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


Posted by at 18:26
Tags: , ,

49 comments:

  1. Unknown3 October 2018 at 03:10

    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. Zach3 October 2018 at 03:23

    The middle position of the switch is both channels jumpered.

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  3. Unknown5 October 2018 at 22:29

    @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. Zach6 October 2018 at 04:18

      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.

      Delete
  4. Lightcatcher13 October 2018 at 07:01

    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. Unknown22 October 2018 at 20:12

    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. Zach22 October 2018 at 21:04

      This is just the preamp.

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    2. Unknown5 November 2018 at 03:09

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

      Delete
  6. cacopanda12 November 2018 at 20:01

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

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  7. Scotchegz_20 November 2018 at 01:56

    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. snorrevonflake16 December 2018 at 19:59

    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. crunk4krist21 December 2018 at 23:35

    what class d amps?

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  10. /Mix/Masters/31 December 2018 at 12:02

    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. cacopanda8 January 2019 at 16:03

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

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    2. /Mix/Masters/10 January 2019 at 21:42

      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. JHSA2 February 2019 at 01:25

      Has someone verified this build yet? :)

      Delete
  11. Anonymous11 February 2019 at 18:38

    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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  12. Unknown12 February 2019 at 02:52

    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. Unknown12 February 2019 at 11:50

      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.

      Delete
    2. Sevrien12 February 2019 at 11:55

      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?

      Delete
    3. Unknown12 February 2019 at 13:38

      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

      Delete
    4. Sevrien12 February 2019 at 13:48

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

      Delete
    5. Unknown12 February 2019 at 15:14

      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 :)

      Delete
  13. Unknown12 February 2019 at 15:12

    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. Sevrien12 February 2019 at 17:42

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

      Delete
  14. Unknown12 February 2019 at 18:06

    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. Unknown14 February 2019 at 17:26

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

      Delete
  15. Zach21 February 2019 at 01:35

    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. Unknown27 February 2019 at 16:01

      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. Zach27 February 2019 at 20:13

      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. Unknown28 February 2019 at 12:23

      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 :)

      Delete
  16. Zach28 February 2019 at 14:27

    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. Unknown1 March 2019 at 10:11

      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. Zach2 March 2019 at 00:34

      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.

      Delete
  17. potierrez8 April 2019 at 15:48

    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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  18. Sevrien28 August 2019 at 10:21

    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. Zach28 August 2019 at 16:44

      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.

      Delete
    2. Sevrien28 August 2019 at 17:36

      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.

      Delete
  19. Sevrien28 August 2019 at 21:03

    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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    Replies
    1. Zach28 August 2019 at 22:42

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

      Delete
  20. Dennis Krigger9 September 2019 at 03:13

    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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  21. Sevrien10 January 2020 at 17:17

    I got the circuit sort of working.
    But, theres a huge gain difference between both channels. Normal is quite weak, bright has instant overdrive.
    I used 2n5457s instead of j201's. Could that be the problem?

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  22. Sevrien15 January 2020 at 17:58

    This comment has been removed by the author.

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  23. Nemo21 February 2020 at 13:41

    Just to be sure: are the resistors labelled "820r" 820 Ohms?
    Thanks.

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  24. NLPC11 April 2020 at 23:57

    Hello,
    In this layout there are 2 inputs instead of the original 4 (Normal Hi Lo, and bright Hi Lo). The 2 inputs that remain are Hi normal, and bright. In this case both 68k resistors (grid stopper resistors) have to be reduced to 34k (33k).
    In the normal or bright channel, these resistors are 68k, in both Hi and Lo inputs. but the both jacks Hi and Lo, are wired such as when the Hi input is used the signal is passing through both 68K resistors in parallel, thus the resistance is 34k. I found this information here:
    https://robrobinette.com/How_Fender_Input_Jacks_Work.htm

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  25. Unknown12 July 2020 at 05:56

    Salut, j'ai pas accès au schéma (!????!!! 😡)... PLIIIIIISE, Help me...
    Merci, Frank

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  26. Sevrien17 August 2020 at 09:39

    Hello, me once again.

    After rebuilding this circuit for the 3rd time, I still have problems.

    When biasing to the voltages given, it does not work. Dialing in way lower voltages for Q1 and Q2 gives me a good signal.
    However both gain controls seem to affect eachother.
    The gain controls also act more or less like volume controls.
    I'd expect clean tones on lower settings and overdrive at higher settings. I got overdrives on almost all settings, just lower volumes when dialing the gain down.

    Tone control also has issues.
    Bass doesnt do anything
    Mid acts more or less as a volume control.
    High works fine (I guess).

    Has anyone verified this except Zach?
    I've built this for the 3rd time, checked a hundred times and cut the traces for a few dozen times. I'm in the dark now.

    Greetings


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  27. Ian2 August 2022 at 19:53

    Did the middle layout, with two inputs, and it is bang on. Superb little pre-amp

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