Sola Sound - Tonebender Mark II Professional [schematic]

[Electric Warrior]

here's the data sheet for an audio transistor package, consisting of an OC81D, an OC81 and an AC127: http://www.vintageradio.me.uk/info/oc81.htm

[phibes]

Out of all the OC81d's I've ever measured I've rarely came across any as low as 40. 60-100 seems to be the average.

[yeeshkul]

They are marked Hfe>25 in the catalogue. As it seems they like to be somewhere between 60-100 ... that's a bit surprising since i supposed they should be either really weak or quite gainy. They are non of that as it seems. Not sure what were the resistor changes for, because this is the range OC75's fit in. The 47k->100k in the Q2 collector should have something to do with gain ... i do not understand the D*A*M guy's statement that they raised the Q2 collector resistance because OC81D had bigger Hfe, it should be other way round. That makes me double-confused. Maybe someone has explanation.

[playon]

The newer DSI OC81D is lower hfe, most were around 40. The old ones seem to have a greater range, the 25 hfe spec is the minimum. Also, the "driver" transistor is the one with the lower gain, not the other way around.

[yeeshkul]

Also, the "driver" transistor is the one with the lower gain, not the other way around.

exactly.

[Electric Warrior]

They are marked Hfe>25 in the catalogue. As it seems they like to be somewhere between 60-100 ... that's a bit surprising since i supposed they should be either really weak or quite gainy. They are non of that as it seems. Not sure what were the resistor changes for, because this is the range OC75's fit in. The 47k->100k in the Q2 collector should have something to do with gain ... i do not understand the D*A*M guy's statement that they raised the Q2 collector resistance because OC81D had bigger Hfe, it should be other way round. That makes me double-confused. Maybe someone has explanation.

yes.. maybe the ones used in Tone Benders were higher gain indeed. I guess specs varied (see page 3).

or somebody thought the OC81Ds just sound better with the 100k resistors, regardless of gain or leakage.

[yeeshkul]

When i am back from holidays i am gonna put together one unit with 3 really weak trannies just to know how it sounds.
My old unit is L-H-H (low gain - hi gain - hi gain) and i like the sound, but i would prefer more control on the fuzz pot. I guess i will have to try the recomended H-H-L variant, just to know if they differ and how.

[Scruffie]

And I'm gunna take readings of my OC81's again this evening when I get the chance, 1 of them shows up as 80 on my multimeter which would make sense if it was very unleaky for the reading I got, but one shows up as 200 now I look so the readings I got from the leakage tester could well be wrong as I thought they both showed up okay...

But I will also be building a Tonebender to put them in, leaky or low gain or not, so we can double check yeeshkuls results, perhaps put a 100k/10k on a switch.

[slim_blues_boy]

When i am back from holidays i am gonna put together one unit with 3 really weak trannies just to know how it sounds.
My old unit is L-H-H (low gain - hi gain - hi gain) and i like the sound, but i would prefer more control on the fuzz pot. I guess i will have to try the recomended H-H-L variant, just to know if they differ and how.

I tried low gain configuration (all low hfe transistors), and it's no good, very weak sound with very low fuzz.
I used OC81D resistor (100K) setting and use transistor with gain around 20 on Q1, 30 on Q2, and 40 on Q3.
I didn't tweak it coz I use low gain transistor just to check the circuit (so if there something wrong and damaged the transistors, I only lose 'useless' transistors) and change them immediately to 'normal' TB transistor configuration.

[slim_blues_boy]

And I'm gunna take readings of my OC81's again this evening when I get the chance, 1 of them shows up as 80 on my multimeter which would make sense if it was very unleaky for the reading I got, but one shows up as 200 now I look so the readings I got from the leakage tester could well be wrong as I thought they both showed up okay...

But I will also be building a Tonebender to put them in, leaky or low gain or not, so we can double check yeeshkuls results, perhaps put a 100k/10k on a switch.

yes, try to build TB and stick those OC81's and look what happen with 100K and 10K resistor.
if only I also have that NOS OC81's .

[yeeshkul]

As i said - raising the base res raises the base DC voltage thus opening the BE and CE junctions for larger stretch of the guitar signal than just the tip of the negative swing. That makes signal more efficient thus the fuzz tone more fuzzy.

Scruffy, thank you buddy for checking those rare units for us. Are you sure you know how to get the Hfe out of the R.G's little aparatus?
1. Adjust the trimpot the way R.G. sais or "hand pick" the resistors so they have THE values
2. adjust your DMM for 2V scale
3. apply the 9V battery
now the machine is ready for some mean testing
4. stick the tranny in at the room temperature and let it "breathe" for at least 2 minutes. Even touching the tranny raises up the leakage and will have a hell lot to do with the final result... which is what we do not want .
5. measure the voltage(U1) with the base lifted - wait until the displyed number gets steady
6. switch your DMM to uA and measure leakage current
where 100uA is common
about 200 happens pretty often
>300uA means the device is nothing special
> 500uA means the device really leaks a lot
7. back to the voltage scale - connect the base and get the voltage U2, again - wait until the displyed number gets steady
8. (U2-U1)/100 = Hfe

Scruffy forget about any numbers coming out of your DMM when you try to measure a Ge tranny straight.

[yeeshkul]

by the way this is what i use for ear-testing of particular transistors, caps and resistors in Fuzz Faces / Tonebenders.
The trimpots and crucial caps are removable, so i can measure the resistance when i am happy with the sound.
more at http://www.diystompboxes.com/smfforum/i ... ic=74130.0

[RnFR]

i remember seeing that. i think it's pretty damn slick! i downloaded the pcb with the hopes to build one myself, but unfortunately never got around to it. it'd be really cool to do this with the mk1/fuzztone circuit as well, and you could even rig up a generic sixties fuzz one for fuzzrite/orpheum variants. pretty cool stuff.

[Scruffie]

As i said - raising the base res raises the base DC voltage thus opening the BE and CE junctions for larger stretch of the guitar signal than just the tip of the negative swing. That makes signal more efficient thus the fuzz tone more fuzzy.

Scruffy, thank you buddy for checking those rare units for us. Are you sure you know how to get the Hfe out of the R.G's little aparatus?
1. Adjust the trimpot the way R.G. sais or "hand pick" the resistors so they have THE values
2. adjust your DMM for 2V scale
3. apply the 9V battery
now the machine is ready for some mean testing
4. stick the tranny in at the room temperature and let it "breathe" for at least 2 minutes. Even touching the tranny raises up the leakage and will have a hell lot to do with the final result... which is what we do not want .
5. measure the voltage(U1) with the base lifted - wait until the displyed number gets steady
6. switch your DMM to uA and measure leakage current
where 100uA is common
about 200 happens pretty often
>300uA means the device is nothing special
> 500uA means the device really leaks a lot
7. back to the voltage scale - connect the base and get the voltage U2, again - wait until the displyed number gets steady
8. (U2-U1)/100 = Hfe

Scruffy forget about any numbers coming out of your DMM when you try to measure a Ge tranny straight.

Yea I know how to use it, set the trimmer to 2.47k or whatever it is just really not got the time so did the measure I took very quickly so might not have let it breathe or whatnot so I wanna take it again when I have the chance to do it properly and if it's a low leakage device I was lead to beleive it should give a decentt idea on the multimeter, which is where my concern came above that I'd done it too hastily and not read it properly but anyway, i will go over them when I get the chance to confirm this and put the question to rest, atleast on my side.

[SPeter]

I don't think the multimeter has to ever be switched to measure micro amps!

"To do the test, stick the transistor in the socket, and read the DC voltage across the 2.4K resistor. The resistor will convert any leakage current from the transistor into a voltage that you can then read on your meter. A 2472 ohm resistor is 2.472 volts per milliamp, so a milliamp of leakage will cause 2.472 volts to display. That is incredibly too much leakage, so any transistor that does that is not going to be useful for a FF. In fact, although it will differ a bit, any transistor that shows more than a few micro amps of leakage is suspect. Because of the resistor scaling, the indicated value on your meter is "false leakage gain" and will have to be subtracted from the total reading that you do next.

To test the total gain, press the switch that connects the 2.2M resistor to the base. This causes a touch more than 4 micro amps of base current to flow in the base. The transistor multiplies this by its internal gain, and the sum of the leakage (which doesn't change with base current) and the amplified base current. If the transistor has a gain of 100 and no leakage, the voltage across the 2.4K resistor is then (4uA)*(100)*(2472) = 0.9888V - which is almost exactly 1/ 100 of the actual gain. Pretty neat, huh? "

Here is the whole R.G. Keen article:
http://www.geofex.com/Article_Folders/f ... fffram.htm

[Scruffie]

I was doing it straight from the R.G keen article anyway, so no issues there.

[Scruffie]

I noticed some talk on the germanium transistor section of Collector paint dots collerating in some fashion to there gain rating... I don't know if it applies here but I do have 1 with a red dot and 1 with a white dot... could this be something to do with this? Or did I take it out of context (couldn't be bothered to read the whole thing at the time)

[playon]

The dots could be gain or it could be production batches... people will have to test them and see.

[Scruffie]

Well they were both out of the same radio and both the same in ware sort of appearances but I suppose that doesn't technically prove anything, just an observation. As you say, have to test and see (I really gotta get round to that)

[WhiteKeyHole]

Redone MKII, previously on strip-board. A PCB just feels a lot more permanent to me.



I applied the layout by hand but drew this up before, as a template. Don't know how useful it is to anyone else as it's spaced pretty specifically for the parts used, but have fun with it.