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Reverb/Vibrato standalone - schematic review request

Discussion in 'Shock Brother's DIY Amps' started by alathIN, Jan 8, 2019.

  1. alathIN

    alathIN Tele-Meister

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    STANDALONE REVERB - TREMOLO

    Latest project is nothing really new or original, basically a combination of long-established circuits that have worked, and been modded, for years.

    The primary goal is to come up with the least cost-effective and least effort-efficient pedal board in the history of music. :D
    As a secondary goal, these are the effects I would most like to have for the effects loop of my Trinity TC-15 amplifier (roughly similar to a Vox AC-15).

    Parts:
    1. Fender harmonic vibrato (tremolo). This is the 5-triode version which has been suggested as easier to de-bug and tweak. Also, the design as a whole came out with an even number of triodes.
    2. Fender spring reverb (mostly taken from AB763 deluxe reverb, with some fairly well-known mods. Except the reverb tone control, which I've only seen one other place. I sure do like reverb tone control though. Ideally I'd like it to be super-bright to start with, and be able to dial down the brightness as needed. I have a MOD 3-spring and an Accutronics 2-spring tank both on hand (both long tank)

    Both of the above are lifted from circuits that go post preamp, pre phase inverter - and I'm going to use them in an effects loop (it's buffered if that matters)

    3. Marshall cold clipper tone stack into EF86 preamp - this will take an instrument input, not a signal from the effects loop send. I know it won't sound like a Marshall, but I do think (hope) I'll get a nice asymmetric distortion from this. I love my TC-15 for everything except super high gain tones; this is an effort to fill that gap.

    review copy 1.jpg

    High res version is in an attachment.

    Questions:
    1. I've got some components with values "TBC" - to be calculated. The values of voltage-dropping resistors are tricky because I don't know what value to put in for amps. I did add up all the heater currents for all the tubes - and there are no power tubes - so I think the rest of the draw will be trivial? Also, when the heater current is quoted for a 12AX7, is that for the whole tube, or just one of its component triodes?
    2. I know all these circuits have worked in other applications, but not confident in how I have put them together. Would appreciate any input about how I've connected the dots.
    3. I'm aware of the ground loop issue. My idea for this was to use 1:1 isolator transformers in both places where a signal goes to or from another amp (shouldn't make a difference on the instrument input). I've never known of anyone to do this for guitar amplifiers, but it's an established practice in the audiophile world. I found some 1:1 transformers from eBay that are 136 ohms and 290mH, 800:800 winds. Would also appreciate any input on this. PS, I'm also planning to use insulated jacks wherever ground isolation is required.

    So appreciate the experience and expertise here, and appreciate anywhere you see I am about to put my foot in a bucket!

    PS - I haven't labelled the tubes v1-a, v3-b, etc.
     
    Last edited: Jan 9, 2019
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  2. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Cool project.
     
    Last edited: Jan 9, 2019
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  3. alathIN

    alathIN Tele-Meister

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    Good suggestion, but as far as I can tell we can't edit titles.
     
  4. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    deleted
     
    Last edited: Jan 9, 2019
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  5. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    The heater current is per tube, so you can just add them up.

    The dropping resistors are pretty trivial because the current is so low. If you figure 1mA per triode section as a ballpark number, each 10K will drop 10V.

    Regarding the 1:1 isolation, I would avoid that. It's just added complexity that shouldn't be necessary. Feed the non-audio sections from the first PS filter (A), the reverb from (B) and the preamp from (C) and it should be fine. It's only Rock'n'Roll so don't get too fancy.

    Have fun with it, and post more as you go.
     
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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    One of the kit makers, it might be Weber, has a combined standalone reverb tremolo. Did you look at their design?
     
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  7. alathIN

    alathIN Tele-Meister

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    Yes. A lot of this is borrowed and/or cross-checked with Weber - which of course is very similar to a lot of Fender circuits. You just reminded me where I got the reverb tone control from - it's on the Weber. Hoffman has something similar also. It was listening to people's audio samples of their Weber and Hoffman units that made me want to do this.
    One difference is that the Weber and Hoffman units are designed to go between the instrument and the amp, rather than plug in before the phase inverter like I have in mind.

    One of the trouble spots I was fretting about is where the instrument-level preamp circuit joins up with the vibrato input. Do I need a switch to disconnect the preamp circuit when I'm using the effects loop input (ie, using the 'parent' amp's preamp?).
     
    Last edited: Jan 9, 2019
  8. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Your vibrato input will have to switch between the preamp shown and the FX Send jack. Just an A-B switch. Then you'll have either preamp available.
     
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  9. alathIN

    alathIN Tele-Meister

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    I'm delighted to drop any unnecessary complexity, but I am not 100% following what you are saying here. I am planning to do the power scheme exactly as you say here - but it seems like there's still a possible ground loop between the "parent" amplifier and this effects unit. If both their chassis are grounded, and they're connected by the patch cables between the amp and effects unit, it seems like a recipe for trouble. I've heard of people disconnecting the third prong from their standalone reverb/vibratos to get rid of hum - but I'm not eager to be a Darwin award winner.

    But I'd be delighted to be talked out of this ;-)

    One idea would be to leave space for these little isolators, but don't put them in unless/until I run in to a problem....

    Done.

    Will do! Thanks!

    PS - I think your harmonic vibrato was the source for some of my mods/tweaks. Thanks for sharing!
     

    Attached Files:

  10. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    I see your concern at the input to each preamp. The connection is complicated by using the external preamp. Many amps uses separate preamps in the same chassis, and your situation might not be so bad. I assume you will channel-jump the two preamp inputs and just switch between outputs, correct? The good news is that only one channel is used at a time, so even with a ground loop created by the channel jump, the unused preamp can be grounded.

    I would consider a relay at the Vibe input to switch between preamp outputs (while also grounding the unused output). It will keep the wiring direct and internal. An external box to switch the relay only needs one cable and does not carry audio. Trying to switch between preamps at the Vibe input with a passive A-B switch box on the floor will carry audio on three long cables.
     
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  11. alathIN

    alathIN Tele-Meister

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    It's an odd arrangement, with two different goals.

    1) an effects unit I can use in the effects loop of my amp (using the amp's preamp channels and bypassing the effects unit's preamp). For this use the input is an effects loop send.

    2) a high-gain preamp, that can also be used with these effects, and feed into my amplifier's power stage. For this use, the input is an instrument cable.

    3) either way, the resulting signal goes back into my amplifier's effects loop return.

    And, yes, I am planning to put the switch that selects instrument-preamp input vs. effects loop send input right on the chassis, right between the two input jacks.

    Of all the obstacles and difficulties I may encounter in this project, one I don't have to worry about is making it convenient for gigging. I'm a prodigal guitarist, having taken 32 years off without playing, and fortunately I do have a day job because I would quickly starve as professional musician.
    I do a little bit of playing at church, in which case I just take an inexpensive (and vastly more practical) Zoom G3n and plug directly in to the board.
    So if I have to plug and unplug stuff to switch between modes, that's OK.

    Apropos of foot vs chassis-mounted switches: what's a good way to deal with two footswitches?
    I was kind of thinking of using a stereo 1/4" jack and plug to get two leads, so I can make a dual-duty (vibrato and reverb) footswitch. On both of these circuits, the switch turns off the effect by connecting to ground, so two conductors and a ground should work.
    I may or may not ever get around to actually building the footswitch - but it wouldn't hurt to have the jack there in case I do.
     
  12. ahiddentableau

    ahiddentableau TDPRI Member

    60
    Jul 8, 2018
    Middle of Nowhere
    Looks like a great project. Kudos! I especially love the switchable cold clipper. So don't expe

    I have a question (which could be interpreted as a suggestion, depending on your perspective). I noticed you're using the pentode for makeup gain at the end of the preamp section instead of in the V1/first position. Why did you choose to put it at the end instead of the beginning? Or to put it another way, is this by design or for the sake of convenience (i.e. just trying to keep the heater current down)? I'm probably stating the obvious, but if you want the particular character of the pentode to shine through you're better off using it as V1.
     
  13. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Western Connecticut
    Unless I'm missing something, I think the isolated jacks / elevated circuit that Jeff Gehring documents, and that I used for my Revibe build would quash any ground hum concerns. It's shown on the layout diagram I gave you a while ago.
     
  14. alathIN

    alathIN Tele-Meister

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    Interesting thought. Now that you ask, I did not consider other spots to put the EF86. I knew I'd be adding gain by using an EF86 anywhere, but a less dramatic departure to replace two triodes with a pentode rather than replacing one triode with a pentode. Some possibly misguided instinct told me that it would be better to over-amp a sweetly asymmetric cold clipped signal, rather cold clip an over-amped EF86 signal. That's about how much electrical engineering went into this decision!

    Now that you're making me think this through, my thought is as follows: I already have a strongly EF-86 flavored channel on my "parent" amplifier, so I'd rather have this one be a very different, high gain, cold clipped channel with a hint of EF86 flavor, than have another EF86 channel with a hint of cold clipped high gain flavor. If anything, I'd be tempted to put the original pair of triodes back...
    [addendum - Now I do actually remember thinking it would be an easy re-wire to put the 12ax7 back for V2a/V2b in place of the V2 EF86 if it didn't work out.]

    Not too worried about heater current - my PT is way over-spec'd.
    But I do have a box full of Soviet EF86s to play with. If there's any economizing going on, that's where it's at.

    I was planning to use insulated jacks (the 'hat' insulating washers that go with switchcraft 11s).
    I didn't notice the elevated circuit you refer to - I'll have to go take another look.
     
    Last edited: Jan 9, 2019
  15. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Here's the relevant bit. And for those following along, the entire layout is attached as a PDF. It's the Revibe I built last year, and is my own layout of Gehring's / Weber's schematic.


    Screen Shot 2019-01-09 at 10.50.03 PM.png
     

    Attached Files:

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  16. alathIN

    alathIN Tele-Meister

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    The isolating transformers would be easier if they worked, but yours is tried and tested; I like that better.
    It's not obvious to me how it works (other than the capacitor filtering radio frequencies).
    What do I look for when I shop for those diodes?

    I'm working on the layout now. It's the first time I've made a layout from a schematic. My plan is to use the original circuits I'm "borrowing" from as a guide.
     
  17. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Just catching up. One thing I didn't see mentioned with the dropping resistors was this:

    Yes, the total current draw is quite small, and that part looks covered. Only thing I would double check is the resulting filter frequency at each node. You want to thoroughly scrub any power supply hum on this by the time you get to the last, or signal path, node. The LFO circuit supply needs good isolation to prevent it coupling into other parts of the circuit, too.

    Sent from my Pixel 2 using Tapatalk
     
  18. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
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    Here's some background. Please note, I have no formal training in electronics. I hope anyone with more knowledge will step in with corrections, as needed.

    The general ground loop problem exists when there are multiple return paths, and multiple circuits of varying potentials. This occurs within a single amp or outboard unit, and various tactics are used to mitigate. The special case, and opportunity for an even noisier ground loop, occurs when there are multiple amps or outboard units, connected together. Each is connected to the wall, and the two are also connected via the instrument cable, forming a large loop. In either of these cases, higher potential power suppy currents will interfere with more delicate preamp potentials, and then of course the preamp amplifies the interference.

    The first step to mitigate loop issues is to stop using the chassis as a common ground plane, and to divide the returns into two groups: higher potential power supply, and very low potential preamp. Often these are still connected to the chassis, and then to ground, but the connection points are few, and are physically distanced from each other. Grounds are no longer connected to the chassis or pot shells willy-nilly. *see note below

    When dealing with two devices, at least one of the units must have the instrument jacks isolated from the chassis ground. The hum-loop-block circuit is inserted between the device circuit, and the chassis/ground, This effectively 'elevates' the circuit so the chassis is at zero volts, and the circuit return paths will be a few volts higher. The low value resistor reduces power supply loop currents. The resistor has a high wattage so it doesn't burn up in certain failure scenarios. The diodes are only there for safety. If the potential builds on the return path, greater than 0.7 V, the diodes will open. The capacitor ensures the higher frequency audio is still connected to the chassis, so it can function as an RFI shield.

    The diodes are spec'd on my diagram. They're commonly available. The cap need be nothing special, as long as the value is approximately correct. I used an old death cap pulled from a SF amp.


    *Note: This can be taken further, if desired, isolating each circuit fed by a different voltage coming off the power supply dropping-resistor chain. Make a star ground arrangement for each, and bring them each back (separately) to a common point very near the PT and wall power location. If taking it to this level, it's necessary to isolate the jacks (input, reverb, footswitch) from the chassis. The chassis is still grounded, both for safety, so it can't become hot, and so it acts as an RFI shield. Merlin shows some examples of this in his grounding treatise, linked below. Also, this is the approach I took with my Revibe. On my layout, each feed/return is color-matched, so you can trace them. Necessary? I dunno, but the unit is extremely quiet, and if you're designing your own layout, it's not difficult to do.


    Merlin - the whole chapter is great stuff, and the hum-loop-block circuit is discussed starting pg 14 of this PDF.
    Aiken
    Gehring - Jeff Gehring created the Revibe, and this is his documented build of the Weber kit. He also mentions the isolation and hum-block circuit, but it's not shown on the Weber layout or schematic. You'll note this page was pulled from archive.org. That's the only reason it's available, as the original pages / site are now dead.
     
    Last edited: Jan 11, 2019 at 11:10 AM
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  19. alathIN

    alathIN Tele-Meister

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    Helpful.

    My Trinity amp kit was designed that way - just one chassis ground point for each section of the amp. Oops, just re-read - on the Trinity, each star grounds to the chassis where it is, instead of connecting back to a common point.

    Appreciate the summary and the links.
     
    Last edited: Jan 10, 2019
  20. alathIN

    alathIN Tele-Meister

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    Do you have a link that explains more about this?
    You have opened the door to another room of the tube amp library that I didn't know existed.


    And I can see your point about the LFO - I'm sure having that signal propagating all through everywhere could produce some really annoying noise.
    How best to isolate it?
    A dedicated filter cap just for the B+ to that triode?
     
    Last edited: Jan 10, 2019
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