Dumbleator build, help with strange voltages

[martin manning]

I think something is loading down the power supply, and that may be the reason for the hum. You should have B+1 about 360VDC with 130VAC input.

[norburybrook]

Ok, I've disconnected the second board and now with just the power board I've got 320v and 280v B+


Does that give any clues?


Marcus

[Charlie Wilson]

Hello Marcus, don't drive yourself crazy right now with coaxial shields and such. You will have plenty of time for that later. It is a little coincidental that you have a hum and your voltages are a little low and I agree with Martin, it could be something in the power supply that is shorting or grounding out. You may have to have a look under the board and make sure a component lead isn't touching something it shouldn't. Also take a magnifying glass and stare at all of your solder joints. I like the divide and conquer approach to finding a problem. You could remove your dropping resistors and starting with your main filters reinstall them one at a time while checking the voltages. You just need to have an idea of what voltages you should be seeing. Like your multimeter leads, it could be a problem you have been staring at but just haven't seen yet.
CW

[norburybrook]

thanks Charlie,

at least by re doing all the earth's at least I've removed that from the list of possible things

So are you saying until I see B+ 1 around 360v there is a problem?


Marcus

[Charlie Wilson]

Marcus, voltages do seem a bit low. 130v x 2 = 260v x 1.41 = 366v
CW

[martin manning]

Lift one end of the 15k resistor following the stacked caps and see what voltage you get.

[norburybrook]

I lifted one end of the 15k cap and had strange voltages which got me thinking about voltage doubling circuits and how they work which I looked into. Very ingenious indeed. this new knowledge then led me to think perhaps I was only getting a voltage from one of the caps, which when I checked was indeed the case , the caps weren't connected.

So a jumper lead added and I'm getting 357v dc now before the 15k resistor and about 320 after. However the schematic shows 368v at this point so I'm still 40v down. I'm still only getting 24 v DC at pin 3 too which is also lower than expected.


anyway to cut a long story short it's working better now but there's still hum/noise.


Marcus

[martin manning]

Get the voltage at the positive terminal on both radial caps. You should be seeing 360-ish on one, and half of that on the other. The 191V only makes sense if you measured in the middle of the stack (so to speak). This is an attempt to isolate the voltage doubler part of the curcuit.

[Charlie Wilson]

Well that 191v is half way there.
CW

[martin manning]

Ok good. 37V over 15k is 2.4mA, which sounds about right for the current draw from both triodes. I'd cut the 15k down to say 3k3 to get the voltage up, and then start looking for the source of the noise and hum. You can easily tell if it's from the send or return stage or both by putting them in the loop individually.

[norburybrook]

Thanks Martin, I'll order a 3k3 resistor tomorrow if I can't find one in my growing box of resistors.

The hum is certainly a lot less now and the extra voltage maybe the final step.


Once again this has been a good learning curve.


Marcus

[martin manning]

The 3k3 value is not too critical. Anything close (2k2-4k7) would be okay to try.

[norburybrook]

Ok I found a 4.7k 2w resistor and dropped that in my voltages are now ball park.


B+1 355 v DC
B+2 260 v DC

V1 pin 3 26.9 v DC



It's still noisy though, way to noisy for a studio amp. There's a background earth hum which disappears if I touch the dumbleator case, but there's a louder general buzz still that's not good.

If I disconnect the return lead it goes quiet, disconnecting the send makes no difference.

I've lifted the return ground and it makes no difference.

if I run my FX without the Dumbleator the loop is silent, plug in the dumbleator and it's noisy.


Any more ideas?

Marcus

[Structo]

I know this is going back, but are you sure the two 100R filament resistors are grounded?

[martin manning]

Make sure that all of the grounds are making good contact with the chassis. Scrape or sand off the black coating, and this includes the pot bushings. The pot cases need to be grounded so that they will act as shields.