Amp builders/designers - please critique my circuit

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by LowRedMoon, Apr 29, 2008.

  1. LowRedMoon

    LowRedMoon Member

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    Greetings, folks.

    I've built many tube-based home stereo amps and preamps in the past 2 years and am getting into building my own instrument amps and preamps.

    I'm working out the final details of what will be my first DIY guitar amp, which those of you who are familiar with amp circuits will recognize as a personalized Frankenstein of SE Fender-copy types.

    What I'm going for here is:
    - Individually cathode biased output tubes with mutually switchable bypass caps, tubes can run as singles or parallel pairs (6V6, 6L6, EL34, etc.)
    - A variety of first gain bypass cap options
    - Input tube options of either 12AX7 or 6SL7 (separate sockets will be installed a la Angela's "Super 6V6"
    - Low ripple, low hum PS with standby switch and bleeders on the first filter cap
    - Master volume via replacing the output tubes' grid leak resistor with a 200K-250K pot wired for 0 -> max signal to ground resistance. The goal is to be able to play either clean and loud or quiet and distorted.

    I'd love to add a switchable extra gain stage but am not yet entirely sure of the best way to plot it out. An effects loop would also be a great addition.

    I urge any suggestions or recommendations regarding this circuit. I'm sourcing the parts and should begin building next week. I'll be building it as a head unit for greater versatility.

    Many thanks,

    Adam

    [​IMG]
     
  2. donnyjaguar

    donnyjaguar Member

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    Well, you asked for it... :)
    Put a 1M5 resistors from the bypass capacitors to ground, otherwise they'll pop when you switch them.

    If you really want to regulate the power supply get rid of the capacitor input section and go with choke input. This way you can make use of the energy storage capability of the choke itself.

    In my opinion there isn't enough gain in the circuit to warrant a master volume. Either add another stage or delete the MV.

    I think EL34 tubes call for 1.5A on the filament. This circuit would exceed 3A once you add the small signal tube into the mix.

    I gotta admit that I wouldn't bother with a circuit like this myself, but if you tell me it sounds brilliant I'd give it a shot. :)
     
  3. VacuumVoodoo

    VacuumVoodoo Member

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    One additional advantage of donnyjaguars suggestion to skip the first PS filter cap is that it will lower the B+ voltage thus making it easier to keep the EL34 etc in strict class A mode.

    Also, tube rectifiers in SE class A give no advantage as there is very little, if any, sag. In your case however they should add some voltage drop which together with choke input PS will prevent your B+ form going to high.
     
  4. LowRedMoon

    LowRedMoon Member

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    Thanks for the replies.

    May I ask why? By all means, if you can recommend a better circuit, I'm all ears.
    My requirements are:
    - pure class A SE - I don't like PP amps, period (my home stereo is even powered with DIY 2A3 single-ended triode amps).
    - No silicon if possible. I prefer the sound of tube rectifiers, whether there is sag or not
    - No EL84's. I have 2 PP EL84 amps already. I want to try something different for a change.
    - Switchable output tubes
    - the ability to play clean or nice and dirty

    As filament rating goes, I originally intended to use only one of the higher power tubes at a time, giving an overall max heater current of about 1.8A. I'll probably end up with a 4A rated tap anyway.

    Where would the resistor connect - on the neg. end of the cap (in series)? Otherwise, it would raise the cathode bias R value.

    I'll try a choke input. Thanks.

    Best,

    Adam
     
  5. BenH

    BenH Member

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    Seems like your trying to make simplistics complex in the form of a test bed.
    Sure, try it all out, plus a whole variety of tone stack options while your at it, see what works ,then strip it down to to minimum again. I'd be tempted to go for the 6SL7 option myself, but then I would go for 807's as power tubes, maybe even parafeed.................sorry, i'm getting carried away again.
     
  6. BLC

    BLC Member

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    I think it's interesting. I myself have played with the 1st stage cathode caps and it is interesting and fun. I also have thought a cathode cap in/out switch for the power tubes, but have not done that. I also plan to play with input jack/ grid stop variations with possibly a switch using the standard 68k/1m and possibly a 10k with say 3.3meg . I might just ask Why? on the 2 5Y3's .......
     
  7. LowRedMoon

    LowRedMoon Member

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    Funny - I actually have 807's and 2.5K & 4K pairs of parafeed transformers in my parts bin.

    With 2 rectifiers, I have a little added control over the B+ voltage range by either using two or pulling one. I expect an 18-22 volt difference between the two combinations. Plus - DUAL RECTIFIERS! I really don't know what I mean by that - sarchasm, mostly. It's just another sound-shaaping option. What sort of this type of amp did you build? What were your results and what modifications did you perform?

    Adam
     
  8. phsyconoodler

    phsyconoodler Member

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    First of all,you have way too much filtering to make this amp sound good for guitar use.It will be too bassy sounding.And two 5y3 rectifiers?Why?If you want two,try two 5AR4's or 5v4's.5Y3's are too lossy for this circuit imho.

    If you must use a tube,one 5AR4 is more than what you need.
    I would use a 40uf first filter cap and three 20uf with decoupling resistors.You can use a choke if you prefer,but chokeless SE amps are raw and sweet sounding.Both work fine.
    I built a single ended amp with two power tube sockets based on a tweed princeton that uses any combination of tubes,or one single KT88 and it is awesome sounding.I put it in a bandmaster three-ten tweed cabinet and it is unmatched for tone.Check the review on harmony central under "clara amps".
    Try and lose the audiophile approach with guitar amps.It doesn't work with guitar amps.They tend to be sterile sounding.
     
  9. BenH

    BenH Member

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  10. phsyconoodler

    phsyconoodler Member

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    807 tubes sound just like 6L6's.They look cooler with the external anode cap,but dangerous!There is no tonal advantage or even a difference,for that matter,from 6L6's.
     
  11. VikingAmps

    VikingAmps Member

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    I would wire the master as a pot instead of a rheostat. I like the 2 recto's, maybe you could switch one out for less voltage/more sag depending on power tube choice, etc. I think the filtering is fine as is, you have 40uf for the power tube plates, 40 for the screens and 16 for the pres. Looks fine to me. You need decent filtering on the power tubes if you're running single ended. You should also have plenty of gain available especially with cathodes bypassed and no feedback. It may be cool to add some tone controls after the last gain stage.
     
  12. mark norwine

    mark norwine Member

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    5 things hit me

    1.) no screen resistors on the power tubes? why not?
    2.) the 68K on the input: If it's there to kill oscillation, put it ON pin 2, otherwise it serves no purpose whatsoever.
    3.) agree with others on the MV...wire it as a pot, not as a rheo.
    4.) agree with others re: rectification.
    5.) why are you bootstrapping the OT's secondary?

    EDIT...

    another observation: In the strictest of conventions, as drawn this amp won't work. Your symbols show a clear differentiation between signal ground & chassis ground. Yet, I see no congruency between the two. What is your employed grounding scheme? How / where do you cojoin your circuits?
     
  13. VikingAmps

    VikingAmps Member

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    I think the 10K going to point B serves as the screen resistor(s). I agree with always putting the grid stopper on the tube socket. I think the resistor across the output may be there to let you hot switch the impedance selector.
     

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