JCM 900 questions...

Vanyu

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738
I've got a bit of a brain bender here.

The amp in question is a late 90's 4100 that's been badly neglected by ignorant owners, and needed a new PT. I bought a Classictone replacement, got it hooked up, and was greeted with voltage in the ground circuit, which as a result, makes D6 and D7 smoke, and damages tracing. The filter caps and HV diodes ground at the same location. Could either of those be leaking voltage to ground and sending voltage to places it shouldn't be?

I'm confident in my wiring job, I was careful to mark each joint with its function, and I'm confident HV isn't making it to the ground circuit due to faulty wiring. It did this before the new PT also.
 
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Vanyu

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738
Bumping.

Could somebody please tell me the function of D6 and D7? Protection diodes? How about C16? Is this supposed to kill off voltage going to ground? Here's the schematic for reference.



Any input would be greatly appreciated.
 

pdf64

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7,364
D6 and 7 prevent excessive voltage between the chassis mains ground and the circuit 0V.
I think they're intended to mitigate ground loops, eg if linking 2 amps together, by there not being a direct short between mains ground and the amp's 0V.
I think that Fender's Reverb RI has a similar arrangement; of course ground loops would be almost unavoidable if mitigation was not used with a mains powered / grounded chassis fx unit http://bmamps.com/Schematics/fender/Reissue_63_Reverb_Service_Manual.pdf
Are you sure that you haven't connected something to the chassis rather than the 0V bus?
Is the parallel 22 ohm resistor R36 ok?
 
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Vanyu

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738
It's funny how these diodes are meant to prevent ground voltage, yet they totally freak out when they see voltage.

The parallel resistor reads normal, but I haven't checked that capacitor yet. I suspect a component failure due to how this problem manifested itself. This problem began after I replaced R8 and R9, and replaced HT and heater fuses and woke up a pair of tubes that hadn't been working for a long time. I'm hoping it's just C16, or something I messed up in the tracing fixes, because if it's not, then it's something upstream, and that can be a lot of different things...

I'm fully confident the transformer is hooked up correctly, I was very cautious in doing so. It had this problem with the old transformer, and another local tech misdiagnosed it as the 6.3v heater windings shorting into the heater CT, and tried to blame it on shorted tubes (which weren't shorted by the way). What I'm not so confident on is some of the tracing work done while the amp was with the other tech. I'm going to redo those traces, some of the joints are really sloppy.
 

Vanyu

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738
I just discovered a short in the heater circuit, blowing F5 with no tubes installed. Tube sockets, maybe?
 

Vanyu

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738
Problem solved! It was one of those silly mistakes. I installed 6550/KT88 tube retainers at the beginning, and one of the heater wires got stuck in a lock nut and was grounding. That explains a lot. Oops!
 

Vanyu

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738
I've got one more question...

The amp is seeing plate voltage with standby on, and even higher with it off. Is this normal? Most amps I've worked on kills PV all together with the standby on. I see how it would be possible in the schematic, but it's just extremely interesting to me that it would be done this way. Just want to verify before I put tubes in and attempt to bias.
 
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pdf64

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7,364
I can't see how there could be any Vdc HT in standby?
The standby switch will prevent any Vac HT flowing.
Perhaps you've used a PT with a CT on the HT winding, and the CT has been connected to 0V? The standby switch would then just break one of the two HT circuits, resulting in a rather saggy, half wave rectified Vdc HT.
If so, you'll need a dual pole standby switch for it to work as normal; but a lowered Vdc HT standby mode seems quite a good idea.
 

Vanyu

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I think I got the HV wires backwards. After looking online at other JCM 900 PT schematcis, I see that they have 360v-0v rails. Classictone didn't specify this, their schematic just had "360v" between the two wires. Oops! Though I do wonder if I used a 3 position switch, if I could utilize the lower voltages... seems like it'd work better than a pentode triode switch...
 
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Vanyu

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738
Found the issue, yet again! Both HT taps carry HV, whereas apparently on the stock PT's one was HV and other was a 0v. Good thing this amp already has a dual standby switch on it already...

One more question... The amp is no longer building up plate voltage in standby, however, it is clinging to it in standby and is very slow to drain. Does this sound normal?

EDIT: Is the voltage supposed to be dissipated through the power tubes? Everything I've done thus far has been with no tubes installed
 
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pdf64

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7,364
What's happening to the bias supply voltage in standby mode? eg increasing?
There's no HT - 0V resistors to discharge the caps, so perhaps if the power tubes are getting shut off in standby then there's only a few mA being drawn by the 12AX7s to drain them.
 

Vanyu

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In standby, I've got -1.8v. Out of standby, -51.5v with the bias pot centered. Back in standby, goes back to -1.8v.

Do you suppose I'm in good shape to load it up with tubes and attempt to bias it?
 
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pdf64

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7,364
Ah, right, with no tubes in place then there's nothing to drain the HT caps in standby, so the HT voltage would stay high.

I don't like that the bias supply collapses in standby; when standby is flipped to operate mode, the HT voltage will rise pretty much instantly, whereas the bias supply has cascaded time constants to charge up, during which time the tubes will overdissipate heavily.
Can you investigate why it's losing bias in standby, eg is C15 still connected to the hot side of the standby switch?
 

Vanyu

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738
The bias circuit is connected to a HT tap before the standby switch. This was how it came to me, and it's how I put it back. I'm wondering if all of this is simply because there's no tubes installed, maybe the tubes complete a circuit responsible for proper bias voltage, HT voltage, etc.
 

Vanyu

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738
I bit the bullet and threw some tubes in there, and was greeted with really freakin high current. I'm gonna go through and replace C12 and see if that helps, I've heard this capacitor often fails and results in bias drift. I may replace R28 while I'm at it, depending on how the C12 swap goes.
 




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