Re: JTM45 good news?
- From: "Ether" <ether@xxxxxxxxxx>
- Date: 3 Nov 2005 19:48:53 -0800
Phil S wrote:
> Relatively good news. Considering I got the tranny for 'bout half a
> sawbuck, and given the bad luck so far, I figured I hadn't much to lose.
>
> I removed all of the tubes. Then, I unsoldered all the secondaries from
> their various connectors, but left the primaries in place. That made it
> very easy to get AC to the primary -- through the switch.
>
> The B+ secondaries were 823v across the two outer legs and 409v one on leg
> to the CT and 412v on the other leg to the CT. (Go figure, lost one volt?)
Excellent.
>
> The filament secondaries were also good, about 5.45v across the yellows, and
> 6.6v across the greens.
Good there, too.
>
> And no smoke or sizzle! Nor did the tranny get the least bit warm to the
> touch in the few minutes it was on.
Good sign...
> So I reconnected the B+ to the rectifier pins 4 & 6, and the CT to ground.
> Then took a bias supply reading, which is now maxed at -60mV on pin 5 of the
> power tube socket. (All tubes out of amp.)
Perfecto.
> So, any hints on what I'm looking for? Am I looking for something that is
> grounded by shouldn't be? Or something that should be grounded and isn't?
More likely the former. Or two lines that shouldn't be crossed, but
are. (As Dr. Venkman said, "That would be bad.")
> Should I reinstall all the tranny wires and power up with no tubes?
Here's what I would do: Test the heater line next. As I said, keep
the rectifier out, reconnect the 6V filament leads, check your heater
wring on the sockets, put in the preamp and outpuit tubes, and test the
heaters. (By the way--do you have a filament center tap, or are you
using an artificial center tap with the two 100-ohm resistors to
ground?)
If your heater works, you'll know that something is amiss after the B+.
Are you sure your rectifier isn't shorted? Maybe try a different
rectifier, too. (This is where it's nice to have a plug-in diode
rectifier and a variac.)
> (Gilbert: caps are generally rated at 630v or 500v, so this shouldn't be a
> problem. No wimpy stuff was used except the first stage bypass cap, which
> is 25v rated.)
>
> If not, I'll have to pull the board (a real PITA now that all the wires are
> soldered in place) and go over the whole thing. Or I could engage in an act
> of stupidity (definition = doing the same thing twice and expecting a
> different result).
Heh...From what you said, I gather that your bias network was correctly
wired to begin with. If everything else is good, I'm afraid you're
going to have to check your circuit-board wiring, PITA though it may
be.
> Thanks.
> Phil
Keep at it!
--E
.
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- JTM45 blows smoke ;~{
- From: Phil S
- Re: JTM45 good news?
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- JTM45 blows smoke ;~{
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