Silverface Vibro Champ Questions

Discussion in 'Amp Tech Center' started by OldPup, Jan 21, 2020.

  1. OldPup

    OldPup Tele-Meister

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    I've got a SFVC with a couple of problems. The tremolo circuit's speed seems ok, but the intensity does not respond until you get past 7. I tried the easy stuff first: swapped out 12AX7s, cleaned pots. None of those were fixes.

    The three ceramic capacitors do not appear stock, but seem to have the correct capacitance ratings. I have not been able to decipher all writing on them. One is labeled with: BC 25u 203Z. The other two are: BC Y5P 103K. I get that 103 and 203 are 10,000 pF and 20,000 pF, respectively. K refers to tolerance. I can't find anything on the rest. Can anyone help me out with that? Are they rated appropriately for voltage? The cathode cap is not original and is rated at 25uF/25V. I plan on replacing it with 10uF/50V. Could it be a culprit as well?

    Edit: there are 2 of the .01uF and 1 of the .02uF caps.
     
    Last edited: Jan 21, 2020
  2. Whatizitman

    Whatizitman Friend of Leo's

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    25uf bypass cap is correct. No reason to change that value.

    I’m not an expert by any means, but I’m doubtful that the other non electrolytic caps have anything to do with the tremolo issue. I don’t have a schem in front of me, though. Could just be the pot. Have you measured it’s resistance through its range?

    You said there are other issues? That could give a clue.
     
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  3. Dacious

    Dacious Poster Extraordinaire

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    The SFVC has a very subtle vibrato. It doesn't do much below 6. If you turn down volume you notice it has more effect. Sometimes changing tubes in V2 can improve it. But it will never be particularly strong below 6.
     
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  4. King Fan

    King Fan Friend of Leo's

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    I always agree with Dacious, and I'm far from understanding trem.

    Still, if you wanted to trouble-shoot, I'd look past the ceramic caps and measure the pot as suggested. Ensure its ground is ±0 ohms to the chassis, and maybe check out all the resistors in the trem circuit. A voltage table might be helpful right about now.
     
  5. Jon Snell

    Jon Snell Tele-Meister

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    Ceramic capacitors are always a problem in amplifiers. They can become microphonic and cause whining or squealing like feedback.
    To put a 10u capacitor back in is a shrewd idea as that shunts the tremolo modulator making it very insensitive. Considering the 25k pot feeding an effective 1k5 load, it is no surprise the control does not start to work until about half way up. It modulates the cathode of the driver section of the amplifier causing the effective gain to increase or decrease with the voltage by adjusting its bias.
    103 is 0.01u or 10nF and 202 is 0.02u or 20nF. I cannot confirm the voltage codes.
    Hope that helps you to understand the workings of the amplifier.
     
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  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    There are three disc caps in the tremelo circuit. The ones that are strung together. (a .02uf and two .01uf)
    Between the top two caps (closest to the pots/faceplate) there is a 1 Meg resistor that is wired to pin 3 of V2 on the other end. Replace this resistor with a 470k or even a 220k. You will have stronger vibrato.

    I probably wouldn't do this until you have tried a good tube. You may just have one of those disc caps going bad. But this mod is good anyway if you like strong vibrato.
    thevintagesound.com/ffg/
     
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  7. OldPup

    OldPup Tele-Meister

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    Regarding the trem - Do I need to disconnect all wiring to the intensity pot to measure its resistance? The resistors in the trem circuit measure within tolerance. I'm a little put off at the notion of making the tremolo less sensitive with the 10uF/50V cap. According to my limited understanding, the reason to swap a cap there is due to voltages commonly exceeding 25V at that cap in this circuit. Maybe a 25uF@50V will suffice and leave sensitivity closer to stock? Any insight there is appreciated. Speaking of stock, I have another SFVC that is bone stock, except someone snipped off the ground at the plug (so I won't spend time on it until I get the plug repaired). When I tested it during the purchase, I recall its tremolo is much, much more responsive. Its effect was noticeable when engaged at basically any level, whereas the one I'm working on has zero effect until an intensity of 7, when it suddenly appears. Maybe I need to get the stock VC out and confirm what my expectations ought to be...

    It might be a week before I get a chance to get voltage readings. I will post an update when I do.

    Regarding the other problems with the amp, they are manifold. In retrospect, this thread may be premature because there are a few potential culprits I should run down first. The amp crackles (leaves rustling) upon start up and has a noticeable noise floor when not being played. The amp's sound is sickly. It has a new Jensen speaker installed by its previous owner, but it's not a clear as other VCs I've heard and the distortion is unpleasant, sloppy, ill-defined. This morning I realized the power tube is microphonic, so a replacement is in order (is there any way the power tube could affect the tremolo? I've read the VC's tremolo is from preamp bias shift utilizing the V2 12ax7, but I figured I'd ask).

    I've tested all resistors in the amp and all but two read within tolerance. The two that seem off are adjacent to one another in the middle of the baord and connect to the speaker via a red wire running along the rear of the chassis to the speaker jack. They're supposed to be 47 and 2700 ohms, but each reads 60 to 90 ohms. Readings for both jump around, so I don't have a lot of confidence in the measurements. I may lift a leg of each to try to get a more reliable reading. Is there any reason those two resistors would go?

    I appreciate any and all help, but it seems like I need to run a few things down before I can get resolute information to you kind people wanting to help out. My goal is to get the amp working properly so I can sell it with a clear conscience. Plus, gaining a familiarity with the circuit will help me maintain the super clean VC I plan on keeping.
     
  8. OldPup

    OldPup Tele-Meister

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    Thanks, I will keep this in mind! I want tremolo that works anywhere on the dial.
     
  9. Ten Over

    Ten Over Tele-Meister

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    I don't think so. The LFO is probably swinging the largest signal possible for that tube regardless of the value of that resistor. Reducing that resistor increases the speed, not the intensity.

    As an example, let's say that the LFO is going at 5Hz. The phase shift with a 1M/.01uF combination is -73 degrees. Changing the resistor to 470K changes the phase shift to -82 degrees at 5Hz and the LFO will not oscillate at 5Hz. Changing the frequency to 10.7Hz makes the phase shift -73 degrees again with the 470K/.01uF combination and the LFO will now oscillate. It's a simplified example, but the general principle is valid.
     
  10. schmee

    schmee Poster Extraordinaire Silver Supporter

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    All I know is it works on a bias tremelo in the Princeton. However, changing the disc cap value does change the speed. I have not experienced that resistor changing speed, just intensity when I did it.
    It is entirely possible that it won't work in the Champ (or any particular amp whose vibrato is driven by bias) I suppose though. Bias settings can eliminate a working tremelo even without mods for sure.
    It's a simple thing to try.
     
  11. Dacious

    Dacious Poster Extraordinaire

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    I think the difference is in the VC the oscillator is working to attenuate the grid bias, so it's a different effect than the PR.
     
  12. Ten Over

    Ten Over Tele-Meister

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    It can be mathematically proven that the resistor value will change the speed. By the seat of your pants, the Speed control works by changing the resistance in a CR pair.

    The tremolo works by having positive feedback with no phase shift. The tube shifts the phase 180 degrees and the three CR pairs shift the phase 180 degrees at a particular frequency resulting in zero phase shift and the tube oscillates like mad. The phase shift of a CR pair depends on the product of frequency x capacitance x resistance. If you reduce the resistance, you must increase the frequency and/or the capacitance to get the same phase shift.
     
  13. Ten Over

    Ten Over Tele-Meister

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    The LFO doesn't know what amplifier it is in and it will respond the same to a change in one of its CR pairs.
     
  14. Ten Over

    Ten Over Tele-Meister

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    You only need to disconnect the wire from the Intensity pot to V1B in order to test that pot. You need to disconnect the wire from the 2.7K resistor to the speaker jack in order to measure the 47r and 2.7K resistors. If you don't do these disconnects, you will be measuring parallel resistors instead of single resistors.

    The impedance of the cathode on V1B forms a voltage divider with the Intensity pot. The impedance of the parallel combination of 1.5K and the bypass capacitor decreases as the frequency increases. The intensity of the tremolo effect will decrease as the impedance of the cathode decreases because of the change in the voltage divider. We don't want that because the faster tremolo speeds will have less intensity. We can use a smaller capacitor that will have a higher impedance at the faster speeds, but this also affects the frequency response of the audio signal from V1B. Fender struck a compromise at 10uF. I would have gone with 5uF, but I wasn't there to make the decision.

    The impedance of the cathode at 12Hz is 1.3K with 5uF, 994r with 10uF, and 500r with 25uF. You would want to use either a 5uF or a 10uF capacitor because 25uF has an impedance that is too low for the voltage divider.

    The 25V rating is for DC, which is way more than the 1.7Vdc in this application.
     
  15. OldPup

    OldPup Tele-Meister

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    Thanks for all that info. I will have to read it multiple times to truly digest it. A supplier mistakenly shipped me a 10uF/100V cap. It's not what I ordered, but it's what I have. Can it be used in this amp?

    A couple of updates on the amp. I swapped out the power tube with the 6V6 from my other VC. That cleaned up the amp's sickly sound. The amp is crazy loud for 6 watts! I have a new 6V6 on the way. I was also able to confirm that the stock SFVC's tremolo is discernible from an intensity of 3 on up. Which confirms something is up with the tremolo circuit in the other amp, which is unresponsive until an intensity of 7. I hope to test the pot and get a voltage chart this weekend.
     
  16. Kitarkus

    Kitarkus Tele-Meister

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    I just got a new (to me) 1968 Silverface Bronco (same as VibroChamp). A lot to like about these amps and specifically the vibrato. I had similar experience as you, however, from playing quietly at night I observed that the Vibrato did in fact add a very mild effect from 2-3 on the intensity dial....albeit VERY mild. I get nothing from 1-2 on the dial. Intensity goes up from there....and not with terrific accuracy/sensitivity as you move up the dial. Speed dial is much more sensitive to changes in the dial as compared to the sensitivity of the intensity knob. Your mileage may vary...I do not have a lot of experience with large numbers of these amps....but thought my experience might help.

    These amps seem to have decent pots....but the pots certainly do not have a good 'feel' to them.
     
    Last edited: Jan 23, 2020
  17. OldPup

    OldPup Tele-Meister

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    Oooh... a '68 Bronco. That would be sweet - with or without the drip edge! Have you done any work on the amp? If so, did you attempt to rein in the current at the power tube?
     
  18. Kitarkus

    Kitarkus Tele-Meister

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    This is a drip edge. I just purchased the amp this last weekend so I am still feeling my way around the amp. I took it to a tech who replaced the lamp cord with a proper 3 prong. He went over the amp and replaced one cap, one resistor, and removed the death cap. He said that his 'readings were all over the board which is typical of these amps but I don't see any reason to replace anything else at this time'.

    I got the feeling that this tech didn't have much respect for this little amp....saying 'it is what it is' etc. I am confident that the amp is safe to run and without substantive defects. It does (volume off) hum just a little louder than my 1971 champ...and it does get hotter than my Champ. I need to get a 6V6GTA put in her as I am currently running a 6V6GT. I put a Jupiter 8SC speaker in it yesterday and am still 'feeling it out'. I replaced the 12ax7's with new tubes. The rectifier tube appears to be original RCA.

    Super cool amp. I have no tech skills and repairs are expensive. Enjoying it for what it is now....and we shall see if it really needs any real work done to it. I had posted about this amp last week during the process of buying if you are interested https://www.tdpri.com/threads/1968-...p-edge-black-line.1001729/page-2#post-9568382
     
  19. OldPup

    OldPup Tele-Meister

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    I think I did see that thread. I need to go carefully back through it. Thanks for the link. A drip edge Bronco has got to be a rarity, right? Especially one as clean as yours. The circuit is not very different from my SFVC, but I noticed a few small things. I will try to post a pic in case you are interested.

    I played a Bronco before I ever heard or played through a Vibro Champ. It was an incredible amp. I would love to have one, but I don't think the wife is gonna be able to ignore it if I acquire another...
     
  20. OldPup

    OldPup Tele-Meister

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    Update: I dropped a Tube Amp Doctor 6V6 GT-STR in the amp. It's cleared the sick sound up nicely. I have not been able to play it much because I only get to work on the amp once my daughter is in bed, but what I heard sounded nice. I have run several bias calculations and come up with dissipation of 13.61 watts, 12.58 watts, and 11.9 watts. The last is from Rob Robinette's online calculator. The tube is rated for 14 watts, which (apart from a number of positive reviews) is a big reason I chose it. I'm satisfied with those results.

    Pin voltages are:
    V1
    Pin 1: 261
    Pin 3: 1.9
    Pin 6: 250
    Pin 8: 1.99

    V2
    Pin 1: 150 - 190 (oscillates)
    Pin 6: 415
    Pin 8: 160 - 190 (oscillates)

    V3
    Pin 3: 412.9
    Pin 4: 413 (not sure about this one. I don't know that I recorded the right pin #).
    Pin 8: 31.3

    V4
    Pin 2: 421
    Pin 4: -4.2
    Pin 6: -4.4
    Pin 8: 421

    I'm wondering if I missed pins on V2 and V3...

    The trem is still acting up. I have not had a chance to dig into it. Any advice on that is much appreciated. Edit: I'm going to pull the intensity pot this weekend and see how it reads, per previous advice. We'll see if that leads to a solution. Thanks for the help, all.
     
    Last edited: Jan 30, 2020
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