freestompboxes.org

Thanks Jose ! I'll start with a 68k in parallel with the 25k pot.

mat

ppluis0 wrote:Hi Mat,

Bear in mind that the values of these 3 pots were an guesstimate due we never seen the original ones.

Nevertheless you can put a resistor across the extreme terminals of the bias potentiometer, say 100 K and you will have the same bias voltage range as if you use a 20 K pot.

Perhaps is a better idea start paralleling your 25 K pot with a lower value (56 or 68 K ?), to see what bias range you have and avoid the risk of damage an expensive EF86...

Cheers,
Jose

Origional POTs on the SAS are: Bias 100K, Volume 100K, Gain 500K and that is according to the ole VOM.

nash-kat wrote:Origional POTs on the SAS are: Bias 100K, Volume 100K, Gain 500K and that is according to the ole VOM.

Hi nash-kat, thanks for the info!
I've been too busy at work to finish the pedal Tomorrow I should have couple of hours to work with it. Definitely will report asap when I have something to report.
I understood that the new ones come with a mod for the volume adjusting. Maybe a log 100k pot ? I was wondering if the 100k bias pot would give a too hot bias voltage for the EF86, not sure - maybe someone else could confirm this.

mat

Just an outside observation- I think this pedal would be far more useful if the EF86 were biased correctly (and not on a knob) and a Matchless DC30 style bass rolloff control were used in front of the EF86. That way you'd eliminate the blocking distortion and take advantage of the harmonic addition the EF86 can provide.

Your mileage may vary.

jamie

imaradiostar wrote:Just an outside observation- I think this pedal would be far more useful if the EF86 were biased correctly (and not on a knob) and a Matchless DC30 style bass rolloff control were used in front of the EF86. That way you'd eliminate the blocking distortion and take advantage of the harmonic addition the EF86 can provide.

Your mileage may vary.

jamie

Thanks jamie for Your suggestion. I'll finish the pedal as in the schem first and if I don't like what I hear I'll start to tweak it.

What do You think of the 100k bias pot. Too hot bias for the EF86 maybe ?

mat

I don't think there should be a bias pot- there should be a 1meg to ground and maybe a 33k to 100k grid stopper on the pentode to help prevent blocking distortion. A more useful version of this pedal would have a bass cut before the pentode and a treble rolloff afterward. Perhaps a tone control afterward? Maybe see how it sounds without it. I don't know. This is all speculation. All I know from all the video clips is it sounds like ass and it needs to be fixed!

jamie

Also you might have to tweak the value of the cathode resistor so the voltage on the pentode's plate is around 2/3 of B+ at rest. (to bias the tube correctly, anyway)

jamie

I should probably clarify- it has a lot of good tones but I think it would be more usable with a bass rolloff before the pentode and less blocking distortion at higher settings.

I guess it's possible that some people LIKE those things! It's up to you, really.

Maybe I should shut up. I don't know. I'm just sharing what I like.

jamie

The extreme gain settings might turn of the majority of the players from the pedal, but I like the 'explodind amp'-sound alot. You are right, there seems to be many usable sounds with the lower gain settings.

After a while I got little time to solder the wires on the build. The only thing missing is the wirings on the tube. Hope to finish the pedal on upcoming week and hope to get something recorded asap.

Still little nervous with the bias pot..

mat

imaradiostar wrote:I should probably clarify- it has a lot of good tones but I think it would be more usable with a bass rolloff before the pentode and less blocking distortion at higher settings.

I guess it's possible that some people LIKE those things! It's up to you, really.

Maybe I should shut up. I don't know. I'm just sharing what I like.

jamie

mat wrote:Still little nervous with the bias pot...

Hi Mat,

When you have finished all the wiring and you're ready to turn on the supply voltage please take the following measurement:

Using an alligator clip leads and a multimeter set to read hundreds of DC volts, measure the existing voltage between the EF86 plate (pin 6) and ground, at both extreme settings of the bias potentiometer.

This way we can know how much current the tube has to deal with, if you post these two voltage values.

This measurement must be done without signal at the input jack or with a shorting wire across the clipping diodes.

After that, release the short, plug your guitar and tell us what you hear.

Cheers,
Jose

Jose, thanks for the tip! I'll post the plate voltage as soon as possible!

mat

Very interestng post! any news?

I havent got a chance to finnish the pedal.. When its done I'll post clips etc.. Sorry for it taking so long.

Mat

diafebus wrote:Very interestng post! any news?

Finally got the wiring done. Very sorry it took so long..

I fired the pedal and first it worked few minutes as supposed (volume and gain pot worked - not sure of the bias pot), only problem was that the volume at full it was only same volume as bypassed (unity). Then the pedal went silent

The voltages are measured to ground (bias pot at half way):
First filter cap - 282V
EF86:
pin1 - 27V
pin2 - 0V
pin3 - 3V
pin4 - 3.6VAC
pin5 - 3.8VAC
pin6 - 98V
pin7 - 0V
pin8 - 0V
pin9 - 2,97V

IC(TL071)
pin1 - 0,14V
pin2 - 4,63V
pin3 - 1,07V
pin4 - 0V
pin5 - 0,13V
pin6 - 1,44V
pin7 - 8,09V
pin8 - 0V

Here are few pics of the build: http://mattitee.kuvat.fi/kuvat/EF86overdrive+pedal/

How does the voltages seem?

Thanks,
Mat

Hi Mat,

The voltages at the tube pins seems to be OK, but the reading at pin 6 of the TL071 must be +4.5Vdc

Take a look at the components associated to pin 3 of this IC: in the node that connects the pair of 100K resistors, the 1 Meg resistor, and the positive side of a 10uF cap, you have to read 4.6 Vdc

Keep us posted about what happens with this fault search.

Cheers,
Jose

Well, after re read the values you posted, if the IC is feed by 8 Vdc, then the bias point of the output (and the node previously mentioned) will be of +4 Vdc

But remember that you are close to componets with high voltages, so be careful when poking your fingers inside the unit

Cheers,
Jose

Hi Jose

The point (where 100K's meet the 1M and positive node of the 10uF) mentioned measures 4,09V.

Mat

mat wrote:The point (where 100K's meet the 1M and positive node of the 10uF) mentioned measures 4,09V.

It's strange to have 1.4 volts at the output pin of the op amp, in that case...

My best guess is that the electrolytic capacitor placed at the output of this IC can develop a short circuit (so the pair of diodes clamp this point to GND) or there is a damaged opamp.

Also check the input cap (47nF) for short or leackage, and assure that the pins 1, 5 and 8 of the chip are really unconnected to anything.

Hope I can help you in any way, so keep searching the fault and keep in contact

Cheers,
Jose

I checked pins 1, 5 and 8 and they are not connected anywhere. I swapped the 071 to new one and realized pin6 getting high readings, so I took the 071 away and measured the voltages without the IC and got strange readings:

TL071:
pin1 - 0V
pin2 - 2,5V
pin3 - 0V
pin4 - 0V
pin5 - 0V
pin6 - 40V and rising !
pin7 - 9,9V<->19V (sailed between)
pin8 - 0V

Something wrong with the low voltage supply I think

Mat

mat wrote:I checked pins 1, 5 and 8 and they are not connected anywhere. I swapped the 071 to new one and realized pin6 getting high readings, so I took the 071 away and measured the voltages without the IC and got strange readings:

TL071:
pin1 - 0V
pin2 - 2,5V
pin3 - 0V
pin4 - 0V
pin5 - 0V
pin6 - 40V and rising !
pin7 - 9,9V<->19V (sailed between)
pin8 - 0V

Something wrong with the low voltage supply I think

Mat

Hi Mat,

Perhaps the fault are somewhere in the string of components placed between pin 6 of the IC socket and pin 9 of the tube socket.

Click an 1K resistor between pins 4 and 6 -directly on the IC socket, for example, and measure what you have at pin 6 in that condition.

I assume that pin 4 is effectively tied to GND so, if pin 6 will develop some voltage, perhaps you have a leaky capacitor or something wrong fom pin 6 to the tube.

I stay tuned to see what you find...

Cheers,
Jose