‘68 Custom Vibrolux Transformer Hum

[Wally]

Hiss in these amps is a combination of the negative feedback resistance being doubled (reduced), the bridging of the two channels to put reverb & trem on the "Custom" channel and the circuit layout. On my '68 CDR I returned the NFB resistance to AB763 spec, removed the jumper that bridges the channels, essentially making the "Custom" channel same as the "Normal" channel on a garden variety DR, and cut the trace to the V2 input from the volume pot and ran shielded cable from the pot to V2. To put the reverb back on the Custom channel I may do Rob Rob's mod, but I have to do some study.

This amp was the noisiest thing right out of the box. The grounding scheme is a bit nuts with loops everywhere. However you can make it work. I made quite a few bulletproofing changes, complete recap with quality capacitors (F&Ts), eliminated the 220/100 filter, uprated the filter & screen resistors to 3w, raised the screen resistors up off the board, moved the heater circuit off the PCB and mounted directly to the power tubes, Mounted the ACT near the PTY, etc. I triedf moveing the power section grounds off the board to points near the PT, but because Fender has the screen cap (also reverb & trem) grounded with the power supply, rtather than the rpreamp ala AB7643, it created a ground loop hum in the reverb. The only fix was to either cut the ground trac e on the main PCB or revert back to stock. Still a work in process, but quiet as a mouse now.

That is a lot of work to make a production amp ‘usable’, imho.

 

[Wally]

One wonders if the 1x12 Custom Pro Reverb avoids these problems?

 

[Michael Smith]

One wonders if the 1x12 Custom Pro Reverb avoids these problems?

I don't own any of the reissue amps, but from what I understand (from watching amp tech videos), all of the '68 Custom series amps are problematic. The '65 series are said to be much better. I agree that it's a shame that a brand new amp needs work just to overcome design flaws.

I expected to have to do some servicing on the 1960's and 70's silverface amps I bought in the last couple of years.

 

[Wally]

Re: the 68 SF customs….imho, a person should expect some noise when the NFB has been reduced or eliminated. Negative feedback cancels signal. When that NFB signal is reduced or eliminated, the amp naturally sends more signal to the speaker…more signal equals more noise.…and more harmonic content, which I like in some cases.
i l8ke to establish a stock NFB situation in such amps…and then add a potentiometer so that if and when a richer sonic is wanted one can reduce that NFB loop’s cancellation factor.

 

[Lowspeid]

Re: the 68 SF customs….imho, a person should expect some noise when the NFB has been reduced or eliminated. Negative feedback cancels signal. When that NFB signal is reduced or eliminated, the amp naturally sends more signal to the speaker…more signal equals more noise.…and more harmonic content, which I like in some cases.
i l8ke to establish a stock NFB situation in such amps…and then add a potentiometer so that if and when a richer sonic is wanted one can reduce that NFB loop’s cancellation factor.

I might just have to live with it, because the amp sounds fantastic.

 

[SoK66]

That is a lot of work to make a production amp ‘usable’, imho.

Indeed. I can't believe these things get out the factory gate like this. It had a bad #1 input jack on the Custom channel as well.

 

[SoK66]

One wonders if the 1x12 Custom Pro Reverb avoids these problems?

Fender insists upon using those wretched IC electrolytic capacitors, which is one foot in the grave right there.

 

[Wally]

I might just have to live with it, because the amp sounds fantastic.

I like to install adjustment pots in the negative feedback loop to decrease the cancellation factor. Noise increases a bit, but the payoff is some versatility to access richer, hotter sonics.
There is an increase in signal noise every time a player steps on a gain pedal And/or turns an amp up. One has to accept certain conditions in order to achieve certain sounds.
Many young players would run away from a rig of the type that was/is used to create the sounds that many of them would want to emulate. When a 100 watt Super Lead Marshall is at ‘speed’, not only is there some signal noise created but pant legs will be moved while standing in front of the cab!