resurecting '68 ampeg v4 &nbs

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pzung
Junior tube assistant
Posts: 7
(4/19/04 2:42 pm)
 resurecting '68 ampeg v4
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 i'm trying to rebuild my friend's old v4 which he has taken to several different shops in different states over the last 8 years but it wouldn't ever stay fixed . it would only play for about ten minutes then make wierd swooshing sounds and various other scary noises . the tubes were supposedly new or nos so i started by changing the filter caps and then everything that was a capacitor at all . the filter caps are in metal cans and didn't look bad but after 30 years i figured they were probably bad . all the other caps were cracking and falling apart . after i did this it seemed to work but when i plugged it directly in to the wall with out the light bulb power limiter rig it would start to over heat and get noisey . i checked the bias and it was only -9v and woudn't go any higher . it should be around -70v . i looked at the power tubes and the plastic bases look like they have been exposed to extreme heat . so i guessed that maybe the bias has just been broken and it would play until the tubes got so hot they expanded the sockets and something would disconnect itself , etc . the plate voltages were around 450 and the voltage coming off the rectifier was around the same . someone had done something really weird to the bias supply and it obscured one of the bias caps so i guess that either that or the diode is bad , but i won't know until the parts show up . my question is :
could something somewhere else cause the tubes to draw mad current (300ma's before i turned it off) and make it look like a bad bias supply ? a short somewhere ?
since i think the diode died , i'm going to replace anything that is a diode so i know they are not broken .

 
PHATamps
I only work on tube amps
Posts: 438
(4/19/04 5:20 pm)
 Re: resurecting '68 ampeg v4
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 Sounds to me like you're on the right track.
 
Tiny Daddy
I will work on all amps
Posts: 623
(4/19/04 10:01 pm)
 Re: resurecting '68 ampeg v4
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 Maybe replace the power tube sockets because if the screen grid or the control grid loose their connection, bad things happen.
 
pzung
Junior tube assistant
Posts: 8
(4/23/04 2:17 am)
 bias supply
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 replacing the diodes didn't resolve the problem ...
someone had tried to change it from the fixed to an adjustable bias . they tried to put a pot in place of r49 with the wiper and one leg connecting to where the negative end of c15 meets r60 and the other leg to ground . with that configuration i don't think the wiper was even doing anything . the bias was still coming off where r60 meets the negative end of c23 .
i think i need to take out r49 and start from where r60 meets c23 . put one pot leg there and send the other leg to ground with a resistor equal to r49 and then put the wiper to the grids .
does that sound right ?
i'm looking at other amp schematics , i'm a little confused by r50 coming off the bias supply diode .
 
Tiny Daddy
I will work on all amps
Posts: 629
(4/23/04 7:22 am)
 Re: bias supply
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 Check out the bias arrangement at:
www.jamesaudio.com/gpsupply.html
Your resistor vaues may be sllightly different to get the correct voltage range with your power supply. Note the 1000V cap, recommended.
 
pzung
Junior tube assistant
Posts: 9
(4/23/04 7:45 pm)
 thankyou
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 i forgot to say thankyou in my last update/question . thankyou for your support and replies .
i still haven't gotten the bias supply to work properly but i'm going to back up and just go through the whole amp as per "how to trace out a circuit problem" because i found a resistor that looked fine but must be burned out inside . it reads open even after being removed . so i can't assume that anything in this amp is okay unless i've checked it first . lesson learned . i guess the expensive part is getting new tubes that i know are okay to check if the problem is in the tubes . i only have spare 6L6's and EL34's , i'm tempted to look for some conversion to see if i can check the amp with some of those tubes and then return it to using the 7027's if it seems okay . i can't decide if i have more time or more money . life is an adventure .
 
 
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