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Vintage threads from the first ten years

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2/12/1998 2:30 PM
Paradiddle7 AMPEG HELP.
Well guys I replaced the preamp tubes to no apparent success,.....I am kinda pissed now because it looks like a power tube has blown,.....Its not lighting up no matter which socket i put it in, it because the tube isn't biased properly?,....I got all new preamp tubes,.....what else should i check?,......What about this tube what could be the problem?  
Thanks alot.......I really appreciate all the help.  
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2/13/1998 7:23 AM
if the tube is not lighting up it's the filament, not the bias circuit. check to see if you've got 6von the filament wires...if yes it's a bad tube...  
2/12/1998 6:15 PM
Re: Ampeg VT22 Cracking/Popping!!!!!!Please help!
Jeff, can't really tell by your posting but it sounds like you may have a problem of motorboating, i.e., coupling between low level amplifier stages. Motorboating is a low frequency oscillation caused by part of the signal feeding back from one stage to a preceding stage that is in phase with the stage feeding back (usually over two stages). The various filter capacitors that directly feed the plate load resistors on preamplifier stages are referred to "decoupling" capacitors. Since your speaker "pops" in and out (low frequency oscillation?) and the frequency varies with the volume control setting (I think?) or with tone control setting you are changing the resistor/capacitor time constant of the oscillator.  
Try bridging a known good capacitor of proper value and rating (40 mfd at 500v I beleive) across the sections of the filter capacitor. My schematic indicates that the crutial on is probably the section marked with the triangle. Second guess would be the half circle. This circuit is poorly decoupled as designed so proper decoupling capacitor value is probably critical. You might even want to use a higher value capacitor and solder it across the triangle section and leave it there (remember, meet, or exceed the rated voltage).  
Hope this helps GW
2/12/1998 2:16 PM
I have just bought some new preamp tubes for my Ampeg vt-22 and put them in. Nothing really changed. I noticed that one of the Kt-88's is not glowing. Now i don't know for sure if it was glowing before but when i remove the pre amp tubes it still doesn't glow!,.....Is there something wrong with the Biaing?,...or is this tube dead>?,...they are all brand new!,......I switched the tubes in different sockets,...and the same tube still didn't glow,......any suggestions?  
2/12/1998 3:38 PM
John Kos

If you are saying that the filament does not glow, then the tube is dead and biasing has nothing to do with it.  
John Kos
2/23/1998 6:37 PM
Re: Ampeg VT22 Cracking/Popping!!!!!!Please help!
Check resistor R20 on the pre board.  
There's a 12DW7 in there just before the 12AU7 that serves as a buffer and return stage for the "preamp out/power amp in" jacks on the back. One half operates as a cathode follower that has a 47K resistor to ground. I've worked on a lot of Ampegs in which that resistor was either just noisey or actually cracked from heat causing popping and crackling. If that is the problem, replace it with one of twice the original power rating(if it's a 1 watt make it 2).  
Good luck!  

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