zaraxisof

Spring Reverb Fender 6G15 emulation
« on: October 05, 2019, 12:11:59 AM »
Hello everybody, i was searching for a spring  reverb built and found a schematic from a site that emulates the Fender 6G15 usinf fet circuity. I could not afford to buy their kit and decided to built "my own" copying exactly this one. After having built it in both solderboard and breadboard i cannot find out where the mistake is. It is smthng small, just a detail i am sure. I ve heard that this schem has some errors, mostly about pins labeling and so.
Is there anybody out there having built this one or can recognise if there is smthng wrong with schem?

I can hear dry signal and loud "splash" sound by tapping on springs but it seems like cannot be "mixed" with signal, no effect produced. So problem must be in input stage (green coloured path in second post)
Tried swap in/out rca rvrb plus, its not there. Swapped some jfets, neither. All connections are triple checked, they are 100% same with this schem. I also get correct voltages at TP1,TP2.

Any thoughts?

« Last Edit: October 05, 2019, 12:16:26 AM by zaraxisof »


zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #1 on: October 05, 2019, 12:18:17 AM »
coloured stages

squarewave

Re: Spring Reverb Fender 6G15 emulation
« Reply #2 on: October 05, 2019, 02:23:19 AM »
The "C10-MOD" cannot be correct. The 440pF with a 10K typical load is a HP cutoff way over 20kHz. Those caps should probably be more like 220nF and not 220pF.

Re: Spring Reverb Fender 6G15 emulation
« Reply #3 on: October 05, 2019, 03:42:06 AM »
Any thoughts?

I assume you have the right reverb pan? It must be an A B type.

The parameter of the 2n5457s are all over the place, they need to be selected. (Voltage at the collector ~ 1/2 VDD)  It is possible that Q4 and Q5 do not amplify the signal enough.

Follow the signal and check where it get lost.  Connect a 8Ohm loudspeaker to the send output after the MOSFETS, can you hear anything? 



Khron

Re: Spring Reverb Fender 6G15 emulation
« Reply #4 on: October 05, 2019, 06:34:08 AM »
For what it's worth, i've seen (tube) guitar amp schematics where the reverb pan was driven from 2-3 parallelled opamps, and a fourth one on the receiving end. Arguably a simpler solution than this, perhaps? Less power-hungry at the very least, anyway...
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RuudNL

Re: Spring Reverb Fender 6G15 emulation
« Reply #5 on: October 05, 2019, 10:19:49 AM »
OMG, original schematic: bias 400 mA!!!  ::)
There is a solution for every problem!

http://www.vansteenisaudio.nl

zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #6 on: October 05, 2019, 01:51:34 PM »
The "C10-MOD" cannot be correct. The 440pF with a 10K typical load is a HP cutoff way over 20kHz. Those caps should probably be more like 220nF and not 220pF.
yeap i ve heard about that so i left that out of the game (optional choice). I just connected lug 2 of Tone pot with lug 1 of Mixer pot, isn't like that?

Quote
I assume you have the right reverb pan? It must be an A B type.
yeap my tank is Type 4, B (8ohms input). The 4AB3C1B.

Quote
Follow the signal and check where it get lost.  Connect a 8Ohm loudspeaker to the send output after the MOSFETS, can you hear anything?
you mean here (red mark) before tank  or here (green mark) having tank in the game?

Before doing so, just noticed that Mosfets get extremely hot, meaning far away than they should normally be i guess, cannot even touch for a milisecond.
« Last Edit: October 05, 2019, 06:02:18 PM by zaraxisof »

Khron

Re: Spring Reverb Fender 6G15 emulation
« Reply #7 on: October 05, 2019, 02:06:10 PM »
yeap i ve heard about that so i left that out of the game (optional choice). I just connected lug 2 of Tone pot with lug 1 of Mixer pot, isn't like that?

That's not a good idea. Use a 100nF for C10.

Before doing so, just noticed that Mosfets get extremely hot, meaning far away than they should normally be i guess, cannot even touch for a milisecond.


12V * 400mA = 4.8W

So when's the smoke-show?
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PRR

Re: Spring Reverb Fender 6G15 emulation
« Reply #8 on: October 05, 2019, 06:56:51 PM »
JFETs today can have VARIOUS pin-outs. First check the "proper" data sheet. But there are many options. It is good to use the diode-test on your DMM to figure which is G (S and D can be interchanged no problem).

It is all 12V. You think you know what part of the plan is losing signal. Not touching any ground, put your finger on various signal nodes. It should buzz loudly. More or less depending on node impedance, but there should be something. Figure where the signal stops.

PRR

Re: Spring Reverb Fender 6G15 emulation
« Reply #9 on: October 05, 2019, 07:02:43 PM »
Yes, over 2 Watts per MOSFET. This is way too much for naked TO-220-- you NEED heatsinks.

OR-- replace the upper MOSFET with 100 Ohms Source to Drain. This will reduce dissipation in lower MOSFET to 1/3rd Watt, safe. It reduces MAXimum tank drive, but should be fine for all but the swampiest surf music, and would confirm/deny other guess-work.


PRR


zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #11 on: October 06, 2019, 11:55:26 AM »
Solved  :) bad alligator clip at input rvrb tank stage  :P
Thank you all guys   :)

Any ideas, general discussion about ways of avoiding mosfets  heating as much i can ?  Indeed i use heat sinks but also think of using metal chassis box as a "big heat sink".. How could i creat my own diy safe isolation tape?

PRR
Quote
OR-- replace the upper MOSFET with 100 Ohms Source to Drain. This will reduce dissipation in lower MOSFET to 1/3rd Watt, safe. It reduces MAXimum tank drive, but should be fine for all but the swampiest surf music, and would confirm/deny other guess-work.
My apologies my english is bad, didn't got it. You mean it should also be good for drippy surfy palm muting classic sound? If so, you mean like this?
« Last Edit: October 06, 2019, 11:59:29 AM by zaraxisof »

PRR

Re: Spring Reverb Fender 6G15 emulation
« Reply #12 on: October 06, 2019, 12:47:10 PM »
Replacing the upper MOSFET with a resistor greatly reduces the maximum drive to the tank. This is more for testing than "drippy" reverb.

I do think this driver is very generous, on the edge of "too powerful". It is about 10 times the suggested current for the tank. Yes, many other reverbs drive the tank past the suggestion. I do think R1 could be doubled or tripled for less dissipation and still give lots of drive.
« Last Edit: October 06, 2019, 06:27:30 PM by PRR »

zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #13 on: October 09, 2019, 06:21:25 PM »
As i said above problem solved, effect works, but i have  a "minor" issue.
In my final output signal i get a  high frequency noise as i turn Tone pot to max. With 50K Tone pot at minimum (bass) it is way quiter, as it gets low pass filtered, when turned to max noise gets maximum .

If also those help:

1.
Quote
That's not a good idea. Use a 100nF for C10.
  with 100nf cap this noise gets louder

2. It is less with 9V supply, instead of  using 12V.

Here is a small recording of the noise. Ignore hum, its from my draft amp oriantation, pickups, cables etc i have on the flow.  With an amp open, i just plug 12V DC and you can hear that noise begins like an "engine machine"     PS.  Amp gain is at about 10 o'clock and interface's input boosted too, in reality its not that loud (didnt passed any signal as it would be too loud and difficult to hear the noise) - think that the "splashes" are generated only by just touching plugs and pots  :P)
Note how it gets decreased as i turn Tone pot at minimum (to bass)

https://we.tl/t-zV7dVcM9Eg

Pic: Look at those overheating spots on  MOSFETS  :(  :(

Any thoughts?
« Last Edit: October 09, 2019, 07:47:01 PM by zaraxisof »

Khron

Re: Spring Reverb Fender 6G15 emulation
« Reply #14 on: October 10, 2019, 11:52:27 AM »
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zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #15 on: October 11, 2019, 09:08:55 PM »
https://sound-au.com/heatsinks.htm
Thanks Khron it was interesting and informative reading this for sure, learned a lot of things. Couldnt imagine how important and critical could the right usage of heatsinks be, specially at A  or AB class amplification.
First n obvious mistake was i had placed them wrong lol.  But even with esp's advice followed and using a lil white thermal paste, heatsinks seem to get still too hot, i think less than before though.


Khron

Re: Spring Reverb Fender 6G15 emulation
« Reply #16 on: October 12, 2019, 04:00:11 AM »
Well, the die (which creates the heat) is in the black epoxy part of the package; there's only SO much heat that the mounting-hole area of those thin tabs can transfer.

And it wasn't meant to solve all your problems - can't really polish a turd (turd in the sense of "major and pointless design flaws"). More like a pointer towards "best practices".
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Gus

Re: Spring Reverb Fender 6G15 emulation
« Reply #17 on: October 12, 2019, 09:01:15 AM »
It looks like they wanted a class A output for the 8 ohm input of the pan used in the tube amp.

Something like this could be a power amp IC for the class A pan driver and opamps for the rest of the circuit.

Good links
http://www.accutronicsreverb.com/main/?skin=sub05_08.html
https://sound-au.com/articles/reverb.htm

zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #18 on: October 12, 2019, 05:54:04 PM »
Yeap i know guys
Gus i am building esp's (2nd link) circuit too, one for low impedance input as i have the 4AB3C1B, but i stopped as i couldnt find in local electro shops the BC640, have in home 5532, bc639 and other stuff but i wait for the 640 from ebay  :P
It seems a nice circuit drive and recovery and a well explained thing as always from esp. It does not give "that" huge surf drippy splash like the above but i ve heard it can be nice for some blues or other signals not only for guitar.
Do you think guys i could use it as an external "pedal effect" unit for my Farfisa Foyer? It a classic farfisa organ and shares same circuit as in the Compacts plus using full tube circuity (1rect,3pre,2out tubes) and gives a mono/headphone output.
Its spring reverb unit was damaged while transporting it to my house and cannot fix it till now (bit lazy after have it full recapped, changed some germaniums, cleaned etc. though having many projects running..)

http://claviers.mezei.fr/dossiers/FarfisaFoyer.A53-124/FarfisaFoyer.A53-124.html

« Last Edit: October 12, 2019, 06:12:38 PM by zaraxisof »

zaraxisof

Re: Spring Reverb Fender 6G15 emulation
« Reply #19 on: October 12, 2019, 11:45:18 PM »
About ESP's reverb circuit, as you can see it needs +- 15V rails.  I have some PCB's for the Green Prea Amp PSU that i ve sent for manufacturing back then but never found time to built the Green. Could, by terms of Amperage, this PCB Power Supply work for the ESP's reverb circuit using only +15  0  -15 from a 2x18Vac trasfo 1A?
I had used this schemo

https://www.lorcan.me/green_preamp/


 

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