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previous: Joe Z. I had the good fortune of picking u... -- 4/24/2000 1:31 AM

Re: Vibro Champ Questions

4/24/2000 6:52 PM
MBSetzer	Re: Vibro Champ Questions I have an OJ Vibrochamp still in near-mint condition, to me it still is the best sounding stock amp I have ever heard of any size or manufacturer. The old Sprague filter cans on the BF seem to be a longer lasting cap than the later Mallory's, my BF is still good, but my later SF all needed the Mallory's replaced. Since yours was replaced in the '70's probably with a Mallory, it likely could use another new can. You can tell for sure by temporarily substituting new axials to see if there is an audible difference. If you do upsize the first cap section from the BF 20MF to the SF 40MF, it will allow somewhat better bass and less hum. Other than that, to me a complete cap job then means replace only one other: the 25MF 25VDC cathode bypass on the 6V6. The schematic that came with my amp shows this as an electrolytic cap, but usually it is a paper cap instead. Age does take its toll on nonpolarized paper caps, plus this was inadequately rated by original design, there is already 21VDC according to the schematic, and when you crank the amp, it can add another 20VAC on top of the DC. So best to use at least a 50VDC rated part for replacement, I use a reduced value of 15MF (for less bass farting) with a 400VDC film cap, the Fastcap type from Angela will barely fit, but you will probably think it is worth it. Of course your 6V6 bypass cap could be a small white molded Mallory polarized solid tantalum like the preamp tubes normally used for BF & early SF, I would still replace it with the film type, but save the white Mallory, they don't make anything quite like them anymore. These are polarized, but not wet tantalum types, they can easily last decades with no degradation. What you don't want is a cheap aluminum wet electrolytic type here. To see a picture of a larger Mallory of the same type I just cut open: http://members.aol.com/setzeramp/mallory.jpg This was a 150MF 35VDC part from a vintage lab instrument board. I have used a couple others from this board, but this one showed some splitting of the plastic like you might expect as an electrolytic expands to leak almost like a dry cell battery. Of course there was never any liquid electrolyte but now under destructive examination it appears that the tantalum plug was off-center enough to make the molded plastic cover a little too thin at the point where it had cracked. Probaly was still good, and the cracking could have been just due to shrinkage of the plastic over the decades. It was a little rough on the hacksaw, the amount of tantalum is massive compared to today's solid tants of the same capacity & rating that look like little hot air baloons. These white Mallory's to me sound excellent and I would never replace one unless it was proven defective or I had decided to use a different value for tone shaping. Reducing the MF will roll off the bass to some degree on the preamp tubes as well as power tube. You can also check them for drift, which is very likely, they will still last decades more, just have different values from that originally marked on them. Lower is better. The rest of the caps could last indefinitely, I would only replace a proven defective high voltage cap with an Orange Drop as the closest current similar component. The original high voltage signal caps in the BF amps are considered by many to be superior to today's equivalent parts though. It may seem counter intuitive to roll off bass using smaller bypass caps, but this will make what you have tighter and you definitely want to protect an 8inch speaker from subsonics it can not produce anyway. This will leave more room for audible frequencies that are within the speaker's range, and the result is less muddy. I would check your 470ohm 6V6 cathode resistor for drift, I stick with a stock value even if it runs the NOS 6V6GT with the plate glowing slightly, a higher value like 510ohm (which is within 10percent of the 470) is OK if you have to run the tube cooler, but it's nice to hear these amps as intended to an extent. Unless the 470 seems perfect, I would replace it with another 470 that was 2Watts like the SF instead of the 1Watt size used on the BF, or if you really want to take the heat without stress, use a 5Watt wirewound 470ohm. Then for reduced hum you need a pair or matched resistors to create a virtual center tap to ground for the heater winding like seen on the larger Fenders of this vintage. 100ohms is common, but I have good luck with a 220ohm pair, soldered to the lamp assembly, provided it is isolated from the chassis (check it first!). First you will have to remove the ground connection for your heater winding and all 6.3VAC heaters (in other words all but the rectifier tube) and run new wires back to the lamp area. More of a pain would be to star ground the whole amp but that also helps reduce hum further. Then check other resistors for drift, the 100K preamp plate resistors can sometimes be a source of hiss or crackle, NOS carbon comp 100K's are the vintage replacement, but 100K metal film type are lower in noise. If you want more bass & midrange, then I would get a 2 x 12 speaker enclosure with 8ohm cones wired parallel to give the Champ a proper 4ohm load. Make sure to use fairly thick speaker wire soldered to an RCA plug on the amp end, if you try an ordinary shielded RCA to Phone plug cable, the inner wires will be too thin to work well for speaker currents. For even better performance, rewire a 4 x 12 Marshall 16ohm cab into a 4ohm configuration, sounds like a hassle, but when I say its worth it you know that's serious. Components not mentioned are not under nearly as much stress, so most other original caps & resistors should probably stay. Spray out the pots, get a few varieties of new NOS tubes, drain the filter caps each time before touching anything, remember never to solder on a tube socket before pulling the tube, probably a good idea to put in a 3-prong plug. Also it will run fine without the vibro tube in place, the vibro tube is to provide oscillation instead of being a tone tube like the other 12AX7, so use your lowest noise/best tone tube in the preamp spot, the vibro tube doesn't have to be the best, but I still like US NOS. When you get this amp working right, you will probably notice that the vibrato sounds better than the majority of full-size Fenders through history, since this is one of the few amps to use a full 12AX7 for vib and built without the less toneful LDR arrangement. This could keep you busy for a while . . . Hope this helps, Mike

4/24/2000 6:52 PM

MBSetzer

Re: Vibro Champ Questions

I have an OJ Vibrochamp still in near-mint condition, to me it still is the best sounding stock amp I have ever heard of any size or manufacturer.

The old Sprague filter cans on the BF seem to be a longer lasting cap than the later Mallory's, my BF is still good, but my later SF all needed the Mallory's replaced. Since yours was replaced in the '70's probably with a Mallory, it likely could use another new can. You can tell for sure by temporarily substituting new axials to see if there is an audible difference. If you do upsize the first cap section from the BF 20MF to the SF 40MF, it will allow somewhat better bass and less hum.

Other than that, to me a complete cap job then means replace only one other: the 25MF 25VDC cathode bypass on the 6V6. The schematic that came with my amp shows this as an electrolytic cap, but usually it is a paper cap instead. Age does take its toll on nonpolarized paper caps, plus this was inadequately rated by original design, there is already 21VDC according to the schematic, and when you crank the amp, it can add another 20VAC on top of the DC. So best to use at least a 50VDC rated part for replacement, I use a reduced value of 15MF (for less bass farting) with a 400VDC film cap, the Fastcap type from Angela will barely fit, but you will probably think it is worth it.

Of course your 6V6 bypass cap could be a small white molded Mallory polarized solid tantalum like the preamp tubes normally used for BF & early SF, I would still replace it with the film type, but save the white Mallory, they don't make anything quite like them anymore. These are polarized, but not wet tantalum types, they can easily last decades with no degradation. What you don't want is a cheap aluminum wet electrolytic type here. To see a picture of a larger Mallory of the same type I just cut open:

http://members.aol.com/setzeramp/mallory.jpg

This was a 150MF 35VDC part from a vintage lab instrument board. I have used a couple others from this board, but this one showed some splitting of the plastic like you might expect as an electrolytic expands to leak almost like a dry cell battery. Of course there was never any liquid electrolyte but now under destructive examination it appears that the tantalum plug was off-center enough to make the molded plastic cover a little too thin at the point where it had cracked. Probaly was still good, and the cracking could have been just due to shrinkage of the plastic over the decades. It was a little rough on the hacksaw, the amount of tantalum is massive compared to today's solid tants of the same capacity & rating that look like little hot air baloons.

These white Mallory's to me sound excellent and I would never replace one unless it was proven defective or I had decided to use a different value for tone shaping. Reducing the MF will roll off the bass to some degree on the preamp tubes as well as power tube. You can also check them for drift, which is very likely, they will still last decades more, just have different values from that originally marked on them. *Lower is better.*

The rest of the caps could last indefinitely, I would only replace a proven defective high voltage cap with an Orange Drop as the closest current similar component. The original high voltage signal caps in the BF amps are considered by many to be superior to today's equivalent parts though.

It may seem counter intuitive to roll off bass using smaller bypass caps, but this will make what you have tighter and you definitely want to protect an 8inch speaker from subsonics it can not produce anyway. This will leave more room for audible frequencies that are within the speaker's range, and the result is less muddy.

I would check your 470ohm 6V6 cathode resistor for drift, I stick with a stock value even if it runs the NOS 6V6GT with the plate glowing slightly, a higher value like 510ohm (which is within 10percent of the 470) is OK if you have to run the tube cooler, but it's nice to hear these amps as intended to an extent. Unless the 470 seems perfect, I would replace it with another 470 that was 2Watts like the SF instead of the 1Watt size used on the BF, or if you really want to take the heat without stress, use a 5Watt wirewound 470ohm.

Then for reduced hum you need a pair or matched resistors to create a virtual center tap to ground for the heater winding like seen on the larger Fenders of this vintage. 100ohms is common, but I have good luck with a 220ohm pair, soldered to the lamp assembly, provided it is isolated from the chassis (check it first!). First you will have to remove the ground connection for your heater winding and all 6.3VAC heaters (in other words all but the rectifier tube) and run new wires back to the lamp area.

More of a pain would be to star ground the whole amp but that also helps reduce hum further.

Then check other resistors for drift, the 100K preamp plate resistors can sometimes be a source of hiss or crackle, NOS carbon comp 100K's are the vintage replacement, but 100K metal film type are lower in noise.

If you want more bass & midrange, then I would get a 2 x 12 speaker enclosure with 8ohm cones wired parallel to give the Champ a proper 4ohm load. Make sure to use fairly thick speaker wire soldered to an RCA plug on the amp end, if you try an ordinary shielded RCA to Phone plug cable, the inner wires will be too thin to work well for speaker currents. For even better performance, rewire a 4 x 12 Marshall 16ohm cab into a 4ohm configuration, sounds like a hassle, but when I say its worth it you know that's serious.

Components not mentioned are not under nearly as much stress, so most other original caps & resistors should probably stay.

Spray out the pots, get a few varieties of new NOS tubes, drain the filter caps each time before touching anything, remember never to solder on a tube socket before pulling the tube, probably a good idea to put in a 3-prong plug.

Also it will run fine without the vibro tube in place, the vibro tube is to provide oscillation instead of being a tone tube like the other 12AX7, so use your lowest noise/best tone tube in the preamp spot, the vibro tube doesn't have to be the best, but I still like US NOS. When you get this amp working right, you will probably notice that the vibrato sounds better than the majority of full-size Fenders through history, since this is one of the few amps to use a full 12AX7 for vib and built without the less toneful LDR arrangement.

This *could* keep you busy for a while . . .

Hope this helps,
Mike

Replies:

Rebel420 q{I have an OJ Vibrochamp still in ... -- 4/25/2000 3:12 PM
Joe. Z Thanks to everyone! I have enough i... -- 4/25/2000 3:22 PM