Sunn Beta Bass preamp schematic

Tube or solid-state, this section goes to eleven!
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DiscoVlad
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Post by DiscoVlad »

My suspicion, from a manufacturing standpoint is that the "common" schematic is the newer one as that has the least components (only 4 capacitors) different between the Bass and Lead version (ignoring the reverb driver section of course). The differences are all in the EQ so building/simulating both to compare shouldn't be difficult...

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Post by DiscoVlad »

This is from the Bass version FWIW - but as far as I can see in LTSpice the different EQ sections have subtly different response curves, mids seems to be the most pronounced (more scooped at 0º notably), but the Bass and Treble controls aren't terribly different to each other, also the bass and mid peaks are slightly lower frequencies than the "common" service manual version.

The alternate version has 6 different components between the guitar and bass versions - 4 resistors, and 2 caps, vs the 4 caps of the "common" one. Overall it probably doesn't matter? Which one is better is not a hill I want to die on. :mrgreen:
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Alternate schematic EQ differences between guitar and bass
Alternate schematic EQ differences between guitar and bass

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Post by georg667 »

Hey guys,

after a couple of months collecting dust I am trying to bring this build to an end. My aim is building a 19" rack two channel version. I tried to figure out how the footswitch section is working and as I am a total newbie in the veroboard creating business I wanted to ask if someone is able to check my version if it's correct.

thanks in advance!!
georg

PS: of course feel free to use the layout for projects!
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Schaltungsboard.png
Beta_Schematics_eingefärbt.jpg

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Tassieviking
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Post by Tassieviking »

Where are you going to get the MC14506 IC from ?
I thought they were obsolete.
I drew up an alternative circuit but I haven't tested it, I used info from the NET on how to replace the MC14506 IC.
I would breadboard it before trusting it.
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Alternate Control Circuit NOT VERIFIED
Alternate Control Circuit NOT VERIFIED

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Post by Tassieviking »

Here are the info I found on replacing the MC14506
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MC14506_replacement_a.GIF
MC14506_replacement_a.GIF (8.46 KiB) Viewed 5460 times
MC14506_Layout2_a.GIF

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Post by Tassieviking »

Please note that if you are using the older circuit that uses the 2uF caps (obsolete)
You can use 2.2 uF instead.
There are also 2 differences between Channel A and Channel B
There are 2 places where the resistors and capacitors are reversed.
It might not make any difference but I thought I might mention it.
All circuits available from Fender:
https://support.fender.com/hc/en-us/art ... Schematics
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Sunn Beta Bass Circuit Vintage A & B Differences.jpg

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Post by DiscoVlad »

Tassieviking wrote: 27 Sep 2020, 05:53 Where are you going to get the MC14506 IC from ?
I thought they were obsolete.
I drew up an alternative circuit but I haven't tested it, I used info from the NET on how to replace the MC14506 IC.
I would breadboard it before trusting it.
I see I'm not the only one with this idea :wink:
4069 doesn't work for this, my suspicion for why is either the load current is too high for it, or it doesn't like the levels around the LED drivers. You have to use a 4049.

The breadboard pics show a test with one channel to check that the muting works, while the soldered protoboard version is 2-channels - this was done to rule out potentially faulty breadboard connections as the cause.

4069 still doesn't work in this case.
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2muted.jpg
1active.jpg
dualchannel.jpg
Last edited by DiscoVlad on 01 Oct 2020, 04:06, edited 1 time in total.

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Post by DiscoVlad »

Here's my version of the switching schematic which has been tested to work, and has additional musing about an updated foot switch to work with modern high efficiency LEDs.
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BetaSwitching.pdf
(223.24 KiB) Downloaded 299 times

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Post by georg667 »

Tassieviking wrote: 27 Sep 2020, 05:53 Where are you going to get the MC14506 IC from ?
I thought they were obsolete.
I drew up an alternative circuit but I haven't tested it, I used info from the NET on how to replace the MC14506 IC.
I would breadboard it before trusting it.
thanks man for the reply!
As I am located in Europe I just searched vie ebay and maybe I am lucky but I got a batch from Motorola via ebay. Here you can find the datasheet:
https://html.alldatasheet.com/html-pdf/ ... 506UB.html


I Also got the 74C04 Hex inverter and hope that it will work out fine...
@DiscoVlad: Do you believe that 74c04 are also going to work?
Couldn't try to test it yet though.

I got another little issue with the main curcuit. It sounds great and matches almost perfect with the original amp. My circuit just lacks a bit of drive. There seems to be a little more "roar" in the amp version. Any suggestion where I could tweak the circuit a bit? :-D

thank you guys!
georg
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Post by georg667 »

Tassieviking wrote: 27 Sep 2020, 06:42 Please note that if you are using the older circuit that uses the 2uF caps (obsolete)
You can use 2.2 uF instead.
There are also 2 differences between Channel A and Channel B
There are 2 places where the resistors and capacitors are reversed.
It might not make any difference but I thought I might mention it.
All circuits available from Fender:
https://support.fender.com/hc/en-us/art ... Schematics
Thankyou for mentioning it... Do you think it's on purpose that the caps and resistors are reversed?

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Post by DiscoVlad »

georg667 wrote: 02 Oct 2020, 18:00 I Also got the 74C04 Hex inverter and hope that it will work out fine...
@DiscoVlad: Do you believe that 74c04 are also going to work?
Couldn't try to test it yet though.
I haven't got any 74c04, so no idea sorry.
Tassieviking wrote: 27 Sep 2020, 06:42 Thankyou for mentioning it... Do you think it's on purpose that the caps and resistors are reversed?
It's likely that it was to make the pcb routing fit better, changing the order of series components between two nodes doesn't (and shouldn't) make any difference.

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Post by Tassieviking »

I have no idea why they swapped the components in the B preamp circuit, you would think that a small capacitor and a small resistor would fit in either place, so it makes no real sense to me.
It might have been done by mistake or lack of care, I have no idea if it makes any changes to he overall sound.
I only mentioned it because some people like to match the original 100%.
I bet someone has even tried getting 50 year old components thinking it will make it sound more original.

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Post by Tassieviking »

:applause:
DiscoVlad wrote: 01 Oct 2020, 04:01 Here's my version of the switching schematic which has been tested to work, and has additional musing about an updated foot switch to work with modern high efficiency LEDs.
Thank you for that circuit, saved straight to my Sunn folder for future use.
I really want to make myself a Sunn Beta Amp one day.
I have made some vero layouts for myself, but I keep putting it off till the day I learn how to make up PCB's layouts.
I want it small enought to fit 2 PCB's in a 1590BB box but versatile enough to make a complete fully working amp.
All the PCB's out there seem to be purely aimed at StompBoxes, you can't join 2 channels with the switching circuit, and no output for a reverb if you build the Lead one.
One day I'll get to it.

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Post by deltafred »

Tassieviking wrote: 03 Oct 2020, 13:34 I have no idea why they swapped the components in the B preamp circuit, you would think that a small capacitor and a small resistor would fit in either place, so it makes no real sense to me.
It might have been done by mistake or lack of care, I have no idea if it makes any changes to he overall sound.
I only mentioned it because some people like to match the original 100%.
I bet someone has even tried getting 50 year old components thinking it will make it sound more original.
It will make no difference to the sound which way round you put a series resistor and capacitor and it is not through lack of care or a mistake that it is drawn like that, it simply does not matter.

The person that designed the circuit knew that and probably thought that the only people who would be looking at it would be service engineers.

The service engineers would either also know or simply not care as long as they got the amp working and off their bench.

Then along come a bunch of pedal freaks who try and recreate circuits to the last tiny detail thinking that it is vitally important to capture the sound of the originals. (Which were usually built with components that had a far greater tolerance than those available today so would have greater variation of sound from one unit to the next.)
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Post by DiscoVlad »

Beta Pre-amp.pdf
This is very much unverified!
(432.59 KiB) Downloaded 313 times
This layout took a lot longer than I was expecting, and it still needs a little tidying up, but it's... done?

2-channels, switching, board is 100mm x 150mm.
The connectors are all Molex Mini-fit Jr.
Rendered
Rendered

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Post by DiscoVlad »

IMG_0158.jpeg
So, I got some boards made and spent several hours soldering...

In the initial testing (after plugging in the ICs of course)
The channel switching logic appears to be working fine :mrgreen:
All of the supply and ground pins for the op-amps measure correctly... except the Channel A 4069 is reading 13.2V on pin 14 (and the other pins which should be 8V) which isn't right...
The 470Ω resistor and 680µF capacitor of both sides measure correctly, and the soldering looks good.


This is how the 4069 supply is hooked up on channel A:
chanA_4069.png


This is how it's connected on channel B:
chanB_4069.png
To make the routing easier the two channels have different unused gates, I don't think that is the cause of the problem but it is possible.
My thinking that the chip is bad as it's probably the one I was using for testing the channel switching redesign and it may have been damaged during that. I'm gonna sleep on it, and swap another chip in tomorrow... of course if it's a bad chip, maybe the 4069 does work for the switching logic :o

Alternatively there could be some dirt/flux/etc. in the socket.
Any other possibilities?


Update: The chip is bad, I put another one in and everything measures correctly now. :mrgreen:

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Post by Tassieviking »

Really nice looking board Disco Vlad
I made a one channel PCB for a stompbox.
I made 2 small mistakes but easy to fix, one cap marked back to front.
I also got a trace to pin 1 of the charge pump that should not be connected, I just cut the leg of the IC socket.

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Post by Tassieviking »

Here are some pics of my first build of the beta in 1590BB, It was the first PCB I ever made up myself.
I still have 9 more PCB's left.
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WP_20201223_18_00_50_Pro.jpg
WP_20210201_18_16_52_Pro.jpg

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Post by DiscoVlad »

I'm still waiting for an enclosure (and to get a power amp sorted out) here, so everything is just mounted onto a board, and powered with a plug-in transformer at the moment.
IMG_0181.jpeg
Originally the high and low EQ pots were labelled wrong, but that's fixed now.
Plugged into the loop return of one of my other amps, It's REALLY LOUD :mrgreen:
I'd previously changed the bad 4069 out, but there's still something not quite right with Channel A, it sounds there's some gating going on, there's some hiss/crackle, and the voltages around the gain stage look wrong...

Channel B however: sounds amazing, it's just a full on thick awesome DOOM roar.

At least with one channel working, I've got something to compare the bad one against. :wink:

The schematic is updated to fix some errors I made in the build - pots are now oriented/labelled correctly, and unused 4069 gates have been tied to ground, rather than the +8V bias that my layout has.
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Beta Pre-Amp 0.1d.pdf
Redrawn hierarchically from factory schematic.
(774.88 KiB) Downloaded 296 times

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Post by DiscoVlad »

DiscoVlad wrote: 28 Feb 2021, 09:17 but there's still something not quite right with Channel A, it sounds there's some gating going on, there's some hiss/crackle, and the voltages around the gain stage look wrong...
I think I've figured out what the "problem" is:
After trying a bunch of things - Reflowing the solder joints, trying different chips, measuring everything etc., I breadboarded out the 4069 section and all the components connected to it, then started changing resistor values to see what happened...

With the feedback resistors in, the gates tend to bias their input/output at around half VDD, so far so good.
Higher/lower values of feedback (resistance) turn the gates on differing amounts and how much current the chip draws, which for me was the clue I needed.

Now, the 8V "supply" to the CD4069 VDD pin is dependent on the amount of feedback present (i.e. the value of the resistors/pot) turning on the MOSFETs in the gates/inverters slightly, (remember that they're a P and N channel MOSFET between VDD and VSS -> they essentially work together as a variable resistance from the negative feedback) which acts as a voltage divider in conjunction with the 470Ω resistor.***

So after measuring all the fixed (the 330k/22k/68k chain) feedback resistors, and their total (it should add up to ~510k) and finding them identical on both channels, the only thing left was the 100k and the 1M potentiometers, so I swapped them between channels... and the sound and voltage levels moved with it.

Turns out that the two pots I used just happened to be at opposite ends of the tolerance band, the "good" sounding one was about 10k over 1M, and the "bad" one was just over 900k. There may have also been a small... accident with some flux being spilled when I was hooking up the leads, if cleaning it out doesn't help I'll replace it with one closer to the channel B pot. :oops:

*** There may be some scope for tuning here by putting a zener diode (6-8.2V maybe?) across the 680uF capacitor, or changing the value of the resistor.


That was longer than I expected... Anyway, it's fixed now :mrgreen:

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