Champ 600 MV

Discussion in 'Shock Brother's DIY Amps' started by bad_rudy, Nov 30, 2019.

  1. bad_rudy

    bad_rudy TDPRI Member

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    Hi All,
    Quick question regarding an issue with installing this. My father is in town and wanted some mods on his Fender Champ 600. I disconnected the pseudo tone stack, disconnected the nfb, and replaced the weird speaker cloth with something more acoustically transparent. Dramatic improvements with those alone. However, the master volume I installed (250k pot where “R4” or the 220k resistor was coming off of the 6v6) is behaving strangely. At minimum, it works as expected - no output. Turning it up slowly works as expected - the amp gets increasingly loud. At what should be max output, however, it suddenly gets quiet again and then silence. So, the output is cut off at both sides of the dial. Any thoughts?
     
  2. LowCaster

    LowCaster Tele-Holic

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    Sorry, not ansering to your question, but for mine I chose to use a simple attenuator (« L-Pad » 100w 8ohm) instead of a master, it costs 20 buck but it is modular and simple and works well.

    I made some of those tone mods too. The most usefull improvement however was to replace the two crappy input jack sockets (and pcb) with switchcraft ones. You might want to wait that it doesn’t work before fixing it, but... it won’t be long.

    Nice little amp, have fun!
     
  3. bad_rudy

    bad_rudy TDPRI Member

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    My personal 5f1 also uses an lpad attenuator that I am fairly happy with. For this one the taps are 4 ohm and I couldn’t find a 4 ohm lpad so we opted for the MV. Wasn’t sure the implications of putting an 8 ohm lpad between a 4 ohm OT and 4 ohm speaker.
     
  4. LowCaster

    LowCaster Tele-Holic

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    Good question too, I forgot about this. All I know is that it works fine since two years. I used the amp with a 12 » 8ohm speaker too and it worked fine.
     
  5. dan40

    dan40 Tele-Afflicted

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    Do you have one leg of the MV pot properly grounded? Can you post some pics of the install?
     
  6. bad_rudy

    bad_rudy TDPRI Member

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    Fixed the particularly weird no volume at either end of the pot issue. Just required some troubleshooting with multimeter and a quick rewire.

    Now perhaps a more common problem? With a 250k audio pot, this thing is basically an “on/off” switch for getting some distortion at bedroom levels. Ie. The bottom has to be just a slight bit above it’s minimum to effectively bring the volume down, and it gets crazy loud again beyond that. Would a linear pot help with this?
     
  7. dan40

    dan40 Tele-Afflicted

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    You can get different tapers in audio pots (10%, 15%, 30% etc..) which may help. Linear usually makes the issue worse and bunches everything up on the low end of the pot. 10% audio would likely be your best bet in this case. The PEC audio pots are a bit expensive but they do seem to have one of the smoothest tapers. The problem with achieving low volume overdrive in this amp is the lack of gain stages. All of the overdrive is coming from the power tube in this circuit and by adding a MV, you effectively cut the signal voltage down before it reaches the power tube. With only one 12ax7 in the preamp, you only have two gain stages to work with and they will likely not be enough to produce any preamp overdrive. Most MV amps are set up with three or more gain stages before the MV to help achieve the overdriven sound from the preamp section while the power amp section is barely working. The idea of the attenuator is a good option with this amp because it allows you to push the power amp into overdrive and then cutting the volume before it reaches the speaker.
     
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  8. otterhound

    otterhound Poster Extraordinaire

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    My Champ came with a 6L6 in it . I learned this only after a planned replacement with a used 6V6 .
     
  9. bad_rudy

    bad_rudy TDPRI Member

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    So far he is fairly happy with the overdriven tone using the MV. The only problem is as I mentioned it’s extremely sensitive and goes from reasonable bedroom levels to quite loud very quickly. Seems like maybe the 10% taper is the solution. Attenuator was preferred, but as I mentioned it wasnt clear to me the implications of putting a 8 ohm Lpad between and 4 ohm amp and speaker.

    thanks for replies so far.
     
  10. dan40

    dan40 Tele-Afflicted

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    Most run of the mill audio pots will be 10% already. You normally have to hunt down the other tapers when they are needed. The PEC pots do seem to have one of the best tapers for volume controls but they are a bit pricey.

    https://www.tubesandmore.com/products/potentiometer-precision-electronics-audio-slotted-shaft

    Rob's quarter power or 10% power switch mods might be a useful addition along with the master volume. Even when the master volume is only allowing 1 watt of output, the amp will still be very loud as you have found. Rob's mod will bring the final power down under 1 watt which can then be fine tuned with the master volume. Added bonus of getting some power amp overdrive in the final tone also.

    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#10%_Power_Switch
     
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