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Thread: Safe to put V1a coupling capacitor on a switch for variety?

  1. #36
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    Cool

    Like this?amazing-switch.png

    Man, I need to quit music altogether and go into graphic design. A bit redundant using a DPDT, but I have some of those. I guess I should order more switches.
    Last edited by Mr. Bill; 01-07-2018 at 07:48 PM.

  2. #37
    Supporting Member Chuck H's Avatar
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    Too complicated. Think simpler

    EDIT: Imagine the amp is built on an eyelet board (as I assume it will be?). You just have your parallel components in place on the board and run two leads from either end to the switch as shown.

    And more to keep in mind. When adding leads in the signal chain that fly from the board to the panel you'll want to design your layout so that these leads can be kept short and away from like other phase circuits if possible. If you get too jiggy with switchable doodads and don't pay careful attention to lead dress you can end up with an unstable amp very quickly.
    Attached Thumbnails Attached Thumbnails smpl1.png  
    Last edited by Chuck H; 01-07-2018 at 08:08 PM.
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  3. #38
    Supporting Member Chuck H's Avatar
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    And... You mentioned having the three way switch on hand.?. If it is a three way toggle you'll want to be certain it's an on/on/on since those are sort of hard to find. Most three way toggles are on/both/on or on/off/on and would require more circuit consideration for wiring.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    I just found an SPST in the drawer, so that simplifies things and is less waste. I'm on mouser right now ordering some more along with some rotary switches. This is going to be one crowded chasis for sure. I think at this point I'm going to build it in a 4 hole box and eliminate the low sensitivity jack to make room. That'll give me 1 Jack and 3 rotaries in the 4 hole and I'll drill for a few mini toggles.

  5. #40
    Supporting Member Chuck H's Avatar
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    Like I mentioned above. It's a dangerous business running control leads back and forth across the circuit to the front panel. Controls should occur on the panel as they occur in the circuit as much as possible and layout should be designed to keep leads as short as practical. This is less critical in non cascade type vintage amps, but the 800 type circuit IS a cascade preamp AND you have a hotter cathode resistor in V2 which increases gain. Too much spaghetti and your amp will whistle like it forgot the words.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    I absolutely hear you on that one. If I eliminate the low sensitivity jack that I've never seen used anyway, then there is zero extra wire length to run the first two switchable mods. Those wires are already running to the face plate anyway. The depth switch, likewise can be right next to the Z selector needing no extra wire length or routing.
    coupler-switch.png

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    Quick silly question, maybe? Doesn't the 2203 cold clipping stage clip the negative lobe of the waveform? I'm Working in LT Spice as suggested by Chuck, and it seems to be clipping the positive lobe in my simulation. I guess dumb question #2 is....I'm supposed to probe the plates to see the output waveform of each stage, correct?

  8. #43
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Mr. Bill View Post
    Quick silly question, maybe? Doesn't the 2203 cold clipping stage clip the negative lobe of the waveform? I'm Working in LT Spice as suggested by Chuck, and it seems to be clipping the positive lobe in my simulation. I guess dumb question #2 is....I'm supposed to probe the plates to see the output waveform of each stage, correct?
    The plate of the triode will be the "output" of the transfer function, so yes, probe the plate to see the effect the stage has on the signal. Keep in mind that each triode stage is inverting, so the plate signal appears with the effect of cutoff on the positive half of the waveform. I think you're doing it right.
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    If it still won't get loud enough, it's probably broken.
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  9. #44
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    If it's cold biased the bias current is lower so the plate voltage is closer to B+ voltage and it will clip on the positive lobe when you probe the plate.

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    Thanks. And if I may add, is there a way to tell the program to keep the heaters warm so you don't have to wait a long time for it to run each simulation?

  11. #46
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    You could use tube models without heaters. I think the Duncan modals say 'NH'
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  12. #47
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    Thanks Dave. Supplying direct voltages for each plate seems to quicken the response time as well. Does this look correct for V1a and V1b plates plus tone stack output? That seems like a lot of signal loss to me, but I've never put an amp on a real scope before. Again, pardon my greenness.1k-test-wave-fixed.png

    edit: Caught my first mistake. Still tone stack question.
    Last edited by Mr. Bill; 01-09-2018 at 10:14 PM.

  13. #48
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    That looks about right to me for three gain stages (Marshall 2204?) The tone stack can have 20dB loss at 1kHz depending how the controls are set. 20dB is for a 100k slope resistor. It will be less than that if it's 33k.
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    I hope it's okay to continue to derail my own thread, but since Chuck got me on this spice thing, I have another question. Does this seem like a reasonable way to simulate a 1M log gain pot set around noon?
    half-turn-pot.png
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  15. #50
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    Looks OK to me. It's what I do.
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  16. #51
    Supporting Member Chuck H's Avatar
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    That's what I do too. But since I use the cheap Alpha pots I go 150k/850k (they're 15% @ half rotation). The only 10% log pots I know of right now are the Bourns "guitar" pots. If you buy them you have to be sure and check the torque specs. They offer an appropriate torque for amps, but you need to choose it. Otherwise your volume knob will be loosey goosey. I like the 10% pots for the bass control on BF type topography amps. It feels and "looks" more intuitive when adjusted than the typical CTS 25% pots.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    I didn't know that about Alpha pots, thanks for the info. Is there any accuracy problem in supplying direct voltage to each plate so you don't have to get the entire rail correct when messing with preamp values? That's what I've been doing so far.

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    So earlier in the thread I mentioned the possibility of making the plate resistor on V1a switchable. Is this not advisable? Again, this is just a prototype that will never leave the room, so it would only be switched while off or on standby. If it's safe to do so, what rating switch would I need? I'm just trying to save myself from having to solder and unsolder parts in while I experiment a little.

  19. #54
    Supporting Member Chuck H's Avatar
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    A switchable plate resistor is something I wouldn't even do on a personal amp. It is unavoidable to put very high voltage on a switch for that. Any switch failure could result in an HV short to the chassis (resulting in analogous part failures) or worse, if the switch has a metal insulated actuator you could have HV on your HAND while holding your guitar (ground). ZAPPPP!!! Nevermind that the amp "would be off or on standby" whenever the switch is flipped. Danger lurking at a misstep! Not going to build it. Find something else to covet
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "Shut up, you big dumb poopy-head!" Justin Thomas

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    Loud and clear! I'll just tack those in after discharging while experimenting. I have liked messing with this value in the past, so I'm sure I will do it. I'm working on my layout right now and getting that nice and thorough. I'll run it by you guys before I actually begin the build and see if you catch any other mistakes.

    I have ran into yet another question that I can't seem to find the answer to, so I'll just keep bugging you guys. If I add a gain stage and use the 2 jacks, one going to the first gain stage and passing back through the second jack which will give me the stock input....Will I cause some problem or get some type of bleed from the "out of circuit" first stage when using the second jack?

  21. #56
    Lifetime Member Enzo's Avatar
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    There are already commercial amps that do exactly that.
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    Education is what you're left with after you have forgotten what you have learned.

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    This is a work in progress and is missing all values at this point. Will I be okay with the cathode value switches on the rear panel where the run is super short? Does anyone feel the need to slap me yet?

    layout-parts-rough.png

  23. #58
    Lifetime Member Enzo's Avatar
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    Sure, I will slap you now... I for one would REALLY like a schematic rather than a layout drawing. The schematic shows the circuit and electrical relations, while the layout shows where things sit in the chassis.

    Just my opinion, but I think you will find things like cathode resistor values to be a circuit tweak, rather than a feature. In other words a little time spent experimenting will find you the value you like best, and just install that. You can wire up switches, sure, but in most cases, you will find one setting you like and never change it again. With shielding and lead dress, you can probably get away with a lot of this stuff, but every switch and wire run you add is just another opportunity for the amp to act weird.

    A friend once had me modify his amp to be switchable between fixed bias and cathode bias. I did. For a week or so he tried switching back and forth, then decided he liked one way best, and never switched it again.
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  24. #59
    Supporting Member Chuck H's Avatar
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    Well in this case the cathode switch changes the amp from a cold clipper ala standard 800 to a hot rod type preamp mod. So it WOULD be an instant gratification switch and not such a subtle tweak.

    And now I have to own up to something I said that, in this case, was misleading. That is the cathode circuit impedance is low enough that you actually CAN get away with longer leads there. So you actually could put that switch on the front of the amp if you like. You still want to practice good lead dress and just sort of keep the leads in their own lane. My experience with back panel switches for preamp voicing is that it's a PITA and a bad idea. One design I make has the "boost" switch on the back for the purpose of lead dress. This is also the model I own and use. One of my customers is always forgetting the switch is there. He's called me to ask why the amp lost gain (when he leaves it unboosted) or why he can't get a clean tone (when he leaves it boosted). I set myself up in a chair to play for awhile, with amp controls just in reach, and now I have to stand up whenever I want to flick the boost switch. In fact I'm redesigning the circuit to eliminate these inconveniences. My Mouser order should be here in a few days.
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    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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  25. #60
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    Quote Originally Posted by Chuck H View Post
    And now I have to own up to something I said that, in this case, was misleading. That is the cathode circuit impedance is low enough that you actually CAN get away with longer leads there. So you actually could put that switch on the front of the amp if you like.
    I've actually put the switch on the floor with a low gain amp (18W type) and it worked fine. There's a jack socket on the rear panel and I used a 10ft guitar lead to a metal box footswitch. Inside the footswitch there was also a LED and 9V battery connected to the other pole of the switch so you could see when it was in boost mode.

    The amp also has a "Contour" switch on the rear panel because I didn't want to drill a hole in the front. You're right, what a PITA it is.
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  26. #61
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Dave H View Post
    I've actually put the switch on the floor with a low gain amp (18W type) and it worked fine. There's a jack socket on the rear panel and I used a 10ft guitar lead to a metal box footswitch. Inside the footswitch there was also a LED and 9V battery connected to the other pole of the switch so you could see when it was in boost mode.
    Absolutely! In fact I have a mid boost design that does what you propose. I haven't used it yet because I haven't run across a situation where it could be put seamlessly into a design. But it's there on the shelf and ready when I want it.

    I'm starting a new thread this morning in the mods/tweaks forum that proposes something similar for the reverb switch on ab763 type circuits to reduce noise.

    So yeah, cathodes can handle some lead length and can be forgiving with lead routing.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "Shut up, you big dumb poopy-head!" Justin Thomas

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    Nice Chuck!

    Here's a rough schematic without power as of yet. I know I could do the cathode switches more correctly to make them functional at all times, and I'm well aware this is overkill. I do want to clarify 2 things though. This is a tinkering amp that will be used in a recording studio by many guitar players. It will never leave my room. I plan to use it to further hear combinations on the classic circuits that I like or don't like with input from other musicians. I am aware that I will have to switch some of these with the amp off or on standby and that it could be laid out better......but, these are the parts I have on hand and the wife just may murder me in my sleep if she sees me on Mouser's website again within the next few weeks.

    The only thing missing here is the NFB cap switch, which I've done before and is no biggie.

    Also, putting these on the back panel let's me add a gain stage without drilling or making a new board. This thing is so tacky already I might just paint it red with white chicken head knobs, ha!

    overkill-rough-schematic.png

  28. #63
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    Quote Originally Posted by Mr. Bill View Post
    So earlier in the thread I mentioned the possibility of making the plate resistor on V1a switchable. Is this not advisable? Again, this is just a prototype that will never leave the room, so it would only be switched while off or on standby. If it's safe to do so, what rating switch would I need? I'm just trying to save myself from having to solder and unsolder parts in while I experiment a little.
    If you must do it I'd use a plastic rotary switch like the one below. A safe way to switch between say a 100k and 220k plate resistor would be to connect a 100k and 120k in series. Connect the free end of the 100k to the B+ node, connect the free end of the 120k to the plate and put the switch across the 120k. Make the 100k 1W or more and it won't matter if the switch shorts to chassis as the current will be limited to a few mA by the 100k.

    rotary-switch.jpg
    Last edited by Dave H; 01-16-2018 at 04:05 PM.
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    Dave, I think I've decided against that at this point. Plus I'd have to drill for it and I just want to get started. Pretty easy to just tack that in and do a comparison and pick one.

  30. #65
    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Dave H View Post
    If you must do it I'd use a plastic rotary switch like the one below. A safe way to switch between say a 100k and 220k plate resistor would be to connect a 100k and 120k in series. Connect the free end of the 100k to the B+ node, connect the free end of the 120k to the plate and put the switch across the 120k. Make the 100k 1W or more and it won't matter if the switch shorts to chassis as the current will be limited to a few mA by the 100k.]
    Smart and eloquent. I like it
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

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    Here's the current progress on the actual chassis layout. I'm not sure about the voltage divider after the first stage, might want to put that on a pot. Also undecided on a third value for stage 2. I don't want to just leave it open, so I'm open to ideas.

    Also, is the only thing I have to do to make the cathode switches functional live is strap a high value resistor from input to ground?

    Anybody see anything here that could get me into trouble? Thanks for everything.

    layout-parts-rough-values2.png

    Current schematic a few posts up.
    Last edited by Mr. Bill; 01-16-2018 at 07:14 PM.

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    Would someone mind verifying for me that this is the correct wiring for the cliff jacks to do what I want them to do? I would like for the first jack to hit the extra gain stage I'm adding, and the second jack to be The stock 2203 "high" input for the rest of the amp. Thanks again and again.
    cliff-jacks-dual-high.png

  33. #68
    Supporting Member Chuck H's Avatar
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    Big kudos. That looks like it should work, assuming...

    That the switch arms are from the left (duh, but just being sure) and that you have a proper circuit (coupling cap and hopefully a voltage divider/pot) between stages one and two.

    I don't know if you just worked that out for yourself or if you got it from the web, but that arrangement looks like it even grounds the front stage output when the stock amp jack is used. Very nice. One stage in front won't always bleed, but sometimes it can depending on a few things. That extra insurance grounding it is a lot better than letting it float.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

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    Thanks Chuck. I looked at the original 2203 layout and then looked at the cliff jack spec sheet to understand how they work better. Then I modified that to give myself 2 high Independence inputs that would lift the ground on the second jack with the first jack inserted and ground the extra stage when the second jack was plugged in. I have a new respect for cliff jacks now. Deceivingly complicated, haha.

  35. #70
    Supporting Member Chuck H's Avatar
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    I've had to work out a couple of switch diagrams that nearly popped a vein in my head. Sometimes it helps me to draw the switches in schematic form so I can see the function without having to imagine it. I do this in ink. Then I work in pencil so I can temporarily draw in or erase contact in action for each switch function (or each jack) with the actual traces explicitly shown.
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    "Shut up, you big dumb poopy-head!" Justin Thomas

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