Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on September 17, 2011, 12:58:56 pm

Title: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: LooseChange on September 17, 2011, 12:58:56 pm: I have a Fender Custom Vibrolux Reverb on the the Bench. The owner wants more clean headroom out of this amp. I did the NFB mod and changed V6 to a 12AT7.
The amp breaks up pretty quickly. I like the breakup for some stuff but it should get a bit more clean.
> Does this type of tone stack attenuate less than the AB763 tone stack?
> My thought is that V4b is getting hit too hard.
> I removed the cathode cap on V1b - helped a bit.
> I found an undocumented part on the circuit board (R71). It is a 10k grid stop on pin 7 of V4b.

Any thoughts on cleaning this up a bit more?

Keep in mind that this is a PC board and a bit difficult to mod extensively.
Thanks!
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: LooseChange on September 17, 2011, 01:05:04 pm: In addition, The amp is extremely close to the Vibroverb 6G16

http://www.el34world.com/charts/Schematics/files/fender/vibroverb_6g16_schem.pdf (http://www.el34world.com/charts/Schematics/files/fender/vibroverb_6g16_schem.pdf)
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: tubenit on September 17, 2011, 03:47:32 pm: Use a 5751 in V1. Use a 4.7uf or 5uf instead of 22uf (or 25uf) cathode caps on the early gain stages.

With respect, Tubenit
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: LooseChange on September 17, 2011, 04:09:06 pm: I removed the cathode cap and I tried a 5751, 12AT7 and a 12AU7.
Interesting thing, when I put the 12AU7 in V2, I get nothing out of the Normal channel.
I think there is just too much gain due to V4.
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: VMS on September 17, 2011, 04:35:32 pm: Quote from: LooseChange on September 17, 2011, 04:09:06 pm
Interesting thing, when I put the 12AU7 in V2, I get nothing out of the Normal channel.

It looks like V1b and V2b together make a 'tweed mixer'. Using different type tubes for V1 and V2 probably alters the function of this mixer. :dontknow:
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: Cups on September 17, 2011, 05:08:11 pm: Wouldn't changing v1b and v2b's plate to 100k and cathode to 1k5 (in other words "usual" fender values) reduce gain?
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: phsyconoodler on September 19, 2011, 11:49:41 am: Google 'Moyer Mods' for Custom Vibrolux. They work well.
Title: Re: Fender "Custom" Vibrolux Reverb - Did the mods and need more clean. (?)
Post by: jlaj on April 02, 2021, 07:02:58 am: Hey guys, looks like I’m VERY late to this topic but I just picked up a CVR, as is being discussed and after going through some of these mods, the one I did end up reversing was the reverb mixer with the 3.3M/10pF cap that swaps out the 470k resistor. That had the most effect on reducing the gain so I actually put the 470k back in as I preferred the earlier breakup.

As mentioned, late to the party but when I was researching these mods, I couldn’t find much info regarding how each mod will affect what, so I figured why not put some information out there in case someone is looking later on.
Cheers.