Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leftu2 on April 17, 2024, 02:54:49 pm
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I was playing the amp, it was working perfectly, and started losing volume slowly and power tubes lost power, no volume output.
The preamp tubes still light up. I'm wondering if part of the power transformer is bad for the heaters??
-all fuses check out good, slowblows register under 5 ohm
-the voltage at the yellow wires (they come straight from the transformer) at connector J7 on the power supply board, register 27.52v on both sides (schematic says these should be 6.3v) and I get that voltage reading through all the J-connectors all the way to pins 2 & 7 at all the power tubes.
- the B+ voltage is 492v to all the power tubes
I'm ruling out a power tube issue as I cant believe all the power tubes went bad at once, I did try swapping them around but no change.
Here's the schematic:
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Check ribbon cable and connections between preamp and power amp board?
The 27V at the heaters seems alarming. How are you measuring that? AC, or DC voltage?
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Ok, sorta scratch that.
Your heaters are elevated, and (I believe) should be at 28 voltish DC.
Your grids on your power tubes should be at 56 voltish DC.
Unplug all your tubes and take some measurements.
Do you have continuity between ground of the power amp board and ground to the pre amp board?
Measuring DC to ground at pin 5 of your power tubes, are you measuring 56 volts?
Measuring DC to ground at pin 2 of your power tubes are you measuring 28 volts?
How about at your preamp tubes at pin 9 in DC voltage to ground?
Measuring AC between pins 2 and 7 of your power tubes are you measuring 6.3 volts?
How about your preamp tubes between pin 4 OR pin 5 to pin 9 in AC voltage?
Seems like a ribbon cable thing, but until you can make sure all connections are good, (mainly grid voltage), you don't want tubes plugged in.
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Check ribbon cable and connections between preamp and power amp board?
The 27V at the heaters seems alarming. How are you measuring that? AC, or DC voltage?
lol...I was measuring DC by mistake, thanks for pointing that out. I'm getting 6.3v at one of the yellow wires from the transformer. When measuring the other one (it goes to the 8 amp fuse at F4), the meter is all over the place and won't settle on a reading, that's happening at the 2 & 7 power tube pins as well. I also get 6.3v at pin 1 on J4 but not pin 2 which is the other 6.3 heater fill.
I'm also getting continuity between both yellow wires at J7 and at both heater circuits at J4. If I pull the J7. If I pull the J7 connector the continuity goes away does that mean there's an issue with the transformer?
I'll check all the stuff you said to check and let you know....
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Sounds like a bad trace somewhere. You'll have to use your R-meter to check for DC-continuity to all heater pins
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I doubt it's your transformer, because your pre amp tubes still light up. Continuity over that winding doesn't necessarily equate to a short. Measure it with your meter set to ohms, and it will likely show some resistance.
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Sounds like a bad trace somewhere.
.....and that what it was.
The plastic connector that feeds the heater voltage from the pre amp board to the pwr amp board burnt up a while back, as they all do it eventually. I cut the burnt part of the connector off and used some single connectors and wire to repair it. When it burnt up, it burnt the pcb trace at the pin on the pre amp board side on pin 8 of the connector. I tried to use some copper tape and re-soldered the pin. Before I posted on here, I checked to make sure I had continuity between both sides of the 2 wires i repaired and I had it, so I thought the connections were still good, nope, the repair on the red wire at the pre amp board was the culprit. I learned continuity doesn't necessarily mean a good connection.
What's the best way to repair the burnt trace other than what Ive done? I definitely don't want to create more issues.
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There’s no ‘best way’ to repair a trace IMO. I’ve repaired a fair few and every time you repair one, you unleash a little more entropy into the universe. If you jump the trace with a wire, you’ll have to completely cut the old trace so it doesn’t become an antenna- even then, if you leave the cut trace on the board, it still functions as an inductor mucking up the mojo of adjacent signal traces. When you try to repair the trace to keep it alive by scraping back the insulation to expose a bare strip of copper to solder a jumper to, you stress the trace and the board. Etc etc
Speaking of entropy, and regarding the use of slide-on terminal connectors, eventually a layer of oxidation builds up on each surface between the connector components, which introduces resistance. Now if the connectors are for heaters, this resistance can trigger more entropy because it makes heat (and the power in heater circuits makes quite a bit of heat - which over time can lead to fire ) you need to periodically remove the oxidation, or better still, remove the slide/on bit and solder the wire directly to the terminal (which fixes it for good. But you have to do that before the heat destroys the trace).
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When you try to repair the trace to keep it alive by scraping back Speaking of entropy, and regarding the use of slide-on terminal connectors, eventually a layer of oxidation builds up on each surface between the connector components, which introduces resistance. Now if the connectors are for heaters, this resistance can trigger more entropy because it makes heat (and the power in heater circuits makes quite a bit of heat - which over time can lead to fire ) you need to periodically remove the oxidation, or better still, remove the slide/on bit and solder the wire directly to the terminal (which fixes it for good. But you have to do that before the heat destroys the trace).
Thanks, I'll do that now. I was just trying to keep things plug-n-play.
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+1 with Tubeswell
or better still, remove the slide/on bit and solder the wire directly to the terminal (which fixes it for good. But you have to do that before the heat destroys the trace).
That's exactly what I do. :icon_biggrin:
Turns an unreliable Peavey into something more manageable. This was especially useful for EVH5150, and atrocious heater wiring design.
Can I say that Leo did really good with heaters :laugh:
Regards
Mirek