Bright channel issues

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FuzzFaceBuilder
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Bright channel issues

Post by FuzzFaceBuilder »

I have a 50watt shared cathode Bass head that the bright channel isn't working on. I'm on the road away from the amp until next week, but, it's been bothering me. Everything works except for the bright channel. EQ, GFB circuit, etc.

On the tube side of the V1b plate resistor, I'm getting 0V on my fluke. Yet on pin 6 I'm getting 290v!

Any thoughts on this?
Last edited by FuzzFaceBuilder on Sat Jun 24, 2017 6:37 am, edited 1 time in total.
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pompeiisneaks
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Re: Bright channel issues

Post by pompeiisneaks »

By dead short do you mean that with one probe to ground and the other at the lead of the anode resistor you're getting continuity? That sounds like the but has shorted somehow internally. Is there a schematic? I'm just not visualizing in my head the difference between 'the tube side of the V1b plate resistor' and 'on pin 6' because pin 6 is the anode and the 'tube side' of that resistor should connect directly to pin 6 on the tube no? i.e. there's no difference between those two locations?

Or are you saying on the board on the lead of the resistor before it goes over to the tube you get the voltage but you don't get it on the pin? That means to me that either the connection between that resistor and the tube is open, or that means the tube isn't conducting anything so theres no voltage dropping over the resistor I'd guess? Either the cathode isn't able to conduct for some reason (Cathode resistor has shorted dead?) or the tube's gone out?
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romberg
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Re: Bright channel issues

Post by romberg »

Resistors almost always fail in an open condition (resistance approaches infinity). But I've read that they can (rarely) fail in a short (approach zero resistance). It is a long shot but it would explain everything mentioned:

1) The resistor measures a "dead short", It is not clear exactly what this means. A failed short resistor would measure zero resistance or a short.

2) The measured voltage on the plate is very close to what one would expect at the rail 290v. Zero resistance would drop zero volts. The only way to measure 290v on the plate is for either the tube to be conducting nothing (mentioned above) or for it to be conducting but have no dropping/plate resistor.

3) Channel passes no sound. If the plate resistor is zero then no voltage will occur across it no matter how much the tube conducts. So, no AC/signal ever leaves that stage.

But this is a long shot. Chances are that it is something more expected. I'd first swap V1 for a good known working tube. I'd also measure both the plate resistor and the cathode resistor and see if they are both within spec. The volume pot for that channel (along with the mixing resistors) would be easy to check as well.

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Re: Bright channel issues

Post by FuzzFaceBuilder »

Excuse the confusion. I'm getting 0v reading at the circled turret and 290 at pin 6 of V1.
1986 V1b plate.png
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sluckey
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Re: Bright channel issues

Post by sluckey »

I bet your probe did not make a good connection on the turret.
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Re: Bright channel issues

Post by FuzzFaceBuilder »

Changed V1 tube and still measured 0V, replaced Rp and reflowed all solder in the area. Still no dice. Hmmmm...
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Re: Bright channel issues

Post by sluckey »

FuzzFaceBuilder wrote: Sat Jun 24, 2017 11:10 am Changed V1 tube and still measured 0V, replaced Rp and reflowed all solder in the area. Still no dice. Hmmmm...
The voltage originates on the board. If you have voltage directly on pin 6 then there must be voltage on that turret. There's only a 2" blue wire between that turret and pin 6.
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pompeiisneaks
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Re: Bright channel issues

Post by pompeiisneaks »

yeah the only way what you're seeing is possible is one of three scenarios:
1. you're not pushing through the built up crust on the turret to get a good connection
2. Somehow the power is coming from the other half of the tube, which would mean a bad tube
3. Somehow you've accidentally jumpered something underneath to the wire and that wire isn't touching the turret you're testing on?

That's about all i can think of. The normal test of voltage at that pin means it has to come from the connected turret through the anode resistor on the board.

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Re: Bright channel issues

Post by FuzzFaceBuilder »

Finally arrived at home and pulled the chassis. Triple checked the Rp voltage and reflowed the area. RE-installed the original tube and reflowed the input jacks. Issue resolved! What a moron. Should've thought to do that first! Thanks for the help guys.
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Re: Bright channel issues

Post by pompeiisneaks »

Outstanding, glad that did it

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