Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on June 11, 2018, 10:00:57 am
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High voltage on chassis:
Just went through an Ampeg SVT V9 cleaning pots,switches,tubes sockets,bias,etc. I plugged into the first channels input jack and it knocked the crap out of me.Totally surprising experience. I was making ground connection to the amp through the input cable in my hand. Very distorted and low volume. The amp was re-capped a few years ago from what i can see.The bias is adjusted for 072 volts at the test points. Not a problem there. I will go no further at this point until i can understand how or where the high voltage can be on the chassis. Nothing looked out of place or unusual. There is no surge cap on the standby switch. I am thinking one of the PS caps is dead shorted to ground.
I have to open it up again.
This weighs roughly 90lbs and my back is out now. I can not stand to open it up again. Problem*
I normally would have a guitar already plugged in to check out the amps sound. Good thing i didn't.
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Have the power amp out of chassis and tonestack cable plugged in. Power on and then flipped standby with oven mitten. I have 460VDC on the ground nuts of the switchcraft input jacks. Talk about a death wish!
This is a perfect example of taking things for granted and not expecting something like this. Ultra Dangerous!!!
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EQ on bench.
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Went back over testing the preamp tubes again. V1 in the power amp has a 12AX7 and is shorted on V1B side.Though the chart says that a 12DW7 goes in the V1 power amp position. I saw my short light turn on and then go off. I flicked the tube to shock it and the short light is staying on now.
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I find it really hard to believe that this amp was built without a dedicated ground wire to mate both the preamp and power amp together.There is a ground for the fan which is then attached to the metal back panel and then to the power amp.
I thought that there would be a ground wire in the harness connecting both preamp & power amp when connected together. NOPE!
Why would they build any amp with separate power and preamp and not have a ground wire in the harness grounding both on the same rail? They wouldn't. But that's what i have here.
I understand that you want to isolate your power amp from your preamp grounds on the chassis and where they are located.
I am continuing to check for continuity on all the harness wire's. There has to be a ground lead for the EQ panel. So far i don't find one.
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Though the amp says V9 on it, the customer says it is not a V9 and one of the odd ball amps during some transitional stage. He has a bunch of them and is more familiar with them than i am. I am assuming that there must have been a flat braided lead wire at some point in it's life that provided ground for the EQ. That's all i can come up with. Maybe the last person to recap it forgot to put it back on. I have no idea.
I ran a wire from the EQ panel and then soldered to the power amp chassis. Preamp voltage are right now and the amp sounds strong.No more voltage on the EQ panel.