Switching between filter caps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Nolatone Ampworks, Aug 31, 2008.

  1. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Hey folks,

    I've been kicking around this idea of having two different values of filter caps for the OTCT and Screen supplies, and switching between them via the grounds.

    I'm trying to think of the danger of doing this and it's not obvious to me there Is any. There's only one cap charging at a time, so tube recitfier current limitations shouldn't be an issue.

    I'm not clearly visualizing what will happen if something like this is switched with the amp out of standby (B+ applied to circuit). I imagine it would create a sag in volume while the new caps charged. It would be intended to be switched in standby anyway.

    Anyhow, anyone ever tried this? Any caveats to consider?

    Thanks,

    Paul
     
  2. phsyconoodler

    phsyconoodler Member

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    I guess my question is for what purpose?If it's to get more or less bottom end,then there are lots of other things you can do to get that.
    What are you hoping to gain by doing this?
     
  3. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    One thing I've noticed in experimenting is that larger filter caps in the first two stages change the *feel* of the amp and the consistency of the bass. Larger caps give a "firmer" feel along with the tigher bass IME.

    Ever feel a Maz Jr? It's got 80uf filter caps! That's a big part of that firmness that amp has.

    My model has a great Marshall crunch thanks to the Radio Spares OT, but feel'wise it's more bouncy Fender, which I love, but I'd like to give the player the option to firm up the feel if they want more of a Marshall type feel.

    I figure switching the first two 30 uf caps with 50's would make a noticable difference. I know going from 22's to 30's did.
     
  4. stratman_el84

    stratman_el84 Member

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    Hi Paul

    I think the major thing to decide is *how* you're accomplishing the switching.

    A DPDT toggle might work, but then there's the players' safety factor along with contact arcing, long, noisy, uneven switching times, and more. This can be abated somewhat by spike and noise reducing/bypassing capacitors and other methods of switch "softening".

    A relay solves the player-safety factor, and the slow/uneven switching times to some extent, as well as shortening PS wiring. However, the problems of contact-arcing and noise still remain.

    There are various electronic switching circuits that could be used, but then you have the tube amp cork-sniffers going "eww". It's not like it's in the signal path anyways.

    If you've got no problem with using MOSFETs or SCR's or HV switching diodes, then you do manage to solve most of the design problems associated with electromechanical and hand switching. Components can be chosen to match voltage and current, as well as to match the desired switching times and characteristics.

    The components to accomplish something like that are getting cheaper and cheaper, which is why most "big" amp makers have started incorporating more solid-state components into their main lines. It seemed the major tube-amp makers had dropped almost everything solid-state in their mid- and hi-end amps except for rectifiers and footswitch LEDs for may years after the horrible "Solid-State '70s" era when fully solid-state and hybrid amps were attempted to be marketed and mostly fell flatter than Vista.

    I'm sure with increased competition from China and other foreign imports, the large US and UK makers will be under more and more pressure to cut costs, so I'd expect to see more. Solid-state just has too much appeal from a manufacturers' standpoint, as in one move they've managed to decrease both parts and labor costs, increase warranty-period survivability percentages, and offer more/cheaper features. Downsides are the same as with much solid-state circuitry, such as typically not receiving any clues that it's getting old/worn/damaged/abused and ready to blow until it does.

    But, that's usually after the warranty period, so certainly not a large design factor for the bean-counters at the major manufacturers.

    Cheers!

    Strat
     
  5. PaulC

    PaulC Member

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    I did something like that with the Heritage Colonial. The "punch/sag" switch changed transformer B+ windings,rectifiers,filter caps,bias supply, and pwr supply dropping resistors. The goal was to get as close as I could to a real JTM45 type of supply, and then be able to switch over to a more modern metal face type of supply. What's a drag though is heritage got a patent on that, but since I quit Heritage I can't use what I came up with:BITCH

    Later, PaulC
    myspace.com/paulcauido
     
  6. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Hey Jade,

    That does sound cleaner to add a parallel cap instead of switching between two. Cheaper too!

    And to clarify stratman_el84's suggesting about user safety, like Jade stated, switching ground sides shouldn't pose any high voltage risk since the lead going to the.

    I'll let you guys know how it works.

    BTW Jade, your Twister clips sound smokin' :dude
     
  7. phsyconoodler

    phsyconoodler Member

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    It seems to me that a switch between diode and tube rectification would get more result than switching filter caps.
    Maybe even preamp cathode bypass caps and mid-boosts.I am thinking that switching filter caps has not enough effect on tone to make it worth the effort.
    Then there's a switch between cathode bias and fixed bias,a low power switch,etc,etc.....All have more effect than switching filter caps.
    As far as 'sag' is concerned,adding a switchable sag resistor is much more effective than larger or smaller filter caps.
    I am not thrilled with the sound of the DR Z MAZ Jr anyway.It is really easy to get that feel without an 80 uf filter cap.I really don't think that's the 'secret ingredient' in getting the feel out of that amp.
    I build an 18 watt lite amp that makes a MAZ Jr sound kind of lame.The 'secret' is solid state rectification and higher voltage.I use a 32uf filter cap can.The punch comes from the higher voltage.And the speakers can hugely affect the way an amp sounds.
    If you think it's worth your time to develop this switching device,then go for it.I'm just saying there are many ways to skin a cat.
     
  8. Jackie Treehorn

    Jackie Treehorn Member

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    I made an amp that has a switch for a 33mf cap in parallel with another 33mf for the OT tap. I switched the B+ side and there's no problem flipping it while the amp is on. It's not significantly different than a standby switch, really. I did it more to achieve an underfiltered amp tone...ghosting, intermodulation distortion, etc. It gets slightly softer on the bass and top, too...a little more compressed. I might go 60/20 next time.
     
  9. PaulC

    PaulC Member

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    The Colonial does it (among other pwr supply things), and so does the Vox Custom amp. If I remember right the vox amp changes the value of both the plate and screen supply caps. Bad cat had an amp the switched rects and the main filter bank. There's a few others I've seen.

    As for the ss/rect switch... Can't do it because boogie has a patent on it, and they've gone after several builders (some big names). Also there's a patent on the fixed/cathode bias switch. Patent laws allow you to make one for yourself, but you can't sell it, or go into production.

    But there's big feel changes that can be found here, and imho it's a good spot to dink with.

    Later, PaulC
    myspace.com/paulcaudio
     
  10. phsyconoodler

    phsyconoodler Member

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    I don't think it would be a problem at all to do.If you look at most amps that have a standby switch,what is switched out?The recitified voltage from the rectifier to the filter caps,right?What difference would it make if when you turn that standby switch back on whether it went between a 32uf or an 80 or even 100uf cap? This is a totally doable mod and if guys think it will make a difference in feel,I say go for it.
    A DPDT switch is all you would need.One side goes to the small caps and the other goes to the bigger caps.Center is standby.Just one wire to prevent the smaller caps from staying in the circuit when the switch is thrown.
     
  11. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Strangely enough, it doesn't seem to be making a difference. You may be right psyconoodler!

    I'm going to need to experiment a bit. Might need to try it switching a choke in and out. I don't have a choke on this design.
     
  12. phsyconoodler

    phsyconoodler Member

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    One thing you have to realize is that every amp does not benefit from extra filtering.And a choke is another thing that may not be a plus either.I build a killer sounding Overdrive Rocket amp that is 50 watts,fixed bias and has no choke.The tone is superb without it.I only notice a smoothing of the tone when the amp is cranked with a choke.
    This is a *umble clone amp.
    I would suggest you concentrate on other areas for features in your amp.When a mod or feature doesn't make much difference,it should be abandoned and your time spent on areas that do make a difference and would be something your customers would really appreciate.
    Sometimes the basic tone of an amp is all that it needs,and the 'swiss army knife' approach only takes away from a good thing.
     
  13. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    This amp has plenty of sag. I'm trying to add a switchable "firm" feature.

    A sag resistor isn't going to get me that, unless I'm misunderstanding something.
     
  14. phsyconoodler

    phsyconoodler Member

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    Firm is usually associated with higher voltage.A diode rectifier will give you a firmer feel.
    If your amp has lots of sag now,you have underdesigned your power supply.Are you thinking about what 'sag' really is?When you crank a Tweed Deluxe for example,the volume doesn't really increase,but the sound gets squishy and compresses a lot.
    I build another amp that has a cathodyn phase inverter,no choke and a pair of 6L6's.It is punchy and dynamic all the way up and only gives it up at 3/4 on the dial.No sag happening here.
    Is that the thing you are looking for?
    A larger OT can give you some oomph,as can a stiffer power supply.
    I use diode rectifiers whenever possible and add a 120 ohm 25 watt sag resistor that simulates a sagging tube rectifier when switched in.You get exactly the same feel and don't get stuck with crossing the 'dual rectifier' patent line.
    I think your amp is real cool looking.Don't ruin it by 'overdesigning' it.Simple can be best I say.
     
  15. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    It's possible I'm using a bogus point of reference for pushing this idea.

    I came up with the idea Saturday when I was cruising around town with the 22 Tango demoing it.

    I had it lined up next to a Germino Club 40 2x12 combo and a Bogner Duende 1x12 combo and that's when I became aware of the big difference between the feel of my amp and a "proper" Marshall, which I consider Germino amps to be!

    There are lot's of other factors as to why that amp would feel more firm than mine, true, but that's what got me to thinking about this (the Duende didn't feel any firmer, BTW).

    The feel is based on intentional design decisions. I intentionally don't use large filter caps or a screen supply choke because I want a bit of that Deluxe type feel. It's completely different than a 5E3 though, but it has a bit of the squish. I doubht it's a squishy as I've probably led some folks reading this thread to believe!

    I'd just thought it'd be cool to have that extra firmness on tap if asked for.

    I may just try your SS rectifier/sag resistor thing. Very cool of you to share.

    Wonder how Germino gets around that patent? The club 40 has both solid state and tube and is switchable.
     
  16. phsyconoodler

    phsyconoodler Member

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    Not a big enough production?I guess Messy Booger didn't want to go after Germino.
    Or he doesn't advertize it.That may be enough.
    As far as patents go,messy has patents on everything,even stuff that has been around since the 40's.It's all in what you can get away with I guess.
    He has some very inovative ideas but I am not any closer to wanting any of them for my amps.Any good amp tech can open up one of my amps and figure out how to fix it.Try that with a booger in the boonies.good luck!That's probably just the way he wants it too.
     
  17. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Maybe that's it. I guess if you get MB coming after for patent infringement, you've "arrived" :messedup
     
  18. vibroverbus

    vibroverbus Member

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    That's exactly how I rig a switchable 'sag' circuit... standby in the middle and 'firm' and 'sag' options on either side. Very clean wiring vs. having standby and sag resistor switches in the PS, in fact I don't understand why anybody goes with a two-switch setup...
     
  19. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    That's a great extension to this idea.

    I think people use standby switches though so they don't have B+ coursing through the circuit when not in use. Plus, if you use a solid state rectifier, you need a delay after power up so the filaments have time to warm up.
     
  20. Jackie Treehorn

    Jackie Treehorn Member

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    You could try the "Torres Diode Trick" of adding a 1n4007 in series with the B+ after the screens which has the effect of reducing sag in the preamp.
     

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