Pro Reverb Volume drop off, desperately need help

[Rykard]

Hi guys, this is my first post, I just laid my hands on a late '70's early '80's Fender Pro Reverb, the amp is in mint condition, it has really been taken care off. I have serious confounding problem in that the volume drops off when I turn the volume knob up, now this is sudden after a sweet spot at say 4-5, it's kinda like a phase cancellation thingy, the lows get completely cut out and the tone is lost completely.
I have a friend of mine that builds amps and knows what he's doing, we did alot of trouble shooting and went through the amp step by step/stage by stage substituting components and looking for results but still could not find the problem, though this would result in the amp suddenly working fine until it's turned on again, with or without the substitutes. We found one anomaly though where the plate resistor on the first channel show a drop in voltage when the volume is turned up, which obviously is the reason for the drop in gain, but why????
I love this amp it's my first vintage amp waited a long time for it, can someone help me.

[pdf64]

This happens on both channels? Is this the one with the master vol and pull boost?
Has your buddy used a scope on it to check for ultrasonics? The later silverfaces had higher gain and lead dress problems.
Have you subbed in known good tubes, all through?
Good luck - Peter.

[Rykard]

Hi peter, it was really nice of you to write in, yes my amp is the master volume, push pull boost one and the problem did occur on both channels, though it began suddenly with only one. The good new though is that we found and solved the issue. It was the ceramic caps (never suspected them) in the High's EQ section of both channels. We never checked them before 'cause they don't normally go bad and surely not on both channels, I guess this is a really rare case. So, we changed them and voila!!! the amp was singing/sparkling brighter than the stars in the nightsky, I am so happy, I'm gonna test it for a few weeks though before I'm satisfied.

I had some more queries though on the caps and stuff, all the caps are stock Sprague caps, wanted to know if I should change them since the amp is over 25 yrs old, will it make the amp sound any better 'cause by my exp it's sounds amazing, if I should, do I go with Sprague or should I try more high end stuff like Allesandro, I am kinda worried that anything else other than a Sprague would take away the Fenderness of the amp, I don't want a change in character, I'm also planning to change all the ceramic caps especially the EQ section ones to silver mica ones, any specific brands I should look for. I await you reply

By the way the amp is in pristine condition, it was really well kept, I post some pics if I can, it seems like it came out of a time machine in perfect condition. Thanks so much.

[Rob Mercure]

If you are referring the the filter and cathode bypass capacitors - yes in an amp this old they probably need replacement. If you're referring the DC blocking and tone shaping caps I'd leave them alone unless known to be defective. Filter caps won't take away the "Fenderness" nor give it - I suspect that you're coming under the evil influences of some HiFi-ish marketer who will do his best to extract as much of the content of your pockets as possible. You do need good quality caps - and Sprague have been such, in general, over the years - but changing brands won't make a great difference in your amp although you might notice some slight tonal difference as you bring the voltages, decoupling, and filtering up to original design ('da funny ting is dat sometimes customers are suprised that their amp sounds different when returned to factory specs - but you'd be suprised how quickly one becomes accustomed to driving with bad alignment and how different a car feels once it's realigned!).

Most of the characteristic of an amp is determined by circuit topology - whether HiFi or MI (musical instrument) - and changing small parts or the tubes won't make as much difference as the salesmen want you to think. And a good tech should be able to tweak an amp's circuit parameters to accomodate different components so, again, don't get caught up in "thou must use brand QXXsplat or thy amplifier will grow chicken legs and spin in place!" Probably the most telling components in an amplifier is the output transformer. Oh, and fer the record, your particular amp, while being a good one, really isn't considered a paragon of "Fenderness" - depending on the person (and we all love our own dogs, cats, guitars, trucks, shot guns [for the HiFi salesmen] and amps) this would probably be either the tweeds or the black faces but I find something I like in most of them (and cast cooties on the Marshall crowd <grin>).

Rob

[T-bone]

lol

good read rod !!. always appreciate a fenderhead T-bone

[Rykard]

Thanks for the input Rob, especially the space bound chicken, lol, I have to change the filter caps I guess 'cause there is a very soft (ear to the speaker) motorboating sound which become kinda audible only when the amp is turned up to 10, and is drowned out by the guitar, but i guess it can wait 'cause I still can't hear a hum. And Sprague is what I'll go for, right?

[Rob Mercure]

Thanks Guys,

Actually, I'm not so much a "Fender head" - although a '67 Vibrolux is my main stage amp along with a Bogen mixer based "acoustic" amp - as much as I am a "not Marshall head." But this has a lot to do with my guitars - '60 Strat and '79 Martin D-25K - if I likes to lift weights and use Les Pauls I might like the Marshalls but to my ear they're "tinny" but a lot of this is the speakers. Celestions seem to go well with many Fenders - 'specially Tweed - while Marshalls do better with a darker JBL. But I'd rather have a Beemer than a Harley (noisey children's toy), a Toyo P/U than a "Vette," a good Pad Thai or Taihli than a steak, grits before cream of wheat, stone before glass, hate quiche ("quitchy") love cornbread and believe life "begins at 2,000 feet!" - ummmm jes ahllllll mmmmmixdddddup! (Elvis knee shake) <grin>!

[Rykard]

Hey I was gonna buy one (Vibrolux) recently when it came my way but didn't have the cash, what's the amp like

[Dave Curtis, dB AudioTech]

Hi peter, it was really nice of you to write in, yes my amp is the master volume, push pull boost one and the problem did occur on both channels, though it began suddenly with only one. The good new though is that we found and solved the issue. It was the ceramic caps (never suspected them) in the High's EQ section of both channels. We never checked them before 'cause they don't normally go bad and surely not on both channels, I guess this is a really rare case. So, we changed them and voila!!! the amp was singing/sparkling brighter than the stars in the nightsky, I am so happy, I'm gonna test it for a few weeks though before I'm satisfied.

I had some more queries though on the caps and stuff, all the caps are stock Sprague caps, wanted to know if I should change them since the amp is over 25 yrs old, will it make the amp sound any better 'cause by my exp it's sounds amazing, if I should, do I go with Sprague or should I try more high end stuff like Allesandro, I am kinda worried that anything else other than a Sprague would take away the Fenderness of the amp, I don't want a change in character, I'm also planning to change all the ceramic caps especially the EQ section ones to silver mica ones, any specific brands I should look for. I await you reply

By the way the amp is in pristine condition, it was really well kept, I post some pics if I can, it seems like it came out of a time machine in perfect condition. Thanks so much.

Hi Rykard,

Glad you found the problem. I have a '76 Pro Reverb (also in XLNT shape), and will advise that you leave all the small signal caps alone, unless one goes bad. Just replace the electrolytics on the board and in the power supply and bias circuits. Another thing you might want to check that not too many techs know about: the AC convenience outlet on a great deal of older Fenders was wired wrong. The white neutral wire from the cord should be connected to the wide slot on the outlet and the hot black wire should be connected to the narrow slot.

BTW, this amp is even better with a Pro Sonic OP transformer ;o)

[T-bone]

hi

gday, just a quick question, how do you start a new post? anyone?

[Rob Mercure]

About my Vibrolux

Rykard,

To my ears my '67 Vibrolux Reverb is much like the same year Deluxe Reverb but louder. Of course the 6L6GC has it's characteristic sound and at times I've thought about running a JJ 6V6 in the beast to see exactly how close to the Deluxe is. I've got the slightly uncommon AB864 whose schemo seems unavailable but which only differs slightly from the available AA964 (and I wonder what the 1-764 variants were) - it has the Jensen 10" ceramic speakers. For a Blackface Fender it breaks up rapidly which I like but my pedal using friends have complained about lack of headroom.

I have made - to me - minor reversible mods on the amp. I've modded the Channel II (reverb/trem) second input with a 1M series, 3.3 M grid return resistors to accomodate piezo pickups directly and also modded the Channel I second input to feed out the Channel I signal to the mixer tube when interrupted. I can then take a short guitar cord (12") and feed the Channel I signal into Channel II (padded down) for greater distortion and I've cobbled together a food switchable relay box to allow me to have either clean Channel II or the both Channels in series. In addition, since I added a properly grounded three wire cord, I added a variable NFB control which sits in the ground reverse hole - which I really never change - and a mini-switch in the tremelo foot switch hole since I don't have a tremelo foot switch and rarely use it. I've also added variable bias controls for the output tubes as well as cathode current sense resistors. Oh, and I replaced the two prong auxillary outlet with a grounded one.

While these changes might seem a bit extensive none entails new holes in the visible chassis (variable bias punched two small holes one near each 6L6GC) and all can be reversed in about a hour or so (not counting chassis removal and restoration time) if I ever wanted to sell the amp. But since it is my main stage amp and it was given to me by the wife of a deceased friend I suspect that I'll have until my own demise.

If you can pick on up for a not ridiculous price - and you like a BF Fender with an early Delux-ish break up (which could also be tweaked fairly easily) - I'd recommend getting one. Being a 35-40 watt Fender their not really that loud as compared to a Super or such but with an external speaker (I use a 12" Peavey 120 W "something") they compete well on stage volume wise and, really, high volumes are what the PA is for. Tone is what the guitar amp if for.

Hope this helps

Rob

[withnail]

And of course the BF vibrolux had a PT with a lower HV secondary, (315v ) than most other BFs, resulting in sooner breakup/less headroom.

[Enzo]

Tbone, go to the index page for one of the forums where you want to post - this one is Maintenance etc. Above the upper left of the index listings is a button marked New Thread. CLick on it.

[T-bone]

cheers

thank you enzo! x

[Rykard]

Wow Rob you really have one nice custom amp there, I bet it feeds your ears with what you want to hear, thanks so much for the info