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VibroKing EL84-reverb problems


 
10/13/2003 10:38 PM
Chris ( CMW amps )
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VibroKing EL84-reverb problems
Hi  
 
A guy did ask me to help him with his early ( sn. 8 ) VibroKing amp .  
The EL84s are burning :) , did check an early schematic and the plates/screens voltages are insane for that tube ( single ended ) .  
Anyone got's some good toughts about curing this problem , I guess I need a lower voltage to feed the EL84 plate and screens but I'm looking for a good/reliable way to do that .  
 
Any AmpageExpriences are welcome !  
 
tia ,  
 
Chris
 
10/13/2003 10:48 PM
Dai Hirokawa
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I've heard of that problem with the VK before, and my recollection was that indeed, the way to solve the problem was to lower the voltage. Sorry, I don't remember the particulars.  
 
Dai
 
10/14/2003 1:24 AM
Ray Ivers
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Chris,  
 
Instead of connecting the EL84 plate/screen to points Z and Y in the power supply, try connecting them with clip leads to points Y and X instead and see how the reverb works, and if anything gets too hot (like R38 or - less likely - R67). You'll probably have to replace R38 with a different value, and possibly higher wattage as well. 470 Vp/380 Vs is just ridiculous for an EL84.  
 
If you end up with voltages you can live with, you should obviously tweak R69 if needed to set the EL84 idle current, and upping the value of R70 to 1K ohms/3W wouldn't be a bad idea either IMO.  
 
Ray
 
10/14/2003 2:34 AM
Major Pain
Why not just email Bruce Zinky since he designed this amp? Im sure he would gladly help with the problem.  
Just a thought.  
 
M.P.
 
10/14/2003 3:55 AM
PAULH
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Hi Chris, (Hoi?)  
 
Fender would later shelf this design. (They went to a 6V6.) But since you have one with the EL84 driver, I recall when I asked Fender about this, they recomended using a certin type of EL84. I don't recall what they recomended but at the time it worked. I do think the "silver bullet" is going to be to get that plate voltage down. What kind of EL84 are you using? Modern EL84's do vary quite a bit between different manufacturers. Another option might be to add a drop resistor from the exsisting supply node with a cap to create a new node at a lower voltage. Cheers! -Paul
 
10/14/2003 7:05 AM
John G
Chris,  
I got an e-mail from Zinky on this one, if my memory serves me right I think he said you must user a Sovtek EL84. This was quite a number of years ago now, I will see if I have kept a hard copy of it somewhere.  
John G
 
10/15/2003 8:43 AM
Chris ( CMW amps )
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and thanks to all !!!
Hoi ( Hi ) Paul ,  
 
groetjes uit Driebergen ( greetings from Driebergen , The Netherlands ) ! :)  
 
The EL84 is used as reverbdriver but the owner went through lots of them . Just did download the schematic from mrgearhead ( http://www.mrgearhead.net/faq/schematics/fender/Vibro-King%20Schematic.pdf ) to compare it to rev. D . I do have JJ/Tesla and Ruby EL84s over here and some Sovtek EL84Ms .  
Imagine a VOX AC4 working with a B+ from about 470 Volts ..... Fixed bias / push-pull EL84s can take wayyy higher voltages like some Boogie and the mentioned early piggyback Tremolux .  
 
I'll try to get the "official Fender repair papers" on this from a fellow tech and please email me if you have those too .  
 
Chris
 
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