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

  1. #421
    Bent Member Chuck H's Avatar
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    Yes. That .002 cap was one of the first things I considered. I was timing it out for the post that would cover any other changes I saw.

    And Helmholtz is absolutely right. I've never met a Fender reverb amp that didn't have plenty of reverb. On that note I (like Helmholtz) think it's possible there may be a problem with the tank or, less likely, the reverb transformer. I suspected the transformer earlier in this thread because your previous distortion artifact sounded similar to some noise I've heard on similar amps when the reverb transformer was running into an open load. That affect would more likely be caused by a bad connection or a bad tank. I think we've dealt with all the connections in that circuit, so I would suspect the tank first.

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  2. #422
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    Quote Originally Posted by TelRay View Post
    ...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)...
    I’m not sure that Helmholz meant it like that; rather the 470 ohm cathode bias resistor resulted in much higher idle current and dissipation, of the tube and transformer.

    Assuming that there are no underlying issues, my experience is that combining a bypassed V3 and a (type 4) master vol results in nasty preamp distortion, as the V3 grid becomes the point with the lowest clipping threshold. A grid stopper on V3 helps to prevent that clipping from getting into the dry signal path.

    Regarding the trem depth, if V5 and its cathode bypass caps are good, maybe the optocoupler is wonky, so consider replacing it.

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    a (type 4) master vol
    What is a type 4 master vol and who invented this arbitrary numbering? Is it just the standard pre PI master (like with a Marshall 2203)?
    Would a PPIMV mask the spill-over distortion?

    A grid stopper on V3 helps to prevent that clipping from getting into the dry signal path.
    I guess this means that V3 is regularly driven into grid conduction?
    What is the minimum recommended grid stopper value (as a high value will reduce treble content in the reverb drive by Miller effect)?

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  4. #424
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    Apologies, I assumed most would be familiar with Ken Fisher’s The Trainwreck Pages https://robrobinette.com/images/Guit...searchable.pdf
    I think that Aspen Pittman’s Tube Amp Book used some bits of it.
    See p41 onwards for the master vol diagrams, but yes, the type 4 is the regular 2203/4 pre phase splitter.

    Regarding a V3 grid stopper, at least 10k helps, but anything is better than nothing.
    With control settings suitable to promote preamp clipping and eliminate power amp clipping, as signal level is increased from zero, the 1st / earliest / lowest headroom point is V3 grid. For high master vol settings / non master vol amps / types 1, 2 or 3 master vols set low, the lowest headroom point moves to the phase splitter plates / power tube grids, so the unpleasant V3 grid clipping gets ‘masked’.

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  5. #425
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    Quote Originally Posted by Chuck H View Post
    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)
    No, I have removed only couple of wires (in light gray in the schematic), No components removed.

    Quote Originally Posted by Chuck H View Post
    If I point it out in the schematic, do you think you'd be able to locate it in the amp?
    I should be able to do that by now

    Quote Originally Posted by Helmholtz View Post
    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.
    Richtig! this is like that since post #349 (as shown in the schematic below)

    Quote Originally Posted by Helmholtz View Post
    If that isn't enough I would try a different tank. Any BF or SF amp should have reverb in abundance
    Quote Originally Posted by Helmholtz View Post
    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.
    I get what you say. That tank was in and out of the cabinet several times, different cables used, cleaned, etc
    As I said, everything changed when I reverted the circuit to stock and started messing with the SPEAKERS wiring.
    In any case, at the time the problem was existing, the voltage divider did the trick of eliminating it.

    Quote Originally Posted by pdf64 View Post
    Regarding the trem depth, if V5 and its cathode bypass caps are good, maybe the optocoupler is wonky, so consider replacing it.
    Yepp, I suspect the ROACH too. However I am comparing the tremolo depth to another amp I have (a 1968 Ampeg Jet 12) that is pretty deep.
    It's not that the effect is not noticeable, I just like a deeper one like in the Ampeg.

    Maybe I'll try to do a couple of recordings tomorrow and post them here. However, again, amp's sounding great. We are "tweaking to taste" here.

    See schematics below.




    PS: and... yessss, the death capacitor is gone too. Plus the in the BALANCE section, that 15 KOhm resistor was replaced (explained in another thread) by a 10 KOhm resistor plus a 25 KOhm trim pot

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  6. #426
    Bent Member Chuck H's Avatar
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    Ok... Kudos to Pete and Helmholtz for conferring on the matter pdf's personal experience with the issue is SO relevant here and Helmholtz interest in it's relevance is too. Maybe two of the most significant minds on such issues on the WWW right here on the matter at hand. Consider yourself blessed

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Just to note that the original 470 ohm V3 cathode bias resistor may have drifted up in value, well above its tolerance limit, thereby mitigating the hot bias condition somewhat.

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    I think that Aspen Pittman’s Tube Amp Book used some bits of it.
    Thanks.

    Pittman's book I have and now I remember having seen the Trainwreck pages containing MV proposals (probably 20 years ago - must have become a kind of bible to the internet community in the meantime). The MV circuits all looked straightforward and I didn't pay attention to the numbering. Seemed to be just an arbitrary listing.
    (As the "type 4" is by far the most common, I would have labelled it "type 1".)

    With control settings suitable to promote preamp clipping and eliminate power amp clipping, as signal level is increased from zero, the 1st / earliest / lowest headroom point is V3 grid.
    I understand, so the "grid stopper" actually serves to isolate the dry signal path from the V3 grid, being a relatively low impedance, non-linear load (caused by grid current) at higher signal levels/input vol. settings.

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    Last edited by Helmholtz; 11-03-2019 at 05:26 PM.
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  9. #429
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    Quote Originally Posted by Helmholtz View Post
    ...(As the "type 4" is by far the most common, I would have labelled it "type 1".)...
    It may be that when he came up with the system (eg early 1970s) master volumes weren't common at all. And with regular preamps, eg Marshall 1987, a type 4 master vol doesn't have sufficient gain behind it to get much preamp overdrive, hence types 1-3 would be the simplest to fit to an amp and achieve a useful degree of overdrive.

    I like to use the Trainwreck classification system as it's the simplest way I'm aware of to refer to the different master volume types; the 'PPIMV' acronym is kinda useless, as it fits 'pre' and 'post' master vol types.

    Quote Originally Posted by Helmholtz View Post
    ...I understand, so the "grid stopper" actually serves to isolate the dry signal path from the V3 grid, being a relatively low impedance, non-linear load (caused by grid current) at higher signal levels/input vol. settings.
    Yes, that's it. My guess is that this V3 grid conduction / clipping is the reason why Fender removed the bypass from V3 cathode when they fitted (type 4) master vols in the 70s.

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  10. #430
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    sure this place is full of "eminences", I am very grateful I could understand about 10% of what has been discussed
    I even met Mr Fahey face to face back in the late 80's when he was manufacturing amps in South America, I had one of his Bass Heads for years.

    To PDF's question about the state of the 470 Ohm Resistor: it is a new one and measured before placing in the circuit (in fact I do not recall now one component that has not been refurbished for a new higher grade and lower tolerance component)

    OK, not wanting to go to Guitar Center with my guitar and just trust my ears to be able to compare accurately what I've heard at home against a new TWIN '65 reissue I came up with a "virtual" solution and used the Fender '65 Twin Reverb from the Amplitube plugin. I dialed a pretty close clean (yes, always a little edgy) sound on my TWIN and the VIRTUAL one and proceeded to compare both (NOTE: both use a REAL SM57 and a VIRTUAL one).

    Easy to follow in the video, however it follows the sequence below

    1) Clean sound of MY TWIN (Chords)
    2) Clean sound of the VIRTUAL TWIN (Chords)
    3) MY TWIN LEFT Ch + VIRTUAL TWIN RIGHT Ch
    4) REVERB sound of MY TWIN (Chords)
    5) REVERB sound of the VIRTUAL TWIN (Chords)
    6) REVERB MY TWIN LEFT Ch + VIRTUAL TWIN RIGHT Ch
    7) REVERB sound of MY TWIN (guitar line)
    8) REVERB sound of the VIRTUAL TWIN (guitar line)
    9) REVERB MY TWIN LEFT Ch + VIRTUAL TWIN RIGHT Ch

    REVERB CONCLUSION:
    the VIRTUAL TWIN seems to have a bit more REVERB (Wet mix)
    however maybe the more noticeable effect is in the REVERB TONE when playing the guitar line (not chords) with more mid lows around 250 Hz (I do not particularly like the boomier sound)

    10) TREMOLO sound of MY TWIN (Chords)
    11) TREMOLO sound of the VIRTUAL TWIN (Chords)
    12) MY TWIN LEFT Ch + VIRTUAL TWIN RIGHT Ch
    13) TREMOLO sound of MY TWIN (guitar line)
    14) TREMOLO sound of the VIRTUAL TWIN (guitar line)
    15) MY TWIN LEFT Ch + VIRTUAL TWIN RIGHT Ch

    TREMOLO CONCLUSION:
    the VIRTUAL TWIN TREMOLO goes definitely deeper (INTENSITY) than MY TWIN

    PS: I know someone is going to say: "you are comparing a BLACKFACE virtual resproduction with a SILVERFACE amp". Yes, I am. I believe once you get them to sound almost identically (tone-wise) the REVERB and TREMOLO features shouldn't be too different... right?
    PS2: @Chuck you wanted me to take a look at something in the circuit? was the question solved by the schematic I posted?


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  11. #431
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    Are the signals from the amps in opposing polarity? As when both together it seems quieter.
    The virtual TR's reverb quality seemed better than any real one than I've heard, no trace of 'boing', more like a hall effect
    The virtual trem speed seems slower; due to the rise/fall characteristics of the optocoupler, a slower trem will be deeper than a faster one. Whatever, assuming that the circuit driving the opto is good (?), the trem depth and 'shape' is pretty much 100% down to the individual opto in there; they aren't super precision devices, especially after 4 decades use.

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  12. #432
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    - The virtual Twin sounds much cleaner. The distortion heard with the real amp might be due to the recording. But any real Twin starts to distort above a vol setting around 4. (The numbers on the pot dials actually don't mean much, e.g. pots typically have a tolerance of +/- 20 % not including variations of taper. ) If the distortion is real/live (and you don't like it), I would try the V3 grid stopper as recommended by pdf64. His reasoning makes a lot of sense. And/or just increase MV and decrease the input vol.

    - The reverb of the real Twin sounds quite normal, not seemingly lacking treble. The reverb of the virtual amp sounds deeper/fuller. A 6-spring tank would get you much closer.

    - Do you really like the staccato quality of the virtual amp's tremolo??

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  13. #433
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    Thank you guys so much for the "reactions" to my video I was proud of it until I read your comments... hahahahaha!

    One VERY IMPORTANT thing first, the AMP's tone is not the issue I am trying to address. I intended to make the VIRTUAL TWIN to sound as close as possible to MY TWIN to have the same baseline to be able to compare the REVERB and the TREMOLO.

    From Hr Helmholtz I guess I did not do as good as a job as I intended However, you get the point.

    POLARITY (question from Mr PDF), I played my guitar through MY TWIN and recorded it (using an SM57). Afterwards I recorded the same guitar phrases (actually playing the guitar again) through my soundboard straight to the PC and applied the PLUGIN effect (which gives you the option of choosing a mic, I've chosen an SM57). I think that phase cancelling is not likely to happen. I did do hard panning (extreme LEFT for my TWIN and extreme RIGHT for the VIRTUAL one) and maybe that sounds a bit "hollow" right at the center.

    DISTORTION, I have no trouble with the sound of it. For me the sound of the amp is good and is back to the sound I expected from a TWIN. I am even more convinced after comparing it with the VIRTUAL one.

    I'm surely open and will even welcome any comments on tone, etc. However that was not my focus

    Now, to the two specific topics:

    REVERB:
    If I compare the two REVERBs (VIRTUAL vs REAL) I agree that the REAL sounds better (speaking about TONE) to my ears and with good HIGHs.
    I was saying before I wasn't sure if I wanted more WET REVERB signal or a more HIGHs. Now, after this exercise, I am almost convinced I just need more WET REVERB (the boomy effect of the VIRTUAL TWIN was kind of too muddy).
    We can consider this a mod (if as Helmholtz says, the REVERB sound is normal for this amp) or a fix (if, as I think, there is less WET REVERB in the mix than on the VIRTUAL TWIN).

    TREMOLO:
    "assuming that the circuit driving the opto is good", I've changed all components and remember measuring some voltages (VIBRATO on and off). However one can never be 100% sure
    From what PDF is saying I think that I cannot be sure I will get a deeper tremolo even if I get a new roach (octocoupler).
    I am looking for something more like a SINE wave here, maybe my amp sounds more like a TRIANGLE and the VIRTUAL amp goes somewhere in between SINE and SQUARE.
    "Do you really like the staccato quality".... hahahha... no I don't but it might become handy to have that sound available. I do not necessarily need to always drive the TREMOLO with the INTENSITY at 10. However I only like the TREMOLO with REVERB and that tends to smoothen the SQUARENESS or STACCATONESS (?!) of the sound.

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    Last edited by TelRay; 11-05-2019 at 06:50 PM.

  14. #434
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    Quote Originally Posted by TelRay View Post
    ...POLARITY (question from Mr PDF), I played my guitar through MY TWIN and recorded it (using an SM57). Afterwards I recorded the same guitar phrases (actually playing the guitar again) through my soundboard straight to the PC and applied the PLUGIN effect (which gives you the option of choosing a mic, I've chosen an SM57). I think that phase cancelling is not likely to happen. I did do hard panning (extreme LEFT for my TWIN and extreme RIGHT for the VIRTUAL one) and maybe that sounds a bit "hollow" right at the center...
    Speaker polarity is a variable - there was no regulation / standard / convention that manufacturers had to comply with. These days the 'positive voltage produces positive pressure' convention has become the norm I think, but I'm not aware of any international standards that cover it.

    Whatever, I think that good practice when there's a source with more than one signal path is to try the polarity both ways.

    Are the 2 channels of the virtual amp in opposing polarity (with each other)? It may be that software plugins follow 'don't flip polarity' convention, even if they're supposedly mimicking equipment that does flip.

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  15. #435
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    Thank you very much for the reply, I think I was not clear enough (that happens a lot, I guess "my ingles no bueno")

    Quote Originally Posted by pdf64 View Post
    when there's a source with more than one signal path is to try the polarity both ways
    I would say, there is not A SOURCE but 2 sources:

    - guitar played through the REAL TWIN
    - guitar played through the soundcard for the VIRTUAL TWIN

    to be more clear, the second guitar was played and recorded minutes after the first one.

    I understand what you say if the case was that I put a signal splitter in the guitar and played it at once through the REAL TWIN and the SOUNDCARD. But this is not the case.


    Quote Originally Posted by pdf64 View Post
    Are the 2 channels of the virtual amp in opposing polarity (with each other)? It may be that software plugins follow 'don't flip polarity' convention, even if they're supposedly mimicking equipment that does flip.
    This i need to check, but I am almost sure the signal of the VIRTUAL TWIN should be ONE CHANNEL (signal path) / MONO as there is only one VIRTUAL SM57 in front of the VIRTUAL SPEAKER

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