ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Sam This amp has got a buzz when playin... -- 10/10/2000 1:28 AM

Re: 68 Fender Vibro Champ problem

10/10/2000 9:24 PM
MBSetzer	Re: 68 Fender Vibro Champ problem Tim & poboy have some good advice. Not only should all the wires to your speaker connection be good, but you may need to polish the old RCA jack and plug tip, plus bend in the ground prongs, and maybe adjust the tension on the signal connector from inside the jack. Sounds like you may already have that covered if you get the same performance problem using an external 4ohm speaker load. After checking the simpler things like connection quality, then maybe check the bypass cap on the 6V6 cathode resistor. This component was not originally rated for high enough voltage in the original amps to allow them to be fully cranked without damaging the cap. It is a 25mf/25VDC paper cap usually, even though the preamp was using the white Mallory solid tantalum polarized type of the same value. On an early SF I have seen a white Mallory on the 6V6, it is also like a BF since it has only a 1watt 470ohm resistor, that part should also be 2watts minimum IMHO. Find the cap in parallel to the 470ohm, unsolder at least one end of the cap then check the resistance of the 470ohm, if its anywhere close to spec leave it for now so you can make a good comparison before & after. The amp will run real well without this cap at all, but have lower output, you are going to be checking for tone while trying to disregard differences in volume. Or you can try and see how low a volume setting you can hear the rasp now, then disconnect the cap and see if you can turn it up beyond that volume without raspiness, it would take a bit more on the volume knob, so you would need to kind of disregard its setting in that case and just listen. If it sounds better without it, you have identified at least one problem, and a likely one. Then acquire a replacement at least with a 50VDC rating, a nonpolarized cap will usually be harder to find but sure beats the tone of a cheap aluminum electrolytic. If you do put an eletrolytic, observe polarity, plus to the tube socket, minus to ground. Today's closest standard value would be 22mf to replace the stock 25MF, you would probably not be able to hear the difference, and there is a large tolerance around exact mf value anyway. There is some agreement that this is more MF than needed to produce the amount of bass frequencies for regular speaker cabinets, so it really seems to make even more sense to use lower MF with an 8inch speaker in an open cabinet. It does reduce bass farting at higher volumes if you use a 10 or 15MF replacement, modern values of 8 or 16mf would be therefore recommended. For the multisection can cap mounted next to the tubes, these do have a limited lifetime so it would be likely if yours is original you will benefit by replacement. This is a much more expensive cap and more difficult to replace. Like Tim says it could very well be your only problem, real serious failure of this part will normally be also accompanied by annoying hum or buzz without playing the guitar. These are cool little amps, don't even think about using amateur soldering skills just because its less expensive than a big amp. Plus the safety issues regarding the high voltage and the current stored in the filter can even after the amp is unplugged, all tend to make me emphasize that my advice is really intended for you to take to an experienced bench tech, but this first simple possible repair could then be made by someone without direct guitar amp background. You could do the listening tests but leave the inner chassis work to a solder jockey, unless you are really good at that yourself. Anyone even thinking about working in an open tube amp should spend a dozen or two hours reviewing safety information, even if they have good electronic background & soldering skills already. Good stuff is posted by R.G.Keen and Randall Aiken among other recognized 21st century experts, along with traditional decades-old cautions. Plus a good tech will have Deoxit or equivalent professional contact cleaner for the jacks & pots, do not use any on the tube sockets though. If you get to the point of working on a hot open chassis, then you might want to post the measured value of the 470ohm resistor, as well as the operational voltage across the resistor, tube bias can be figured from that, simple advice is to make sure it is close to 470ohms or higher, if it is too much lower the bias can be too hot even though the resistor is still within the +/- 10% original tolerance. Nothing wrong with putting in a new 470 that tests on or above the intended ohms, or if the tube runs too hot use 510ohms instead. Hope this helps, Mike

10/10/2000 9:24 PM

MBSetzer

Re: 68 Fender Vibro Champ problem

Tim & poboy have some good advice.

Not only should all the wires to your speaker connection be good, but you may need to polish the old RCA jack and plug tip, plus bend in the ground prongs, and maybe adjust the tension on the signal connector from inside the jack.

Sounds like you may already have that covered if you get the same performance problem using an external 4ohm speaker load.

After checking the simpler things like connection quality, then maybe check the bypass cap on the 6V6 cathode resistor. This component was not originally rated for high enough voltage in the original amps to allow them to be fully cranked without damaging the cap. It is a 25mf/25VDC paper cap usually, even though the preamp was using the white Mallory solid tantalum polarized type of the same value. On an early SF I have seen a white Mallory on the 6V6, it is also like a BF since it has only a 1watt 470ohm resistor, that part should also be 2watts minimum IMHO.

Find the cap in parallel to the 470ohm, unsolder at least one end of the cap then check the resistance of the 470ohm, if its anywhere close to spec leave it for now so you can make a good comparison before & after.

The amp will run real well without this cap at all, but have lower output, you are going to be checking for tone while trying to disregard differences in volume. Or you can try and see how low a volume setting you can hear the rasp now, then disconnect the cap and see if you can turn it up beyond that volume without raspiness, it would take a bit more on the volume knob, so you would need to kind of disregard its setting in that case and just listen.

If it sounds better without it, you have identified at least one problem, and a likely one. Then acquire a replacement at least with a 50VDC rating, a nonpolarized cap will usually be harder to find but sure beats the tone of a cheap aluminum electrolytic. If you do put an eletrolytic, observe polarity, plus to the tube socket, minus to ground. Today's closest standard value would be 22mf to replace the stock 25MF, you would probably not be able to hear the difference, and there is a large tolerance around exact mf value anyway. There is some agreement that this is more MF than needed to produce the amount of bass frequencies for regular speaker cabinets, so it really seems to make even more sense to use lower MF with an 8inch speaker in an open cabinet. It does reduce bass farting at higher volumes if you use a 10 or 15MF replacement, modern values of 8 or 16mf would be therefore recommended.

For the multisection can cap mounted next to the tubes, these do have a limited lifetime so it would be likely if yours is original you will benefit by replacement. This is a much more expensive cap and more difficult to replace. Like Tim says it could very well be your only problem, real serious failure of this part will normally be also accompanied by annoying hum or buzz without playing the guitar.

These are cool little amps, don't even think about using amateur soldering skills just because its less expensive than a big amp.

Plus the safety issues regarding the high voltage and the current stored in the filter can even after the amp is unplugged, all tend to make me emphasize that my advice is really intended for you to take to an experienced bench tech, but this first simple possible repair could then be made by someone without direct guitar amp background. You could do the listening tests but leave the inner chassis work to a solder jockey, unless you are really good at that yourself. Anyone even thinking about working in an open tube amp should spend a dozen or two hours reviewing safety information, even if they have good electronic background & soldering skills already. Good stuff is posted by R.G.Keen and Randall Aiken among other recognized 21st century experts, along with traditional decades-old cautions.

Plus a good tech will have Deoxit or equivalent professional contact cleaner for the jacks & pots, do not use any on the tube sockets though.

If you get to the point of working on a hot open chassis, then you might want to post the measured value of the *470*ohm resistor, as well as the operational voltage across the resistor, tube bias can be figured from that, simple advice is to make sure it is close to 470ohms or higher, if it is too much lower the bias can be too hot even though the resistor is still within the +/- 10% original tolerance. Nothing wrong with putting in a new 470 that tests on or above the intended ohms, or if the tube runs too hot use 510ohms instead.

Hope this helps,
Mike