Sunn Beta Bass preamp schematic

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

On my PCB I used a 9v1 Zener across the 680uF capacitor just in case, as the caps I have at home are rated at 680uF 16v.
I also made provision for 2 smaller caps to replace the 680uF cap, ie. 2 x 330uf in parallel.
I put 1 x 680uF and 2 x 220uF and 9v1 Zener in parallel in the circuit, on the PCB layout I put the Zener and 2 x 220uF on top and the 680uF under the 2 x 220uF.
I can put 2 x 220 - 330 uF on top or 1 x 680uF on top using the same footprint on the PCB.
I also put a 1N5817 in series with the supply to the charge pump, then just in case I want to use a higher voltage I added a 7809 regulator across the 1N5817.
If I use the 7809 I reverse the 1N5817 to protect the 7809 from back EMF. (In case I run higher voltage to box with built in amp for practice setup).
Im having a bad morning so I think I'm even confusing myself with my explanation, old age and chronic pain, but veryyyy good painkillers from my Doc. =}

You can sort of see what I mean from the layout, I cant add DipTrace files here for some reason (.dip .dch) or I would put them up too.
I might zip the DipTrace files up later and add them.
Why can't we add DipTrace files here ?

Ok, I think I got my updated Diptrace files zipped ok. They have not been tried or checked since I fixed pin 1 on chargepump IC U1 and reversed polarity on C11
I believe they are correct.

If someone wants to use the updated Diptrace files, just go for it. Please note however that the build doc in the zipped file was for my prototype board and not the fixed board. You do not need to cut leg 1 of U1 IC and you do not need to reverse C11 with the PCB or circuit in the zipped file.
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Sun Beta Complete Corrected ZIPPED.zip
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SUNN BETA Circuit 5.jpg
Sunn BETA   Components  layout 2   .jpg
Sunn BETA   Components  layout .jpg
Last edited by Tassieviking on 11 Mar 2021, 08:08, edited 1 time in total.

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Post by Tassieviking »

Disco Vlad
Have you thought about using a 7808 voltage reg for 8volts across 680uF capacitor ? or is a slightly variable voltage part of the sound.
Have you considered using log pots for drive and level ?
What would happen if you reversed one part of the dual pots, Ie decrease clean as you increase distorted.
When I turn the drive up I have to lower the level a lot
I'm not switched on enough yet to do a sim in a Spice program, but I might have to learn.
I'm just curious, maybe curious enough to have to mess up my board I built to find out.

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Post by DiscoVlad »

Tassieviking wrote: 08 Mar 2021, 14:34 words
  • I don't know if it's part of the sound, but the 8V at the capacitor does vary depending on the gain pot position. It would be easier/cheaper to just change the value of the 470Ω resistor (or put a trimpot/extra pads in to make it adjustable).
  • given how they're connected I don't think audio pots would help: they're in parallel with a fixed resistor which tends to give a reverse log curve. I did use (and would recommend using) pots with a center detent for the EQ section though.
  • Pots going opposite directions - In my sim this looks like it's just louder/quieter versions of a similarly distorted signal which doesn't appear to clean up - it's two wires, swap them around and see what happens :)
  • Splitting the gain into two separate pots could be interesting?
  • Yes, it is a VERY loud preamp. The original manual does mention something about using the channel level control to balance the volume for each channel and the master volume to set the overall level. A single channel pedal version might benefit from altering the output level section for less gain, or adding some form of attenuator.
  • LTSpice is free, and there's plenty of tutorials/examples online to learn from.

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Post by Tassieviking »

I wonder if someone could help me with some dimensions for the full size amp front panel ?
Size of metal fascia panel, Distance between pots, jacks. leds etc.
Sort of like the picture.
I am thinking of building a full size BASS and LEAD amp since I still have 9 single channel PCB's left over.
I might have to build 4 amps since my daughter might just "borrow" one of each.
Any help is greatly appreciated.
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Sunn BETA Lead Front Panel Dimensions.jpg

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Post by Tassieviking »

Damn it DiscoVlad, now you got me interested in Spice and KiCad, I'm just drawing up the full circuit of the Lead amp in KiCad since Diptrace free wont let me use more the 300 points for free.
KiCad and spice should give me some nice headaches before I get the hang of them.
I wonder where I can get an old "Folded Line" by O.C. Electronics, type number 56 reverb pan from ? I was told on another site (REDDIT) this is what was original.

I guess a 8FB2C1B tank should work (Medium delay) or a 8FB3C1B (Long Delay) if you stick with a short tank
Otherwise a 1 or a 9 as a first number for the longer ones, as long as the 5th symbol is a "C" for insulated input or "D" for insulated in+out.
The second and third symbol should be FB for the right impedance.

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Post by Tassieviking »

Test Point 1 WTF
I am trying to find the correct test point locations, and on test point 1 it is shown as R143 on the BASS version, but its clearly on R146 on the lead version.
I'm pretty sure it should be in the same place on both boards as they most likely only used one PCB design for both versions.
The Lead amp looks like the best spot, the output of IC102 (Pin 1), the Bass drawing seems to show it's between R143 and R144 (Drive Pot).
I am going with the Lead Amp picture as it seems more likely to me, does anyone have a clue ? Anyone know whats going on ???

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Post by DiscoVlad »

TP1_sim.png
Nice spotting! :mrgreen:
I never compared the Lead and Bass boards when doing the reverse engineering, but the output of the opamp (pin1) definitely looks like the most likely spot.

*makes more changes to schematic/layout.

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Post by Tassieviking »

DiscoVlad wrote: 18 Apr 2021, 21:35 TP1_sim.png

Nice spotting! :mrgreen:
I never compared the Lead and Bass boards when doing the reverse engineering, but the output of the opamp (pin1) definitely looks like the most likely spot.

*makes more changes to schematic/layout.

Speaking of making changes DiscoVlad, what do you make of the modification mentioned in the service mods on page 8 paragraph 4 of the service manual?
"4. ....will have a 2.2uF capacitor added across pins 1 & 2 and across pins 5 & 6 of I.C. 103. "
All I can see on the schematic is a 10nF capacitor in those locations ( C164, C165 ). That's between the FET's in the channel switching circuit.
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Sunn_Beta_Series_Manual.pdf
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Post by Tassieviking »

If anyone else is thinking of trying to trace the power amplifier circuit from the poor quality Beta Service Manuals you find on the net, including the ones from Fenders pages, DON'T
Use the one in the Alpha Service Manual as they are the same, at least as far as I can tell.
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Alpha Series.pdf
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Post by DiscoVlad »

Tassieviking wrote: 29 Apr 2021, 12:56

Speaking of making changes DiscoVlad, what do you make of the modification mentioned in the service mods on page 8 paragraph 4 of the service manual?
"4. ....will have a 2.2uF capacitor added across pins 1 & 2 and across pins 5 & 6 of I.C. 103. "
All I can see on the schematic is a 10nF capacitor in those locations ( C164, C165 ). That's between the FET's in the channel switching circuit.
The caps are between the gates of the switching fets so they probably make it less prone to false triggering. 2.2uF seems like overkill, and now (on my layout at least) you need another pack of of the more expensive caps.

The pre and power amps use 20x2.2uF between them and the ones I used come in bags of 20 (or 100, or a box of a some number of thousand)

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