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GK rails in a pedal?

#243
happy new year everyone.
i finally got the newest V4 board built up.

0001-GKV4PCB-02.jpg


I just need to wire it to the pots and foldback switch and I can take it for a test drive.

test results soon to follow.
 
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#245
I was looking at the 800RB preamp schematics and there is one other thing we may need to consider for the authentic GK ‘rails’ sound.

Q89 (J113) JFET amplifier is on the return side of the effects loop, essentially making it part of the power amp circuit in a way. This Q89 amplifier has a gain of approx 15 and can easily distort with a 1Vp-p input signal. This Q89 amp will distort long before any of the other preamp circuitry when boost is enabled.

So, I am thinking that this GK ‘rails’ sound may need a distorted input to the GK amp simulator. Where our amp simulator should get us very close to the characteristics of the power amp, we may need this one last piece of dirt from the preamp.

-Frank
 
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#247
johnk_10 said:
I discovered a few errors in my latest V4 PCB (it didn't work) so I just finished redrawing it with the corrections, so i'll just have to build another one for testing.
do you think you want to make any changes before I etch the corrected one?
Click to expand...

It's up to you. The V4 as it stands, IS a very good simulator of the 1982 GK power amplifier. But if you want the 800RB from the effects return jack to the speaker output then we should change the input buffer circuit.

It seems to me that you guys are looking forward to mating it up with other preamps, so I would say let's keep that theme going forward and let V4 be the true amp simulator. You could easily tack on the Q89 JEFT circuit as an output into any dual supply preamp that you desire for GK authenticity.

I will draw up a "Q89" circuit that would allow you to tap into this V4 at C3 so you can keep the V4 copper the same.

-Frank
 
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#250
John, this circuit will replace C3 on your V4 PCB... so you may want to socket C3 on your V4 amp simulator. This way you can test to see if the GK preamp has anything to do with the 'rails' distortion.

***IMAGE REMOVED- WE HAVE A NEWER VERSION LATER IN THIS THREAD***

-Frank
 
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#254
boomertech said:
I was looking at the 800RB preamp schematics and there is one other thing we may need to consider for the authentic GK ‘rails’ sound.

Q89 (J113) JFET amplifier is on the return side of the effects loop, essentially making it part of the power amp circuit in a way. This Q89 amplifier has a gain of approx 15 and can easily distort with a 1Vp-p input signal. This Q89 amp will distort long before any of the other preamp circuitry when boost is enabled.

So, I am thinking that this GK ‘rails’ sound may need a distorted input to the GK amp simulator. Where our amp simulator should get us very close to the characteristics of the power amp, we may need this one last piece of dirt from the preamp.

-Frank
Click to expand...

I remember Robert Gallien saying something like that in another thread on this topic, something along the lines of there being a preamp in the power amp or after the power amp or something.
 
#255
ryansalmond said:
I remember Robert Gallien saying something like that in another thread on this topic, something along the lines of there being a preamp in the power amp or after the power amp or something.
Click to expand...

The common thought on the effects return jacks on amplifiers is that they run directly into the power amp section. This is somewhat true, but in the 800RB they have this Q89 boost amplifier after the return jack on the preamp board. If people have jacked into the effects return with a different preamp to just use the GK power amp, they are also using boost amplifier circuitry from the preamp.

This will be a great test to see if this 'rails' sound is 100% power amp, a mix of Q89 boost and power amp or 100% Q89 boost with nothing to do with the power amp. Hopefully John will be able to determine exactly where the 'rails' sound is created and everyone following this thread will have the correct answers.

If we can get those answers, then we can potentially design something that is a lot simpler circuit wise.

-Frank
 
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#256
boomertech said:
If people have jacked into the effects return with a different preamp to just use the GK power amp, they are also using boost amplifier circuitry from the preamp.
Click to expand...

Interesting!
This will be a great test to see if this 'rails' sound is 100% power amp, a mix of Q89 boost and power amp or 100% Q89 boost with nothing to do with the power amp. Hopefully John will be able to determine exactly where the 'rails' sound is created and everyone following this thread will have the correct answers.

If we can get those answers, then we can potentially design something that is a lot simpler circuit wise.

-Frank
Click to expand...

So excited :)
 
#258
okay. I got the latest V4 built and tested and it works (and IMO, works properly), even if slamming the input hard with an external booster, there's no more BLOWN LF353's (YAY!)
for the BJT transistors i'm using 2N4401's and 2N4403's (both having hfe's at from 250-260).

right now, there are a three issues with it though that (I feel) must be addressed.

first, the preamp gain pot acts very strange. when you adjust the gain it from any setting, up or down, the volume dips to zero and then comes back on (it sounds like it is voltage surging to me). there's also slight static on the pot when rotating it. (DC on the pot?)
second, the distortion doesn't really sound like 'rails' clipping to me, since it's decay has an ugly fizzy top end when the note fades.

and third, the feedback switch has no effect at all (which is probably a matter of adjusting the 10K/100K resistors as Frank already stated. I socketed those two so tomorrow i'll experiment with their values.

IMO, we're getting there, but it's not quite right yet.

it's after 2:30 am here, but i'll post some sound clips of it and take some voltage measurements tomorrow.

the good news is, if you don't run it into clipping (or just have very light clipping) it sounds big, full and fat and can be used as a fat clean boost.

BTW, this circuit only draws a total 20ma when run on 9V (with the LT1054 providing the bipolar + & - 9V to the circuit), with a maximum of 30ma when driven very hard with and external booster.

here's a pic of my test circuit:

0001-GKV4PCB-03.jpg
 
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#259
Great John!!

For the pot issue, change C3 to 0.1uF... we will still have a corner freq of 16Hz at that node.

For the fizzyness we might have to strap a cap across the output pot to roll off the freq to match a speaker response. But first you will need to find the value of R7 that gives you what you want for max volume before we can calculate the cap value.

Please give me the DC pin voltages around U1 and TP3.
What are the DC voltages across C11 and C12.

If you get a chance, look at the output signal with your scope (DC coupled) at TP3 with and without foldback, what are the clipping voltages?

Your currents look real good! Get some sleep, you deserve it!

-Frank
 
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