Tube Amps / Music Electronics
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|1/10/2001 7:29 PM|
|Jack A. Zucker
||Vibroking Reverb Sucks!|
Sorry to be so inflammatory. I have a recent Vibroking (the newer one with a 6V6 driver tube) and I'm looking for ways to improve the reverb which is very "crashy". It's not nearly as sweet as my '70 Super Reverb or my outboard reissue reverb unit. I've experimented with an AT7 in V1 and a 6K6 in V3 which improves things a bit leading me to believe the reverb tank is being driven too hard. Has anyone done any circuit changes which make the reverb in this unit passable?
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|1/10/2001 7:52 PM|
Reverb tanks are mass produced. The assembly is actually quite complex for how simple it looks, and contains close tolerances at the coil/armature interface. Occasionally you get a poor performing or defective reverb tank. Before you do anything to modify the circuitry, try using some extension phono cables and hook up your Super Reverb's reverb pan over to your VibroKing amp. If the reverb sound quality improves, you found the culprit. If the sound is still poor, then you could rework the resistor values on the reverb circuit's input voltage divider for less drive.
|1/10/2001 7:59 PM|
|Jack A. Zucker
I should have mentioned that I already tried the Vibroking's reverb pan in the Super Reverb (sounded wonderful) and the Super Reverb's pan in the Vibroking (sounded awful) so the pan is not the culprit here.
Regarding reworking the voltage divider on the reverb circuit's driver input, would I be better off doing that or changing the cathode bias of the 6V6? I don't have a schematic to the Vibroking but I found a Dual Professional schematic which had different cathode bias parts than mine does. Mine has a 330ohm resistor bypassed with a 100 mfd cap.
I believe the Dual Pro uses a 1kohm and 100 mfd cap. (I don't have the schematic in front of me)
|1/10/2001 8:02 PM|
Drop the supplied B+ voltage to the 6V6 or 6K6 tube a little and make sure the tube is not idling too high or producing too much power for the reverb OT.
You can go as high as a 470ohm cathode resistor too. Try a smaller value cathode bypass cap... 5uF or so.
I've replaced the reverb OT with a bigger one in couple that were killed by too much power.
Is the 606-XXX, B0047605 OT? Pretty decent OT for the 6K6GT if so.
Wire in a some negative feedback to the driver from the output jacks.
IMHO, they NEVER will sound like a BF Reverb.
Lots of players love them, so we could just be in the minority.
|1/10/2001 8:07 PM|
|Jack A. Zucker
Thanks Bruce. I'll check the reverb OT and get back to you with the part #. Which OT did you replace the ones you were looking at with? Any suggestions for resistor values for neg feedback or for lowering B+.
I realize it'll never sound like a BF reverb but I'm hoping to at least get close to a 6G15 which would make me very happy. I just don't understand why Zinky would bother to use that Reverb circuit instead of just sticking to the BF design but adding a few tweaks such as better vibrato, effects loop, fat switch, etc.
I'll try bumping up the Cathode resistor. I saw a schematic for a dual pro and I believe it was using a 1k coupled with a 100 mfd.
|1/10/2001 9:24 PM|
|pumpstein||Cathode Bypass cap|
Bruce is pointing you in the right direction...
I worked on a b/f Twin Reverb once and the owner had subbed in a 50uf/50v cap for the stock 25uf cathode bypass on the 12AT7 reverb driver tube, and it resulted in a very crashy, boomy sound that the owner loved for its outlandishness, but which I thought sounded like doo-doo.
You may save yourelf some headache by simply de-soldering the 100uf cap and clipping in a 5uf or some other small value electrolytic. This will alter the frequency response of the tube and send far less mids/lows to the tank. Notice how the coupling cap feeding the driver stage of a standard b/f is typically smallish - like 500pf. This is so the tank doesn't get swamped with all those (conventionally) undesirable freq's, which many seem to agree 'cloud up' the reverberated signal.
I would probably change the 330-ohm cathode resistor to a 470-ohm as Bruce also suggested, because, really - and I can't imagine what Fender was thinking when they designed this reverb circuit - what's the point of trying to get so much gain out of this stage? If anything, I'd reduce the gain - I can rarely use ANY stock b/f or s/f reverb, set anywhere past 3.5 - there is so much gain already that its really kind of inefficient, IMHO - the useful range of the pot is quite small. I know some guys have swapped out the 1M pot for a lesser value, like 500k or 250k. I'd probably opt for 500k, as I seem to typically set it @ around 3.5, or roughly 330k.
BTW & FWIW, a friend has been after me to "fix" his VK reverb for a while now - maybe now I'll have him bring it in...you've got me curious.
Good luck -
|1/11/2001 3:15 AM|
Jack I looked at trying to modify my 1999 VK. After talking to a lot of people and looking at possible schem. mods. I sold the VK and purchased A Super Reverb. I sent it to Bruce for a tune up. This amp gives me chills everytime I play it.
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