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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
reverb_6g15_schem.pdf
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.
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.
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!.
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!
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
reverb_6g15_schem.pdf
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.
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.
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!.
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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