vibro champ

[tonalvision]

I just acquired a 1978 vibro champ. I have it on my bench to fix a problem with oscillation or something similar. I really am not sure how to describe the problem. It manifests itself with the note that is played ending in a fizzling sound. Thus far I have tried: Replacing all tubes, clean and tighten tube sockets, replace every capacitor save two(therefore re-soldering just about all points on the board), checked the value of each resistor (values ok), attempted to clean up the lead dress, tried playing through a new speaker known to be good. The problem is still there. It seemed to lessen somewhat when changing 6v6, but is still there. The amp is much louder now then when I received it probably due to the new filter caps. Any suggestions appreciated. Thanks

[Gtr_tech]

Change the output tube cathode resistor and cap? How about plate load resistors in the front end?

[Zouto]

I had similar problems with my '72 VibroChamp.Besides old filter caps a semiblown OT was the main troublemaker. Replaced it with a new one with 8Ohms tap, put in a better speaker, Sovtek GZ34 and TungSols and enjoyed the fine sound ever since.
Zouto

[Enzo]

AMen. Did you connect the amp to a different speaker. Your description fits a rubbing voice coil.

[tonalvision]

vibro champ

I really wish that it were a bad speaker. I tried a substitute and it did not seem to sound any better. Thanks to everyone for your observations. It really isn't that frustrating since I am enjoying the work. I would, however, like to get this one fixed. I had thought that maybe it could be a bad transformer. Can someone advise on testing? I can check for certain voltages at test points. I do not have an o-scope. I would not mind trying another OT. The PT looks kind of suspect as it has melted black material clinging to it here and there, as if it has oozed out from inside. Another thing that is bothersome is the eyelet board and components have a waxy substance on them. (I think it IS wax). Any help appreciated.[IMG][/IMG] [IMG][/IMG]

[captntasty]

That PT looks a little scary... have you ever seen RG's Super secret transformer tester? You'd need a neon lamp (NE-2 I believe but you might be able to sub in something else). It's only going to tell you if you have shorts...

edit: I forgot to mention - I have a homebrew 5F1 champ - same problem. Sounds like a blown speaker but it's not. I've smoothed it out several different ways - made preamp cathode bypass caps switchable(reduces gain) and fiddled with more or less negative feedback. They both help but bring the amp to the point where it doesn't get that nice fully cranked small wattage-SE overdrive - which is exactly what we want! I have also heard that OT's that have no air-gap can produce this trailing sputter. I have a hammond 125j (I think - 125 series anyway) and have considered giving their 125 SE series a try in this one - specifically for SE amps. I built two others with the 125 SE series and don't have this problem...

edit #2: have you considered taking the bell off the PT to visually inspect?

[willard]

... The PT looks kind of suspect as it has melted black material clinging to it here and there, as if it has oozed out from inside. Another thing that is bothersome is the eyelet board and components have a waxy substance on them. (I think it IS wax). Any help appreciated.

My new 78 Vibro Champ has the same ooze on the PT. I'm not a tech so I'd also like to find out how to test this.
[edit] It doesn't have the trailing note problem that you have though. [/edit]

The waxy stuff on the eyelet board is wax. It's from the old caps; they were sealed with it. The heat from normal operation inside the amp causes a "wax vapour" to form, spread and settle inside the chassis. The breakdown of the wax seal is what make these types of caps become leaky. [this just summarizes what I've read elsewhere on the web about the wax]

[52 Bill]

Welcome Willard!

The caps that you are talking about were not used much in the 1970's. They were more common back in the 40's and 50's.

Fender would dip the entire component board in wax as a way to stabilize the components and to keep out moisture.

[willard]

Welcome Willard!

The caps that you are talking about were not used much in the 1970's. They were more common back in the 40's and 50's.

Fender would dip the entire component board in wax as a way to stabilize the components and to keep out moisture.

Thanks for the welcome. Figured I should post right away and stick my foot in my mouth early

So if I end up replacing most of the components on the board and am happy with the tweaks, should I do something similar?

[ThermionicScott]

You could always try adding a grid-stopper to the 6V6.