Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: Davidg on December 22, 2012, 10:29:01 pm
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I have been taking in some repair jobs lately and a guy brought in a Ampeg Svt6 PRO with a totally fried output board and mosfets along with the dreaded JCM2000.The Marshall was done in a day but I cant even find a schematic for this Ampeg and am not very familiar with solid state as I told the customer but would see what I could do.Only info I have gleamed is that this is a fairly common problem with these and that they use a hybrid design with 12AU7s driving voltage and the FETs driving current which I do not even understand.So I hear that LS electronics in St Louis is the place to send these but I am unsure about recommending someone I dont know so the question is is anyone familiar with these and this problem of spontaneous combustion and should I go ahead and give up so I can start on my 18 watt without feeling guilty? Happy Holidays to All!
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> they use a hybrid design with 12AU7s driving voltage and the FETs driving current which I do not even understand.
This amp is 1,100 Watts in 4 ohms.
That is 66V at 16 Amps.
A 66V signal.... hey, a 12AU7 can easily swing that! Might need a good high B+, but no big deal.
16 Amperes..... well, a 12AU7 can deliver 16 _milli_Amps. Cool, we only need a thousand tubes!
However _ONE_ big modern MOSFET can dump 16 Amps easy. Doing it at high voltage and power might need a dozen or more (or less).
12AU7 are $10 each, so $10,000. Big MOSFETs are $10 each, so $120.
The MOSFETs are (probably) working at unity voltage gain. Giant "cathode followers".
True, we could have got that 66V with solid-state driver. But then it would be no tubes at all, and competes with a million other all-sand boxes.
I think I have seen the guts of a related amp. My thought? I would not stick my hand in there. The 12AU7 run at lethal voltages. The MOSFETs can pass enough current to weld your rings to your finger-bones. The topology was not simple, fault diagnosis would be a mind-mess. And if like many other modern products, the physical layout makes no sense, was not intended to be serviced.
> should I go ahead and give up so I can start on my 18 watt without feeling guilty?
I say yes.
> LS electronics in St Louis is the place to send these but I am unsure about recommending someone I dont know
Google turned up LS' lame website, and one review, which is unfavorable, but the customer was really in the wrong place, and the owner replied nicely.
http://plus.google.com/108743721928408826604/about (http://plus.google.com/108743721928408826604/about)
They say they are authorized to warranty-repair Ampeg, which means they can't have a lot of outstanding complaints.
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Thank You for your reply and for explaining that in a way that made perfect sense PRR.I guess that would explain the huge toroidal PT that looks to be about the same size as the hubcaps on my wifes Hyundai! This one is definitely over my head so I guess I will try to get hold of LS after christmas. I heard of them while researching this amp on the TalkBass forum-supposedly several ex-Ampeg employees work there including the head tech who was supposed to be in on designing this SVT wanna-be.From what I gather the total meltdown is from losing bias voltage and these are very tricky to dial in correctly and if not done right will suffer the same fast death eventually.I figure the best I could do for the guy is se that he can get it done right or if it is too high buy another one-or better yet buy a real SVT! Thanks again and Happy Holidays!
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I have worked on a bunch of SVT's if the problem is on the power amp/power supply board then it's a major pain. Also if it's 1 or more of the MOSFET's good luck ( I would back away from it) several of the ones I have worked on these are glued to the heat sink.