Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: suncrest on April 11, 2020, 06:30:33 pm
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Hi forum folk. First time caller, long time listener. Thanks in advance for any suggestions.
During this downtime, I built a version of Rob Robinette's Deluxe Micro using a bunch of old parts and a few new ones.
Fired it up today, and it's an awesome little amp.
*But during a post-trial voltage check, I metered between the heater circuit and the chassis, and saw about 150VAC....didn't check this during initial start up for some reason...
Not sure where this could be coming from, so I'm looking for some ideas.
When I measure across the heaters or pilot light, I see +/- 6V, as I would expect.
There are some deviations from Rob's posted schematic, but overall it's pretty close.
What's different:
no-name scrap heap PT
6X4 rectifier instead of EZ81
10uF filter caps
1R0 5W dropper in heater string
I bench checked the PT for shorts, then checked unloaded secondary voltages before i installed it.
They were:
Red/RW/Red: 325-0-325V
Blue/BW/Blue: 28-0-28V capped off
Green/Green: 7.3V
Yellow/Yellow: 7.8V capped off
No idea what current capacity it has, but the wiring and build are fairly stout. I hope/think it can handle this circuit.....PT warm after 30 min trial run, but not excessive heat by any means.
I don't do drawing programs, so I am attaching a cocktail napkin schematic of the power plant as built, can add pictures if anyone is willing to take a look.
So, to summarize my problem:
Any ideas what this 150VAC between heater circuit and chassis means?
Does it even matter?
I inspected every connection I made, nothing is shorted as far as I can tell.
The current draw through my Sencore unit is about 30mA at idle, creeps up a bit when the amp is pushed.
Here are the tube voltages at idle with 8R dummy load, no input signal @125VAc in:
6X4: pin 1 and 6: 327VAC, pin 7: 390VDC
12AU7: pin1,6: 337V, pin 3,8: 13.5V
12AX7: pin 1: 149V pin 3: 2.3V pin 6: 157V pin 8: 2.24V
Thanks again for any thoughts. :smiley:
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and saw about 150VAC
only stabbing here;
re-measure to the ground wire from wall, at the recept EIA connector back if SAFE
or at least OHM from there to chassis, then you'll know ground is ground
I've never built an amp without a CT or fake CT, so I've never seen more than 3.~2v to ground.
one problem with your issue tubes have a spec for Vdif between cathode n filament
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Your filament winding is floating and has no reference to chassis. What you see is a phantom voltage. Don't worry about it. It has no current capacity. However, for hum reduction, you need to have a reference to chassis. Do this by connecting a 100Ω resistor from each side of the filament winding to chassis. That's two resistors. Now when you measure filament voltage in respect to chassis you will measure about half the voltage you measure between the two filament wires.
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Added 2 100R, problem solved. Changed out 1R for .51R also, now 6.3V between legs, 3.15V to chassis.
Thanks shooter and sluckey! :worthy1:
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And 100 ohms resistor make the amp more quiet.