thanks Guys, my voltages are around 264 vDC and I've got 191v DC going to the valve pin 1.
Everything works as expected but I have a terrible hum though, seems to be on the output as its affected by the output level pot.
I've swapped valves and checked the shielded cable runs. they're all connected at one end only and I tried pulling the shield from the out jack all together but it made no difference as does chop sticking the cables etc
I have some of my shields going to the bus bar rather than the pots, could that make any difference?
Is it possible it's the caps making such a bad hum?
Marcus, a couple things. The output level is the output for the entire unit so your hum could be anywhere in the circuit. Is that chassis painted black? In which case running your coaxial shields to the back of the pots does not ground them. Brandon(Bludotone) did a Dumbleator post a long time ago and he talked about how he ties the back of each pot to the buss bar and runs the coaxial shield drains to the buss bar. You may do a search and find that post. Finally, and I don't want to sound like a broken record but those filter caps can not be trusted at this point.
CW
Charlie, I have all my shields going to the buss bar and the earth legs of the pots tied to it as well. The earth points are scraped clear of paint so I'm pinning my hopes that the hum is coming from the bad caps as I can't think of what to try next.
they should be here tomorrow so I' replace those then see how things stand.
Markus. What does you PT secondary voltage read ?
The PT I had made is 138.7 VAC no load to achieve 255v on pin 1
I used a 12k 3 watt and a 56k 3 watt replacing the 15k and 100k
I don't recall what I changed the 100k on pin 6 to get 235v.
I'm sure each PT is different. And you want 30v on pin3.
One of my most memorable negative voltage experiences was on my first rewiring for a donor amp, I put the bias cap in like every other cap with the negative to ground.
They are loud when they explode. Covered me in white powder and little bits of paper like a cheap firecracker. My wife was sure I'd shot something. Good thing I was wearing reading glasses.
Hello, while we are on the subject of Dumbleators. Does anyone know what the orange drop cap sitting all by itself is. Does this unit have a ground switch?
CW
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Charlie Wilson wrote:Hello, while we are on the subject of Dumbleators. Does anyone know what the orange drop cap sitting all by itself is. Does this unit have a ground switch?
CW
Death cap.
The 1-N-2 ground switch is the one located furthest to the right on the front panel of the chassis.
Structo wrote:Yep, we used to blow up caps by connecting them to a extension cord from the wall.
Of course I had a switch on it so we could be a safe distance away.....
BOOOOM!
Structo,
Were you in my class at electronics school in the early 80's?
Did the exact same thing! Rigged a bunch of small e-caps to extension cords plugged into bench power strips in the lab classroom. Instructor comes in and turns on the power and....
norburybrook wrote:OK, this is really embarrassing ...
the reason I had negative voltages was................
I HAD THE LEADS FROM MY MULTI METER THE WRONG WAY ROUND
I'll get my coat....................
Marcus
Don't worry, you will never forget about this lesson... And you are a BIG man to admit something like that got you. Besides, you had it put together correctly, right?
Tony
"Education is what you're left with after you have forgotten what you have learned" - Enzo
