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Fender Vibrochamp recap/mods?

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Posted on January 14, 2010 at 15:47:37
Blooze
Audiophile

Posts: 378
Location: Texas
Joined: August 1, 2005
 I've got an old 1971 Silverface Vibrochamp that I've had for several years that I'm sure is needing some attention. I bought it from an older gentleman (late 70's) that used it to practice for a church band he was in. The only thing I can see he changed was he had a 12AU7 running the pre instead of the 12AX7. I've never had a problem with it. The one thing I don't like about it is the vibrato settings on it are either barely on or full speed it seems like, without much real adjustment.

Any suggestions on recapping or any reversible mods?

Oh, and my mains voltage here tends to run 123-126V, if it's of consequence.

 

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RE: Fender Vibrochamp recap/mods?, posted on January 14, 2010 at 16:25:11
FenderLover

On the cicuit board are three cap discs in a row, by the middle 12AX7 (0.01 mfd, 0.01 mfd, 0.02 mfd). Change one 0.01 mfd cap to a 0.02 mfd and you'll slow down the vibrato section. Make sure that the vibrato is being driven by a 12AX7 type tube and not the 12AU7.

Replace ALL the electrolytic caps with new ones. Go to 50VDC on the bypass caps (to compensate for your house voltage). New 'lytics, like ATOMS are as small as the original 25 VDC rated one, so should be no space issue. Keep the main power supply filter caps at 20mfd/450 VDC (ATOMs are fine).

I also recommend changing the NFB by to blackface, changing the power supply resistors to black face values, and bypass the poor tone stack of this amp. I can explain those steps, if you wish.

 

More mods, posted on January 14, 2010 at 17:01:29
FenderLover

I had to had this later, as I didn't realize there is a time limit for posting.

To bypass the tone control, you are going to change is the 250 pico-farad cap off pin 1 of the first 12AX7. Take the right hand wire off the VOLUME control (tape it). Take the right hand lead off the TREBLE Control. Solder the right hand lead from the TREBLE pot onto the VOLUME control.

On the first 12AX7 you’ll see the 250 pico-farad cap connected to two 100K-ohm resistors. Remove the 250 pico-farad cap and replace it with a 0.02 mfd/400 VDC coupling cap (Orange Drop 716P is fine). Remove the 100k-ohm resistor that lies horizontal on the circuit board. At the bottom right of the circuit board.

You now have the tone pots bypassed. Which will give the amp much better midrange, as the tone pot arrangement robs the amp of the great 6V6GT mids.

To change the feedback loop to black face standards, find the 2700-ohm resistor running from the speaker connection wire. Replace this resistor with a 22k-ohm, ½ watt resistor. Solder it DIRECTLY to pin 8 of the 12AX7 (eyelet just to the right of the speaker lead).

The silver faced Vibro Champ runs the 12AX7 too high voltage, which give a strident voice to the amp. Drop it back to blackface standards by replacing the 10K-ohm, 1 watt resistor with a 22k-ohm, 1 watt resistor. The resistor is between the second and third filter cap on the circuit board. Now, the voltage on the plate is back down to black face standards. If the GAIN is still a bit too much, you can remove the bypass cap on the first 12AX7 (far right 25mfd/25 VDC eletrolytic on the board). This tube did not have a bypass cap, on the original black face Champs.

Last thing I did with my Vibro Champ is get a good speaker. I chose a Alnico Vintage 8-inch from Weber Speakers. Any good 8-inch, for the cab or a good 12-inch for an external cab will be better than the cheap 8-inch Fender put into their silver faced Vibro Champ.

I’ve personally had very good success changing the 5Y3GT rectifier for a 5V4G. Gives better bass and maintains the nice brown midrange.

 

one other thing, posted on January 14, 2010 at 17:11:34
FenderLover
 Fender did not put a resistor inline with the screen grid. This was prolly because 6V6GT were as cheap as resistors (and solder) in those days (??). I always put a 470-ohm, 1 watt resistor between the power supply and the screen grid. Keep the resistor leads as short as possibile. Just solder the 470-ohm resistor between pin 4 and pin 6 (unused). Then, solder the power lead to pin 6.

This will drop the screen voltage about 5 volts from the plate voltage on the 6V6GT. Helps to keep the screen from rising above the plate volts, during heavy usage.

I've done each of these mods to several silver faced Vibro Champs (tremolo slow-down, bypass tone control, dropping C+, speakers, screen resistor, etc) and each person's amp that I've done it to, has said the amp sound MUCH more blackface (good). Yet, still has good volume (very much speaker dependent. So get a good speaker, makes a big difference). I've used mine nearly daily for years and it sound pretty dang good. Very reliable, even when played at 6+ all the time. The only amp I have at this size that I think sounds better, is my very early Champion. Which has a much browner, sweeter tone. And perhaps better iron (Triad WW2 surplus items) and uses a 6SJ7 gain tube.

 

RE: Fender Vibrochamp recap/mods?, posted on January 14, 2010 at 18:00:28
Blooze
Audiophile

Posts: 378
Location: Texas
Joined: August 1, 2005
 Thanks for the reply, Fenderlover. I started looking up the date codes and I guess it's a 1976 (A51098 stamped on the chassis). For some reason I thought I had looked it up quite a few years ago and it was a '73. Oh well.

I would definitely appreciate any steps you're willing to provide! Especially with the values, types/brands of parts, etc....Seems like a pretty simple circuit.

I don't suppose a speaker change would be in order as well somwhere down the line?

I've heard one of the tweed Champ reissues and thought it was fantastic, but I'm not sure that sort of conversion is possible? I've never heard one of the original Blackface Champs. All in all, this amp is pretty good for the $25 I paid for it.

 

I'm slow on the draw (nt), posted on January 14, 2010 at 18:03:16
Blooze
Audiophile

Posts: 378
Location: Texas
Joined: August 1, 2005
 .

 

RE: Fender Vibrochamp recap/mods?, posted on January 14, 2010 at 18:47:56
FenderLover
 Check all the mods below. Blackface is one of the best sounding Champs. Try this first before going to the Tweed Circuit (which would mean removing the Vibrato section). A good speaker makes a WORLD of difference. Go for Vintage Blues style Alnico. IMHO, better sounding in this amp than a ceramic magnet speaker.

I run Tungsram ECC83, Raytheon JAN 6V6GTY, and RCA 5V4G is my Champ. I have not tried the Russian made 6V6GT, though many ppl like it.

 

Dear Fender Lover you are my Hero....so many questions, posted on January 14, 2010 at 22:01:23
-3db


 
I am starting my own guitar amp after so many years do hifi...you have answered several of my posts in the past about tubes and the like and I thank you....so,

1) you like the 716P, generally so do i, but some don't like it as much ...too 'bright' etc, but to me it always seemed smoother, less bright and deeper than any polyfilm ... Many people are liking MOJO, or Cornell Dublier WMF series....I would what you think...

2) I am looking at doing both of two front ends...one a Champ just like above, and another based on the Marshall 12AX7 front end, and both with the Fender/Marshall 5F6A tone stack....so I am thinking of doing the Marshall 12AX7 with battery bias and wonder what you thought of this for guitar....I also will actually be using 12BZ7, because this is what I like, and what I have 2 of...

3) I am looking at the cathode follower side driving the tone stack , and looking at using 6n1P, because I have one, and like the ways it sounds....any comments on this...

4) I have Polyprop capacitors in the standard 20uf for the power supply, any comments on sound versus the electolytics??/

5) I have an extra small Fender choke which I was going to use between the plate power supply tap, and the choke Power supply tap along with the 1K WW resistors instead of your 470ohm ones....any thoughts or comments on this???


Thanks so much for putting yourself out in the guitar world ahead of me...

Big Peace,
-3db


 

No hero... but a caveat!, posted on January 15, 2010 at 01:26:33
FenderLover
 No hero stuff, just school of hard knocks and mis-guided enthuiasm.

kk...

1.) I like a small variety of cap types. For the Vibro-Champ or any of Leo's single ended 6V6GT amps, the 716P is fine. 715P works fine, too. Keep your selection of caps simple. You want to maintain the tone. Keep as many OEM coupling caps in the unit as you can. Each change-out changes the tone. Often for the worse.

2.) Which model Marshall are you thinking about using a battery bias. One thing you prolly already know, that even head unit get bang about quite a bit. So the batteries would need to be quiet secure, in mounting.

3.) I've converted a couple Fender amps (a silver face Bandmaster and a silver faced Showman Reverb to use the 6N2pi-EV in first gain stage. Much quieter than usual 12AX7 type. Tone was ok. Problem was I had to give owners a couple two, three replacement tubes. So they wouldn't need to rewired the amp back to be to take 12AX7, when it came to replacing tired tubes. The 6N2pi-EV seem to last a bit longer than 12AX7, too.

4.) You can try it. For regular Fenders (TV front, Tweed, Blonde, Blackface, Silverface) I've had good results using Sprague electrolytics. They may not have the refinement of tone more expensive caps, but the Sprague ATOMs last so I haven't experimented much with other brands or types.

5.) I haven't tried this. So no comment.


I should state that my goal is to take an old Fender (or any vintage amp) and restore it as close as possible to the specific Fender circuit which gave the amp the best or most famous TONE. For instance, I'd take a Super Reverb and try to get it back to the early to mid 60's tone which Jazz and C&W guys really like. That clean, shimmering tone, with lots of headroom. Ok mids, but tight bass with clean high notes. Good harmonics.

Or a Deluxe and revert it back to that great brown tone of the early black face models. Yet, the ones that maintained good bass and slow onset of overdrive. More Bluesy style.

I do not experiment much with Fenders. I head for an endpoint I think is most desirable with the guitarists looking for these specific vintage amps. These amps are worth a LOT once they've been re-capped and altered to get the tone that made them famous. AND are made to be reliable. I've had several offers of well over $2K for my 1967 Princeton Reverb, prolly because few amps really can make the kinda tone of this amp... except PERHAPS the $5K boutique jobs out there.

So, my advice is do not stray too far from a specific tonal goal. Keep all old parts so you can revert the amp back to stock (I see silverfaced Vibro Champs going for over $1K now... if in original OEM form). Keep the mods as inconspicuous as possible. Every change to coupling caps or resistors shold be for a reason. DO NOT do wholesale change outs unless the unit is pure trash. Work for that vintage tone and RELIABILITY. You'll have guys wanting to drop off their amps all the time.

I've had more than my share of amps come to me, from other DIY/guitar amp freaks. They spent a ton of money on boutique parts and/or tried the latest "miracle" radical mod to the circuitry. And found the amp sounded like the "south end of a cow ending north." And then, they couldn't get the tone back to the starting point. Unfortunately, many of these amps sit in my "cave" to be used for parts.

 

Crate VC508, posted on January 15, 2010 at 04:50:57
FenderLover
 This is a GREAT little amp to start out with. A few modifications and a good speaker (I once again used an Alnico Weber 8-inch Vintage line speaker) & you have a very nice practice amp for guitar and Blues Harp. It is simply a 12AX7 driven EL84 single ended amp. I modded it more-or-less to what the LINK has, but I did put in better IC's in the front end.

Best thing I like about this amp is it has separate GAIN and VOLUME controls. So, you can real get the thing to fuzz out at very low volumes.

Anyhow, easy to modify. I use a 5751 and the Reflector 6N14pi-EV tube. Coupled with the Weber speaker and the mods you get a very aggressive, dark sounding Bluesy amp whose tone is just as good as many "boutique" amps that cost 5+ times as much. Good enough for studio work or miking on gigs.

I used to be able to buy these amps for around $100, used. In very good shape. I've modded and sold 5 of them to friends. The blues Harp guys LOVE the thing. Gives that really great Little Walter or Paul Butterfield tone. If you can find one for a good price, think about it. You can do the mods and get a good speaker for under $200. And you'll have a nice little practice amp with an attitude.

 

RE: Fender Vibrochamp recap/mods?, posted on January 15, 2010 at 09:58:54
zarniwoop
Audiophile

Posts: 590
Location: NY
Joined: November 28, 2002
 the silverface circuit is not changed from blackface in this amp, correct? higher B+, it seems.

I personally would not tweed-mod it. my SF amp is great. better than a gibson copy of a tweed champ i have. Plus, the vibrato is the bias-modulation type on the driver tube and is probably the best vibrato ever. I have slowed it down with cap value change and thats nice.

thumbs up on the weber speaker. they usually blow away some of the cheaper 70's speakers fender used.

I would caution using the 5v4 rectifier. Gives significantly higher voltage and the SF is already running high for a 6v6. in particular, watch screen voltage and dissipation as that is where the 6v6 will get abused in a cranked champ. can add a larger value screen stopper resistor to help.

 

RE: Fender Vibrochamp recap/mods?, posted on January 15, 2010 at 10:48:43
Blooze
Audiophile

Posts: 378
Location: Texas
Joined: August 1, 2005
 I don't know about the B+ or any differences between the two. I'd think lowering the B+ might not hurt, which is what I thought the Blackface mods did?

I have debated doing any mods to it, other than recapping and a speaker change, over the years. I don't think I'd ever sell it except maybe if I financially had to for some reason, but keeping things true to themselves is something I like to do.

What I'd really like to have is something like the '57 Tweed Deluxe. They are soooo expensive though, I could never justify it. I love the sound, though.

I wonder if you can build a Tweed Champ from scratch without breaking the bank?

 

RE: Fender Vibrochamp recap/mods?, posted on January 15, 2010 at 11:48:52
FenderLover
 You're right. The black face had the later silver faced circuitry where for some reason Fender, tried to make the Champ more linear sounding than the tweed version (5F1). They ran higher plate voltage on the 12AX7 gain stages, which made this amp sound brittle and strident. And added a funky tone stack. Plus, didn't quite get the vibrato (really the tremolo effects) right. The only difference is the plate voltages are higher on the silver face (as you said) versus the black face.

Perhaps leave the tone stack in and see how you feel about the tone. My bypass can be easily reversed (all the sugggested changes can be, except for the speaker). My amp sounds much better bypassing the tone controls. Browner and louder. I also removed the bypass cap on the first half of the first 12AX7 to drop the gain a bit.

For sure change out all the 'lytics and go to 50VDC rated ones for the cathode bypass.

I've run the 5V4G for quite awhile with no problems. I'm using NOS tubes. So maybe you are correct about sticking with the 5Y3GT, if you plan to use the new Russian made 6V6GT (or its equivalent).

My mods are to set the circuit back to the 5F1 (tweed) circuit in some of the critical sections.

Thanks, for the clarifications!

 

RE: Crate VC508 - That's Crazy, posted on January 15, 2010 at 11:51:01
Chris O
 I had a VC508, horrible little amp. So much easier (and not as expensive after the extensive mods), to build a little 5 watt head and hook it to a small cab.

 

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