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

  1. #281
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    I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
    The pull boost was just horrible and no one ever used it (except as 'horrible' effect).
    But the master was also not very well liked, many either ran it up full all the time, or removed it. In that era, Fender did not know how to do distortion.
    I would say that you are no longer dealing with a 'fault', just trying to optimize the quality of the distortion. That is ok to do, but you may be at a point where it is as good as it gets without a major re-design. If you ever have a chance to try another SF Twin w/Master, compare it at similar settings.

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    Quote Originally Posted by g1 View Post
    I'm not hearing that farty glitching anymore. You did say this was with the ch.vol. around 7 and lower master setting, so I'm going to suggest what you now have is normal.
    I agree with you Mr G1.... partially
    I like the current slightly overdriven sound I am getting from the amp, it's pushy and clear and I would never expect any more grit coming from a Twin. That has improved noticeably in the last 100 posts
    But... I'd like to get the same great sound with the REVERB tank connected, and that "dirt" I hear around 9 KHz in the latest videos (the one with the overdrive on post #268 and the clean tone with the single coil pickups on post #278) prevents me from considering this case closed. While this could be "just tolerable" for live performance it is still not suitable for studio work.
    After I have solved this issue I will consider I have restored this amp to its former glory and will eventually think about any tweaks in the future.
    Would you agree? Please say "Yes" because I still need your help

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  3. #283
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    Ok. I missed the part that the reverb was not connected. Carry on with those mods and post your results.

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    trying to get things in order to dig into the amp again today, I wanted to prioritize what to do and how to check that the modification is effective

    1.- Add 20 uF / 500 V Cap

    Reason: there shouldn't be signal voltage at the power supply
    Verification: place scope on point "A"


    2.- .0022 uF Cap try the different positions described on Fender Schematics

    Reason: the cap across the REVERB OUTPUT transducer regains some TREBLE - especially when using the 560pF cap at the V3 plates, which produces a LOW PASS effect. It also attenuates unwanted HF content with this wiring.
    Verification: scope at the SPK OUT


    3.- 560 pF cap try the different positions described on Fender Schematics

    Reason:
    Verification: scope at the SPK OUT


    4.- Install a Voltage divider replace 1M resistor by 2x 500K

    Reason: Reduce grid drive
    Verification: scope at the SPK OUT

    5.- add Resistors (around series with the 560 pF cap

    Reason: extracts energy from the HF resonance by dissipation and thus damps or even kills resonance
    Verification: scope at the SPK OUT

    6.- Faraday Cage add Aluminum foil to the enclosure

    Please jump in if you have any other scientific verification methods

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    You might want to avoid aluminum foil. Torn or loose foil could be a problem. Aluminum coil stock is available at any home center, sold as flashing.

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  6. #286
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    Perfect and reasonable order/sequence of steps

    Regarding "6.- Faraday Cage add Aluminum foil to the enclosure", you may tack some household aluminum foil to a large enough cardboard and place it on the chassis (making sure it makes contact) to test its effect outside the cabinet.
    The additional shielding may actually reduce oscillation and unwanted coupling issues, so also check with scope at SPK OUT.

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    Aluminum coil stock is available at any home center, sold as flashing.

    I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?

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  8. #288
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    Quote Originally Posted by Helmholtz View Post
    I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?
    I don't think the heavy foil is flashing. At least here, flashing is a building material, most often seen at roof-edge, so more like the marshall stuff.

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    TE=Helmholtz;529871]I understand the thicker/heavier foil is called flashing, right?

    What would you call the alu sheet metal used in Marshall tops?[/QUOTE]

    In the States aluminum foil is thin (easily torn by hand) rolls used to cover a casserole or a potato for baking.

    Flashing is a mm or two thick with some strength and rigidity to it.

    I could imagine aluminum foil torn/ unglued hanging down by the high voltage parts. That was my concern.

    Sorry for any confusion.

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    Quote Originally Posted by Helmholtz View Post
    Regarding "6.- Faraday Cage add Aluminum foil to the enclosure", you may tack some household aluminum foil to a large enough cardboard and place it on the chassis (making sure it makes contact) to test its effect outside the cabinet.
    The additional shielding may actually reduce oscillation and unwanted coupling issues, so also check with scope at SPK OUT.
    Great, will definitely try it like that unmonted. I Did before but have not seen a change in the oscillation, now that the 560 pF is doing part of the job flattening it out I will give it another try.

    Quote Originally Posted by ric View Post
    You might want to avoid aluminum foil. Torn or loose foil could be a problem. Aluminum coil stock is available at any home center, sold as flashing.
    thanks Ric for jumping into the conversation. Any particular (affordable!) one you might know of? A quick search on Home Depot shown results of $80 or more (and I don't really need that much Aluminum)

    https://www.homedepot.com/p/Spectra-...W187/205009266

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    alright, people. First reports back from the front:

    1.- Add 20 uF / 500 V Cap
    I got the cap soldered to point "A" ('+' side) and with an alligator clip to the ground side of the V3 grid resistor for a quick A/B comparison



    Note that since post # 85 the 470 Grid resistor has been replaced by a 25 uF/25V cap and a 2.2K resistor, so it looks like this:

    (meaning that the alligator clip, or '-' cap side is attached to the ground shared by the 2.2K resistor and the 25 uF cap)

    I believe that maybe there's one test that I forgot to do after post #240 in which I placed the scope at V3 pin 6 when there was still no 560 pF cap (added on post #262).
    I think should have placed the scope BEFORE and AFTER the 560 pF, to see what is the effect of the cap. If I do that now, I get the following:



    POINT 1: before the 560 pF cap, I get the same oscillation we've seen on post #240



    this does not change when connecting the newly introduced 20 uF / 500 V cap

    POINT 2: after the 560 uF cap, point "A". Completely flat line, either with the 20 uF / 500 V cap connected or not



    So, given the fact that there was no signal after the 560 pF I moved the scope to the SPKR OUT to see if there was any benefit in introducing the 20 uF / 500 V cap. The answer is: no. The same bump place where the oscillation used to be before introducing the 560 pF cap is still there.



    Additionally tried disconnecting the .0022 uF cap from the REVERB INPUT, leaving it disconnected and connecting it to the REVERB OUTPUT but have seen no change.

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    Last edited by TelRay; 05-21-2019 at 12:04 AM.

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    POINT 1: before the 560 pF cap, I get the same oscillation we've seen on post #240
    What do mean with before and after the 560p cap?

    POINT 2: after the 560 uF cap, point "A". Completely flat line, either with the 20 uF / 500 V cap connected or not
    Confused. Didn't you report and even show a scope picture of point A having signal voltage of 90mV? (Scope input should be AC coupling and mV range.) What changed?
    Do (did) you always have the reverb control at zero?

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    Last edited by Helmholtz; 05-20-2019 at 10:59 PM.
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    [QUOTE=Helmholtz;529969]What do mean with before and after the 560p cap?

    I edited the post for clarity, there was a paragraph placed in the wrong section. Sorry.

    What I mean is that: yes, I have placed the sope on Point "A" before and seen the oscillation. At that point in time, placing the scope on point "A" or at V3 pin 6 meant the same thing as there was no component in between.

    Now it's different with the 560 pF cap so there is a difference in placing the scope in point 1 (between V3 pin 6 and the cap) or point 2 (between the cap and point "A").

    When I introduced the 560 pF cap into the circuit (as per the schematic) I have measured the effectiveness by placing the scope at the SPKR OUT (not at point "A" anymore) and seen how the oscillation got flattened into a bump.

    Now, if I place the scope before and after the 560 pF cap I see signal + oscillation on the V3 pin 6 side but a flat line on the point "A" side (which is something I have not done, unless I am wrong, after the introduction of the 560 pF cap).

    Hope is clearer now with this image



    NOTE: need to check on the AC coupling setting and REVERB at 0 (in any case the two photos here were done with the same setting s on the scope and the amp)

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    Last edited by TelRay; 05-21-2019 at 12:21 AM.

  14. #294
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    Quote Originally Posted by TelRay View Post


    thanks Ric for jumping into the conversation. Any particular (affordable!) one you might know of? A quick search on Home Depot shown results of $80 or more (and I don't really need that much Aluminum)

    https://www.homedepot.com/p/Spectra-...W187/205009266
    Looks like Lowe's is selling unpainted mill flashing rolls @ 6"x12' for $13 and change/ 6"x50' for $17 and change here in mid Michigan. 24" is considerably more.

    Also saw it advertised as being at Wal-Mart, Ace Hardware and around here it's a fair price @ Menards, if you have one of those out there.

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    Quote Originally Posted by ric View Post
    Looks like Lowe's is selling unpainted mill flashing rolls @ 6"x12' for $13 and change/ 6"x50' for $17 and change here in mid Michigan. 24" is considerably more.

    Also saw it advertised as being at Wal-Mart, Ace Hardware and around here it's a fair price @ Menards, if you have one of those out there.
    we do have an Ace store a couple of miles away... great tip, man!!!

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  16. #296
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    At that point in time, placing the scope on point "A" or at V3 pin 6 meant the same thing as there was no component in between.
    Definitely no! V3 pin 6 is the reverb driver output (must have signal voltage) and point A is connected to a power supply filter cap (should show no signal as this is an AC ground) - no matter if the 560p cap is connected or not. Now I wonder what the vid in post #255 actually shows.

    Now it's different with the 560 pF cap so there is a difference in placing the scope in point 1 (between V3 pin 6 and the cap) or point 2 (between the cap and point "A").
    A tiny 560p cap influences HF but could never eliminate signal voltage from the power supply (point A).

    (Saying "before" and "after" the cap is confusing. Please always specify the circuit point you mean.)

    BTW, 1=1000000 (1million) times 1p.

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    Last edited by Helmholtz; 05-21-2019 at 04:23 PM.
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  17. #297
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    thank you guys for being patient (my learning capabilities allow me to barely keep pace with the overwhelming support you are providing).
    I deep dived into the scope measurements around point "A", V3 pin 6 with and without the 560 pF cap and with and without the 20 uF 500V ecap.
    I believe that my statement before about a "flat line after 560 pF cap" is wrong due to not changing the SCOPE's settings to actually "amplify" and see what that flat line really was (vertical scale needed to change from 5V/div to 10 mV/div as Helmholz correctly pointed)
    This is what I would see if I leave the settings on the SCOPE at 5V/div and 0.5 ms/div



    in the following video you can see:
    - scope placed at V3 pin 6 with REVER INPUT RCA connected
    - scope placed at V3 pin 6 with REVER INPUT RCA disconnected
    - flat line on SCOPE when measuring POINT "A"
    - change in SCOPE settings from 5V/div to 10 mV/div



    - keeping the same scope settings at 10 mV/div added the 20 uF 500 V e cap




    Quote Originally Posted by Helmholtz View Post
    Definitely no! V3 pin 6 is the reverb driver output (must have signal voltage) and point A is connected to a power supply filter cap (should show no signal as this is an AC ground) - no matter if the 560p cap is connected or not. Now I wonder what the vid in post #255 actually shows.
    totally right, I incorrectly mentally deleted the 560 pF from the circuit but kept V3 pin 6 and POINT "A" connected by an imaginary wire

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    Last edited by TelRay; 05-21-2019 at 07:51 PM.

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    Sorry, this is too much and confusing.

    - There is no sense measuring at the open end of the 560p.
    - With 560p connected at both ends, do you see a difference when connecting the 20 (I would actually prefer the 20 soldered at both ends, as this ensures lowest contact resistance). Please read the peak-to-peak voltages without changing controls' settings or signal level.
    - Whenever posting a scope picture, specify the scope settings.
    - All comparisons must be done at exactly the same controls' / scope settings and signal level.

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  19. #299
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    Quote Originally Posted by Helmholtz View Post
    Sorry, this is too much and confusing.
    Sorry about that
    The intention was to clarify that my mistake was to keep the same settings on the SCOPE when measuring V3 pin 6 and POINT A

    Quote Originally Posted by Helmholtz View Post
    With 560p connected at both ends, do you see a difference when connecting the 20µ (I would actually prefer the 20µ soldered at both ends, as this ensures lowest contact resistance). Please read the peak-to-peak voltages without changing controls' settings or signal level.
    That's a good question, the video shows a very unstable and jumpy signal. I will try to take a photo so I can freeze it and count. Is that instability normal or I am doing something wrong?


    Quote Originally Posted by Helmholtz View Post
    Whenever posting a scope picture, specify the scope settings.
    Good point. I think I did by saying the first batch of photos were at 5V/div and 0.5 ms/div and then (only in the video) there is a gradual change to 10 mV/div (will make sure is clear every time)

    again (and again) thx!

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  20. #300
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    That's a good question, the video shows a very unstable and jumpy signal. I will try to take a photo so I can freeze it and count. Is that instability normal or I am doing something wrong?
    The "jumpiness" is normal and caused by the underlying 120Hz ripple voltage. Only look for a significant difference.

    I think I did by saying the first batch of photos were at 5V/div and 0.5 ms/div and then (only in the video) there is a gradual change to 10 mV/div (will make sure is clear every time)
    Too much guesswork for me. I prefer big scope pictures with precise settings information, because we need to use the scope as a measuring instrument.

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    Last edited by Helmholtz; 05-22-2019 at 07:53 PM.
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  21. #301
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    hope this is more understandable. Due to the irregularity of the waves I have looked for the voltage between the highest peak and lowest valley.
    I would say the 20 uF eCap reduces the voltage in half


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  22. #302
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    I would say the 20 uF eCap reduces the voltage in half
    Agree, signal as well as ripple appear to be reduced by aproximately 50%. That's just what you would expect from doubling the filter capacitance at node A.

    Now I wonder what you see doing the same with the MV at zero, meaning zero output. This should reduce the "jumpiness" (ripple voltage) and avoid signal injection by the power tube screen currents. The idea is to separate/isolate the influence of V3 on the power supply signal voltage.

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  23. #303
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    Quote Originally Posted by Helmholtz View Post
    Agree, signal as well as ripple appear to be reduced by aproximately 50%. That's just what you would expect from doubling the filter capacitance at node A. Now I wonder what you see doing the same with the MV at zero, meaning zero output. This should reduce the "jumpiness" (ripple voltage) and avoid signal injection by the power tube screen currents. The idea is to separate/isolate the influence of V3 on the power supply signal voltage.
    thx, mister.
    1.- Add 20 uF / 500 V Cap we'll have to wait to see the influence of the MASTER VOLUME as I had to put the amp back together for a show tonight

    3.- 560 pF cap I have some update on placing the 560 uF in different positions that I will post later on today.

    6.- Faraday Cage in case this is of use for someone else in the future, I have found at Lowes a nice mill flash roll (thanks for pointing me in the right direction because I did not know this denomination and would have only search for Aluminum foil). 14 in by 10 ft 200um thickness) for $14

    https://www.lowes.com/pd/amerimax-14...iABEgLIhfD_BwE

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  24. #304
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    3.- 560 pF cap: moved it around following the evolution of the Fender schematics. The conclusion is that there is a difference between having the cap or not in the circuit but the effect shows to be exactly the same regardless of the cap being connected to POINT "A" or the GRID RESISTOR



    2.- .0022 uF Cap try the different positions described on Fender Schematics, already did that and the position never showed to influence the oscillation

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    I guess the pictures are taken at the amp's output?

    The conclusion is that there is a difference between having the cap or not in the circuit
    Looks as if the 560p mainly lowers the resonant frequency.

    but the effect shows to be exactly the same regardless of the cap being connected to POINT "A" or the GRID RESISTOR
    Yeah, that's what I expected after you mentioned that you changed the CATHODE (not grid) resistor from 470R to 2k2 in parallel with 25F. The result may have been different with the single 470 Ohm cathode resistor. ( Not easy to keep track of your modifications.)

    2.- .0022 uF Cap try the different positions described on Fender Schematics, already did that and the position never showed to influence the oscillation
    Ok, but having the cap at the reverb output jack should improve/boost the treble response of the reverb signal.

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  26. #306
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    Quote Originally Posted by Helmholtz View Post
    I guess the pictures are taken at the amp's output?
    cooorrect... this was the agreed verification method for this specific modification (560 pF) on the post in which I listed all the recommendations in order
    (Verification: scope at the SPK OUT)

    Quote Originally Posted by Helmholtz View Post
    Yeah, that's what I expected after you mentioned that you changed the CATHODE (not grid) resistor from 470R to 2k2 in parallel with 25F. The result may have been different with the single 470 Ohm cathode resistor. ( Not easy to keep track of your modifications.)
    hahahah... I know, and I am even changing things around at a much slower pace than you guys are suggesting.
    just to re-cap:
    The amp originally had the 2K2R in parallel with the 25 F cap, when I bought it (they were modern components so I understand this was a modification done by a previous owner). As I was having problems in that area I reverted to what the schematic showed, placing the 470R.
    On this thread it was suggested to go the "blackface" arrangement of 2K2R in parallel with the 25 F cap.
    I was actually thinking of placing the 470R again in the circuit and see what happens moving the 560 pF cap around.

    Quote Originally Posted by Helmholtz View Post
    Ok, but having the cap at the reverb output jack should improve/boost the treble response of the reverb signal.
    yessss, the REV is a bit dark. but today I cannot use it at all as I had to play the gig last night with the REVERB cables disconnected to avoid those "farty speaker cone" sounds. I guess we will worry about this later

    Today the amp is a beautiful sounding FENDER TWIN. With time it will become a FENDER TWIN REVERB...
    again guys... thank you!

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    4.- Install a Voltage divider replace 1M resistor by 2x 500K

    Reason: Reduce grid drive
    Verification: scope at the SPK OUT

    Before trying this one out I wanted to make sure that what I have understood from Helmholz's recommendation (proposal #1) is correct (as I was reading a bit about out and found out there is an easy mistake to make)


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  28. #308
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    I proposed a real voltage divider as shown in your proposal #1, as it mainly reduces drive voltage. Your proposal #2 doesn't do much to reduce drive voltage. You may vary the individual resistor values, but the sum of the resistances should be close to 1M= 1000K.

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    Last edited by Helmholtz; 05-25-2019 at 01:37 PM.
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  29. #309
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    To illustrate the evolution of the oscillation and make it easier to historically track changes, please see the photo below.
    The conclusion is that reverting to the 470R kills the oscillation at the settings I would normally use when playing: VIBRATO CHANNEL @ 6.5 and MV at 3-4 (keep in mind the .002 uF cap is still at REVERB IN as seen on the schematic extract above every oscilloscope photo).
    If I push the amp above a CHANNEL VOLUME of 7.5 or more... the ""bump"" re-appears. If I keep the CH VOL at 6.5 the ""bump"" does not re-appear by an increase in the MV settings (went up to 10)



    NOTE: the "bump" above CH VOL 7.5 is not changed by placing the .002 uF cap at the REVERB OUT or changing the 560 pF cap to V3 cathode (pins 3-8)

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    Last edited by TelRay; 05-26-2019 at 06:14 PM.

  30. #310
    Bent Member Chuck H's Avatar
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    I looked through every page, but I couldn't find a clear shot of the phono/rca jack wiring. The one photo I could find wasn't clear enough to be sure, but it looked like some stuff may have been wired wrong. I'm sure some things have changed since then, but... Can we get a shot of the reverb and footswitch jack wiring?

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Quote Originally Posted by Chuck H View Post
    I looked through every page, but I couldn't find a clear shot of the phono/rca jack wiring. The one photo I could find wasn't clear enough to be sure, but it looked like some stuff may have been wired wrong. I'm sure some things have changed since then, but... Can we get a shot of the reverb and footswitch jack wiring?
    sure, mister.
    in theory you shuold be able to see this








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    Last edited by TelRay; 05-27-2019 at 03:09 AM.

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    @ Chuck, were the photos OK?
    (just let me know if you want something else)
    thx!

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    The photos are fine. And (unfortunately?) wired correctly as far as I can tell. So I'm confused as to why disconnecting the reverb tank input would squash the problem. It's definitely a parasitic oscillation because when it first comes on you can see the time lag due to phase error on the one lobes return and then it's that area that develops the hash at higher gain. So that's a positive feedback. I only have observations at this time, not solutions.

    Since removing the load from the reverb transformer secondary also reduces current through the reverb transformer I think you may want to investigate any daisy chained grounding or ground locations for specific circuits relative to the reverb. Try separating daisy chained grounds in the signal path for the reverb recovery and the mixer stage and make sure the reverb driver tube circuit and transformer secondary are grounded with their power supply filter and NOT the preamp. Except for the 1M resistor on the grid of the reverb driver tube. Keep that grounded with the preamp (but not sharing a ground lead with another circuit).

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    As the reverb transformer secondary is grounded to the chassis via the RCA jacks, it is important that these make perfect chassis contact. Also I don't like some of your solder joints.
    I may make sense to dismantle the RCA jacks and clean the contact parts/chassis area with Deoxit (on a Q-Tip). The ground lugs are easier to solder when removed from the chassis.
    Also clean and tighten the outer "crown" of the RCA plugs.

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    I didn't want to say anything But yeah, probably time to think about cleaning up the overworked solder from the repeated work.

    As to the actual problem, I actually think it may be "normal" for that amp. Two channel Fenders with reverb and vibrato are spaghetti monsters and that's why you see many HF ground caps and such in the silver face line. That doesn't mean we shouldn't try to "fix" it

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "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

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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