blackface vibrolux. what to replace?

[imo]

I have recapped my '63 vibrolux recently and was going through the circuit to make sure everything looked ok. Here are some things i was looking into doing. I would love any info on thoughts.
1. Replacing bias balance control with a bias level control

2. Replace old ceramic caps with poly caps. if so are there caps i would be wasting by doing this?

Any other changes to consider.

I will also say that this is my gigging and primary studio amp. I want it to sound as good as possible. I am not looking at extra vintage. I also keep all the old parts in case i was going to sell as a "vintage amp"

Thanks guys
Ian

[52 Bill]

1. Replacing bias balance control with a bias level control

If your amp is a 1963 model, the bias control is a level control, not a balance control.

2. Replace old ceramic caps with poly caps. if so are there caps i would be wasting by doing this?

Yes, the ceramic caps in the tremolo oscillator circuit. If you replace the ceramics in the amp, limit yourself to the signal path caps. The really small value ones will probably be best replaced with silver micas.

I'd also suggest checking the 470R screen grid resistors on the power tubes and the 100K plate resistors on all of the pre-amp tubes.

[imo]

Bill,
I really appreciate your info. Do you know this circuit pretty well? As i was looking through the various schematics i noticed a number of changes.

Do you have particular aspects of ones you like, weak spots, strong points?


I am also trying to learn more about the amp as i go. I checked out all the resistors, a couple were a bit high. I replaced the ones that were off, but everything else seems pretty cool.
The bias voltage cap seemed pretty old. I replaced it. It was a 50U, all i had in that voltage was a 100U, It seemed that this is just a resevoir cap, so going a bit bigger seemed fine.

[52 Bill]

The bias voltage cap seemed pretty old. I replaced it. It was a 50U, all i had in that voltage was a 100U, It seemed that this is just a resevoir cap, so going a bit bigger seemed fine.

The real important value here is the voltage rating. For some unknown reason, Fender used a cap rated at 50v in the bias circuit that routinely placed 55-60 volts on that cap. Most people will upgrade it to at least a 63 volt cap. As for capacitance value, some people would recommend keeping the value smaller as they feel that the higher the value, the longer the cap needs to charge up, allowing the output tubes to start up with too little bias voltage. This should not be a problem if you use the standby switch, as the bias circuit is always on.

First, check and let us know what model your amp is. It may be marked on the paper label/tube chart. Even if it is marked, don't be surprised if the actual circuitry is a different version from the marked version.

All blackface Fenders share very similar circuitry. The basic circuit elements are all the same. There are minor value tweeks here and there, but they share a lot of the same circuits.

[Rob Mercure]

Imo,

I play through a 67 V'lux and love the beast - but I've "slightly" modded it to be able to slave the first channel into the second for some nice distortion. A mod that can be reversed in about 45 minutes. In general the ceramic tremelo caps in these old Fenders rarely fail so I wouldn't replace them unless the tremelo fails to fire - and if you don't use the tremelo then does it matter?

Since you've got a tube recto - should be a GZ34 - there is no way that you'll build up B+ before the bias supply charges even with somewhat higher bias filter caps - and this is a 1/2 wave single cap filter set up. I like 100 uf/100WV which gives you more filtering and higher breakdown voltage while still not increasing the charging time enough to make any significant difference.

Rob

[imo]

Rob,
I've heard that done. I really love the sound of the amp.. Just rehearsed today, replaced a failing rectifier tube, put some film caps in place of the ceramics..
Sounded really great. The rectifier was ganky, so part of it was that, but the amp was cleaner (replaced the drifted plate resistors (up around 112k) with 100k's) and had a really nice punch..

What about the reverb section? I find mine to be a bit bright and springy. I was thinking about putting in a smaller cap before the reverb pot (forms a high pass) to allow more low end through. Anyone done this?
right now it is a .003...Maybe a .002...Or should i do this somewhere else?
Thanks
Ian

[Rob Mercure]

Do what? A smaller cap would more emphasize treble - the opposite of what you want. But reverbs really don't like to reproduce lower frequencies that well and some of the artifacts are "boomy." Generally I like the reverb sound - it is the classic 60's "Fender Sound" - especially when driven by my '60 Strat - but tastes vary. While you could put in a larger cap I'd suggest instead following the route that CBS chose by putting a small cap across the reverb output coil from the 7025 grid to ground. You can solder this right across the reverb foot switch jack so it's easy to get to/reverse. Do a quick search of later '60s V'lux models for the value that Fender used but off the top of my head I think it was 220 pf.

The mod I used on my V'lux was to send the output from Channel one to the second "input" (no longer) jack for that channel. Then I padded down the Channel two second input with 1M series, 3.3M parallel resistors (this was originally done to directly accomodate piezo P/Us but then I gave up on piezos - tone suckers). So I send the Channel 1 output to the second Channel 2 input giving me 4 stages with two volume and 2 treble & bass controls for distortion - I cobbled up an external relay switching scheme as there was no way that I easily create an internal one without "hacking" up the amp in a non-reversible manner. What this gives me is either channel stock by using the first input jack and switchable between Ch 2 stock and Ch 1->Ch2 for gain. Flexible but generally stock.

Rob

[imo]

Hmm. I think the reverb in this particular amp is a bit springy. I have a couple of other blackface fenders with beautiful reverb ('65 pro reverb, '63 vibroverb) where you can turn the reverb all the way up and get a usable, very washy surfy sound. This one becomes pretty unusable after about 2-3 on the dial. I like a lot of the surf, early 60's stuff as well, and i would love to be able to get that sound.
Perhaps it is the reverb pan then. I may try one of the others and see..

That mod sounds awesome. When i have some experiment time i will check it out. What a gorgeous amp..I find it to be my fave gigging amp and along with my AC30 to cover almost all recording sounds i would want
Best
Ian

[Bruce / Mission Amps]

Hmm. I think the reverb in this particular amp is a bit springy. I have a couple of other blackface fenders with beautiful reverb ('65 pro reverb, '63 vibroverb) where you can turn the reverb all the way up and get a usable, very washy surfy sound. This one becomes pretty unusable after about 2-3 on the dial. I like a lot of the surf, early 60's stuff as well, and i would love to be able to get that sound.
Perhaps it is the reverb pan then. I may try one of the others and see..

That mod sounds awesome. When i have some experiment time i will check it out. What a gorgeous amp..I find it to be my fave gigging amp and along with my AC30 to cover almost all recording sounds i would want
Best
Ian

When you say you recapped it did you also replace all the electrolytic caps on the eyelet board?
What is the cathode biasing resistor value on the 12AT7 which drives the reverb output transformer?

[imo]

Bruce,
Thanks for the reply
I had actually replaced all of the electrolytics, including cathode caps, a few years back. The only caps i replaced recently were the ceramics.
I will check the resistor and get back to you
Thanks
Ian

[Rob Mercure]

Maybe it is a bit springy - like I sed addding the cap across the input will cut the highs and might work for you. Cutting the drive a bit also helps with some machines so you could increase the 12AT7 cathode resistor a bit - from 2.2K to 2.5 or 3.3 and see if that cuts the "sprong." Generally taming works best on the drive side of the springs.

Rob

[imo]

love the info. I've had these amps for years and being able to look at them as dynamic instruments, which they are, is really nice. I will check the cathode, possibly try the cap, and report back,
You guys are being really helpful. Thanks for that
Ian

[imo]

I wanted to say that i had some pretty nice results from changing the "reverb" tube from a 12AT7 to a 12AU7. Still a bit springy, but much nicer and usable. I'm thinking of getting a new reverb tank down the road, but i have a much more workable amp.
Things i did:
-Changed and balanced the 2 1 meg resistors on the driver grid.
- Changed and balanced the output caps (this made a huge difference) on the driver tube
-Changed all signal path caps to metal film( bar for the trmelo).Left cathode caps alone..Replaced small caps with mica
--Put in a couple of plate resistors early in the preamp close to 100k. I left the later ones that had drifted to 110k..I like the way they sound in the circuit
--Replaced the .1 and .047 tone caps with new caps
Replaced .1 cap on phase inverter with a higher voltage cap
I used all Orange drops.
Super happy with the changes. About $10 worth of parts and a huge difference in sound