Under the hood of a Vibrochamp

SamBooka

Member
Messages
2,222
I posted this on TDPRI but thought I would get some more opinions on this

I have a 77 Vibrochamp I haven't used in a couple of years. Fired it up last week and it was humming more than usual (bad cap can.. no biggie).

All is well. I have already changed the other electrolytics a couple of years ago and none of the coupling caps seem leaking.

Sounds good and I am about to seal it up again BUT I wanted some opinions.

As routine maintenance what do you do when you work on an amp as in "I've got it open, I might was well clean/tighten the socks. Squirt the pots and jacks etc etc etc"

Like I say the amp sounds fine now and I am not looking to do mods but "while I am in here I might as well check _ _ _ _ "

Thanks

Drew
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Blue Strat

Member
Messages
30,194
Increase the value of the 470ohm 6V6 cathode resistor to 800-1000 ohms. Then it will have at least a slight chance or running in spec rather than in continual melt down mode. If you haven't already changed the cap across that resistor, you should now. Use something with at least a 50 volt rating.
 

SnidelyWhiplash

Silver Supporting Member
Messages
8,444
Mine certainly did. I replaced the 470 ohm with a 1k resistor to keep from
burning up my power tube.
 

SamBooka

Member
Messages
2,222
Well I have to say that personally I LOVE the 470 Resistor. The original owner of the amp had issue with it and sold it cheap. I bought it, fired up amp and snap crackle kaboom the powertube was going off like crazy. New power tube and it was fine for 6month and the that one went south. Both were EH.. The tungsol ri that has been in there 5 years (wow.. 5 years already) has been fine but I will change the resistor.

Thanks
 

LPVM

Member
Messages
2,520
The problem with that resistor is it tends to get hot and since it sits up against the 25uf 25v cap it cooks the cap until it pops. The standard fix with that cap is to replace the 25uf 25v cap with a 25uf 50v cap and put a little gap between the resistor and the cap so it doesn't toast it anymore. My resistor actually drifted down to something like 330 ohms. I was routinely burning up power tubes and if I turned the amp up north of 6 I could take it out in a matter of hours. Once that cap went off like a party popper I changed both and haven't had a problem since even with all controls dimed and the amp boosted. I kept mine at 470 ohms and bumped up the resistor size (watts) so it wouldn't get hot anymore and bake the cap next to it. I changed the filter caps in mine a year or so ago as well but need to go back in and change the remaining 2 electrolytics.
 
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Prattacaster

Member
Messages
1,369
I always replace the 470 ohm with a fresh 470/5w cathode resistor. I think raising the value impacts the tone more than some other things you can do to get that 6v6 running cooler. Typically the plate dissipation is around 17w or better for the average 6v6gt in this amp. I feel less than 15w would be acceptable.

Lowering the B+is my preference. Stick a 390-470ohm resistor between pin8 of the recto and the first node. Plate voltage will drop from 380ish to 340ish. Cathode current will drop also. Yes you will lose a little less than 1watt of headroom. You also lose some trebles, both of these are a good thing in my opinion for a vibrochamp.
 

LPVM

Member
Messages
2,520
Lowering the B+is my preference. Stick a 390-470ohm resistor between pin8 of the recto and the first node. Plate voltage will drop from 380ish to 340ish. Cathode current will drop also. Yes you will lose a little less than 1watt of headroom. You also lose some trebles, both of these are a good thing in my opinion for a vibrochamp.
I really like the sound of this mod. You are absolutely right, both of those things would be a good thing in a VC. I'm definitely trying this.
 

SamBooka

Member
Messages
2,222
Thanks Pratt... Will try that. When I had replaced the bypass cap I had already used a 50V and left a gap between them.
 

tmuka

Member
Messages
751
Thanks for the thread, SamBooka, I had similar questions about my '76 vc. I bought the replacement can cap a few years back but haven't put her in yet... Love the sound of this amp, just don't want it to explode and fry the transformers!
 




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