Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: guitarhvac on January 25, 2011, 11:13:31 am
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I was wondering if you guys could tell me what is responsible for the major difference in the sound of the vibrolux vs. the twin reverb.
I have a twin and to me it sounds horrible next to a vibrolux. Maybe it's just my preference, but the vibrolux has so much natural compression and sustain!! My twin has none and just sounds too............not good for lack of better explanation. Anyways, I wanna make my twin sound as close to a vibrolux as possible. Any help is much appreciated!!
Thanks a lot!!!
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Vibrolux or Vibrolux Reverb? Silverface, blackface or ??? for each amp?
Knowing nothing else, think about the schematic and/or the amps in front of you.
Twin: 12" speakers, 100w, feedback loop of 820 and 100 ohm resistors.
Vibrolux: 10" speakers, 50w, feedback loop of 820 and 100 ohm resistors.
The different-size speakers can be a different sound on their own, but the lower-power amp might also get turned up more than the higher-power amp, probably causing more speaker break-up. You might be hearing a bit more distortion in the smaller amp all around.
The feedback loop also looks-same, but isn't. Both use the same resistors, going to the same places, but each amp has a different amount of power (we are assuming blackface Twin Reverb vs silverface Vibrolux Reverb here). So what? I'm glad you asked...
Because each amp has the same 4 ohm speaker load but outputs different power, there is a different amount of voltage present at the OT secondary, assuming each amp is turned up to maximum clean power. The point here is that for something like a similar volume-knob setting, there is a each amp outputs a different amount of power.
Power = Voltage2/Resistance, so Power*Resistance = Voltage2. Take the square-root to find the voltage present at the speaker terminals.
SqRt (100w * 4 ohms) = 20v
SqRt (50w * 4 ohms) = 14.14v
After being reduced by the 820/100 feedback voltage divider, the Twin has 2.17v of feedback, while the Vibrolux has 1.54v of feedback. That's not a big difference (in volts or dB), but probably enough to help the Twin have a cleaner, more authoritative sound.
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The Twin Reverb has a 460VDC power tube plate 340-0-340 PT
The Vibrolux has a 410 VDC power tube plate using a 325-0-325 PT
For starters you would need to change the PT out for a lower B+ to get the lower preamp & power tube voltages
Your twin does not have a rectifier tube so your plate VDC may still be a bit higher using rectifier diodes
Is your Twin Reverb vintage or a reissue?
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Difference between Twin reverb and Vibrolux? about 20 lbs. :wink:
http://www.ampwares.com/fender.asp : SF Twin Reverb: Weight: 69 lbs. SF Vibrolux: Weight: 50 lbs.
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My twin reverb is a silverface early 70's!
Thanks for the help!
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Assuming the bias is set correctly, I'd change the 820Ω NFB resistor to about 3300Ω, and disconnect the tremolo. You could replace the 10kΩ midrange pot with a 50kΩ pot. If that doesn't open it up you could rebuild the PS and replace the PT and OT- not really a cost effective option. You can get a Twin Reverb to sing, but only at ear-splitting volumes. It isn't a 135w Twin Reverb, is it? You'll never get that to sound like the Vibrolux, sell it to someone who wants a high-powered clean amp and buy a Vibrolux (or perhaps a Pro Reverb.)
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turn your twin into a vibroverb? :undecided:
http://www.el34world.com/charts/Schematics/files/fender/vibroverb_ab763_schem.pdf