EA ELECTRONICS - Amplifier Article #2
Modding a Custom Vibrolux Reverb
Warning! Tube amps operate with lethal voltages. If you are not absolutely certain about what you are doing leave this kind work to qualified technicians.
This mod will most surely void your warranty!
First of all, credit where credit is due, to Mark Moyer who created the instructions for the mod. My contribution was just to add the visuals. I bought my Custom Vibrolux Reverb (CVR) back in 2003 and it sounded OK in the store (where there was a lot of background noise). When I got it home in a quiet room I realized that it had a major hiss problem. So I went online and found Moyer's post on the Fender Forum.
The type CSR8 CVR has the "reverb on both channel", an interesting feature if you don't mind the hissing noise and shallow sounding reverb. Another interesting fact about the CVR is that unlike virtually every other all-tube Fender amp, it has no feedback loop which is only going to make the noise worse. A lot of people do mind and would gladly give up this "reverb on both channel" for a quiet amp with big clean reverb tone. Here's how to do it:
Note: you can use 1/2 watt resistors and 600 volt capacitors unless otherwise specified.
On the CVR a blue wire (indicated by the dashed line below) connects the plates of V1B and V2B together. Unsolder this wire from pin 6 of V2 and solder it to the hole marked 6 in the V1 section of the printed circuit board (PCB). Change R23 from 47K to 82K (1 watt).
Install 220K resistors in positions R11 and R35. Replace the 500 pfd cap at location C16. Moyer recommends a .0033uF/1kV silver-mica cap. I didn't have any in stock but a .002uF/600V orange drop works well.
Next; adding the feedback loop. This involves changing the value of R37, installing R41, R42 and running a wire out to the speaker jack.
Next, Moyer recommends adding C20, a 100 pF cap which is across the plates of the phase inverter. According to Moyer, this capacitor helps to reduce parasitics (which are never a good thing) and has no effect on the sound. The parasitics are an inherent problem with circuit boards, which is why you won't see these caps on vintage Vibroverb schematics since they did not use printed circuit boards. I should probably add this cap on my CVR.
Another thing Moyer recommends is removing the 3kV surge protection diodes (CR6, CR7) on the 6L6 tubes. The reason he gives are a) the diodes tend to cause the amp to blow fuses when switching from standby to on, and b) they tend to drain off some of the high frequencies.
Here is where I disagree with Moyer, my CVR has never blown any fuses when switching from standby to on and the diodes protect the output stage from high voltage spikes. I'm keeping the diodes on my amp. Moyer is correct in saying that none of the vintage amps had these diodes, but in the 1950s and 60s these 3kV diodes may have been either unavailable or very expensive.
The final step Moyer recommends is changing the termination resistors on the 6.3 VAC filament lines from 100 ohms to 47 ohm/1W. This is to reduce the hum and hiss even further. On a historical note; all the vintage amps used 100 ohm resistors for this (except the ones which had a grounded center tap on the 6.3 VAC windings and didn't need the resistors). These resistors provide a low impedance path to ground for the filament winding. I didn't make this mod on my amp but I don't see any harm in trying it if you are still getting noise after the previous mods.
On my CVR I added a couple of RCA jacks so I could easily disconnect the reverb tank when pulling out the chassis for servicing. I also replaced all the plastic input jacks with Switchcraft metal ones.
Final note, in order to do these mods, you need to get access to the bottom of the printed circuit board. This means unmounting all of the jacks and pots on the front panel and disconnecting a lot of the push-on connectors. This is major part of the job and be sure to tag everything so you can put it back together the same way!
Warning! Tube amps operate with lethal voltages. If you are not absolutely certain about what you are doing leave this kind work to qualified technicians.
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