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Thread: Fender SF Revrb Unit conversion to 6G15 Circuit

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    Supporting Member nevetslab's Avatar
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    Fender SF Revrb Unit conversion to 6G15 Circuit

    One of my clients who handles all of our piano tuning needs stopped by with a Fender SF Reverb Unit that he’s had stored away in his garage. Very dark sounding, lacking life and sounding absolutely NOTHING like the original 6g15 Reverb Units from the days of Dick Dale and all the other surf bands in the early 60’s. He was interested in having it modified to sound like that original unit.

    I took on the challenge. I made up a parts list to go by, and marked up the SF list with what changes someone else had made years ago, having changed the coupling caps and tone caps to Sprague Vitamin-Q oil-filled paper types, hermitically sealed caps

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    reverb_6g15_schem.pdf

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    Wanting to use a 6K6GT Driver tube, I looked into the Reverb Driver Transformer, not finding much to go by, I ended up ordering a W022905 from Ted Weber’s company, along with his W022699 9H 120mA choke. Also ordered 250k Audio & 250k Lin taper pots for replacing the Dwell and Mixer pots, along with some 100nF polypropylene caps, just in case I’d need them. I decided to leave the Vitamin-Q oil-paper caps in place to start with.

    Parts arrived. I was curious to see the difference between the physically larger Fender 012990 Reverb Driver Transformer and that of the Weber W022905 I can post those details later, but did find the Weber Xfmr in the 7k primary impedance from 100Hz-4kzhz, while the Fender Xfmr was 3.6k-4.1k over the same range. So, the Weber was a better match for the plate circuit of the 6K6GT. Smaller core size, so LF saturation point would be higher, though not a concern here.

    With marked-up documentation, I un-wired the tube sockets, pulled all the parts being changed or eliminated. I installed all the new parts on the tag board first. Then, drilled a new mounting hole for the Weber Driver Xfmr and the choke, which fit nicely inside next to the fuse post. I pulled the power supply compartment open, re-wired the SF circuit and installed the input choke ahead of the 2nd filter stage.

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    Then, wired up the Octal socket with the new wiring. I had decided to use small Mogami shielded coax in all the grid and plate wiring and connections to/from the front panel controls & jacks. I replaced the two panel pots, adding the new wiring as I went along. The only change I made to the 6G15 circuit was a 390pF polystyrene cap from the Tone pot wiper to the Mix pot, in place of a 250pF ceramic cap. I left the 10nF Vitamin-Q cap on the tag board, it being the other tone cap in the circuit, connected with the original long wire.

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    I wired the output jack from the wiper of the Mix Pot as is done in the 6G15 circuit.

    When I first fired it up and verified all the circuits were nominal DC-wise, I gave it a listen. Still very dark and murky sounding, which surprised me. I extended the cables of the two reverb tanks, having the Fender 6g15 Re-issue unit in the shop with me for reference. No difference noted in the tanks…both sounded fine with the 6G15, and both sounded dark and murky in this re-build to that of the 6G15 circuit.

    Lots of things were rolling around in my head at this point. Tag board layout is different. Added a fair amount of shielded cable, so added capacitance. Old Tag board itself. The chassis showed signs of oxidation from being stored in a cold, non-ideal environment, so….has the tag board changed in it’s dielectric properties? I’ve heard stories about them going bad, or becoming questionable. Heaven forbid, looking at what it would take to replace it!!

    First thing I changed was removing the Vitamin-Q caps from the coupling circuits & tone circuits. Some improvement, but not much. Then, replaced the 390pF polystyrene cap with a 220pF ceramic cap. That helped, but still lacking top end, still dark sounding. I added a 120nF cap across the 2nd stage cathode resistor, giving a HF lift at around 880Hz. That helped a bit more. I also added a 1500pF polystyrene Brightness cap across the Dwell pot. That also helped. Finally removed the 10nF mylar Tone cap from the tag board and installed a 10nF ceramic right off the tone pot to Gnd. Major improvement!.

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    Fender 6G15 Revised Schematic.pdf

    I was now pretty close to the sound of the 6G15 Re-issue now. I had thought about trying to replace the bottom insulator sheet below the component tag board. I backed the screws out as far as they would go, without removing them, and lifted up the component board to give a listen.

    I’ve been using burst pink noise as a signal source during the re-build, 2 sec on, 3 sec off, so there’s enough time in the cycling to listen to the tone and tail of the reverb, comparing it with the Re-issue unit. I didn’t hear any change in lifting the tag board up. Still doesn’t mean I don’t have issues with that board material. This afternoon, my client is coming back in, so we'll get a chance to listen again to the 6G15 Re-issue, this converted SF unit and the Vibro-King, which has a nearly identical sounding RU circuit built in, with him playing a Fender Stat.

    I finally listened to different tubes, That didn’t yield any improvement, though I did like the 12AY7 as input tube.. I hadn’t yet listened to the output buffer tube, the one thing I liked conceptually about the SF RU. It was still using a Vitamin-Q coupling cap, and still wired up the same, other than having replaced the shielded cable to the input grid bias resistor. I liked the sound of that stage added in., so I punched a hole in the chassis for an insulated phone jack for that output, having left the Output Jack on top wired to the Mix pot wiper as per stock 6G15.

    The two don’t sound identical, but it now has most of the character and sound quality of the 6G15 Re-issue. Took me 3 days labor to get it there, so needless to say, another expensive education while my client gets a hell of a deal and new life out of his former murky-sounding SF Reverb Unit.

    Also interesting was finding the foot pedal, it having a shielded polyethylene core cable, it ADDED top end to the Reverb when plugged in and engaged!

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    Last edited by nevetslab; 11-04-2015 at 01:01 AM.
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  2. #2
    Lifetime Member Enzo's Avatar
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    I have to ask, did we try making the existing circuit work up to spec before tearing it out?

    Did we demag the heads? Accumulated magnetism is one cause of loss of high end definition.

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    Quote Originally Posted by Enzo View Post
    Did we demag the heads? Accumulated magnetism is one cause of loss of high end definition.
    Oh Enzo!

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    I read up on what others had done in attempt to make the SF Reverb Unit sound better, and only those that committed to gutting the circuit appeared to get there. Why I still ended up with lack of high end and dark, murky tone still isn't clear. I did change back to ceramic caps in the tone circuit for their edginess, and it sounded like a good call. Not normally a fan of ceramic, but they did render the sound I was after, having listened to other dielectrics (mylar, polypropylene, polystyrene, polycarbonate).

    One attribute I didn't defeat, as far as magnetics are concerned, is the field radiation of the power transformer, being picked up by the reverb tank's Return Transformer. Both the 6G15 Re-issue and this SF RU have the same ugly waveform. Moving the tank out of the cabinet eliminated that. Thus far I haven't cut any shielding material to cover the open pan.

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    Lifetime Member Enzo's Avatar
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    Oh man, well I hadn't had anything to eat, and I had been discussing an Echoplex elsewhere, and... and...

    So you know, those old silverface Echoplexes... and ...and...

    Um, how about them Kansas City Royals?

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    You got peanut butter in my chocolate .

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    Supporting Member nevetslab's Avatar
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    ......well, it seemed like a good idea to go back out and pitch the 9th. opps. oh man, how did Cain get on 2nd?

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    Deleted the HF pre-emphasis caps

    After the guitarist had a chance to play with both the 6g15 Re-Issue and the modified SF Reverb Unit, wired up like a 6g15, with the added HF boost caps I added, we found the pre-emphasis was too much. He brought it by today after living with it over the weekend, and we confirmed pulling the Brightness cap across the Dwell pot and the 120nF across the 1.5ik cathode resistor of the 2nd stage tamed that high end down.

    We also listened to the difference in the 10nF Tone cap to Gnd...bottom of the Tone Pot. I added a 10nF mylar to the same ground, and clipped in either the Ceramic or the Mylar while he was playing. For what was being rendered in the Reverb signal....the 'tail' or decay character, the Ceramic added more brightness to it, while the Mylar cap was darker. Ceramic measured 11nF, with DF of around 0.6%, while the Mylar measured 9.8nF and DF around 0.3%. Regardless of steady-state measurements, we stuck with what sounded better to the client's ears.

    Still puzzled about the overall darkness of the SF RU in general. We're dealing with a 40 yr old SF unit built on the tag board, only had a Re-Issue 6G15 to compare against, built on questionable-quality circuit board material, like that found in the Hot Rod series.....beige epoxy glass, I think. Different layouts of course.

    If I were to do one again, I'd be inclined to build from scratch a hand-wired board on FR-4 & stacked-in terminals, and probably follow the parts layout of the original 6G15. It was, of course, easier to work off an existing component board, not knowing the overall darkness remained even after wiring it up with the 6G15 circuit.

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