Tuesday, 2 October 2018

Fender Bassman 5F6 Preamp

Alright ladies and gentlemen I finally had some time on my hands to relax and kick out a bunch of new layouts and since a lot of you out there have been playing around with those little Class D amps I thought I would be nice to have some more preamps based on amps we all know and love. First off the amp that gave birth to the likes of Marshall, Laney, Vox, and many others. It's the Fender Bassman 5F6. If you are unaware this is the amp that has essentially been copied and modified by every amp company.



With a switch to go between channels


With 2 separate inputs like the amp


With both channels jumpered


41 comments:

  1. If I build the one with the switch, then add another switch to jump over the cut in row D, will that act the same as the both channels jumpered option?

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  2. The middle position of the switch is both channels jumpered.

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  3. @zach These are all VERY cool! Thanks for doing them. Will these work as stand alone pedals if not used with class D amp? Thanks again!

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    1. Definitely. Just it like an overdrive or run it in your effects loop so you bypass your amps preamp, basically making this the preamp to your amp.

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  4. So... you just feed the output to your favorite mono amp board, then out to a nice guitar speaker, or two, or four, and BAM! ?

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  5. is this the wiring for the amp itself or for a preamp pedal that is like the amp? Like could i build a cabinet, add a speaker and use it as an amp?

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    1. This is just the preamp.

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    2. So could I house this in a pedal and use it in place of an amp?

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  6. Can I pair this with a tda7293 power amp module? with fx loop in between?

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  7. Treble pots wired backwards changed it to 100k I also changed the Bass to 500k just wasn't much adjustability there, then again I am using cheap pots.

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  8. Which class D amps are you talking about, was looking for something as a practice amp with pedals like those

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  9. Why would the switchable channel DPDT be On-On-On and not just On-On? Is it not just one channel or the other?

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    1. You could blend both channels in the middle position of the DPDT.

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    2. Ah that is cool! With the wires going to multiple pots, I guess I can just run a jumper between the different pot lugs, eg Treble 1 and Bass 2 and 3.

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    3. Has someone verified this build yet? :)

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  10. What about the trimmer settings? What am I aiming for?
    I also don't really understand the switch wiring, but that could be my inexperience.

    thanks


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  11. I've build the board on the first picture with channel switch. There are three problems with this preamp:
    1. The Treble control is not working on my build. When I turn treble to max (to "treble 3") then I loose much output signal and the sound does not really seems to have more treble frequency. I've already checked for short circuits and also compared to schematics I have found on the internet. I do not find the error.
    2. The general output of the preamp is very low. You have to set level to max and gain (bright/normal) to 3/4 to achive the same output as in bypass mode.
    3. As I understand the schematic the 100pf should be located between "Gain Normal 2" and "3" but it isn't.

    Maybe someone could help me with this problems.

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    1. I've found a solution:
      The main problem was, that I had connected it directly to a passive DI with speaker emulation. Because of the construction of the circuit this is a hughe problem. The circuit is build to go directly from tone control to output and in that case the input impedance of the next stage affects the tone control.

      If you want to build this preamp I would recommend to add a buffer to the circuit or, even better, a booster if you want to run it without distortion.

      I've seen that Zach has modified the bright and normal channel so my point 3. isn't relevant.

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    2. perhaps a dumb question but should I put the booster before or after the preamp? My guess, after, but I'd like to be sure.
      BTW, what are your trim settings?

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    3. Yes, after the preamp. If your booster has a fixed boost you can connect the input of the booster to Treble pot lug 2 and the output of the booster then to the Volume pot lug 3.

      Voltage have to be set to 4.5V

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    4. aha, great,
      That 100pF still has to be moved? or is the layout correct?

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    5. The layout is correct, he has just modified the circuit (I don't know why exactly but you can leave it as it is). I've now set a fixed booster afterwards. Now the padal works brilliant :)

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  12. The layout is correct, he has just modified the circuit (I don't know why exactly but you can leave it as it is). I've now set a fixed booster afterwards. Now the padal works brilliant :)

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    1. Wat booster did you user? I was thinking about the BBE boosta Grande.

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  13. I've build the booster/buffer a long time ago and do not have the source anymore. I think it was based on the schematic on page 19 https://www.tonefiend.com//wp-content/uploads/DIY%20Club%20Project%203%20v02.pdf.

    The BBE boosta Grande should work fine I think. Any linear buffer will work.

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    1. so would you call it verified? or is that booster part mandatory? perhaps it can be added on the layout?

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  14. I have to clear up some miss information that's been posted recently. There is no need for a booster after this circuit. If built properly and used properly it will work as intended. Keep in mind, this is a conversion of a the tube preamp to use FETs. You need to make sure the FETs are biased correctly. The interaction with the DI mentioned above seems really odd to me, because people have used the other preamps that are similar without issue. This leads me to believe that either the DI used has a specific input impedance or there's something wrong with the build. I've uploaded corrected layouts with the suggested FET voltage biases and a different value bias trimmers that will be less finicky to allow for easier bias. Also, I marked it verified because I've built the other preamp layouts which are identical aside from value differences due to being from different amps.

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    1. alright cool! thanks Zach.
      I recently bought myself a tiny mono power amp board with the TPA3118 and i intend to build this preamp soon to go along with that power amp. Since you mentioning working with class D amps would you say output impedance is sufficient? would it work?

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    2. You’ll have no issues. Many of us have done the same thing with mono power amp boards like that. Two things to keep in mind. To get the most out of the power amp you want to run it at 24V. Second the speaker impedance will affect the output. If I’m not mistaken, those boards have max output wattage with 2ohm speaker impedance, so anything more than that will have lower output.

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    3. thats nice! id love to see some demos. alright i imagined that about the voltage as initialy i was thinking about 12v battery. perhaps with a charge pump just for the power amp board up to 24v would do the trick?
      If thats the case it will be really hard to find a proper 2ohm guitar speaker. any ideas?
      i bought it on ebay and there it's specified as:
      Applicable Speaker Impedance: 4-8 ohm
      but then perhaps this is all wrong. i'll have to read the datasheet of the TPA3118.
      Thanks for the hints and tips thought :)

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  15. Then the max output would be at 4ohm. I couldn’t remember if it was 2ohm or 4ohm. I would go with what is stated in the auction. As for getting 24V, you can do a charge pump, I posted on here that will take 9V and bump it to 24V, or you can use a 24V power supply. I would go for a 24V power supply because you need enough amperage to power everything, preamp and amp. I don’t have any demos of a build with this preamp, but I posted on of the Mini Sunn Model T I build.

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    1. What would you say is the average requested amperage for such combo?
      Ill take a look on your charge pump and sunn model t build.
      Again thank you so much for all the work you'r providing us!
      Cheers!

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    2. I didn’t use a charge pump in mine. I have a dedicated 24V powering the board. I run 6A in, which is more than enough.

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  16. Hello!!
    I am interested in a circuit for a blues junior tank reverb and i am thinking in a 6G15 but instead valves transistors. I have not find any information about it. Is there any reference outhere to do this? Thank you

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  17. still issues:

    I'm experiencing the same volume and tone issues as mentioned above.
    I run a passive bass thru the preamp going to a home build practice amp with a Kemo mono amp module, speaker is 8 Ohm Jensen

    https://www.kemo-electronic.de/en/Light-Sound/Amplifier-Splitter/Modules/M031N-Amplifier-3-5-W-universal.php

    volume is really low and almost no tone control.

    when I put a Palmer pocket amp (bypassed) in between the preamp and the power amp, I do get more volume but still no tone-control.

    I've check the voltages on the transistors and those are exactly 6.5V on Q1 and Q2. 5V on Q3, 9V at Q4.

    Outboard wiring is correct and the board is checked for bridges and faults.

    Any idea?
    The booster buffer-story is false?

    greetings

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    1. You have some sort of build error if the tone controls aren’t working. I looked at the link you posted for the amp you’re using. The amp is a 3.5watt amp at 4ohm and given 12V and probably half that if you’re running it through an 8ohm speaker, so you’re not going to get a lot of output.

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    2. I know I won't have a lot of output but without the preamp it sounds louder, so something is wrong.
      I also used 22nF caps for the 20nF and 25nF's since I didn't had them in stock but I don't think that would cause problems.

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  18. The gain issue seems to be solved, a cap was misplaced.
    The tone-problem remains. When turning the treble clockwise, volume drops. Lows and mids do nothing.

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    1. Go through the whole layout. You have another build error. The tonestack will work properly if it’s built correctly.

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  19. i think the 100pf capacitor of the bright channel should be across the 2 and 3 legs (according to the schematic), in that way it's just in parallel with the 250k resistor getting of the leg 2.

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