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Thread: Screaming Bright Switch Cap??? - 1974 Fender SF Twin Reverb Master Volume Push Pull Switch

  1. #386
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    duly noted!

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Chuck H View Post
    Your playing circumstances can be sensitive to phase. Sitting in front of an amp is different from facing away from the amp when it's on stage. Ever notice how players chasing feedback and acoustic interaction turn and move to get the affect they're after? I encourage anyone here to try this for themselves. Hook up a switch to invert speaker phase, set up your amp and A/B the two polarity options (EDIT: As per Helmoltz post, do this at some volume level that imparts physical interaction between the instrument and the amp). That isn't to say that one way will be better for everything, but one way might be better for, say, just sitting in front of the amp and noodling rather than playing live.
    Facing towards or away from the speaker has nothing to do with speaker polarity, and everything to do with the human ear being more sensitive to sounds coming from in front of the head as opposed to sounds coming from behind. The typical stage "back line" volume level is - almost always - loud & obnoxious to the audience, plus the FOH mixer who's trying his best to get vocals heard over that blare. (A perpetual problem I've had to face zillions of times.) In situations where there's enough space and equipment and personnel to mix back line speakers into wedge monitors, or even better, in-ear monitors, there's a much better chance of coming up with an overall quieter AND well balanced mix.

    With the guitar feedback trick, just like a microphone, many guitars will start up a howl, often involving the strings, when pointed at the speaker from only a couple feet away. A phenomenon, when it's kept under control, sometimes used to good effect by the guitarist, but when it's unexpected - not. Whereas, with the player's body between the guitar and speaker, and pickups pointed away from the amp, less chance of setting up that feedback loop. Polarity of the speaker or pickup again has little to nothing to do with the fb loop. Instead proximity, directionality and frequency response are the guiding factors.

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    Enjoy. Every. Sandwich.

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    Bent Member Chuck H's Avatar
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    Carlos Santana use to have places marked on stage that gave him his famous sustain during sound check. Every working guitarist has done gigs where they just couldn't get their sustain and feedback mojo working (with much stomping and gnashing of teeth). I'm sensitive to your perspective as a sound guy, but any unexpected feedback for guitarists is most often due to an equipment failure. Microphonic pickups. Otherwise I don't hear guitarists complaining too much about their guitar communicating with their amp in a more musical, sympathetic way. Acoustic guitars being an exception. I've had 'em almost shake apart when some noob walks away from their instrument without turning it down. Anyway...

    I've done the same test as Helmholtz with a speaker polarity switch. At harmonic feedback levels of gain and/or acoustic interaction levels of volume it absolutely made a difference in my tests. Since this was my experience it's impossible for me to dismiss. Being as I know it actually happened and all.

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    At harmonic feedback levels of gain and/or acoustic interaction levels of volume it absolutely made a difference in my tests.
    I have no reason to doubt your results (didn't perform this experiment myself). The desirable(?) positive feedback requires to the PU signal to be/oscillate in phase with the sound.
    But...

    Carlos Santana used to have places marked on stage that gave him his famous sustain during sound check.
    ...with a sound wave phase not only changes over time but also with distance from the source (speaker). E.g. for a 1kHz soundwave phase reverses about every foot distance from the speaker, thus turning positive into negative feedback at this frequency. So changing the players position determines if sustain of the note played will increase or decrease.

    In other words, with reversed speaker polarity, the places of constructive feedback will be at different locations.
    The same holds for PUs having reversed signal polarity.

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    Last edited by Helmholtz; 10-07-2019 at 05:39 PM.
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    Hello guys, I'm sorry if I sometimes derail the conversation and we end up chatting about "mojo hunters" and Santana feedback / sustain spots but I also find that part of the conversation interesting. Thank all for you input.

    Now, back to the subject of the problem here (let's call it "farty noise in loud passages with reverb tank connected") I decided to take a step back reversing the circuit to a previous known as faulty state (reproducing the anomaly) before starting the mods proposed. And... I could not reproduce the failure. This is good news, but also bad news as I cannot understand why. Let me list all the variables.

    1) Speakers rewired (white cable is now + to + terminals and black is - to -)
    2) Polarity reversed (+ DC applied to + plug terminal moves speaker cones outwards)
    3) New Speaker Plug and cable (needed a longer cable to hook the cabinet to the amp head)
    4) Eliminated voltage divider and reverted it to the 1 M Ohm resistor (see image below)
    5) eliminated the 5,000 Ohm + 560 pF between V3 1-6 and point "A" (red Reverb transformer primary) - see image below



    6) the Reverb tank is out of the enclosure (something that was also done in the past)



    The current layout is shown below (distance from the enclosure to the amp head is about 50 cm / 2 ft, reverb tank is partially below the amp head).

    I've tried different guitars (humbucker, single coils), going to 10 with the CHANNEL VOLUME, TONE and MASTER VOLUME settings.
    Connected and re-connected the Reverb tank several times.

    I was not able to reproduce the failure.

    Decided to reverse the speakers polarity again and still no failure reproduction.

    I'm starting to think this is pure witchcraft... :/

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    Is the reverb tank housing well grounded?

    I also recommend to ground the speakers' chassis.

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    thank you man!

    Quote Originally Posted by Helmholtz View Post
    Is the reverb tank housing well grounded?
    verification method being... probe continuity between the reverb chassis and the amp head chassis?
    grounding is provided by the RCA plugs ring (connecting TANK to AMP HEAD), correct?

    Quote Originally Posted by Helmholtz View Post
    I also recommend to ground the speakers' chassis.
    any specific recommendations on how to do it?

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    verification method being... probe continuity between the reverb chassis and the amp head chassis?
    grounding is provided by the RCA plugs ring (connecting TANK to AMP HEAD), correct?
    Correct. I never rely on old mechanical contacts.

    any specific recommendations on how to do it?
    I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.

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    Last edited by Helmholtz; 10-07-2019 at 10:29 PM.
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    Quote Originally Posted by Helmholtz View Post
    I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.
    like this? and done independently for every single speaker, right?


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    Supporting Member The Dude's Avatar
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    I'll add to the discussion: There are amp manufacturers that wire speaker polarity so that amp input phase matches amp output phase, assuming the user wants the phase relationship of input to output unchanged. So, the speaker can sometimes be wired + to ground to retain phase relationship between input and output. In this case, speaker wiring will depend on amp design- number of gain stages and other design choices.

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  11. #396
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by TelRay View Post
    I'm starting to think this is...
    The harder a symptom is to troubleshoot, the harder the solution is to name unless it's stumbled upon and recognized in the debug process. Once fixed, there may be no way to reproduce. Bad solder joint? Bad component that was swapped out and trashed? Just make sure you wrap this up before Halloween

    ...pure witchcraft. :/
    edit: just remember the other 99,999 of this model worked more or less OK.

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  12. #397
    Bent Member Chuck H's Avatar
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    Sometimes an amp just needs to know it's loved. Pay some attention to it and it starts behaving

    It happens. I once had the guys here chasing (what I thought was) an odd oscillation in one of my builds that turned out to be a cold solder joint that would only become intermittent at two frequencies. It wouldn't have been out of the realm of possibility that in the course of troubleshooting I could have resoldered that joint without knowing that a cold joint was what I was chasing. In that case the problem would have seemed to spontaneously disappear. That or I might have thought that whatever change I made while resoldering that joint had fixed the problem. Even though if I reversed it the problem wouldn't recur. See? Not witchcraft, just coincidence. The way I see it something about your speaker wiring was causing a problem. There's still potential for induced feedback loops even with the tank out or the reverb turned all the way down. It might have been how the leads were routed? Who knows. Short tale...

    On a rather complicated build, that just happened to be a 2x12 with reverb, I had a whistling oscillation when the reverb was turned above five. The only thing that fixed it was grounding the speaker frames. There may have been another solution or design flaw that wasn't diagnosed before I grounded the frames. I don't know and I don't much care. Once you figure in all the variables of induced and invisible circuits that exist within a guitar amp, which even for vintage designs is a very high gain circuit, there are so many electronic principals happening and interacting all at once and differently in any given circumstance that it's near impossible to account for everything ahead of time or even to know where pitfalls might be before they happen. As a guy that pretty much only builds his own designs that's my $.02 on witchcraft.

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    Quote Originally Posted by The Dude View Post
    I'll add to the discussion: There are amp manufacturers that wire speaker polarity so that amp input phase matches amp output phase, assuming the user wants the phase relationship of input to output unchanged. So, the speaker can sometimes be wired + to ground to retain phase relationship between input and output. In this case, speaker wiring will depend on amp design- number of gain stages and other design choices.
    thx man, I get that. I guess it might be useful to ask other 70's silverface twins how are the speakers wired


    It might be that... witchcraft, lead dressing, cold solder joint, etc, etc. What I am afraid of is assembling this thing back together.... hahahahahahah

    Just to make sure of what I was hearing as "good" sound I turned the VIBRATO Channel to 10 and made a video with LEFT Channel REVERB TANK disconnected and connected on the RIGHT channel. I could not hear any difference... which is a good thing



    Just for info, the sound has not been manipulated in any way (no EQ, compression, etc)

    Thiking about it, the only other difference I notice (apart from the anomaly not being reproduced) is the REVERB being quieter in terms of hum. I Remember in the past trying to reduce the hum by flipping it 180 degrees in the cabinet, moving it away from the speaker magnets and the amp's transformers, etc. Could that be pointing in the direction of TANK grounding as Helmoltz pointed out?

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    Supporting Member The Dude's Avatar
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    To the best of my recollection, Twins were/are always jack tip to speaker positive. Again, to the best of my recollection, the factory wired them that way regardless of speakers used in the amp.
    Here's the rub. Some of the old Twins had factory installed JBL D120F speakers, which were "backwards" according to standard speaker terminal marking practice. In other words, a positive voltage applied to the positive terminal caused the speaker cone to move inward- not outward. This would indicate to me that Fender didn't care much about speaker phase as long as the speakers were in phase with each other.

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    like this? and done independently for every single speaker, right?
    Not quite. Washer and lug should be on the flat edge of the frame under the nut. You can't have anything between speaker and baffle.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I use a solder lug over one of the frame mounting screws secured by a serrated washer between lug and frame, solder a wire to the lug and connect to the grounded speaker terminal. And I verify contact by measuring.
    Quote Originally Posted by Chuck H View Post
    The only thing that fixed it was grounding the speaker frames.
    This gets a little interesting as to whether we are talking ground, chassis, or common. As far as I know for this model, the speaker jack is not insulated from chassis. So Helmholtz method is essentially connecting speaker frames to chassis ground. But what if the jack was insulated? Would it be better to connect the frames to the neg. lugs, or to amp chassis?
    Chuck, in the scenario you mentioned, did you 'ground' the frames to the spkr. neg. terminals, or to amp chassis?


    Quote Originally Posted by The Dude View Post
    Here's the rub. Some of the old Twins had factory installed JBL D120F speakers, which were "backwards" according to standard speaker terminal marking practice. In other words, a positive voltage applied to the positive terminal caused the speaker cone to move inward- not outward.
    Maybe in those units they wired speaker plug tip to speaker neg. terminals like Telray found in this guy.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    Chuck, in the scenario you mentioned, did you 'ground' the frames to the spkr. neg. terminals, or to amp chassis?
    OT final, and therefor speaker negative (and now frames), are grounded to the chassis.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by g1 View Post
    ......Maybe in those units they wired speaker plug tip to speaker neg. terminals like Telray found in this guy.
    I don't recall taking apart a plug on a Twin to see, but I do recall battery testing Twin cabinets with JBL's and the cones did indeed move inward with +battery to jack tip. I remember thinking it was odd that they didn't wire them differently. It's also possible that someone rewired the amps or changed speakers before I worked on them and inadvertently changed polarity.

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    Would it be better to connect the frames to the neg. lugs, or to amp chassis?
    Always to amp chassis/ground. So it's essential to identify which speaker terminal connects to amp ground. The principle is to use the grounded frame for shielding the voice coil's electric field. It actually matters only (sometimes) in combo amps.
    If none of the speaker leads is connected to amp ground (very rare) both speaker wires are floating and none of them can be used for frame grounding.

    Interestingly some older speakers already had a riveted connection between the minus lug and the frame. This may seem a good idea but prohibited series wiring.

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    I don't recall taking apart a plug on a Twin to see, but I do recall battery testing Twin cabinets with JBL's and the cones did indeed move inward with +battery to jack tip.
    As already written I can confirm that (some?) JBL D-series speakers were opposite phase.

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    Supporting Member The Dude's Avatar
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    Yes. I'm aware of that. All JBL D series were opposite phase, unless they were reconed with an aftermarket kit. What's in question is how Fender wired them.

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    Just for some more derailing, here is another "phase mystery" from the past:

    Starting from 1960 the magnet and signal polarity of Fender Strat PUs was reversed, so that older and newer PUs were no longer compatible.
    Now one might speculate about a relation to changed speaker phase...(just kidding).

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    hi all, just to let you know I put the amp back together (without adding the ground connections to the speakers yet) and everything is workings fine.
    As I said earlier on the thread the only additional (good) symptom I see (hear, really) is no hum when engaging the reverb.

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    I am still just curious about the REVERB VOLUME increase modification proposed by Chuck... and as I already have the needed resistors and caps I am most certainly going to give it a try.
    However, there is still something that can have an impact on the reverb sound and is something Fender changed when going from the MASTER VOLUME model (RIGHT Schematic) into the PUSH PULL MASTER VOLUME one (LEFT Schematic):




    What I currently have is the version on the LEFT (the 470 Ohm Resistor, though I have removed the 560 uF capacitor that appears faded in the pic).
    What change should I expect if I implement the schematic on the RIGHT (25 uF cap across the 2.2 K Resistor)?

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  25. #410
    Bent Member Chuck H's Avatar
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    There are some principals involved with the two circuits you're comparing. I'm not studied in electronics or specific formulas involved, but I'll do my best in laymans terms...

    The circuit you have now uses a considerably hotter bias (DC relationship between the grid and cathode) than the earlier circuit. This has the affect of making the grid more sensitive to the input signal. The colder circuit can significantly improve gain with the bypass cap because the cathode elevation is much higher. But it also doesn't amplify as much to the same amount of signal at the grid. The seesaw action isn't obvious, but it's there.

    Since you've had to decrease grid drive to the reverb as part of the stabilizing effort I actually think it's a good idea to try the colder, but cathode bypassed circuit IF you also know that this may mean increasing grid drive again. Who knows anymore with the trouble you've been through. But maybe using the colder bias would allow you to increase grid drive to stock levels without instigating the problem again. Just work carefully so that the advantage you've gained doesn't get lost if you want to reverse any changes.

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    thx Chuck!!! great explanation

    only one reminder:
    Quote Originally Posted by Chuck H View Post
    Since you've had to decrease grid drive to the reverb as part of the stabilizing effort
    I am not currently using the voltage divider or anything else than what’s on the original schematics (with the exception of having removed that 560 pF cap going to point “A”)

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  27. #412
    Bent Member Chuck H's Avatar
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    Ok. Good? So why the desire to change the reverb drive tube cathode circuit? If the amp isn't misbehaving anymore and the circuit isn't strangely altered, that's great. But what are you NOT getting from it? I'm thinking more reverb was hinted at. Do you need it or is it just a case of wanting the option of too much so you know you have enough Because that's TOTALLY me. Or is it the tone of the reverb? The character of the reverb (doesn't sound boingy enough, or whatever). This isn't just a repair forum. We've been doing mods here since I can remember (read- a long time). It's cool to ask about circuit differences if you just want to know what it means. If your amp is missing anything you should post about it. I'll get to the point...

    The amp to practice mods and learning on probably shouldn't be one you already like and rely on. I think a lot of members here have made that mistake. You get gear mod/tweak syndrome and before you know it you can't find your way back and, though you know you've lost something you're not savvy enough yet to pin it down and it's gone forever. Ok, maybe that's just me. But it's good advice.

    So, if you want your amp do something better than it does now, ask about it specifically. If you want to play with amps you should try to find success in every project and never risk undoing it.

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    Grid stoppers still on V3?
    Is there still a master volume control?

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    Thanks Chuck and PDF... the circuit is in fact back to stock (as per Fender schematics, apart from the minor exception of the 560 uF cap not being there anymore) and everything is working well (the anomaly has disappeared and I keep my fingers crossed because it is not clear when / why it disappeared, however it was around the time in which I started questioning the speakers phase / polarity) [that means grid stoppers, voltage divider, etc... all gone]

    I remember that back on post #86 there was a brief discussion on the topic and Helmholz expressed:

    "I much prefer the BF version with 2.2k/25µ. Changing the cathode resistor to 470 Ohm unnecessarily increases V3 plate and transformer current by a factor of around three. It also strongly increases the driving power to the reverb transducer. I would expect more "crosstalk" with the 470 Ohm version."

    I read "driving" more as "compressing/saturating" more... which is good because I like a "dense" reverb. If that is the case, I will leave it as it is with the 470 Ohm resistor (which is what the schematic for this specific model shows).

    My question was more towards if that difference (470 vs 25/2.2) had any influence on the REVERB tone.

    To the point made by Chuck "what are you NOT getting from it?" I would say, two things:

    - I'd like a bit more of REVERB or more BRIGHTNESS in the REVERB (need to try both to decide). (Even though when I reverted to the original schematics by taking the voltage dividers out the REVERB recovered some volume, as expected)
    - (point not discussed yet) I'd like a deeper VIBRATO (I always thought the tremolo was not reaching - let's say - 0 dB before coming bouncing back to the set volume even with the INTENSITY knob at 10)

    Again, the amp is sounding great and it is back to stock. And what I am discussing is clearly a mod. I could gladly start a new thread however as the increase REVERB volume mod was something proposed during this discussion I wanted to ask about the 470 vs 25/2.2 versions before doing anything else.

    As always, relying on your wisdom

    Thx!

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    Master vol?

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    Quote Originally Posted by pdf64 View Post
    Master vol?
    yessir, MASTER VOLUME was always in the circuit (what I removed a long time ago was the couple of wires providing the PULL ugly distortion... and never put them back)

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    Bent Member Chuck H's Avatar
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    That's how the switch should have been labeled, "PULL UGLY".

    If you go back to page one... You posted a schematic that is accompanied by a "wiring diagram". Can you tell us if your amp circuits look to be consistent with either one. And if yes, which one? They are quite different.

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    hahahaha... sometimes “ugly” is good but i haven’t found a use for “that ugly” yet

    quick answer: the schematic that reflects the current amp is the white one (that’s the original Fender one, god knows which good intentioned soul came up with the blue layout) with the two exceptions mentioned before:
    - no 560 uF cap to point “A”
    - no “PULL” wires

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    Quote Originally Posted by TelRay View Post
    with the two exceptions mentioned before:
    - no 560 uF cap to point “A”
    - no “PULL” wires
    Ok. It's hard to say for sure from the schematic what remnants may be left over from the old pull switch. I know you removed the wires. did you remove anything else? There is likely other circuitry in the amp that's sole purpose was to facilitate switching to ugly mode and it detriments the clean mode (and reverb). If I point it out in the schematic, do you think you'd be able to locate it in the amp?

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    - I'd like a bit more of REVERB or more BRIGHTNESS in the REVERB (need to try both to decide). (Even though when I reverted to the original schematics by taking the voltage dividers out the REVERB recovered some volume, as expected)
    The single most effective measure to increase reverb brightness is the .002µF cap wired across the 220k grid leak resistor of V4A (as mentioned several times) assuming that the circuit is completely back to stock. If that isn't enough I would try a different tank. Any BF or SF amp should have reverb in abundance.

    While I am happy that the original problem (mysteriously) disappeared, this is one of the more frustrating threads to me as it is not clear what actually solved the problem (maybe a bad ground contact at one of the reverb jacks?) and all the well founded proposals turned out to be superfluous. Wasted a lot of time.

    Lesson learned: Always check chassis ground contacts first.

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