reply #11 on: August 03, 2022, 03:14:02 PM »
I like some silicon power transitors (Tip41, Bd237) for two reasons,
1. hfe falls faster at low ic, a cool feature when turning the guitar volume down.
2. higher internal capacitance --> less highs (but this is a personal taste)
3. they have the "standard" gain.

mac

Hhmmm....gonna have to dig some of those up.  Thanx!
« Last Edit: August 03, 2022, 05:22:08 PM by Sparky »