Is it ok to use a 5U4 instead of a GZ34 in a Vibrolux?

[Humbucker]

I just worked out a trade for a 1969-70 Fender Vibrolux Reverb amp, I just started cleaning her up and I noticed that the tube chart says to use a GZ34 for the rectifier but somebody went and put in a 5U4 instead. Is this a potential problem to the health of the amp or maybe to the way it performs?

I'm not sure if I need to replace this tube right away or if it's okay to use a 5U4 rectifier tube in this amp.

[bob p]

That would depend upon the power transformer in your Vibrolux Reverb. Is it original or a replacement? The VRs that I am familiar with spec a 5U4GB rectifier tube. If you have a Fender tube chart that says GZ34, then the safest thing to use is a GZ34.

The reason for concern is that the GZ34 only requires 2A of current on the 5V filament tap, while the 5U4 requires 3A. If Fender slapped a GZ34 in your amp just because they ran out of 5U4, then you have no problem and you can use either tube. OTOH, if they changed transformer specifications so that you've got a transformer with a 5V tap that's rated at only 2A, then running the 5U4 will be a problem.

The answer to your question, then, is that you haven't provided enough information to provide a definitive answer. It would really help to know more about your PT. Perhaps someone can answer this question off of the cuff, based upon the assumption that you have an original PT -- but in the interim the safest approach is to use the rectifier that's listed in your tube chart until you can prove that your amp's PT will support a 3A load at 5V.

[pdf64]

The flipside view is that a GZ34 will give the amp an increased B+ supply, which will increase the idle dissipation and so make everything run hotter.
The 5U4 only draws 5 watts more power from the transformer than a GZ34, whereas just a few millamps of idle current multiplied by 450v B+ can easily become a more significant factor for the power transformer dissipation.
If the chassis and power transformer isn't too hot to touch then I'd leave it as it is. If it is getting way hot, then take it to a tech to check out (it's probably overdue a re-cap anyway) and seek their advice. Hope that helps - Peter

[stokes]

A quick and easy way to check if the PT can handle the extra current of the 5U4 heater winding is to check the volts on the 5v heater (pin 2 and 8 of the rectifier),if it is below 5volts you are drawing too much current,if it is giving you the full 5v's you're okay.The SF Vibrolux called for a 5U4,while the BF used a GZ34.Fender was famous for using the wrong tube chart if that was what they had on hand when the amp was built,they never wasted anything and it is quite common to see conflicting tube charts as well as parts.Since you have a SF Vibro,I am pretty confident the 5U4 is correct.

[Humbucker]

Thanks for your help guys!

After playing this amp at full volume for an hour or so, I put my hand on the output and power transformers and the chassis too. The amp and transformers are barely warm to the touch, so maybe I'm safe using this 5U4 rectifier tube after all. I'll have to open up the amp and check the bias and also the voltage on the 5V heater of the rectifier tube.

I love the sound of this amp but it's a bit shy on volume, I plan on doing a complete cap job and see if that livens up the amp's output. Maybe the lack of volume is due to the power tubes maybe being weak or maybe not properly biased, or maybe the GZ34 would help?

[Bruce / Mission Amps]

If you can get the power tubes to idle at around 15-16 watts with a GZ34 and higher B+, use that... they sound great.

[stokes]

Just to add to what Bruce said,and I agree 100% with his observation,if you do use the GZ34,you must readjust the bias for the higher plate volts you will get with that rectifier.

[Rob Mercure]

HB,

You mentioned that your amp is a bit "shy on volume" but this may just be a matter of style/taste. I've got a '67 Vibrolux Reveb and while it seems quite loud enough for home practice it's a bit weak on stage - especially if I want a cleaner sound. So I've added an extension cabinet - with an Eminence 12" speaker - that I separate from my V'lux by about 10 feet. This gives me all the stage "volume" that I need (and allows the drummer to "finally hear me" <grin>"). I've seen the output stage of these beasts rated from 35-45 watts, depending on what piece of old Fender literature I've gotten hold of, which can provide a nice volume as long as the rest of the band isn't too loud. Unfortunately bassists don't play through the Bassman amps or B-15Ns that were common when the V'lux was new but now lean more toward the SS equivalent of an SVT!

So external speakers are a fairly effective, easy, and inexpensive "fix" but the real issue may be getting the rest of the band to turn down a bit.

Rob

[Humbucker]

Thanks for chiming in people!

Rob, I was kind of wondering if maybe I'm expecting more volume out of this type of amp than what it can put out. I'm used to my Twin Reverb, Traynor YBA-1 50W BassMaster and Marshall JCM 800 50W!


The way that the amp is now, it's loud enough to keep up with a drummer in my practice room, so I guess that it's probably fine and I'll just have to get used to this type of amp.

BTW, would doing a cap job make any noticeable difference in sound level at all? I'm also curious about trying out a GZ34 in there to see if I like that better, just a little bit curious I guess.

[bob p]

So external speakers are a fairly effective, easy, and inexpensive "fix" but the real issue may be getting the rest of the band to turn down a bit.

Same problem here. Most people would expect the reason for the band being too loud would be the drummer playing too loud, but in my case its the keyboardist. He's the one pushing out dB, and everyone else is trying to catch him. Just as you'd expect, he's playing through the same Super Twin Reverb at practice that he uses at gigs, and he always turns it up until it hits the "sweet spot." If I could only get him to play through a Vibrolux! (or an attenuator)

[Humbucker]

Update:

I opened up the amp and it appears to be an AB 568 circuit and is bone stock by the looks of it(that's a good thing).
Link:
http://www.schematicheaven.com/fende...b568_schem.pdf

There's all this extra junk on the output tubes(150 resistors between each of the cathodes and ground, and a cap between the two cathodes), there are also some capacitors between the control grids and ground too. What's up with these changes, was this to prevent oscillation due to sloppy lead-dress, etc...?

And unfortunately, they used a "bias balance" scheme instead of bias adjust, dang! Is there a way to get the power tubes biased okay with this type of biasing method or should I just go ahead and rewire it for bias adjust?

Since this is a vintage amp, I'm inclined to keep the amp stock rather then start modding it, would you agree with this idea? The amp sounds great to my ears BTW.

I also checked the voltage on the heater filament of the rectifier tube with three different meters(just to be sure) and I'm reading 5.3V with an old Radio Shack Micronta analog meter, 5.1V with one digital meter and with my other digital meter the reading bounces between 4.9-5.0V

So, I guess that I'm safe using this 5U4 tube in my amp. I'm sure that this tube's been in this amp for many many moons and it would have caused trouble a long time ago I suppose. I'll still try out a GZ34 later on just for kicks.

I checked the bias voltage and it's right around -39V, seems pretty safe. It's late now but I'll check the current draw of the power tubes tomorrow and post another update.

Thanks to all for your help!

[stokes]

Some Fenders had a combination fixed and cathode biased arangement,that would explain the 150ohm cathode resistors you see.The caps from grid to ground are common snubbers to supress parasitics.Whether or not to mod the bias control is up to you,and how you feel about keeping an amp stock or modding it to suit your tastes.There are two schools of thought on the matter,those who prefer to preserve the collector value would say not to mod anything,and those,like myself,who are not concerned with any potential resale value,would not hesitate to mod the amp to perform as I like it.But I think modding the bias balance to a true bias adjustment is a very beneficial mod.Its quite common to see deviations from the schems on Fender amps particularly during the "crossover" period when they went from Black to Silver Face.

[Bruce / Mission Amps]

The bias balance is really a good circuit to leave alone!
I NEVER EVER take it out if it there... instead,
take that 15K grounded resistor off and use a 20K trim pot in series with a 6k8 1/2 watt resistor.
Replace the bias supply filter cap with a new one and use a GZ34.
Now you have balance and level adjustment and a more powerful amp.
While you are at it, you can remove all the junk and black face the phase inverter for a little more gas.
Do not fall victim to Net driven BS or what self proclaimed amp gurus say... do not use anything other then 1500pF to 4700pf to drive the PI and make it a great capacitor, not ceramic.
I've only had a couple amps that gave me any oscillation problems dropping all that junk off and blackfacing the PI.

[Humbucker]

Thanks stokes and Bruce!

So, what does this "combination fixed and cathode biased arangement" accomplish? Why did they put this in there and how does it effect the amps overall sound? Was it added for some sort of protection or to help regulate the bias?

Would it be a good idea to wire both cathodes to ground and get rid of that capacitor between the two cathodes? Would this also give the amp more volume? I'll definately remove those two snubber caps.

I like the idea of keeping the balance adjustment and then add a level adjustment. Very useful, thanks!

What type of capacitor would you guys recommend for replacing the ceramic cap in the phase inverter? Polypropylene, Polyester?

[Bruce / Mission Amps]

"So, what does this "combination fixed and cathode biased arangement" accomplish? Why did they put this in there and how does it effect the amps overall sound? Was it added for some sort of protection or to help regulate the bias?"

I don't know exactly what they were thinking there... especially with that AC capacitor going back and forth to the cathodes for some kind of feedback, but that mod I mentioned accomplishes the obvious... idle current level and balance control.
I think tubes of the day were better right out of the box so power tube hum balance was more a concern. Cold idle current in the day was not much of a concern. But, I bet it is also part of the reason why some stock amps killed right out the door and others were cold and sterile sounding

"Would it be a good idea to wire both cathodes to ground and get rid of that capacitor between the two cathodes? Would this also give the amp more volume? I'll definately remove those two snubber caps."

Yes. Dump the resistors and caps. Mod the bias supply. The cross over cap might be some kind of AC NFB thing to help balance the two power tubes in AC output?
After these mods, it won't really make a huge difference in output power but it might make the amp sound a little less compressed and probably bigger, more punchy and muscular.

"What type of capacitor would you guys recommend for replacing the ceramic cap in the phase inverter? Polypropylene, Polyester?"

Polyester or polyprop is OK.
I use .0015uF to .002uF Xicon Metalized polyprops there with very good results.