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heathkit random pop/thump

L

leland331

Active Member
hey, I already have another older thread somewhat describing my issue with the amp but it has changed and I figured that I would make a new thread.

my problem is a random popping and thumping from both speakers when the amp warms up for at least a minute.

I have tried cleaning all the tube sockets and tried all new tubes, checked the wiring many times. the amp still pops and thumps with the 6u8/6an8 out so this leads me to believe it is a power supply or output issue. does anyone have any ideas?
 
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Are the power supply caps fresh? Old ones or bad ones can cause these noises. Bad rectifiers can do this too if they arc internally. It does sound like power supply though since its common to both channels. What amp are we talking about here?
 
Sounds like it's time to pull the bottom off and do some poking around. Sometimes a cold joint will "make" after it warms up (expansion and all). You could use a plastic probe in and around the power supply and output sections, or just re-flow all the joints if that hasn't been done yet.
 
Okay I will get right on that when I can today. How likely would it be that it's a bad resistor? And would it be a good idea or a waste of money to replace all the resistors?
 
Personally speaking, as cheap as good tight tolerance metal films are, I would do the PS and output section for sure. I would also split the cathode resistor into one for each channel instead of one for both channels. It will be a 5 watt cast resistor going between ground and the cathodes of the el84s. I just double the value and go from there after checking tube current. You can also up the cathode R bypass cap at this time by using one for each of the new resistors. I go big, about 470uf x 25v.
 
Okay I will do that as well as I have noticed some odd voltages at the 6u8s. One more thing, I replaced the can caps with axials up top of the cassis and I had to tie two together to get to 50mfd and I had to run a wire from them to the resistor in the power supply. Essentially, my capacitors leads are longer being extended by a wire. Will this setup introduce a lot of ripple?
 
Actually I had plans to do the separate cathode bias as well. Thank you for explaining how to go about that as I had trouble finding info on that
 
leland331 said:
Okay I will do that as well as I have noticed some odd voltages at the 6u8s. One more thing, I replaced the can caps with axials up top of the cassis and I had to tie two together to get to 50mfd and I had to run a wire from them to the resistor in the power supply. Essentially, my capacitors leads are longer being extended by a wire. Will this setup introduce a lot of ripple?
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No, that shouldn't be an issue as long as the wire isn't close to a preamp tube, and even then, I'd doubt it. The ripple (or ac riding the DC) will be the same on both ends of the wire. You can set a digital volt meter to AC and measure your ripple voltage at each end just as an experiment. If it's the first filter stage you will likely see a volt or two of AC.
 
Thanks, I'll do that. I'm interested to see how much there actually is. Later today I will warm the amp up and poke around in it.
 
And thanks a lot for that page. It'll be useful. Also, would it be even better to use a cathode resistor for every tube instead of one for each pair?
 
leland331 said:
And thanks a lot for that page. It'll be useful. Also, would it be even better to use a cathode resistor for every tube instead of one for each pair?
Click to expand...

I have done that, and then put in meter ports from the hot side of the cathode R to an easy place to read the voltage between the port and ground. That way you can pretty much match the tubes in circuit, where it really matters. Some say you need the two cathodes to reference each other, so if you do singles you need to put in a non polar cap between the pair's cathodes. I never have, and haven't had any instability issues. :dunno: Two watt metal films work well for individuals, just multiply your single times 4 for a starting point.
 
I checked my bias and got 33ma and 12w dissipation. reading the bias chart using my plate voltage of 366, it shows that I am way above hot and should be running about 20 not 33.
 
leland331 said:
I checked my bias and got 33ma and 12w dissipation. reading the bias chart using my plate voltage of 366, it shows that I am way above hot and should be running about 20 not 33.
Click to expand...

We can most likely attribute this to two things; todays higher wall voltage (what's yours), and the fact that people who sold amps also sold tubes. :D
366 is higher than I would run a regular 6BQ5, but that's just an opinion. IIRC the book says 300 max, but most everyone blew that spec. off. I'm going to re-read this thread.
 
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