BF Vibro Champ Mod

[zendragon63]

A fellow guitar player and co-worker from corporate sent me his bone stock unmolested Blackface Vibro Champ to mod for him. I have told him that very little if anything should be done to this gem but he just wants to warm up the harmonic content a bit. It does need a cap job so that is first but do any of you guys have a recommendation as to a very basic, reversable mod to try? There are hot rod mods galore for Champs but the only one single thing I can come up with is to replace one of the pots with a SPST push pull to bypass the tone stack. Any thoughts would be appreciated. Regards

dennis

[HotBluePlates]

That's pretty much all he can do. At least without major alterations, like adding a gain stage.

If he doesn't like the amp as-is, tell him to give it to me!! I love the old blackface VibroChamp stock. I've had 3-4 of them at one time or another.

[tubenit]

Only mod I'd made for a VC is slow the tremolo by changing one of the .01 tremolo caps to .02. I owned a VC once also and really loved it.

With respect, Tubenit

[alerich]

I have a switch to add a .03uf cap to the tremolo. Slows it way down in a cool way but easily back to stock at the flip of the switch.

To the OP, bypassing the tone stack will not so much add harmonic content as it will unleash the beast. It's a big change in tone - very raw. One thing to try just to warm it up a bit is to make the NFB switchable. I usually leave it switched out on my VC. Or instead of a full tone stack bypass add a 250K or 500K pot between the mid resistor in the tone stack and ground - the "raw" control. That allows you to variably dial out the tone stack without the harsh in/out effect of a full bypass. I think I'm going to go back and do that to mine instead of the bypass. Just use the low input that nobody really uses to mount the pot.

 

[phsyconoodler]

Try lifting the negative feedback,raw control is great,try NOS tubes,different speaker,change bias cap to 1k from the stock 470 ohm to get the bias a little cooler.
  Make sure the amp is up to snuff everywhere else.

I absolutely love Blackface Champs for clean tones.The eight inch speaker can hold it back,but if you have a good one the amp can really sound great.It can limit the 'harmonics' of the amp and is the single most effective mod to liven them up.
  A Weber Ceramic or Alnico Speaker will kick things up a notch.

[bigdaddy]

What speaker is in there? Does he use the tremolo? I personally hate the tone of the amp without the tone stack, it's distorted but not in a pleasant way, it is RAW and more tweed like. If that's the case get a 5F1. If he wants to make a little monster that emulates the sound of the bigger BF Fenders without really ruining the amp then disconnect the tremolo, take out the NFB and use a 1K/10mfd combo on the cathode of the second gain stage. Lower the voltage going to the the plates of the 12ax7, if it's over 185v then it's too high IMO. Get an old Jensen C8R in 4 ohms from Ebay or a Weber CVC8 which is as close to it as can be.

For tubes there are plenty of RCA 5Y3 rec tubes, a JJ 6V6 and an EH 12AX7, add a 1.5k screen resistor. As for filtering, leave the old can in there and build a little circuit out of turret strips using F&T or Illinois caps, they are small. 30 first cap-20 or 30 for the second- 20 for the third and 10 or 20 for the preamp. add a 100 ohm 5-10 watt resistor after the junction for the rectube/30 mfd cap then a 30-20 mfd cap there, a dropping resistor of 1K 5watt with a 20mfd cap, add a 470 ohm screen resistor, then whatever resistor gets you in that 180v ballpark with a 10-20mfd cap there. The schematic says 10K but you might want to add an extra 4.7K in series. Twist pairs of heater wires and add 100ohm resistors to ground that should take care of some noise with the increased gain. The 15K mid resistor is too high IMO, I actually remove one of the inputs and parallel the existing 68K resistors, I then put a 25KL mid pot there with one of those white/brown or new black smaller fender type knobs and remove the 15K resistor.

All these things are reversible.

Now you have a small amp that sounds as close to a super reverb or similar BF Fender. Believe me I have built so many now using the basic champ as a starting point and finding out which mods work and which IMO are a waste of time. If he want to keep the trem then that's a whole other game and you need to add a few things to make it work better, the tremolo must be cut off or your bias voltage fluctuates too much even with the circuit controls down.

[zendragon63]

Thanks for the responses gang. And welcome back Bigdaddy. I will try the tone stack lift but I kinda doubt that raw or nasty is what he is looking for--at least when it comes to amps!  You know what, it sounds pretty good until it is on for a few minutes and it develops a little trailing note squawk. Caps gotta go. I will get into it in the next day or two. Probably start with lowering the NFB and make sure that 6V6 bias is in the ball park.

I'll let you know what I find--I better get back to work  Again thanks. Regards

dennis

[phsyconoodler]

a trailing note squawk is a speaker voice coil....maybe.

[zendragon63]

I am finally getting to this VC and ended up with a couple of questions. First, the voltages on this are running nearly 40V higher than the those on the file schematic, so running an inital powercheck at the 6V6 idle gave me 384V on the plate and drawing 51 ma at the cathode. JJ's or not, that still seems hot at 384 x .051= 19.5W (!) but wasn't redplating. Hot distortion by itself sounds kinda flubby IMHO. Kinda makes sense to bring it down a bit BUT raising the cathode resistor value brings even a higher voltage to knock down for the preamp tubes. And that tube draws an idle of ~2 ma so I am needing in the neighborhood of a 68K to get that rail voltage down 136V to give me <200V at the preamp plates. Does that sound right? Bigdaddy? Phsyco? Thanks in advance. Regards.

dennis

[HotBluePlates]

The voltages in ALL vintage amps will tend to be higher than on the schematic because the outlet voltage is higher now than it was then. Further, most (all?) Fender schematics state that all voltages are +/-20%.

If you're worried about output tube dissipation, make sure to subtract screen current from your cathode current measurement to see what the plate is really dissipating. If you have a screen resistor, you can measure the value of the resistor with the amp off, then measure the voltage across the resistor with the amp on. Voltage/resistance = current, and you'll see how much screen current to subtract from your cathode current measurement to get plate current.

Yes, if you raise the cathode resistor and reduce plate current, the supply voltage will rise somewhat. I wouldn't worry about too high preamp voltage, but of course you can always increase a power supply dropping resistor to bring the preamp voltage back down.

Most of these amps run the output tube quite hot, maybe too hot. That said, I have owned a bunch of Champs, VibroChamps, a tweed Princeton, etc. They haveall had higher than spec supply voltage due to high wall voltage. I never altered the cathode resistors in them, I did use mostly NOS 6V6's and never had a tube failure or even a worn out power tube.

Maybe I've been lucky, but as long as the output tube is not one of the early russian "forgery 6V6's" I don't think you'll have problems. The tube I'm talking about is a russian tube type that did not meet all 6V6 specs that was imported and relabeled as a 6V6. The high number of failures (usuallyin Deluxe Reverbs) gave russian tubes a bad name for a while, until New Sensor made some changes and other companies came out with a 6V6 variant that performed well.

[alerich]

Maybe I've been lucky, but as long as the output tube is not one of the early Russian "forgery 6V6's" I don't think you'll have problems. The tube I'm talking about is a Russian tube type that did not meet all 6V6 specs that was imported and relabeled as a 6V6. The high number of failures (usually in Deluxe Reverbs) gave Russian tubes a bad name for a while, until New Sensor made some changes and other companies came out with a 6V6 variant that performed well.

I have a set of those that I bought a few years ago out of curiosity after I had read about them. These are Groove Tubes (surprise surprise) 6V6HD tubes that are essentially 6L6 tubes that couldn't cut it at 6L6 specs so they were labeled and sold as 6V6 tubes. Supposedly these were sold at a time when there were not many (or any) new production 6V6 offerings in the marketplace and NOS was getting pricey. This set were old stock tubes from a mom and pop music store. I bought them on eBay for $10 including shipping. They have no OEM markings on them so I am not certain who originally manufactured them. My guess would be Sovtek.

In a Champ or other single tube amp (that has the heater current capability) they might work ok in a pinch. They sound ok in my Vibro Champ. They won't deliver full 6L6 power dissipation in a 6L6 environment. I tried to set them up in my Super Reverb and they just weren't up to the task and they sounded crummy, too. The trouble is that they pull 6L6 heater current so they would not have been a viable option for many 6V6 amps. I guess that was only a minor detail to Aspen.

According to the The Tube Store http://thetubestore.com/groovetubes.html Fender now owns the Groove Tubes name. Oh, how the mighty have fallen.

[HotBluePlates]

I was thinking about different tubes. Here is a reference on Russian Types that are similar to American Types. I had not committed any of these type numbers to memory, so I couldn't think of the right number before. The one I'm thinking of is the 6P6S (6Π6C). You can look at a 6Π6C data sheet at Frank's.

Max plate voltage is 350v (we exceed that in most american 6V6 amps) and max screen voltage is 310v (ditto). We run american tubes hotter than the sheets say too, but supposedly Fender had checked with the manufacturer to find out they wouldn't die right away. We have no such information from russian tube factories.

Also, the forgeries I'm thinking of always claim to be "6V6" or "6V6GT" and often have markings that would make you think they were made by Raytheon, Brimar, etc. The best plausible explanation I've seen is that the initial goal wasn't so much to make money by claiming they were a brand they weren't, but to make money importing cheap tubes at a time when US Customs would look very suspiciously at anything imported from the Soviet Union.

I've also seen tubes claiming to be "6L6GC Made in Germany" which were a russian tube type that also doesn't meet 6L6GC specs.

[zendragon63]

OK guys, mucho appreciates the responses. Even at Fender's standard for 'living toward the edge', 19.5 W still seemed hot and more suited for a cooler biased 6L6. I won't sweat it then but just learn from it. Regards

dennis

PS - HBP: my sincere thanks for your service to our country.

[zendragon63]

Finally got this one on and off the bench and thought I would give a status in case y'all may be interested. (I know, I know, Champ mods are dime a dozen  )

-Changed out the PS and bias ecaps.

-I brought the operating voltage down to 385V from 402 with a 300R 10W off the 5Y3.

-Traded the 470R for a 680R 5W on the 6V6 bias.

-Traded the 10K dropping resistor to 43K to get the preamp plate volts down to 190V.

-Left the tone stack; I really didn't like the sound completely disconnecting the NFB so I just reduced with a 15K and

-biased the V1b with a 1K/1uf. It took a little of the bass stress off the 8" speaker. (Thanks bigdaddy)

Added a 3 conductor power cord, replaced the switch, and various repairs blah blah blah. It does sound better if you want a little more harmonic content--which is what my friend wants--but still cleans up somewhat but just not BF clean. I really tried to stay in the spirit and leave as little of a footprint as possible. I did not rewire the filaments because this one is not that noisy. And I didn't have any cool looking green cloth wire. Anyway, thought I would share my first first Fender Vibrochamp mod rite-of-passage experience; thanks again for the ideas and interest. Regards

dennis

[bigdaddy]

The big issue is the speaker as far as getting that clean BF sound. The 4 ohms makes it hard to find a speaker unless you go with the Weber or Jensen models which should be an improvement over the stock speaker.

If it had an 8 ohm OT and you plugged it into a good 10" speaker with a high efficiency you'd get your BF clean. 8" speakers just don't do clean well unless you have 4 of them even with a 5 watt amp, they just cannot handle it by themselves very well. Even the higher rated ones like a Weber 8F125, it handles the power but won't really do clean. I have a number of weber 8s.

Yeah, ya gotta choke off some of that bass output and bring up the gain on the highs to get a sweet spot. You might even try a .68/820. You could also have tried a 220K plate resistor on the first preamp stage, more gain and lower voltage or even decoupled the stages, so many little things on those amps that can be done to dial in a sound but it's all limited by the speaker.

Did you put a cap at the 300R/5Y3 junction? Just curious.....I don't because I use a choke on most builds before power tube so I have an extra cap, one at the 5Y3 or diode rec then down the line. It's a way of increasing the filtering and maybe tightening the lows up.

[Fresh_Start]

+ 1 on a pi filter (C-R-C) before the plate node on the power rail.  Shows big improvement in ripple reduction in Duncan's PSU, borne out by lower noise to my ears.

Fender used a pi filter before the plate on the Tweed Princetons for a while (5C2 & 5D2) before moving to a choke filter for the 5E2 and 5F2.  Choke vs. resistor is both a cost & taste decision.

That pi filter also lets you get much better filtering with smaller caps.  For example, look at Hoffman's improved bias supply:  http://www.el34world.com/charts/Biascircuits.htm (bottom of page)

Cheers,

Chip

[zendragon63]

Yep, when I replaced the 4x20uf PS cap with one identical, I had one of the sections idle so I hooked it up to the 5Y3 and 300R junction. Just turned out that way.

An 8" speaker will only be an 8" speaker. Again thanks. Regards

dennis